s u s t e l n e t - ecn - your energy. our passion. · s u s t e l n e t policy and ... sent to the...

S U S T E L N E T

Policy and Regulatory Roadmap for the Integration of Distributed Gen-eration and the Development of Sustainable Electricity Networks

REVIEW of

TECHNICAL OPTIONS AND CONSTRAINTS for

INTEGRATION OF DISTRIBUTED GENERATION in

ELECTRICITY NETWORKS

Søren Varming Christian Gaardestrup

John Eli Nielsen

Final Version 1.1 2

ACKNOWLEDGEMENT Support of the European Commission The SUSTELNET project is supported by the European Commission under the 5th RTD Framework Programme within the thematic programme ‘Energy, Environment and Sustainable Development’ under the contract No. ENK5-CT2001-00577. This document or any other document produced within the SUSTELNET project does not represent the opinion of the European Commission. Neither the European Commission, nor any person acting on behalf of the Commission, is responsible for the use that might be made of the information arising from the SUSTELNET project. SUSTELNET project partners in EU Member States: • Energy research Centre of the Netherlands (ECN), Petten, The Netherlands (co-ordinator) • University of Warwick, Coventry, UK • Öko-Institut, Freiburg, Germany • Fondazione Eni- Enerico Mattei (FEEM), Milano, Italy • Tech-Wise A/S, Fredericia, Denmark • Institut für ZukunftsEnergieSysteme (IZES), Saarbrücken, Germany SUSTELNET project partners in Newly Associated States: • Enviros, Prague, Czech Republic • The Polish National Energy Conservation Agency (KAPE), Warsaw, Poland • Hungarian Environmental Economics Centre (MAKK), Budapest, Hungary • EGU Power Research Institute, Bratislava, Slovakia Other organisations contributing to the SUSTELNET project: • Eindhoven University of Technology / Foundation for the History of Technology, Eindhoven,

The Netherlands • Eltra, Fredericia, Denmark For further information: • www.sustelnet.net • Mr. Martin J.J. Scheepers

Energy research Centre of the Netherlands (ECN) Phone: + 31 224 564436 E-mail: [email protected]

ABSTRACT For many years the electric power industry has been driven by the paradigm that most of the electricity should be generated in big power plants, sent to the consumption areas through EHV transmission lines, and delivered to the consumers through a passive distribution infrastructure that involves HV, MV and LV networks. In this paradigm, power flows only in one direction: from the power station to the network and to the consumers. This paradigm is about to change due to a large scale integration of dispersed generators (DG) on the MV and LV networks. The traditional large power plants are now challenged by these generators whose growth will certainly have consequences on the design of future power systems. It is recognized that the distribution networks no longer can be considered as passive appendages to the transmission network, but that the entire network must be designed and operated as a closely integrated unit. This requires for example a more active role of the distribution network operators (DNOs). Typically the distributed generators are non-dispatched and power from wind and photo voltaic genera-tors are even intermittent. The handling of imbalances, between production and consumption in a net-work with a high degree on these generators is analyzed. In this report the general characteristics of different generation technologies are reviewed together with a technical analysis of the connection of these technologies to the network. Several main impacts is also identified in the design and operation of the HV and LV networks with a large amount of dispersed generation and the report gives an overview of the technical options and con-straints for the integration of distributed generation in electricity networks.

Final Version 1.1 3

CONTENTS PREFACE EXECUTIVE SUMMARY 1. INTRODUCTION.................................................................................................................................................... 8

1.1 References ........................................................................................................................................... 12

2. GENERAL CHARACTERISTICS OF DIFFERENT GENERATION TECHNOLOGIES........................................ 13 2.1 Introduction........................................................................................................................................... 13 2.2 The traditional generator ...................................................................................................................... 14 2.3 Combined Heat and Power units (CHP)............................................................................................... 23 2.4 Reciprocating engines .......................................................................................................................... 26 2.5 Wind Energy Converters (WEC)........................................................................................................... 31 2.6 Hydro Power (HP) ................................................................................................................................ 41 2.7 Photovoltaic (PV).................................................................................................................................. 43 2.8 Micro-Turbines...................................................................................................................................... 50 2.9 Fuel Cells.............................................................................................................................................. 55 2.10 Stirling engine....................................................................................................................................... 59 2.11 Conclusion............................................................................................................................................ 62

3. INTERACTION WITH THE ELECTRICITY NETWORK....................................................................................... 64 3.1 Introduction........................................................................................................................................... 64 3.2 Power quality ........................................................................................................................................ 64 3.3 Protection ............................................................................................................................................. 68 3.4 Network stability ................................................................................................................................... 69 3.5 Synchronising, switching operations and soft starting.......................................................................... 70 3.6 Reserve ................................................................................................................................................ 71 3.7 Connection of DG generators to distribution network........................................................................... 71 3.8 Costs of grid Connection ...................................................................................................................... 72 3.9 Comment to network connection rules ................................................................................................. 73 3.10 Conclusion............................................................................................................................................ 74 3.11 References ........................................................................................................................................... 75

4. LARGE SCALE INTEGRATION OF DISTRIBUTED GENERATORS.................................................................. 78 4.1 Introduction........................................................................................................................................... 78 4.2 The Eltra system................................................................................................................................... 78 4.3 Priority Production ................................................................................................................................ 80 4.4 "Smooting" the green electricity............................................................................................................ 80 4.5 System imbalances .............................................................................................................................. 81 4.6 Increased risk of interruption ................................................................................................................ 82 4.7 Mobilising local resources .................................................................................................................... 82 4.8 A framework for new control structure.................................................................................................. 84 4.9 Conclusion............................................................................................................................................ 85 4.10 References ........................................................................................................................................... 86

5. A REDESIGN OF THE GRID ............................................................................................................................... 87 5.1 Introduction........................................................................................................................................... 87 5.2 The Grid and the Net ............................................................................................................................ 87 5.3 Active Networks.................................................................................................................................... 90 5.4 Micro-Grids Based Power Systems...................................................................................................... 96 5.5 References ........................................................................................................................................... 97

6. ACRONYMS......................................................................................................................................................... 98

Final Version 1.1 4

PREFACE Technological developments and EU targets for penetration of renewable energy sources (RES) and greenhouse gas (GHG) reduction are decentralising the electricity infrastructure and services. Although liberalisation and internationalisation of the European electricity market have resulted in efforts to harmonise transmission pricing and regulation, no initiative exists to consider the opening up and regulation of distribution networks to ensure effective participation of RES and distributed generation (DG) in the internal market. The SUSTELNET research project provides the analytical background and organisational foundation for a regulatory proc-ess that satisfies this need. Within the SUSTELNET research project, a consortium of 10 research organisations analysed the technical, socio-economic and institutional dynamics of the European electricity system and markets. This has increased the understanding of the structure of the current European electricity sector and its socio-economic and institutional environment. The underlying patterns thus identified have provided the boundary conditions and levers for policy development to reach long term RES and GHG targets (2020-2030 time frame). It was consequently analysed what regulatory actions are needed on the short-to-medium term to reach the existing medium-term goals for 2010 as well as likely scenarios for longer-term goals. Regulatory Road Maps The main objective of the SUSTELNET project was to develop regulatory road maps for the transition to an electricity market and network structure that creates a level playing field be-tween centralised and decentralised generation and network development. Furthermore, the regulatory road maps will facilitate the integration of RES, within the framework of the liberali-sation of the EU electricity market. Participatory Process To deliver a fully operational road map, a participatory regulatory process was initiated throughout this project. This process brought together electricity regulators and policy makers, distribution and supply companies, as well as representatives from other relevant institutions. This ensured a good connection with current industry, regulatory and policy practice, created involvement of the relevant actors and thereby will enhance the feasibility of implementation. Newly Associated States The SUSTELNET project also anticipated on the enlargement of the EU by providing support to the Newly Associated States (NAS) with the preparation of a regulatory framework and thus also with the implementation of EU Directives on energy liberalisation and renewable energy in four Accession Countries (The Czech Republic, Poland, Hungary and Slovakia) Project Structure The SUSTELNET project was divided into two phases. During the first phase, the analytical phase, three background studies were produced: • Long- term dynamics of electricity systems in the European Union. • Review of the current electricity policy and regulation in the European Union and in Mem-

ber States. • Review of technical options and constraints for the integration of distributed generation in

electricity networks. In the second phase, the participatory regulatory process phase two activities took place, dur-ing which there were extensive interactions with regulators, utilities, policy makers and other relevant actors: • Development of a normative framework: criteria for, and benchmark of distribution network

regulation. • Development of policy and regulatory road maps. This report was produced during the analytical phase of the project and was part of the review of technical options and constraints for the integration of distributed generation in electrical networks.

Final Version 1.1 5

EXECUTIVE SUMMARY For many years the electric power industry has been driven by a paradigm where most of the electricity was generated in big power plants, sent to the consumption areas through EHV transmission lines, and delivered to the consumers through a passive distribution infrastructure that involves HV, MV and LV networks. In this system, power flows were only in one direction. From the power station to the network and to the consumers:

In the conventional paradigm the generators produce both energy and all ancillary services, like voltage control, frequency control and black start capacity. This situation is described in Chapter 2.2 as the baseline for the following analysis. This paradigm is about to change due to a large-scale integration of distributed generation (DG) at either the MV or at the LV levels. Electricity is now going to be produced closer to the consumers:

In the last years this change became more perceptible mainly due to the connection of a lot of generation sources at the MV level. These sources have been mainly:

• Combined heat and power plants (CHP) • Wind Energy Converters (WEC) • Hydro Power stations (HP) • Photo Voltaic systems (PV)

Transmission network

Distribution network

Consumers

Generator

Generator

Generator



Consumers

Generator

Final Version 1.1 6

From the starting point the different sources of DG has different properties regarding regulation of energy production and the supply of ancillary services. Some of the DG sources (Wind, PV and run-of-river hydro stations) are constrained in their regulating abilities due to the depend-ence on fluctuating natural phenomena. A review (Chapter 2) of the possibilities of the different technologies shows that with the proper design most technologies can deliver ancillary ser-vices. A review of the technical standards for connection of DG in the member states (Chapter 2 and 3) shows that the criteria used are very different. A large scope for standardization exists and standardization may at the same time make it easier to integrate DG in the network and at the same time improve the functioning of the internal market for DG-technologies. This new scenario demands the evaluation of the technical impacts in the networks. These impacts can be identified at the different levels of the system:

• transmission • distribution • consumer

On the other hand, the problems that affect the system operation as a result of a large integra-tion of DG can be analyzed from different perspective:

• steady state behavior • dynamic behavior • quality of service (reliability and power quality) • personal safety

This report describes the main impacts under this new scenario, taking into consideration the characteristics of the DG primary sources and the technologies of the energy converters used. As a case study for the large scale introduction of DG is the situation in the Western part of Denmark described in Chapter 4. In this area 1500 MW local CHP and 1900 MW of WEC is introduced in a system with a minimum demand of 1150 MW and a maximum demand of 3800 MW. This means that for large portions of the year will the DG be the main source of electric-ity. Positive and negative experiences are gained from this “experiment”. The most significant positive experience is that is has been technically possible with such a penetration of DG in a conventional grid. With the caveat, that strong international connections have been necessary to balance the system. From a network point of view the risks of serious network failures has increased. From an economic point of view an optimization of the system is also wanted. The key finding is the distribution network can no longer be seen as a passive appendage to the transmission system. In the future the DNO (whether as actual or virtual organizations), will have to use local resources in order to minimize the need for energy and ancillary services from the transmission system. By integrating the DG more systematically in the dispatch sys-tem the value of the DG will increase. A number of ideas have been presented and reviewed that can lead towards this goal. (Chapter 5) DG in large amounts will have major impacts on nearly all parts of the electrical energy supply system. The uncertainty of their power output and sometimes even the lack of knowledge about their installed capacity require strong but also more flexible networks up to EHV, where the need for power and reserve trading may increase. The substitution of energy and power from large units by DG affects network operation, re-serve requirements and procurement and aspects such as scheduling or frequency control. In distribution networks the additional installation of DG may increase the necessary network capacity and cause additional costs for planning. If dispatched their may be a positive influ-ence such as increasing the reliability under certain conditions.

Final Version 1.1 7

For integrating DG in an existing system at its best, set the competition between classical gen-eration and DG on an non discriminatory bases, followed by: • development of standard network connection procedure • development of standard network connection rules • development of tools that consider DG in planning process • development of tools that consider DG in operation • co-operation with DG-operators with regard to ancillary services Precondition in any case, is the exact knowledge about location and type of all DGs. Many open questions still remain. These questions all deserve attention, some at a technical level and others at a regulatory level.

Final Version 1.1 8

1. INTRODUCTION The current electricity networks are the result of technological and institutional development over many years [1]. The evolution of power systems began a century ago, when electrification was first introduced. Thomas Edison started the first power station in New York in 1882. In the early 1900 the same pattern could be seen in many city's. Local entrepreneurs, followed by municipal authorities, building their own generating stations and building an infrastructure to distribute the electricity across their city's. As the consumption grew in the 1930, issues of reserves and economy received more atten-tion. These drivers first led to interconnections between power stations within the municipal areas, and later also between neighbouring city's. In the distribution system these drivers in-fluenced the network structure. A first move away from the radial network structure was taken. But the cooperation with the neighbouring power stations was not enough. At this time it be-came possible to increase the unit size of a power station, thereby improving the fuel efficiency and reducing the cost per kW installed. So after being regionally interconnected it was evident to exploit the economics of scale, and as a consequence many of the smaller generators dis-appeared. As there was a need between city's for cooperation, then there also was a need for regional power systems to cooperate in the areas of building and operating power stations. This was the driver behind the transmission network. This meant that power could be transported over larger distances with minimal losses and power stations could be sited more economically. For many years now the electric power industry has been driven by this paradigm where most of the electricity is generated in big power plants, sent to the consumption areas through EHV transmission lines, and delivered to the consumers through a passive distribution infrastructure that involves HV, MV and LV networks. In this paradigm, power flows only in one direction: from the power station to the network and to the consumers.

Figure 1.1 Paradigm 1 The above paradigm is about to change due to a large scale integration of dispersed genera-tors (DG) coming back to either the MV or at the LV levels. Electricity is now again going to be produced closer to the consumers.



Consumers

Generator

Final Version 1.1 9

Figure 1.2 Paradigm 2 Several main impacts can be identified in the operation of a distribution system with a large amount of dispersed generation: • Voltage profiles change along the network, depending on the power produced by the gen-

eration units and on the consumption levels, leading to a behaviour different from the typi-cal one

• Voltage transients will appear as a result of connection and disconnecting of generators or even as a result of their operation

• Short-circuit levels increase • Losses changes as a function of the production and load levels • Congestion in system branches is a function of the production and load levels • Power quality and reliability may be affected • Utility protection need to be coordinated with the ones installed in the generator's side In solving these problems it is important to keep in mind, that the existing network design stan-dards and regulatory framework is based on paradigm 1. As many countries now believe that a market-driven power system will provide more services at a lower price, the next step is the evolution is deregulation. To ensure that the public service objectives are still met, the government now takes the place as the regulator of the power sys-tem. From this evolution it is evident, that to have a coordinated infrastructure, the following players must have a set of common guidelines: • Regulator (REG) • Transmission system Operator (TSO) • Distribution System Operator (DNO) Increased penetration of distributed generation will also require a more active role of distribu-tion companies in controlling network stability, optimising centralised and distributed dispatch, metering and billing, interconnection, etc. CIGRE and CIRED, two international organizations has investigated the impact of the increas-ing contributions of DG to the power systems and documented this in the reports [3] and [4]. These two reports are at present the most comprehensive reviews available on the subject of distributed generation (DG).



Consumers

Generator

Generator

Generator

Final Version 1.1 10

Table 1.1 Installed DG capacity in EU 1998 Source: CIGRE WG 37-23 [3] As there is no universally agreed definition of what constitutes embedded generation, dis-persed generation or distributed generation. Some care is therefore necessary in interpreting the rather inconsistent data in the two reports. Table 1.1 and Table 1.2 are based on these reports in trying to give an estimate for the installed DG capacity in EU 1998 and the future development in capacity.

Table 1.2 Installed DG capacity in EU 1998 Source: CIRED [4] It can be seen that Denmark and The Netherlands stands out for themselves. Danish energy policies have led to the building of approximately 1,932 MW wind turbines and of approxi-mately 1,560 MW distributed CHP generators in the western part of Denmark by the end of 2001. See Figure 1.3. This means that the generation from the CHP generators and the wind turbines can already exceed the system load during periods of low customer demand.

[1998 MW] [Future MW]

BE

DK

FR

DE

NL

NO

ESP

EU

target

CHP 1600

2000 7260 4100 300

16000

WEC 900 5000

2400 > 3600

150 9000

Other 350

Total DG 2500 7500

9660 4750

% DG/ Installed Capacity

10 20

37

< 5

35

40

1

9

[MW]

AU

BE

CZ

DK

FR

DE

GR

IT

NL

PO

ESP

UK

Diesel 214 977 610

CHP 70 1174 2000 435 2800 3 766 4736 3000 3732 3732

WEC 13 5 1450 8 1545 40 34 427 330

HP 616 97 450 3333 155 2159 37 2008 1500 1494

PV 1 17 5

Other 1938 250 904 252 80 421

Total DG 700 1938 1913 3450 1753 8599 43 3708 5208 5008 4000 5977

System Installed Capacity

14693

12150

114500

9859

70641

18981

33400

50311

68340

System Maximum Demand

11972

6400

68900

6705

43774

12000

23500

27251

56965

% DG/ Installed Capacity

4

13

28

3

1

5

28

15

8

9


Figure 1.3 Production capacity at each voltage level in the Eltra system (Denmark) The European Union has also an agenda to encourage the introduction of new renewable energy schemes. Table 1.3 is taken from a white paper issued by the European Commission in November 1997 [4]. This white paper gives a good insight into where a significant increase in the contribution to energy production might come from. The two main contenders here are wind energy and biomass. Energy source 1995 2010 [TWh] [TWh] Wind 4 80 Hydro 307 355 - large > 10 MW 270 300 - small < 10 MW 37 55 Solar voltaic 0.03 3 Biomass 22.5 230 Geothermal 3.5 7 Table 1.3 Estimated contribution of renewables to EU electrical energy supply [5] The rate of development of the different forms of DG varies. At present the fastest growing of the new renewable technologies is wind energy. See Table 1.3.

[MW] Germany 3 899 Denmark 1 761 Spain 1 131 UK 351 Total 8 349 Tabel 1.3 Installed wind turbine capacity in January 2000 Source: Windpower Monthly news magazine


Chapter 2 gives an overview of the general characteristics of different generation technologies. To give some kind of reference for this overview, the tasks solved by the traditional generator is analysed first:

• Power generation • Frequency control • Load following control • Voltage control • Availability • Supplying a fault current • Faults-ride-through capability

This part of the chapter can be skipped by those familiar with traditional generator technology. In chapter 3 the interaction with the network is analyzed. A review of the technical standards for connection of DG in EU member states shows for example that the criteria used are very different. A large scope for standardization exists and standardization may at the same time make it easier to integrate DG in the network and at the same time improve the functioning of the internal market for DG-technologies. In chapter 4, the large scale introduction of DG is the situation in the Western part of Denmark described and used as a case study. In this area 1500 MW local CHP and 1900 MW of WEC is introduced in a system with a minimum demand of 1150 MW and a maximum demand of 3800 MW. This means that for large portions of the year will the DG be the main source of electricity. Positive and negative experiences are gained from this “experiment”. The most significant positive experience is that is has been technically possible with such a penetration of DG in a conventional grid. With the caveat, that strong international connections have been necessary to balance the system. From a network point of view the risks of serious network failures has increased. From an economic point of view an optimization of the system is also wanted. 1.1 References [1] G. Verbong, E. van Vleuten, Foundation for the History of technology, Eindhoven Uni-

versity of Technology, "Long-term electricity systems dynamics", 2002, www.sustelnet.net

[2] CIGRE WORKING GROUP 37.23, "Impact of increasing contribution of dispersed gen-

eration on the power system", September 1998 [3] CIRED preliminary report of CIRED Working Group 04 : 'Dispersed generation', Issued

at the CIRED Conference in Nice, June, 1999 [4] Commission of the European Union, "Energy for the future renewable sources of en-

ergy", White paper for a community strategy and action plan, COM97, 599.


2. GENERAL CHARACTERISTICS OF DIFFERENT GENERATION TECHNOLOGIES

2.1 Introduction In an electrical power system, the traditional power generators have more functions than only generating electrical power. They can to a certain extent be overloaded, participate with the other generators in the frequency, voltage and unit outage control. The unit outage control takes care of the sudden trip of the largest unit, by ensuring that there is enough spinning re-serve activated. When faults occur, they can generate a fault current. This fault current can be used to distin-guish between a normal and a faulted situation and thereby helping to localize the faulted ele-ment. In case of a fault in the network they also can have the ability to stay connected to the network, while the fault is cleared. In a case of a system outage, they can have the ability to make a "Black start". These functions are normally specified for a given electrical network [1], [2] and [3], and some-times referred to as "ancillary services": • Power generation • Overload capacity • Short term reserve (spinning or standing) • Performance of frequency control • Provision of voltage control (reactive power) • Generation of a fault current • "Fault-Ride-Through" capability • "Black Start" capability (after a system outage)

Figure 2.1 Traditional power plant Source: Elsam This chapter gives an overview of the general characteristics of different generation technolo-gies. To give some kind of reference for this overview, the tasks solved by the traditional ther-mal generator is analysed first.


2.2 The traditional generator 2.2.1 Power generation The main function of a power plant is to generate electrical power or active power. In fossil fuel fired power plants, fossil fuels such as coal, oil and gas, are burnt and the energy is converted into electrical power. The energy is then used to generate mechanical power and a synchro-nous generator is used to convert the mechanical power into electrical power. In Figure 2.2, a schematic diagram of thermal power generation is shown.

Figure 2.2 Schematic overview of thermal power generation Full power output Output values from a power plant are defined in the connection point to the network. The con-nection point to the network is also the point, to which the accounting is attributed. Normally two output values are defined: • Full power output = 100 per cent continuous load • Net power output = Full power output + overload capacity The output value is normally referenced to the main fuel. Overload Capacity Overload means a load beyond the full power output and can only be used to a limited extent because of reduction in efficiency and/or increased lifetime consumption. Fossil-fired units should be prepared for overload capacities only to the extent that intrinsically available sources are present. For steam turbine units this could be the bypassing of high-pressure pre-heaters. The unit including auxiliary equipment should be designed to utilise these overload capacities up to 4 h/day and up to 500 h/year. Minimum Power Output To ensure operational flexibility the minimum power output of the plant should be as low as possible. As a practical guideline, the minimum power output should be 35% of full power out-put for coal-fired units and 20% of full power output for oil-fired units. In this operating point the units should be able to operate continuously and with a normal operational configuration.

Electricity Generator Steam Boiler

Coal

Gas

Oil


House Load Operation House load operation is the power plant operating with its own auxiliary supply as the load. A power plant is normally designed to start in the house load operation point or to change safely over to this operating point from other operation conditions. The power plant able stay in house load operation for at least 1 h. Start-up and shutdown The connection of a synchronous generator to the grid is done by means of an automatic syn-chronisation mechanism. In normal operation the power output can be driven close to zero before making a connection or a disconnection. This ensures that no extra voltage or current transients exist when the power plant is connected to or disconnected from the grid.

Figure 2.3 Performance requirements in relation to voltage and frequency Frequencies and Voltages The power plant must be able to maintain operation at frequencies that occur relatively often and with the least possible load reduction, see Figure 2.3. The power plant must also be con-structed so, that is does not trip for the frequency derivates that might appear in case of net-work faults. Generators The generator reactance must be as low as technically and economically possible in order to support the static stability and reactive power control. Generators with outputs below 500 MVA must have: • No-load short circuit ratio Kc (Saturated) of at least 0.5 • Direct axis transient reactance Xd' (saturated) of less than 0.35 Generators with outputs above 500 MVA may deviate from these values, the allowable limits being Kc => 0.42 and Xd' <= 0.42. Each generator must be capable of operating on the rated active power continuously at power factor down to at least 0.95, under-exitated, and 0.9 over-exitated. This must be possible in connection with voltage and frequency conditions as described elsewhere. However, at under-excited conditions normal grid voltage is applied instead of 90% voltage. PSS, Power System Stabiliser PSS must be included in each generator. The PSS must be tuned to improve the damping of the oscillations of generator and power system, especially the damping of low frequencies (0.2 … 1.0 Hz) inter-area oscillations.

47,5 49 5 0 51 52 53

Frequency [Hz]

Grid Voltage [%]

110

105

100

95

90

85

+- 0,3 Hz

1 h – small reduction

1 h – 10% power reduction

continuos operation

3 min island operation

30 min power reduction max 0% at 49 Hz 15% at 47.5 Hz

30 min – small reduction

5 sek - transient operation


Line Side Faults Thermal power units must be designed so that the turbine generator set can withstand the mechanical stresses associated with any kind of single-, two- and three-phase earth or short circuit fault occurring on the grid on the high voltage side of the step-up transformer. The fault can be assumed to be cleared within 250 ms. No damage, nor need for immediate stoppage for study of the possible consequences are allowed. The unit must also be designed so that it remains connected to the grid and continues its op-eration after isolation of line side fault within 250 ms. Large Voltage Disturbances The unit may be disconnected from the power system, if larger voltage variations or longer durations than those for which the unit has been designed occur, and must, in each case, be disconnected if the unit fall out-of-step. The unit and its auxiliary power system must be designed for such voltage variations that a safe changeover to house load operation can take place after disconnection from the network. 2.2.2 Power control If we look at the load curve in a magnifying glass, we notice that the load varies all the time due to independent switching of load at the consumers. As it is impossible to store electric energy, the actual production must be adjusted to the actual load second by second. Controllability An important characteristic of a power plant is therefore the controllability of the output power. The controllability of the power is determined by: • The power response rate and range • The degree of uncontrollable output power variations A perfect power plant can vary its production between zero and the nominal value at an infinity rate in both directions and the output power does not show any uncontrollable variations. There is no practical technology which makes it possible to change the generated power in-stantly and there always exists some probability of uncontrollable output power variations. Power Response Rate and Range - Coal Coal-fired units are normally designed for a power response rate of at least 4% of full power per minute. This power response rate of change is applicable to any range of 30% between 60% and 100% of full power according to the load schedule. This range may be restricted to 20% in certain cases. The power response rate may be limited to the maximum power re-sponse rate permissible for the turbines or the steam boilers in the range below 60% and above 90%. Power Response Rate and Range - Oil and Gas Oil-fired and gas-fired units are normally designed for a power response rate of at least 8% of full power per minute. The above power response rate of change is applicable to any range of 30% between 40% and 100% of full power according to the load schedule. The power re-sponse rate may be limited to the maximum power response rate permissible for the turbines or the steam boilers in the range below 40% and above 90%. Operational Mode The change of power of a thermal power unit at the rates and within the ranges specified, dur-ing normal control and during disturbances control, is normally activated as follows: • By the unit controller • By manual operation


The control of the active power is based either on the generator speed or the output power. Plain speed control is appropriate only in island operation while in the case of a grid with other generators a control with output MW based droop can be applied. Droop This means that for each generator a share of the required output power change (when the grid frequency is changed) is defined by the droop factor. According to the unit controller must have an adjustable frequency set point in the range from 49.9 Hz to 50.1 Hz. The set point resolution must be 5 mHz or better. The droop set point adjustable in the range from 2% to 8%. The normal operation is generally with setting in the range from 4% to 6%. Adjustable frequency dead band of the unit controller within the setting range of 0-50 mHz is acceptable. It must be possible to disengage this dead band. Power Step Change Limiter The units must also be equipped with adjustable devices for limiting the magnitude and rate of the power change, so that it will be possible to set these set points at any value from zero up to the maximum specified, both for normal condition and for disturbed condition. Normal Operation The frequency values defining normal operating condition is defined by the grid operator. The required power output during normal operation is the manually preset power output, modified by a frequency-sensing unit controller (or turbine governor). Disturbed Operation Power system disturbances are frequency or voltage disturbances and include in severe cases stability disturbances as well. The need for disturbance control is governed by frequency-sensing equipment. Instantaneous Power Response The demand from the power system is that the instantaneous power response must be avail-able within and after 30 seconds after a sudden frequency drop to 49,5 Hz. Half of that power response must be available within 5 seconds after the frequency drop. Power Step Change - Fossil Fuel Fossil-fired thermal units must be designed with an operating mode allowing an instantaneous step-change in output power of at least 5% of full output within the range of 50%-90% when requested. Half of that power must be available within 5 seconds after the frequency drop. Units without or with one re-heater must be designed in such a manner that this power step will be accommodated within 30 seconds. If a unit includes more than one re-heater, a further delay corresponding to the time constants of such additional re-heaters is acceptable. Subsequent Power response rate After the power step changes specified above, thermal power units must also be capable of accommodating a load change at the rates specified earlier. However, the total change in load may then be limited to the values specified. Spinning Disturbances reserves All units of the condensing type are made so that they at times can be used as spinning distur-bance reserves and then perform the above mentioned power variations, if serious distur-bances occur to the grid. Island Operation In case of very serious (and exceptional) disturbances, where the power system is separated into smaller grids, the units must also initially be capable of performing the above mentioned power changes (upwards and downwards), and then achieving stable operation and normal power control capability according to the specifications.


2.2.2.1 Frequency Control In an electrical power system, the total amount of generated power must equal the sum of the load and the system losses. An imbalance between generated power on one side and the sum of the consumption of the loads and the network losses on the other causes a frequency drift. The frequency drift results from a change in the rotational energy stored in the rotating mass of the generators. This change occurs because the rotating mass of the generators is used as an energy buffer that compensates for the imbalance between generation and load. A generation surplus leads to a frequency increase, a generation shortage to a frequency decrease. See Figure 2.4. The frequency drift continues as long as the balance between generation and consumption and losses is not restored. Therefore, power plants continuously monitor the frequency. As soon as the grid frequency starts to drift, they slightly change the generated power in order to stop the frequency drift. This approach is referred to as frequency control. The controller that changes the generated power depending on the actual frequency is called a governor. This is, however, only feasible when the imbalance between generation and load is relatively small. When the imbalance is large, the control range of the power plants that are on line is not large enough to restore the balance. Therefore, new power plants need to be put in operation or power plants must be stopped. It would also be possible to restore the balance by switching loads on or off. The units are designed so, if necessary, that they can participate in following the occasionally varying loads that cause frequency variations on the interconnected power system. This im-plies that the units must be capable of accommodating power changes without intervals by plus or minus 2% of full power output within periods of 30 seconds. The units must be capable of performing these changes within the ranges specified.

Figure 2.4 Keeping the balance = Keeping the frequency Source: Soeder, KTU


2.2.2.2 Load following The following of the steady continuous variation in the load during 24 hours, is named "Load following". The task here is to estimate the load for the following hour, day or week, and allo-cate the production units so, that there will be enough units on the network to cover the load and some reserves to cover errors in the load forecast and a forced trip of a unit. The units are normally designed so that they can be used for load following during certain pe-riods of the year, using the rates of load change specified previously. 2.2.3 Voltage Control For efficient and reliable operation of electrical networks, the control of voltage and reactive power should satisfy the following three objectives: • Voltages at the terminals of all equipment in the network are within acceptable limits • System stability is enhanced to maximise utilisation of the transmission system • The reactive power flow is minimised so as to reduce losses to a practical minimum The problem of maintaining voltages within the required limits is complicated by the fact that the power system supplies power to a vast number of loads and is fed from many generating units. As loads vary, the reactive power requirements of network vary. Reactive power transmission will cause voltage drops and losses, therefore reactive power is not transmitted over long distances. It is preferable that system operation should be organised in such a way that the balance of reactive power will be maintained as effectively as possible on a local basis. Node voltages are mostly affected by reactive power. The reactive power generation of a syn-chronous generator can be changed by changing the current in the rotor winding. This can be done either automatically or manually. A control system that changes the generator rotor cur-rent based on the actual measured terminal voltage is called an exciter. Synchronous genera-tors are normally equipped with an automatic voltage controller, which continually adjust the excitation so as to keep the generator terminal voltage at a given value. Voltage control is affected by using special devices dispersed throughout the system. This is in contrast to the control of frequency which depends on the overall system active power bal-ance. A brief review of the network components from the viewpoint of reactive power is: • Synchronous generators generate or absorb reactive power, depending on the exitation • Shunt capacitors generate reactive power • Shunt reactors absorb reactive power • Overhead lines generate or absorb reactive power, depending on the actual load • Underground cables generate reactive power • Transformers absorb reactive power • Loads normally absorb reactive power Reactive Power Capability The thermal power unit must be able to generate and to consume reactive power in adequate amounts within their capabilities for the voltage control of the power system. The unit must be designed so that the normal grid voltage the generators can be operated at the reactive power output and input within the limits defined by the capability diagrams of the generators or by static stability. At the grid voltages higher than the normal voltage the under-exited capability of the genera-tors must be fully available according to the capability diagram or static stability, whichever is more limiting.


Voltage Regulation The preferred dynamic characteristics for steady state are defined in a measurable way as follows: The 10% step response of generator voltage is recorded in no-load conditions, disconnected from the grid. The set value of the voltage is changed by plus and minus stepwise changes causing change of generator terminal voltage from 95 to 105%, and from 105 to 95%. In both cases step response of the generator terminal voltage must be as follows. • Response in non-oscillating • Rise time from 0 to 90% of the change is 0.2 … 0.3 s in case of static exciter, or in case of

brushless exciter: 0.2 … 0.5 s at step upwards, 0.2 … 0.8 s at s step downwards: • Overshot is less than 15% of the change Additional Voltage Control Equipment Current limiters (for generator rotor and stator) must have invert time characteristics to utilise the generator over-current capability to a good extent for various network conditions. Voltage Control Priority The normal way of operation is automatic control of generator voltage with the effects of reac-tive current statics. In case of needs for different type control, like control according to power factor or reactive output, these additional controls must affect at lower priority than the regulation of voltage. Reactive Power Output at Low Voltage Thermal power units must be equipped with such excitation systems and must be designed for such a power factor that the generator will be capable of providing a reactive power output of about the same magnitude as the rated active power output for 10 sec, in conjunction with network disturbances and at a generator bus-bar voltage of 70% of the rated generator volt-age. 2.2.4 Power Availability Power availability is another important characteristic of a power plant. If we firstly assume that the primary energy source for a thermal power plant is always available, then the availability will depend on the characteristics of the plant itself: • the frequency of plant tripping • the scheduled maintenance • the unscheduled maintenance In contrast, the availability for renewable generators such as wind turbines and photo voltaic generators can be quite different. Wind and sun cannot be stored. Therefore, the availability of the output power of such power plants will depend both on the availability of wind and sun, and on the characteristics of the power plants themselves. 2.2.5 Fault current Capability When a short circuit occurs, it is the task of the protection system to remove the faulted com-ponent. To that end the protection system uses a monitoring device and switchgear. The moni-toring device watches the current and as soon as it observes a current increase, a command is sent to the switchgear. The switchgear then interrupts the current and disconnects the faulted element from the system. In this protection scheme it is essential that a reliable and large current is generated in case of a short circuit. A synchronous generator is able to provide such a large and reliable current.


2.2.6 Faults-Ride-Through Capability Grid faults can reduce the voltage on the transmission system to zero, possible on all three phases. The majority of transmission line faults are of a transitory nature. Therefore, common practice is to re-close the circuit breakers automatically to improve service continuity. The removal of a faulted element requires a protective relay system to detect that the fault has occurred and to initiate the opening of circuit breakers which will isolate the faulted element from the system. It therefore takes some time to clear the fault. It is normally expected that a power plant should be designed so, that it can withstand a gen-erator voltage variation resulting from faults in the transmission system, without disconnection from the grid. An example of such a voltage profile is shown in Figure 2.5.

Figure 2.5 Dimensioning voltage profile for network fault [1] 2.2.7 Summary The ancillary services a traditional power plant can offer the network operator are the follow-ing: • Power generation • Overload capacity • Short term reserve (spinning or standing) • Performance of frequency control • Provision of voltage control (reactive power) • Availability • Generation of a fault current • "Fault-Ride-Through" capability • "Black Start" capability (after a system outage) This functionality is to the largest possible extent verified by a full-scale test made at commis-sioning and by the owner at regular intervals throughout the entire lifetime of the power plant, if it is assumed that characteristics have changed. This verification includes for example: • Full output power • Minimum Load • Overload Capacity • Power Response rate including range • Power step change • Deep voltage transients by short circuit if possible

100 95

2

[%] Voltage in Network

Time [sec]

0 0,25 0,75


• Changeover to house load operation • House load operation for 1 h • Step response of generator voltage • PSS test In the frequency control function, the events that make it necessary to change the output power of a generator are of stochastic nature and occur on a relatively short time frame (sec-onds to minutes). The output power of a generator is changed with small amounts, basically caused by generator trips or load disconnection. Therefore the output power controllability is important for the frequency control function In the load following function, the events that make it necessary to put power plants in or out of operation are mainly of deterministic nature and occur on a relatively long timeframe: the time of the day, the day of the week, etc. In this function, the generator’s output power must be available when it is necessary to put it in operation. Therefore the availability is important for the load following function. 2.2.8 References [1] Nordel, "Operational Performance Specifications for Thermal Power Units larger than

100 MW", 1995, www.nordel.org [2] Eltra, "Power Station Specifications for Plants > 50 MW", 1998, www.eltra.dk [3] E-ON, "Netzansclussregeln - allgemein", 1. Dezember 2001, www.eon.de


2.3 Combined Heat and Power units (CHP) General description By a combined heat and power plant or CHP unit, we will here understand a traditional thermal power plant, were the waste heat resulting from the production of electricity is used for process or district heating purposes, rather than being rejected to the environment. A true co-generation unit is therefore capable of achieving a gross energy conversion efficiency of 85% or more. This type of energy supply is especially useful for consumers with a continuous and steady-going heat demand.

Figure 2.6 Combined Heat and Power plant Source: Elsam The basic principle behind a CHP unit with a steam turbine is given in Figure 2.6. In the boiler the incoming water is transformed into dry steam under high pressure. The steam is transmit-ted to the turbine where it expands and as a result the electricity is produced. The wet steam leaves the turbine and passes through two heat condensers where it exchanges heat with water in the heating system. The water obtained as a result of steam condensation is trans-ferred to the water tank. With the help of a pump the water is forced to the boiler under a proper pressure. Capital and O&M Costs Investment cost for CHP units with a steam turbine depends very much on the utilised fuel. For CHP units based on either natural gas or oil will have capital costs around 1000 €/kWe. For CHP units based on bio-fuel combustion and with an electrical capacity of 10 MW will be around 2000 €/kWe [1]. Smaller units will usually be more expensive.


GSteamturbine

Water tank

Boiler Heatcondensators

PumpWaterheater

Figure 2.7 CHP power plant with steam turbine Efficiency The average electrical efficiency is about 20 to 30 %, but the total efficiency can reach 80-85% depending on the efficiency of the boiler, other losses and size of the unit. Control in normal operation The active power output is linear dependent of the thermal output. Some flexibility can to some extent be achieved by use of heat storage. The reactive power output can be controlled within the operative constraints of the synchro-nous generator. For this reason, CHP units are equipped with a power factor control. Fuel flexibility There are different types of steam boilers and almost any fuel can be used. Thus, there are boilers based on natural gas, fuel oil and bio-fuel. However combustion of waste is very de-manding, the fuel must be prepared for combustion (turned into a homogeneous mass) other-wise the fuel properties, particularly moisture content, will differ considerably, which will con-siderably influence the thermal value of fuel. Emissions Environmental properties of the steam turbine units depend on the fuel, which is used in the boiler and cleansing technology of the exhaust gases. Emissions from bio-fuel based boilers can be seen from [2]. Application of bio-fuel and especially waste requires installation of ad-vanced external filters in order to reduce dust and NOx content in exhaust gases.


Capacity of the boiler [MW]

Type of fuel Oxygen content

Total efficiency

NOx, mg/MJ

Dust, mg/m3 n

1.0 Chips, pellets, briquettes 6.5 % 89.5 % 85 100 1.7 Pellets 5.3 % 90.0 % 59 99 3.2 Briquettes 6.7 % 91.4 % 51 54 3.2 Briquettes 5.2 % 91.4 % 51 54 4.0 Chips 120 150 4.2 Demolition wood 5.7 % 90.4 % 56 28 5.1 Sawdust 6.1 % 88.0 % 50 94 5.7 Chips 5.2 % 85.0 % 37 16 8.0 Sawdust 115 50

15.0 Bark, sawdust 5.8 % 100 118 Table 2.1 Emissions from the bio-fuel based boilers [2] Summary A CHP power plant is a mature, well-tested technology with more than 100 years operation and development experience. It is only the heat dependent production that differs compared to a traditional generator. Some flexibility can here be achieved by thermal heat storage. References [1] Nordel, "Operational Performance Specifications for small Thermal Power Units", 1995,

www.nordel.org [2] Eltra, "Power Station Specifications for Plants < 50 MW", 1998, www.eltra.dk [3] Eltra, "Power Station Specifications for Plants 2 - 50 MW", 1995, www.eltra.dk [4] Eltra, "Power Station Specifications for Plants < 2 MW", 1995, www.eltra.dk [5] DEFU, “Relay protection for local CHP units”, TR293, 2nd edition (in Danish) [6] C. Persson, J. Olsson, “Jämförelse mellan olocka kraftvärmeteknologier”, Rapport SGC

128, ISSN 1102-7371, February, 2002. [7] H. Carlsen, “Status of small-scale power production based on Stirling engines”, Depart-

ment for Energy Engineering, Technical University of Denmark.


2.4 Reciprocating engines General description Reciprocating engines, developed more than 100 years ago, were the first among DG tech-nologies. Both Otto (spark ignition, SI) and Diesel cycle (compression ignition, CI) engines have gained widespread acceptance in almost every sector of the economy. They are used on many scales, ranging from small units of 1 kVA to large several tens of MW power plants. Smaller engines are primarily designed for transportation and can usually be converted to power generation with little modification. Larger engines are most frequently de-signed for power generation, mechanical drive, or marine propulsion [4]. Reciprocating engines are usually fuelled by diesel or natural gas, with varying emission out-puts. Almost all engines used for power generation are four-stroke and operate in four cycles (intake, compression, combustion, and exhaust). The process begins with fuel and air being mixed. In turbo-charged applications, the air is compressed before mixing with fuel. The fuel/air mixture is introduced into the combustion cylinder and ignited with a spark. For diesel units, the air and fuel are introduced separately with fuel being injected after the air is compressed. Reciprocating engines are currently available from many manufacturers in all size ranges. They are typically used for either continuous power or backup emergency power. Co-generation configurations are available with heat recovery from the gaseous exhaust. Heat is also recovered from the cooling water and the lubrication oil [3], [4].

Figure 2.8 Diesel generator [13]


Classification Typically, medium (275 – 1000 rpm) or high-speed (1000-3600) engines are applied for dis-tributed power generation (engine speed classification from [6]). The output of medium-speed engines ranges up to 20 MW while high-speed engines are applied up to 5 MW level.

Figure 2.9 Classification of reciprocating engines [6] Capital and O&M Costs The values given, for the installed costs of reciprocating engines varies greatly depending on the source. The overall range is from 250 to 1500 EUR/kW. For the operation and mainte-nance costs values from 0.005 to 0.015 EUR/kWh are given [3], [4]. Efficiency The electric efficiency is 30 – 50 % for diesel engines and 24-45 % for natural gas engines [3], [4]. In co-generation application, a total efficiency of 80-85 % can be achieved. Grid connection Typically, synchronous generators are applied with internal combustion engines. Reciprocating engines are then connected to the grid in the same way as the traditional generator. Start-up and shutdown The grid connection of a synchronous generator is equipped with automatic synchronisation, which makes sure that no extra voltage or current transients exist when the power plant is connected to the grid. In normal operation the power output can be driven close to zero before making the disconnection to avoid transients. Control in normal operation The output of power quality from engine driven synchronous generators is good. In addition, from the viewpoint of reliability and availability the diesel power plants are relatively good. The active power output is controlled by adjusting the torque produced by the engine. This means in practice the change of air/fuel ratio of the mixture to be burned in the engine [8]. Another control is needed for maintaining the desired terminal voltage at the generator. By adjusting the magnetising current of the synchronous generator also the reactive power output controlled. Depending on the application several alternative modes for these main controls are available.


The voltage controller typically keeps the generator terminal voltage at a given value. An outer loop can also be added to voltage controller that takes care of reactive power balance. The control for active power output is based either the generator speed of the output power. The latter means that the power plant produces the given constant MW amount continuously. Plain speed control is appropriate only in island operation while in the case of grid with other generators a control with output MW based droop can be applied. This means that for each generator a share of the required output power change when grid frequency is changed is defined by a droop factor. Special arrangements such as the one pre-sented in [7] are needed when optimal operation of more than one different type of DG units is required. The power controller also includes a down-kick module that shuts down the fuel injection in the case of sudden loss of load. In practice modern engines also include emission control system [8] that is closely linked to the power controller. Placement, Noise and Weight The generators can be made so compact, that they can be easily transported to the desired location. A reciprocating generator is relatively noisy. Fuel flexibility An advantage of reciprocating engines is that they can be used with different types of fuels. In addition to usual diesel or natural gas also the digester gas from sewage treatment plants can also be applied [3]. An obvious barrier for gas engines is the availability of the gas. For making the plant operation economical a gas pipe is required. For engines using diesel or other fuels the availability of fuel is generally good. Emissions The modern control and filtering systems are lowering the level NOx and CO content of ex-haust gases below the limits but still the level is relatively high for diesel units. However, the engines operating with natural gas have very low level of NOx output. Scalability Reciprocating generators can be found in many sizes, from small units of 1 kVA to large MW power plants. Availability Reciprocating engines are maintenance intensive, but they can provide high levels of availabil-ity [6]. Availability factor is usually between 90 and 97 % depending on the size of engine and the fuel applied [3], [6]. This is a clear benefit for diesel and gas generators, as opposite of e.g. wind power plant. Thus, an opportunity might be to apply diesel generator so that they are combined with e.g. a wind power plant. These kinds of hybrid applications can be found in for example the Greek islands. Standards and recommendations • USA

o IEEE P1547 [10] • UK

o G59/1 [11] This Engineering Recommendation can be applied. Considering the protection requirements given in G59/1 there is a technical report [12] by Electric-ity Association giving some further guidance. A new recommendation G83 is ex-pected to be finished by the end of year 2002.

Application Reciprocating engines are used for either continuous power or backup emergency power.


Summary Reciprocating engines, developed more than 100 years ago, were among the first DG tech-nologies. They are used in many scales, ranging from small units of 1 kVA to large several tens of MW power plants, and are usually fuelled by diesel or natural gas, with varying emis-sion outputs. They are typically used for either continuous power or backup emergency power. Cogenera-tion configurations are available with heat recovery from the gaseous exhaust, the cooling water and the lubrication oil. From an operational point of view, a reciprocating engine is superior in many ways. It can be started very fast and the power output can be controlled to give the required output at any time. This means that they can be used for both frequency control and voltage control. References [1] Jenkins N., Allan R., Crossley P., Kirschen D., Strbac G., “Embedded Generation”, The

Institution of Electrical Engineers, 2000. [2] www.distributed-generation.com/technologies.htm [3] Bining A., “California Advanced Reciprocating Internal Combustion Engines (ARICE)

Collaborative”, 7th Diesel Engine Emissions Reduction (DEER) Workshop, Portsmouth, VA, August 5-9, 2001, www.osti.gov/hvt/deer2001/bining.pdf

[4] Iannucci J., “Natural Gas Fueled Engines as Distributed Energy Resources”, Distributed

Energy Resources Workshop for Federal Facility Managers, San Jose, CA, February 8, 2001, www.eren.doe.gov/femp/techassist/pdf/iannucci.pdf

[5] “DG Technology Series: Reciprocating Engines”, Distributed Generation Monitor, Vol 1.

Issue1, December/January 2001, www.distributedgeneration.com/Library/Monitor_Jan01.pdf

[6] “Technology Characterization: Reciprocating Engines”, EPA, February, 2002,

www.epa.gov/chp/pdf/EPA_RecipEngines_final_5_16_02.pdf [7] Canever D., Dudgeon G.J.W., Massucco S., McDonald J.R., Silvestro F., “Model valida-

tion and coordinated operation of a photovoltaic array and a diesel power plant for dis-tributed generation”, IEEE Power Engineering Society Summer Meeting 2001, Volume: 1, 2001, pp. 626-631.

[8] Guzzella L., Amstutz A., “Control of diesel engines”, IEEE Control Systems Magazine,

Volume: 18, Issue: 5, Oct. 1998, pp. 53-71. [9] “Distributed Generation: System Interfaces”, An Arthur D. Little White Paper, Arthur D.

Little Inc., 1999, www.encorp.com/dwnld/pdf/wp_ADL_2.pdf [10] “Draft Standard for Interconnecting Distributed Resources with Electric Power Systems”,

IEEE P1547/D08, Institute of Electrical and Electronics Engineers Inc., New York, NY, 2001, http://technet.nreca.org/pdf/distgen/P1547StdDraft08.pdf

[11] “Recommendations for the connection of embedded generating plant to the public electricity suppliers’ distribution system”, Engineering Recommendation G59/1, Electricity Association, 1991. [12] “Notes of guidance for the protection of embedded generating plant up to 5 MW for op-

eration in parallel with public electricity suppliers’ distribution systems”, Engineering Technical Report No. 113, Revision 1, Electricity Association, 1995.


[13] www.generatingset.com [14] www.dieselpower.no


2.5 Wind Energy Converters (WEC) General description Modern wind turbines convert wind power to electrical power, with a rated generator power of marketable models currently ranging up to 2.0 MW. Hub-heights reach more than 80 meters, rotor diameters are typically 65 m for 1.5 MW machines. Rotor construction is either • Variable blade angle, or • Non-variable. Conversion from mechanical to electrical energy is via either • Synchronous, or • Induction generators. Synchronous generators are usually equipped with a pulse width modulated converters. Con-trol of these converters is essential for regulating the behaviour of the windmill in the electric grid, e.g. reactive power adjustment. Wind turbines have developed rapidly from unit sizes below 20 kW (fixed speed, stall control) in the seventies to the present sizes of up to 2 MW. In order to withstand the mechanical stress most wind turbines above 1.0 MW are equipped with a variable speed system incorpo-rating power electronics in combination with pitch control. If advanced enough these systems are capable of decoupled active and reactive power control on the grid side and of decoupled torque and generator excitation control on the generator side.

Figure 2.10 Wind Farm at Rejsby Hede in Denmark Single units can normally be connected to the distribution grid at 10 kV and 20 kV. The present trend though is that wind power is being located off shore in larger wind farms that are con-nected to higher voltage levels.


For such wind farms the transmission system operators in several European countries have made new connection rules. These rules imply, that a wind farm must oblige to the same kind of rules, as the traditional generator does. It must for example offer services like: active power regulation, frequency regulation, voltage regulation, and not least be able to survive prolonged periods with low grid voltage. The first offshore wind farm built according to Eltra specifications will be in operation summer 2002. The farm at Horns Rev will consist of 80 turbines with a unit size of 1.5 MW, in total 160 MW. The wind farm is situated 54 km from the transmission network. The offshore wind farm at Horns Rev is a demonstration project and one of the purposes is to test different techniques and to adjust the turbine specifications. The technical rules for the connection to the transmission network will also be examined.

Figure 2.11 The wind farm at Horns Rev in the North Sea [www.hornsrev.dk] Some of the wind farm control functions are controlled centrally for the entire wind farm, whereas others are managed individually by the turbine in question. The production of the wind farm is controlled centrally. The control functions are as follows: • Production limitation set output to a maximum • Reserve operate with a certain reserve downward and/or upwards • Balance control set output downwards or upwards in steps • Grid protection interventions set output to a lower value, in critical situations • Gradient limitations adjust output gradient limit upwards and/or downwards Each turbine handles its own frequency control. In addition, the reactive power can be con-trolled locally or centrally so that the wind farm's total consumption/intake of reactive power is kept within ±16 MVAr at the 34 kV side of the wind farm transformer. Disconnection of the turbines in case of grid faults or too strong winds is also controlled individually. Classification The elements in the currently three most important applied wind turbine concepts are: • Fixed or variable blade angle • Fixed or variable generator speed • Induction or synchronous generator • Converter or no converter • Gearbox or no gearbox


Table 2.2 summarizes the main characteristics of three types of wind turbines which indicate the range of the possible technologies realized today. Fixed blade angle - Stall control The stall control means that the wings are firmly attached to the hub and the wind attack angle of the wings is fixed. Thereby at higher wind speeds the turbulent flow will increase which re-duces the transfer from aerodynamically to mechanical power. Variable blade angle - Pitch/Active stall control The mechanical power can be controlled by introducing a system with flexible coupling of the wings that can change the wind attack angle of the wings hydraulically or electrically and thereby maintaining a laminar flow around the wings also at high wind speed. In a pitch control system this angle can be changed in a large area while in the active stall system the variation is limited. Fixed generator speed Fixed generator speed means that the rotor speed is stuck to the grid frequency and cannot be changed. Variable generator speed Variable generator speed means that the rotor speed can be changed. Converter system If the wind turbine operates at variable rotational speed, the electric frequency of the generator varies and must therefore be decoupled from the frequency of the grid. This can be achieved by an converter system. Concept 1: Stall controlled induction generator (fixed) speed wind turbines The dominating wind turbine technology below 1 MW is the stall controlled fixed speed system. The stall control means that the wings are firmly attached to the hub and the wind attack angle of the wings is fixed. Thereby at higher wind speeds the turbulent flow will increase which re-duces the transfer from aerodynamically to mechanical power. Fixed generator speed means that the rotor speed is stuck to the grid frequency and cannot be changed. The greatest advantages of a fixed speed wind turbine with induction generator are the simple and cheap construction. In addition no synchronisation device is required. With the exception of bearings there are no wearing parts. The disadvantages of induction generators are the high starting currents, and their demand for reactive power. A stall regulated fixed speed system is therefore normally equipped with capacitor banks and a soft starter (Thyristor controller). Concept 2: Pitch controlled double-fed induction generator (variable) speed wind turbines The pitch control means that the mechanical power can be controlled by changing the wind attack angle of the wings hydraulically or electrically and thereby maintaining a laminar flow around the wings also at high wind speed. The wind turbine rotor is coupled to the generator through a gearbox. The generator is a doubly wound induction generator, where the rotor windings is fed using a back-to-back voltage source converter. The stator of the generator is connected to the grid directly. Only the rotor of the generator is connected to the grid by the electronic converter. This gives the advantage that only a part of the power production is fed through the converter. That means the nominal power of the converter system can be less than the nominal power of the wind turbine. In general the nominal power of the converter is the half of the power of the wind turbine. By the control of active power of the converter, it is possible to vary the rotational speed of the generator and thus of the rotor of the wind turbine. Concept 3: Pitch controlled synchronous generator (variable) speed wind turbines The pitch control means that the mechanical power can be controlled by changing the wind attack angle of the wings hydraulically or electrically and thereby maintaining a laminar flow around the wings also at high wind speed. The wind turbine rotor is coupled directly to a syn-


chronous generator. This generator is a low speed multi pole generator, therefore no gearbox is needed. The synchronous generator can have a wound rotor or be exited using permanent magnets. It is coupled to the grid through a back-to-back voltage source converter. This con-verter system is connected between the stator of the generator and the grid, where the total power production must be fed through the converter.

Table 2.2: Wind turbine characteristics Sources: CIGRE WG 37-23 [1], ELTRA Capital and O&M Costs The capital cost is decreasing with size. Typical turnkey costs of wind power projects are from 900 to 1.100 EUR/kW. The yearly operation and maintenance cost is around 1.7 % of the total investment cost. For off shore sites the cost may increase with 50% for the building and grid connection part. The total production cost is believed to decrease 15–25 % until 2010. Efficiency In a wind turbine the kinetic energy of the streaming air is converted to electric power. Nor-mally power is produced in the wind speed range 4 to 25 m/s. The wind aero-dynamical power is proportional to the cube of the wind speed and to the circle area swept by the wings. The mechanical power is theoretically close to 0.6 of the wind power but in practice lower (0.4 to 0.55).

Concept 1 Concept 2 Concept 3

induction generator

directly coupled to the grid,

stall regulation

double-fed induction generator

stator coupled directly to the grid. Rotor coupled via a 4-quadrant ac/ac

converter to the grid. Pitch control of turbine

directly coupled to the grid,

pitch regulation

synchronous

generator coupled via

a pulse-width-modulated

converter to the grid

pitch regulation

Concept ü AG MS C

SG MS WR

reactive-power-regulation

sometimes compen-sated to cosϕ=1

variable cos ϕ variable cos ϕ

active-power-regulation

No regulation Below current available wind power: yes

Below current avail-able wind power: yes

Voltage regula-tion

No regulation by reactive- and active power regulation

by reactive- and active power regulation

Flicker usually high low (reduced by active regu-

lation of rotor speed)

Usually low

Contribution to short-circuit cur-rent

4 ... 8 · Ir 4 ... 8 · Ir if closeby fault

Ir if far-away fault

Ir

inrush current Imax ≥ 2,5 · Ir Imax ≥ 2,5 · Ir Imax = Ir


The yearly aero-dynamical energy content is estimated to around 4000 kWh/m2 for Swedish mean wind conditions at 100 m height. For groups of wind turbines the reduction in energy production is estimated to 2 – 8 %. The yearly production time depends on system technology, hub height and site. On-shore wind turbines may produce during 2100 hours/year, while off-shore wind turbines are expected to produce during 3500-5000 hours/year. Start-up and shutdown: Concept 1 During the start-up sequence the speed of the turbine is increased until the generator speed is close to the synchronous speed. The generator needs to be connected quickly why the soft starter operates for a short period. This in combination with the subsequent capacitor bank connection gives a high current peak followed by current oscillations caused by mechanical oscillations. The voltage dip will be rather large, depends on the SCR (short circuit ratio), due to the need for reactive power from the grid at the start. The shut-down sequence at low wind: The generator breaker will disconnect the generator and at the same time the capacitor banks. The generator is then free wheeling awaiting higher wind (above the cut-in speed 4 m/s) that gives possibility for a new start attempt. The negative power quality (flicker) contribution is in this case only from the step in reactive power, but as the banks are divided in several small units it could well bee close to zero. The shut-down at high wind speed (above the cut-out value 25 m/s) is started with tilting the wing tips to reduce the mechanical power. It's a regulation demand to have two independent brake systems. The following procedure is the same as at low wind to reduce the speed and active power as much as possible before opening of the generator breaker. Emergency shut-down at line disconnection. In this case there is no time for tilting the wing tips. Full mechanical braking is needed which gives a very high mechanical stress to the wind turbine. The worst case is at nominal power as the step in active power will be the nominal value at the line disconnection. The step in reactive power depends on the local compensation degree. Start-up and shutdown: Concept 2+3 The use of pitch control as well as speed control and no need for capacitor banks gives much smoother performance during start-up and shut-down. It gives much less impact on the power quality. At emergency shut-down due to line disconnection the pitch control will be used without need for the mechanical brake. The mechanical brake is then used as a parking brake. Manually there is a possibility to order both pitch controlled braking together with mechanical braking in case of large danger. Power Quality The power quality depends on system design. Direct connected asynchronous generators may contribute to increased flicker levels and relatively large active power variations. Converter connected systems may contribute to higher harmonics levels. The impact of different designs and control strategies differs rather much. Disconnections will always influence voltage and if large enough also the frequency. Very large disconnections due to domino effects or lack of redundancy in the grid design may even endanger the system stability. The concept 1 systems have several disadvantages such as flicker contribution from the tower shadow effect, large 20% active power variations as function of wind gusts that even may acti-vate tap changers, increased voltage dips during faults due to decreased capacitor compensa-tion and thereby larger risk for protection activation and disconnection. In concept 2+3 systems the converters are normally sensitive to disturbances. These have to be designed so that blocking and disconnections are avoided as much as possible. With these


systems it is though possible to smooth the active power variations and assist in grid voltage control. Fault response: Concept 1 During faults, the grid voltage is reduced and the generator will reduce its torque and speed up. The sub-transient short circuit current, including the DC-component, from these systems may rise to values several times the nominal current. The connecting switchyard has to be dimensioned for this high current. As the generator speeds up there is also concern whether the kip torque, very much reduced at low grid voltage, is passed and for excessive consump-tion of reactive power. If the generator is not disconnected before the fault is cleared the active power will increase, when decelerating, to values quite higher than before the fault during typi-cally half a second. The grid protection settings have to be co-ordinated with this. During faults the concept 1 system will physically assist in grid frequency control by its inertia. In some grids with less frequency control strength (or speed) this may be rather valuable and there will be a balance between voltage and frequency protection settings. After faults in some cases the concept 1 systems will not recover meaning that the local grid voltage will remain low. They have to be disconnected. The system design including capacitor banks contribute to this possible negative behaviour. Fault response: Concept 2 The concept 2 systems may reduce the short circuit current significantly by increasing the rotor resistance. This also contributes to reduced active and reactive power flows after the fault is cleared. Fault response: Concept 3 In concept 3 systems the short circuit current may be completely controlled as the whole active power is controlled. Environment Wind power is an environmentally clean production source. The governmental permission procedures for approval of new wind power erections is nevertheless rather lengthy (two years is not unusual). Questions that are addressed are e.g. noise, bird and sea biology interfer-ences, and in many cases maybe most important the visual impact. Availability The technical availability of marketable systems has reached 98 to 99%. Today the manufac-turers believe that wind turbines for off shore applications will manage operation without main-tenance for 1 year. Standards, recommendations and guidelines The following measurement guidelines give rules and requirements for the measurement of power quality of wind turbines: • IEC, "Power quality requirements for grid connected wind turbines", IEC 88/101/CD: 1998-

12, Draft IEC 61400-21, www.iec.ch • MEASNET, ”Power quality measurement procedure”, November 1997, www.measnet.net • Fördergesellschaft Windenergie e.V. FGW, "Richtlinie zur Bewertung der elektrischen

Eigenschaften einer WEA", Rev. 12., Brunsbüttel, Germany In addition to the measurement requirements the draft of the IEC guideline gives methods for estimating the power quality expected from wind turbines or wind farms when deployed at a specific site.


MEASNET is a network of European measuring institutes with the aim of harmonising measur-ing procedures and recommendations in order to achieve comparability and mutual recognition of the measurement results of the member institutes. The German guideline is a national guideline, but is also accepted in other countries. The guideline is different from the IEC-guideline. Thus results from the German guideline and from the IEC guideline are not comparable. The following guidelines give requirements and limited values for the grid connection of wind turbines: • VDEW, "Eigenerzeugungsanlagen am Mittelspannungsnetz. Richtlinie für Anschluß und

Parallelbetrieb von Eigenerzeugungsanlagen am Mittelspannungsnetz", 2. Ausgabe 1998, Germany, www.vdew.de

• DEFU, "Connection of wind turbines to low and medium voltage networks", Komité rap-

port 111-E, October, 1998, Denmark, www.defu.dk • Sveriges Elleverantörer, "Anslutning av mindre produktionsanläggningar till elnätet", Stock-

holm 1999, www.elforsk.se • Eltra, "Specifications for Connecting Wind farms to the Transmission Network", TP98-328,

2000, Denmark, www.eltra.dk These four guidelines are national guidelines. The German VDEW guideline is based on the results on the German measurement guideline. The Danish and the Swedish guidelines are based on results of the IEC 61400-21 measure-ment guideline. There is no specific international guideline, giving limits and recommendations for grid connec-tion of wind turbines. However there are IEC guidelines for special items of power quality, but not especially for wind turbines. The IEC 61000-3-6 gives requirements concerning harmonics and the IEC 61000-3-7 gives requirements concerning flicker: IEC 61000-3-6: 1996, EMC. Part 3: Limits - Section 6: Assessment of emission limits for dis-torting loads in MV and HV power systems - Basic EMC publication. (Technical report) IEC 61000-3-7: 1996, EMC. Part 3: Limits – Section 7: Assessment of emission limits for fluc-tuating loads in MV and HV power systems - Basic EMC publication. (Technical report)


Summary Wind energy is now firmly established as a mature technology for electricity generation and over 13,900 MW of capacity is now installed worldwide. It is one of the fastest growing electric-ity generating technologies and features in energy plans across all five continents, both in the industrialised and the developing world. It differs, however, in several respects from the "conventional" thermal sources of electricity generation. Key differences are the small sizes of individual units, the variable nature of the wind and the type of electrical generator. Small unit sizes: The small unit sizes mean that both wind farms and individual wind turbines are usually connected into low voltage distribution networks rather than the high voltage transmission systems and this means that a number of issues related to power flows and pro-tection systems need to be addressed. Electrical safety is an important issue under this head-ing. Variability: The variable nature of wind is often perceived as a difficulty, but in fact poses few problems. The variations in output do not cause any difficulty in operating electricity systems, as they are not usually detectable above the normal variations in supply and demand. Variability also needs to be taken into account at the local level, to ensure consumers are not affected by "flicker". Appropriate care in electrical design, however, can eliminate this problem. Electrical properties: Early wind turbines followed steam turbine practice with synchronous generators, but almost all modern wind turbines have induction generators. These draw reac-tive power from the electricity network, necessitating careful thought to electrical power flows. Other machines, however, are capable of conditioning the electrical output and providing a controllable power factor. This is an asset, especially in rural areas, where it may be undesir-able to draw reactive power from the network. Table 2.3 gives an overview of the characteristics of wind turbine concepts 1-3. Advances in wind-turbine technology and the results of nearly two decades of research mean that the integration of wind turbines and wind farms into electricity networks generally poses few problems. The characteristics of the network and of the turbines do nevertheless need to be evaluated but there is now a wealth of experience upon which to draw.


Concept 1 Concept 2 Concept 3 Stall controlled induc-

tion generator (fixed) speed wind turbines

Pitch controlled double-fed induction generator (variable) speed wind turbines

Pitch controlled synchronous genera-tor (variable) speed wind turbines

Output power control and frequency control/ short term balancing

Switching wind turbines on and off

Switching wind turbines on and off Control of pitch angle and power electronic converter

Switching wind turbines on and off Control of pitch angle and power electronic converter

Output power availability and long term balancing

Problematic due to dependence on wind as primary energy source



Voltage control Not possible Without Extra Equipment

Possible when converter rating is sufficient and when controlled appropriately

Possible when converter rating is sufficient and when controlled appropriately

Supply of fault current

Inherent to working principle

Difficult due to limited overloading capability of power electronic converter

Difficult due to limited overloading capability of power electronic converter

Fault-ride-through capability

Risk of voltage instability, dependent on actual wind speed, fault duration and grid strength

Problematic due to substantial difficulties in controlling power electronic converter

Theoretically Possible, but Only with Appropriate Control of the Power Electronic converter

Table 2.3 Wind turbine characteristics [16]


References [1] Cigre Working Group 37.23, "Impact of increasing contribution of dispersed generation

on the power system", September 1998, www.cigre.org [2] www.ewea.org, European Wind Energy Association, Belgium [3] www.btm.dk, Wind-energy links [4] www.danmarks-vindmoelleforening.dk, The Danish Wind Association [5] www.windpower.org, The Danish Windmill Industry [6] Fördergesellschaft Windenergie e.V. FGW, "Richtlinie zur Bewertung der elektrischen

Eigenschaften einer WEA", Rev. 12., Brunsbüttel, Germany [7] VDEW, "Eigenerzeugungsanlagen am Mittelspannungsnetz. Richtlinie für Anschluß und

Parallelbetrieb von Eigenerzeugungsanlagen am Mittelspannungsnetz", 2. Ausgabe 1998, Germany, www.vdew.de

[8] DEFU, "Connection of wind turbines to low and medium voltage networks", Komité rap-

port 111-E, October, 1998, Denmark, www.defu.dk [9] "Anslutning av mindre produktionsanläggningar till elnätet", Sveriges Elleverantörer,

Stockholm 1999, www.elforsk.se [10] MEASNET, ”Power quality measurement procedure”, November 1997,

www.measnet.net [11] Eltra, "Specifications for Connecting Wind farms to the Transmission Network", TP98-

328, 2000, Denmark, www.eltra.dk [12] E-ON, "Netzansclussregeln - allgemein", 1. Dezember 2001, www.eon.de [13] E-ON, "Netzansclussregeln - Ergänzende Netzanschlussregeln für Windenergieanla-

gen", 1. Dezember 2001, www.eon.de [14] IEC, "Power quality requirements for grid connected wind turbines", IEC 88/101/CD:

1998-12, Draft IEC 61400-21, www.iec.ch [15] DEFU+Risø, “Power quality and grid connection of wind turbines”, Parts 1, 2 and 3 (in

Danish), Risø-R-853 and DEFU-TR-362, www.risoe.dk, www.defu.dk [16] W.L. Kling, J. G. Slootweg, “Wind turbines as Power Plants”, Delft University of technol-

ogy, P.O.Box 5031, 2600 GA Delft, The Netherlands


2.6 Hydro Power (HP) General description A hydro power station consists of turbines connected to electric generators and the structures necessary to channel and regulate the flow of water to the turbines. Hydro power stations con-vert the energy of flowing water into electricity. The vertical difference between the upper reservoir and the level of the turbine(s) is known as the head. The water falling through this head gains kinetic energy which it then imparts to the turbine blades. The potential energy of water can be stored by constructing storage reservoirs either in natural lakes or in artificial basins made by building dams across the watercourse.

Figure 2.12 Hydro power station in Norway [1] Classification Hydro power stations are classified in two main types of installations depending on the storage capacity: low-head or run of river and high-head or stored stations. Low head Low-head hydro power stations have a small head of water but a large water volume. Since it is difficult to regulate the water flow by means of a reservoir close to the power station the


water generally is used when it is available. The amount of electricity generated therefore rises when the river is carrying more water. High head High-head power stations are generally constructed to utilise a large head of water but a smaller volume than the run-of-river power stations. The power station is usually constructed near the storage reservoirs, and is connected with the reservoirs by tunnels through the rock above or pipelines on the ground. Start up and shut down A hydro power station has the ability to start up quickly and the advantage that no losses are incurred when at standstill. Emissions A hydro power station with dams, reservoirs and power-lines will have both positive and nega-tive effects on the local environment however he generation of electricity in hydro-electric power stations causes no pollution of the environment. Application A hydro power station has great advantages, especially when working in conjunction with a thermal system, to meet peak loads at minimum costs. A pumped storage plant is a special type of hydro power plant. It consists of an upper and a lower reservoir and turbine-generators which can be used as motor-pumps. During peak load hours on the network the turbines are driven by water from the upper reservoir in the normal manner. In low load periods water is pumped back to the upper reservoir. The generators then change to synchronous-motor action and, being supplied from the general power network, drive the turbine which is now acting as a pump. Summary Hydro-power is probably the world’s most important renewable energy resource. The gross theoretical potential for hydropower is estimated to be around 36.000 TWh world-wide. The exploitable potential exceeds 14.000 TWh. Around 18 %, or 2500 TWh of this poten-tial is utilised. Small hydropower plants rated at an installed capacity of 10 MW or less currently contribute with more than 37 TWh/a to about 2.5% of the European electricity market. Based on a fairly stable growth rate of 3% p.a. over the last decades by means of modernisation, reconditioning and exploitation of new sites, about 50% of the remaining small hydro power resources in Europe are expected to be developed by 2015. Besides technically mature and economically attractive mean and high head sites, run-off river low heads installations are expected to contribute substantially to the future development. Im-proved turbine designs, cost effective plant construction in combination with new technologies and improved control and operating strategies have the potential to reduce the high initial costs as one main barrier for the future exploitation. From an operational point of view, a hydro power station is superior in many ways. It can be started very fast and the power output can be controlled to give the required output at any time. This means that they can be used for both frequency control and voltage control. References [1] www.smakraftverk.com [2] www.kraftverk.net [3] www.microhydropower.net


2.7 Photovoltaic (PV) General description Although conversion of solar energy to electrical energy has been technical possible since the late 1930's practical applications first began in the early 1970s, when PV cells were adopted by the US space program. Photovoltaic power systems can be categorized into three application types: • stand-alone, • hybrid or • grid-connected The stand-alone systems generally involve batteries and are used in remote locations which have no access to a public grid. In a hybrid system one or more auxiliary power sources such as wind or diesel generators are added to the PV scheme, in order to supply the load continuously. The grid-connected types normally do not include batteries. Here the public network acts as an infinitely strong system which accepts all available power from the PV system. Connection to the grid is made via an converterer device. In 1990, world-wide photovoltaic sales reached about 50 MW and about 30% of it was related to electric power applications. No actual figure is available but we may assume that it is dou-bled in the meantime. If the current trend of market growth continues, an overall world demand for photovoltaic could go up to 6 GW in the year 2010, with an important portion for decentral-ized electric power applications.

Figure 2.13 Grid connected Photovoltaic system [2] The major barrier to a widespread adoption of PV equipment is its high cost. A number of strategies for reducing costs are actively pursued around the world. Some new technological concepts seem to be promising in this respect. In the past, most of the PV activities for power production were pilot or demonstration projects sponsored by public authorities, like for instance the 1 MW plant in Spain (EU Joule pro-gramme). More and more, the electric utilities are coming in now and start up PV applications.


Capital and O&M costs Photovoltaic systems have high capital costs, but as they are reliable and require minimal maintenance to operate, their O&M costs are small. Efficiency The power output of a PV system is directly related to the surface area of the PV modules, the efficiency of the system and the actual solar radiation, which varies from hour to hour and from day to day. In practice the output depends on the latitude, which determines the path length of the solar radiation trough the atmosphere. Large variations are also due to the varying cloud conditions. The output of PV modules is specified in standard conditions with power density 1000 Wm-2 and cell temperature of 25 °C [3]. PV systems can be single phase or three-phase, with or without transformer. The maximum power output of a PV module is obtained near the knee of its characteristics see Figure 2.14. Since the voltage depends on the cell temperature a maximum power point tracking (MPPT) stage is needed in the converter to always get the maximum power output [3].

Figure 2.14 Typical characteristic of a PV module [3] The maximum theoretical efficiency of a PV cell is 30% [4]. Current units have efficiencies of 24% in laboratory conditions and 10% in actual use. Grid connection Connection to the grid is made via an inverter. A schematic of a small grid connected PV in-verter is shown in the following figure.

Figure 2.15 Schematic diagram of a small PV inverter for grid-connected operation [3] The DC/AC conversion is usually accomplished by a self-commutated PWM inverter that is current controlled. The amount of current depends on the DC power available from the PV array.


In order to avoid DC injection to the grid an isolation transformer is used. Alternatively a shunt or dc-current sensor can be used, that initiates inverter shutdown when the dc component of the output current exceeds the specified threshold [10]. It is also possible to make the trans-former considerably smaller by applying a configuration where it carries a high frequency AC current.

Figure 2.16 Diagram of a PV inverter applying high frequency transformer [14] Considering the whole PV system there are alternative ways to achieve suitable modularity (see Figure 2.17). The central configuration is the most classical for especially residential PV system. There are several PV modules connected to supply one inverter. For reducing the amount DC wiring needed other solutions where adopted mid-nineties. String inverters are connected to one string of PV modules so that the junction box for parallel connection can be omitted [13].

Figure 2.17 Different photovoltaic system configurations [13] An AC Module is defined in as an integrated combination of single solar module and inverter. The inverter converts the DC energy from the module into AC energy and feeds this energy into the AC network. The main advantage of AC Modules is modularity. Plug and play type if grid connection is the objective. It is expected that AC Modules will be used worldwide within a few years [6]. Power Quality PV units potentially cause harmonics. On the other hand, they may be sensitive to harmonics. Start-up and shutdown Inverters are typically equipped with automatic control that enables smooth start-up and shut-down.


Control in normal operation As with harmonic distortion, the older line-commutated inverters generated current waveforms with poor power factor, but modern PWM inverters can generate power at unity power factor (i.e., the output current is exactly in phase with the utility voltage). Inverter designs that generate other than unity power factor (and so can be used for power factor correction), are possible. However, these units must necessarily store energy through part of each cycle and are thus generally more expensive and less efficient than unity power factor inverters [6]. IEEE standard 929-2000 says that the PV system should operate at a power factor > 0.85 (lagging or leading) when output is > 10% of rating [10]. Utility-interconnected PV systems do not regulate voltage, they inject current into the utility. Therefore, the voltage operating range for PV inverters is selected as a protection function that responds to abnormal utility conditions, not as a voltage regulation function [10]. Protection Short-circuit current from the PV system may cause malfunction of over-current relays and fuses. It would be necessary to develop a new fault detection system [9]. Present PCU designs typically include the following protective relaying functions in their control circuitry [6]: • Under/Over Voltage (typically line voltage ±10%) • Under/Over Frequency (typically line frequency ±1 Hz) • DC Current Injection (if no 60 Hz isolation transformer is used) Any deviation outside these limits will cause the inverter to shut down and disconnect from the utility line within a few cycles. Protective functions recommended by the IEEE Power Systems Relaying Committee Working Group C-5 include [6],[7]: • Transient overvoltage suppressors • AC & DC undervoltage/overvoltage trips • Current overload and short-circuit protection • Under-frequency and over-frequency trips • Abnormal current flow in grounding conductor • Loss and return of utility line voltage (reclosing) • Over-temperature. There is also an IEEE standard 929-2000[10] that specifies the following normal operation ranges: • Voltage: 88 % - 110 % • Frequency: 59.3 - 60.5 Hz (at 60 Hz system) According to this standard the dc current injected to the network must be limited to 0.5 % of the rated inverter output current [10]. Perhaps the most critical protection issue is “islanding” that can be defined as follows [9]: Islanding is the continued operation of a grid-coupled inverter (or generator in general) in cases where the utility grid has been switched off, cut off or the distribution lines have been damaged so that no electric energy is delivered from the utility side. In such a situation, the safety of persons and/or the safety of equipment might no longer be guaranteed.


Many anti-islanding methods have been identified in the literature and have been tested in practice. They can be divided into 2 groups [9]: • Passive methods: a detection circuit monitors grid parameters (e.g. voltage, frequency,

voltage phase jumps, and voltage harmonics); these methods do not have any influence on grid quality

• Active methods: a detection circuit deliberately introduces disturbances (e.g. active or re-

active power variation, frequency shift) and deduces from the reaction to these distur-bances if the grid is still present. The grid quality is somehow affected; however, ordinary devices like TV sets have a much bigger (negative) effect.

IEEE standard 929-2000 defines a test procedure for “non-islanding PV inverters”, which veri-fies whether an inverter will cease to energise the utility line under certain conditions [10]. There is also a German standard DIN VDE 0126 [15] that covers one-phase grid connected PV inverters. It prerequisites that anti-islanding protection is to be based on the monitoring of grid impedance and the disconnection is made by an independent switch. Standards and recommendations • Germany

o VDE 0126 [15] • USA

o UL 1741 [11] o IEEE 929 [10] o IEEE P1547 [12]

• UK o Engineering Recommendation G77 [5] defines test for inverters for PV generators.

If the inverter passes these test, it is pre-certified as an 'Approved Inverter for PV Generators' and it may be connected without further testing

o The type testing includes the following tests: A1 CE Marking and certification: the compliance with

European CE Mark Regulations is checked. A2 Testing of automatic protection A2.1 Over/Under Voltage A2.2 Over/Under Frequency A2.3 Loss of Mains Protection A2.4 Re-connection A3 Power Quality

A3.1 Harmonics A3.2 Power Factor A3.3 Voltage Flicker A3.4 Electromagnetic Compatibility A3.5 DC Injection A4 Standards

• IEC o IEC TC-82 Solar Photovoltaic Energy Systems is working on the subject. The grid

connection of PV systems, are also considered in the following IEC standards: IEC 60364-7-712: Electrical installations of buildings - Part 7-712: Re-

quirements for special installations or locations - Solar photovoltaic (PV) power supply systems

IEC 61727: Photovoltaic (PV) systems - Characteristics of the utility inter-face

IEC 61173: Over-voltage protection for photovoltaic (PV) power generat-ing systems

Applications PV system could operate, in the future, as active filters to reduce harmonics in the distribution system [13].


Summary Photovoltaic generation of electricity or direct conversion of sunlight to electricity is a well es-tablished technology for power supplies to sites remote from the electricity network. In the past, most of the PV activities for power production were pilot or demonstration projects sponsored by public authorities. More and more, the electric utilities are coming in now and start up PV applications. The major barrier to a widespread adoption of PV equipment is its high cost. A number of strategies for reducing costs are actively pursued around the world. Some new technological concepts seem to be promising in this respect. Islanding seems to be the most controversial topic with grid-coupled PV systems. However, theoretical studies show that islanding can only happen under very special and unlikely cir-cumstances if basic safety methods are implemented. These methods are: • monitoring of grid voltage • monitoring of grid frequency [9]. Common international guidelines addressing the problem of islanding do not yet exist. Dan-gerous situations are very unlikely, but the consequences could be grave. One of the main questions is: “Which measures lead to an acceptable degree of security?” To reach an inter-national consensus the following aspects have to be clarified: • Definition of voltage and frequency limits for the operation of inverters • Definition of the allowable duration of islanding • Definition of standard test methods for islanding-prevention devices. As with other types of DG many issues are discussed between the PV owner and the utility: • Interconnection procedure • Contracting • Connection costs • Rates • Metering • Standard connection


References [1] www.arcon.dk [2] www.fortumsolenergi.com [3] Jenkins N., Allan R., Crossley P., Kirschen D., Strbac G., “Embedded Generation”, The

Institution of Electrical Engineers, 2000. [4] WWW-site: http://www.distributed-generation.com/technologies.htm [5] WWW-site: http://www.distributed-generation.com/technologies.htm “Recommendation

for the Connection of Inverter-Connected Single-Phase Photovoltaic (PV) Generators up to 5KVA to Public Distribution Networks”, Electricity Association: Engineering Recom-mendation G77, UK, 2000.

[6] Wills, R.H., “The Interconnection of Photovoltaic Power Systems with the Utility Grid: An

Overview for Utility Engineers”, SAND94-1057, Sandia National Laboratories, Albuquer-que, NM., June 1994, www.sandia.gov/pv/Interconnect.doc

[7] “Static Power Converters of 500 kW or Less Serving as Relay Interface Package for

Non-Conventional Generators”, IEEE Power System Relaying Committee, Working Group C5.

[8] “Analysis of Photovoltaic Systems”, Report IEA-PVPS T2-01, April 2000,

http://www.task2.org/public/download/Rep2-01f.pdf [9] “Utility Aspects of Grid Connected Photovoltaic Power Systems”, Report IEA-PVPS T5-

01, December, 1998, www.oja-services.nl/iea-pvps/products/download/rep50_01.pdf [10] “IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems”, IEEE

Std. 929-2000, Institute of Electrical and Electronics Engineers Inc., New York, NY, 2000.

[11] “Static Inverter and Charge Controllers for Use in Photovoltaic Systems”, UL 1741, Std

1741, Underwriters Laboratories Inc., Northbrook, IL. [12] “Draft Standard for Interconnecting Distributed Resources with Electric Power Systems”,

IEEE P1547/D08, Institute of Electrical and Electronics Engineers Inc., New York, NY, 2001.

[13] Woyte A., Belmans R., Nijs J., "Grid-Connected Photovoltaic Systems," International

Conference Power Generation and Sustainable Development - AIM, Liège, Belgium, October 8-9, 2001, pp.233-238, www.esat.kuleuven.ac.be/~woyte/publics/AIM01-AW.pdf

[14] Trzynadlowski A.M., “Introduction to modern power electronics”, John Wiley & Sons,

Inc., 1998. [15] “Selbsttätige Freihaltstelle für Photvoltaikanlagen einer Nennleistung ≤ 4,6 kVA und

einphasiger Paralleleinspeisung über Wechselrichter in das Netz der öffentlichen Versorgung” (Automatic disconnecting facility for photovoltaic installations with a nomi-nal output ≤ 4.6 kVA and a single phase parallel feed by means of an inverter into the public low-voltage mains), DIN VDE 0126, April 1999.

[16] www.napssystems.com [17] www.scanwafer.com [17] www.bpsolarex.com [18] www.sunwind.no [19] www.solenergi.no


2.8 Micro-Turbines General description Micro-turbines operate on the same principles as the traditional gas turbines. Typical is a very high number of RPM of the turbine and the generator, such as 70.000 to 120.000 RPM. A micro-turbine can have a size of a refrigerator and generate 30 kW of electricity and aggre-gated into one unit it can generate MW of electricity. The generator produces high frequency AC power that is converted to 50 Hz by power elec-tronics. Typical in the range from 25-500 kW, although units may be directly interconnected to provide, up to several MW. The technology varies between the different manufactures [1]-[6].

Figure 2.18 Principle components of micro-turbine [1] The main components of a micro-turbine are: • Gas turbine and recuperator • Permanent magnet generator • Electrical system • Exhaust gas heat exchanger • Supervision and control system • Gas compressor The most micro-turbines uses a turbine mounted on the same shaft as the compressor and a high-speed generator rotor. The rotating components can be mounted on a single shaft that spins up to 120 000 rpm. The micro-turbine could be equipped with oil-lubricated bearings or air bearings.


Figure 2.19 Principle components of micro-turbine [4] Capital costs Capital costs are expected to range in the 500-1.000 EUR/kW range. Efficiency Electrical efficiencies range from 27-32%. Operated in the combined heat and power mode, utilising the exhaust heat can improve the overall efficiency up to 80%. For each produced kilowatt-hour the micro-turbines will produce two kilowatt-hour of heat. Capacity kWel Efficiency Capstone 28 23% Capstone 60 25% IngerSoll-Rand 70 29% Elliot 80 28% Bowman 80 26% Turbec 100 30%

Table 2.4 Micro-turbines Capacity and Efficiency Grid connection An example of an electrical system for a micro-turbine is presented in Figure 2.20. The gen-erator produces AC power. This power is rectified to DC and then afterwards converted to AC power again. An inductor stabilises the AC output and an EMC filter protects the operation to prevent generated interference. The electrical system is entirely controlled and automatically operated by the micro-turbines power controller.

G ~= ~

===

Generator Rectifier/Start converter DC bus Converter Line filter EMC filter

Main circuitbreaker

Figure 2.20 Electrical system for a micro-turbine.


Start up During the start-up sequence the grid power is used to motor the micro-turbine. Power flow through the power controller is then reversed. When power is available from the turbo-generator, the power controller converts the generator output to 3-phase power synchronised with the utility grid. Shut down Shut down procedures differs for normal and emergency stop. During a normal shutdown the following sequence of events occurs: • Fuel flow to the turbo-generator stops • Rotation sped of the turbine decreases • Output power flow ceases • The power electronics switches off. When an emergency shutdown is initiated by one or more protective relay functions the follow-ing events occur: • Output power flow ceases • The power electronics switches off • Fuel flow to the turbo-generator stops. Control in normal operation Micro-turbines are controlled and supervised with automatic control systems. In case of critical distortion the system automatically shuts down and records the fault. The power output de-pends on the type of the unit and selected mode of operation. Power factor The power factor is normally set to one. But there is a possibility to adjust the power factor between for example 0.8 leading and 0.8 lagging. Protection The protection of a micro-turbine normally includes the following functions: • Over and under frequency trip • Over and under voltage trip • Over current and fault current protection • Anti-islanding protection. Placement, Noise and weight The micro-turbines can be placed indoor or outdoor. In the case of indoor solution it might be a problem to find space inside in buildings for example in boiler room. The micro-turbines need some space. For an indoor installation noise levels in the range 70-80 db is not expected to be a problem. But for outdoor placement the requirement could be, that the noise should be less than 45 db. Capacity

kWel Noise

At a distance of Weight

kg

Capstone 28 65 dB(A) 10 meter 500 Capstone 60 70 dB(A) 10 meter 760 IngerSoll-Rand 70 73 dB(A) 1 meter 1 860 Elliot 80 65 dB(A) 10 meter 860 Bowman 80 77 dB(A) 1 meter 1 900 Turbec 100 70 dB(A) 1 meter 2 000

Table 2.5 Micro-turbines Capacity, Noise and Weight


Short time from project to operation One other important opportunity is the short time from decision to production of power and heat. It is expected that new power from micro-turbine can be in operation a couple of weeks from decision. The capacity can then be increased in many small steps. Fuel flexibility Natural gas, town gas, biogas from landfill or sewage, oil, methanol is some of the fuel that can be used. Emissions The emission can vary a little between the manufacturers and also depends on the fuel. Val-ues in the range of NOx< 15 ppm/v and CO<15 pm/v are expected. These low emissions is to the advantage of the micro-turbine. Scalability Micro-turbines have the opportunity to aggregate more units into one unit. This means that many distributed generators can work together as one unit with a very high availability.

Figure 2.21 Micro-Turbine scalability Remote control and dispatch Micro-turbines can be operated manually from the display panel at the front of the unit, or by serial modem connection. All the communications and control features of the system are avail-able by either method. Standards and recommendations The general standards and recommendations for CHP connection to the network can be ap-plied for micro-turbines.


Application The micro-turbine can be used for the following applications: • Combined heat and power production • Combined cooling and power power production • Stand-by power • Un-interruptible power • Peak shaving • Load following unit • Stand alone. Summary The micro-turbine is a reliable, environmentally beneficial solution for power generation. The micro-turbines can be used for peak shaving stand-by power, capacity addition, stand-alone and others. In the case of capacity addition the short time from decision and order to operation can be a heavy argument distributed generation with micro-turbines in the future. For the manufactures the market for micro-turbines can be huge especially if they can produce large numbers of standard engines. The units should not require special permits and should have low maintenance and low investment costs. References [1] www.capstoneturbine.com [2] www.bowmanpower.co.uk [3] www.irpowerworks.com [4] www.turbec.com [5] www.elliott-turbo.com


2.9 Fuel Cells General description Fuel cells are able to convert fuels and oxygen into electricity, heat and water. Fuel cells are similar to batteries in that they both use an electrochemical process to produce DC current. The fuel cell is not a new technology, the principle has been known for two hundred years and development has been done for forty years. Fuel cells are very versatile and can potentially be used for every application needing power from cell phones to multi MW power plants. Both batteries and fuel cells consist of two electrodes separated by an electrolyte. Unlike bat-teries, fuel cells electrochemically convert the energy in a hydrogen-rich fuel directly into elec-tricity and operate as long as the fuel stream lasts. The individual fuel cell only gives a voltage of 1 volt, so it is necessary to connect a large num-ber of cells in a series, giving a so-called cell stack with the desired voltage. The many differ-ent types of fuel cells are usually named after their electrolytes. The primary types are: The Alkaline Fuel Cell with an aqueous KOH electrolyte, the Phospho-ric Acid Fuel Cell, the Solid (Polymer) Proton Conductor Fuel Cell, the Molten Carbonate Fuel Cell and the Solid Oxide Fuel Cell, whose electrolyte is a ceramic oxygen ion conductor. Depending in the electrolyte the fuel cell operates between 80 and 1.000 C, ignoring this pro-duced heat can raise the efficiency to over 80%. As hydrogen usually not is directly available most fuel cells are fuelled with hydrogen which is processed from available hydrocarbons, e.g. natural gas. Normally the fuel processing is an integral part of the fuel cell system.

Figure 2.22 Fuel cell system [1] Operating characteristics All fuel cells generate a direct current, the voltage depending on cell voltage and the number of cells in series. Furthermore the voltage varies with the load and also to some extent with time as the fuel cell stack ages. Typically a fuel cell supplying AC current has power-conditioning equipment handling DC to AC conversion and current, voltage and frequency control. Apart


from supplying power to the external point of supply the fuel cell also has to cover some inter-nal power needs, e.g. pumps, fans and control system. As fuel cells are in a development stage it is difficult to make general statements about operat-ing characteristics as for example operating procedures tends to be on the cautionary side. Capital and O&M Costs Cost targets for fuel cells are commonly given as 800-1500 EUR/kW with O&M costs in region of 0.005-0.010 EUR/kWh. Today’s costs are an order of magnitude larger but is on the other hand for small, practically hand built series Efficiency There are four major fuel cell technologies with somewhat different characteristics. The main apparent difference is the electrolyte, which also have far reaching effects on the design and operating characteristics of the fuel cell. In Table 2.6 these four technologies are listed with some key characteristics. PEMFC (PEFC)

PAFC

MCFC

SOFC

Electrolyte Protone Exchange Membrane

Phosphoric Acid

Molten Carbonate

Solid Oxide

Operating temperature (°C)

80 200 650 800 – 1000

Electric efficiency based on na-tural gas (%)

30 – 35 35 - 40 45 - 55 45 – 55

Table 2.6 Major fuel cell technologies Grid connection Fuel cells can be grid independent or grid parallel as well as a combination of the both. In the latter case a grid failure means that fuel cell disconnects from the grid and works as grid inde-pendent. A typical layout of a fuel cell system can be seen in Figure 2.23.

Figure 2.23 Typical power conditioning system of a residential fuel cell In this system the DC/DC converter creates and maintains a constant DC voltage, here called DC Link, from the varying output from the fuel cell. A constant DC input to the inverter means that the latter can be efficiently designed and, furthermore, is of-the-shelf product from many manufacturers. The grid connection is principally the same as for other DG technologies especially for large fuel cells. For residential fuel cells there is need to keep to costs down with regard to the small


allowable unit cost. As development is ongoing it is not clear to which extent the need for cheap interconnections will confirm with the demands of grid operator. Start up and Shut down Inverter must include automatic synchronisation to the grid. To reduce transient resistive dummy loads can be used at start-up or shutdown. Start-up time depends on type of fuel cell and type of fuel processing system. A low tempera-ture fuel cell (PEMFC) with partial oxidation could probably be started in a couple of minutes while a high temperature fuel cell takes 3-4 hours due to the need of avoiding thermal stresses during warm up. Generally speaking, high temperature fuel cells are not suited to start-stop operation. Control in normal operation The load controls power on the DC-side as fuel cell acts similarly to a battery (with high inter-nal resistance). In a grid-connected mode the fuel cell feeds in power against the full load of the grid, which means that there are special demands on the inverter in order to control the load. Protection Standard protection as for other DG technologies includes protection against grid distur-bances, over-under voltage, etc. As there is additional risk for islanding for DG equipment connected through inverters there are possibly a need for more sophisticated protection for safety reasons. For residential fuel cells there will probably a demand for an external switch to ensure personal safety for grid maintenance personnel. The connected power distribution system is protected by conventional equipment isolation in case of an over-current malfunction on the fuel cell side. Emissions Fuel cells are virtually emission free. With hydrocarbons as fuels there is, apart from the inevi-table carbon dioxide, trace amounts of NOx and CO from the fuel processing system. Reliability Fuel cells have a potential for high reliability as the number of moving parts is low and consists of auxiliary equipment such as fans and pumps but this remains to be proven. The target for life length of fuel cells is usually given as 40000 h for the stack and at least twice the number of hours for the system. This target has been reached for a small number of fuel cells but in general still remain to be proven. Scalability Fuel cells are very versatile and can potentially be used for every application needing power from cell phones to multi MW power plants. For DG purpose fuel cells from 0.5 kW and upward are developed. Standards and recommendations • USA

o A specific residential fuel cell recommendation is in [4]. o The US standard for connection of fuel cells is in [5].

Applications Proposed applications include continuous generation, cogeneration or power only, remote power, backup power etc. A fuel cell in it self can facilitate nearly instant load changes. However, a fuel cell system has a limiting factor in the fuel processing system which has a certain time lag (varying depending on type) and a truly load following system would need a buffer, e.g. batteries or hydrogen storage capacity. A typical turndown ratio of a fuel system is about 1:5 and high efficiencies are kept to at least half load.


Summary As the situation is today, it will probably take some more years for fuel cells to become a com-petitive commercial technology. Due to the development within gas turbines, fuel cells are expected to become competitive only for units below approx. 20 MW. On the other hand there are numerous activities on-going with the objective of putting fuel cell systems on the market in the 2005-2010. While the development covers the entire range up to 1 MW an emphasis can be seen on so called residential fuel cells. Residential fuel cells are systems in the 0,5-10 kW range which are targeted against single or multi family houses, small enterprises etc and planned to be pro-duced in very large numbers. Obvious obstacles or rather uncertainties are the economic and technical challenges that lie ahead for fuel cells. If these are solved there are also questions regarding installation, techni-cal support and business model that needs to be addressed. The first two items is about building up a new service structure and the last is how to handle the at least initial technology risk. A barrier for fuel cells is the availability of fuel, primarily natural gas although other fuels as propane, biogas and diesel are feasible. For residential fuel cells there is also a need for stan-dards and codes with regard to the installation in buildings. As a highly efficient, versatile and environmentally compatible new technology there is an abundance of opportunities for fuel cells. The potential market for stationary fuel cells in DG applications has been estimated to billions of dollar in various studies, which of course is a major attraction for the involved companies. For governments fuel cells represent one impor-tant component in reducing green house gases due to its high electric efficiency and large number of possible applications. References [1] www.siemenswestinghouse.com/en/fuelcells/demo/index.cfm [2] www.scanwafer.com [3] www.ercc.com/site/products/commercial.html [4] Torrero, E; McClelland, R. "Residential Fuel Cell Demonstration Handbook". NREL/SP- 560-32455. Golden, CO: National Renewable Energy Laboratory. July 2002, http://www.eren.doe.gov/distributedpower/library.html. [5] “IEEE 1547: Standard for Interconnection Distributed Resources with Electric Power Systems”, http://www.eren.doe.gov/distributedpower/library.html


2.10 Stirling engine General description On September 27, 1816, Robert Stirling applied for a patent for his Economiser at the Chan-cery in Edinburgh, Scotland. By trade, Robert Stirling was actually a minister in the Church of Scotland and he continued to give services until he was eighty-six years old. But, in his spare time, he built heat engines in his home workshop. Lord Kelvin used one of the working models during some of his university classes. In 1850 the simple and elegant dynamics of the engine were first explained by Professor McQuorne Rankine. Approximately one hundred years later, the term "Stirling engine" was coined by Rolf Meijer in order to describe all types of closed cycle regenerative gas engines. Today, Stirling engines are used in some very specialised applications, like in submarines or auxiliary power generators, where quiet operation is important. Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot Cycle efficiency.

Stirling engine G

Air

CombustionFuel

Heatingsystem

Exhaust

Cooling

Figure 2.24 Co-generation system based on Stirling engine Stirling engines are powered by the expansion of a gas when heated followed by the compres-sion of the gas when cooled. The Stirling engine contains a fixed amount of gas, which is transferred back and forth between a "cold" and a "hot" end. The "displacer piston" moves the gas between the two ends and the "power piston" changes the internal volume as the gas expands and contracts. The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet. The Stirling cycle uses an exter-nal heat source, which could be anything from gasoline to solar energy to the heat produced by decaying plants. No combustion takes place inside the cylinders of the engine. The best working gas in Stirling engine is hydrogen. Helium is working nearly as good as the hydrogen, but it is much more expensive. The cheapest alternative is air, but it has much worse properties than the other two gases. The example of the co-generation system with a Stirling engine is shown in Figure 2.24. The heat source for the Stirling engine is situated outside the engine. The fuel is burned in the combustion chamber in presence of air. The hot gases which appearing in the combustion process warm up the Stirling engine. At the same time water or the surrounding air cools it down. Evidently, water is preferable because in this case the engine can be used also for heat-


ing purposes. The exhaust gases on their way out first warm up the air coming into the com-bustion chamber and then water for the heating system. Capital and O&M Costs As an example, the investment cost for a complete micro-CHP from SOLO-Kleinmotoren is about 1 800 €/kWe. Annual operation and maintenance costs are estimated at about 6% of the total investments cost. Efficiency Electrical efficiency of the engine is about 25%. The total efficiency in co-generation applica-tion is as high as 90%. If water is condensed from the exhaust gases the total efficiency can approach 100%.

Figure 2.25 Fuel flexibility from Stirling Motor Fuel flexibility Since the combustion is external almost any fuel is suitable for utilisation in Stirling engines. The most typical fuel is natural gas, but also bio-fuel and even solar energy can be used. There is a requirement that the natural gas should be under high pressure. The required pres-sure depends on the type of the engine, but typically is between 100-200 mbar. For bio-fuel there is a limitation concerning the moisture content – big variations in moisture content should be avoided. Emission From the environmental point of view Stirling engines are relatively good. According to the 10 kWe Stirling engine fuelled by natural gas exhausts in average 15 ppm Nox at λ=1.4. This corresponds to about 10 mg/MJ NO2. Summary Stirling motors. They are external combustion engines where fuel is burnt continuously to heat one part of the engine cylinder. This is unlike internal combustion engines, where fuel is in-jected into the cylinder intermittently and then ignited. They require very little maintenance, are non fuel specific and have high efficiencies. Moreover their size can be highly reduced. But they are very expensive and it is difficult to tell how much the cost will be reduced by a large production due to deregulation in many coun-tries. Therefore Stirling engines are still being developed and will not be available until at least 10 years.


For wide commercial application of Stirling engines more documented experience from the installed units is required. Right now this experience is still insufficient in order to get serious attention to the technology. Advantage of Stirling engine is relatively high efficiency, good operation possibilities with par-tial load, good modularity and quit operation. Today there are developed Stirling engines available for commercial application. However the interest for Stirling engine for co-generation is still low. Therefore, it is important to continue research concerning application of Stirling engine for co-generation and perform more tests on real installed units. Another important research field is related to the fuel for Stirling engines. One of the promising alternatives is bio-fuel. Thus in Denmark there is a test installation where the combustion chamber is replaced by the bio-fuel boiler. The engine is 35 kWe and has the electrical effi-ciency 19% and total efficiency 87%. Referencer [1] www.sterlingenergy.com [2] www.senertec.de [3] www.stirling-engine.de [4] www.energystoragecouncil.org/ [5] www.eren.doe.gov/der/energy_storage.html [6] H. Carlsen, “Status of small-scale power production based on Stirling engines”, Department for Energy Engineering, Technical University of Denmark.


2.11 Conclusion The overview of costs in Table 2.7 gives an indication on the competitiveness of the different kinds of distributed generators. It has to be pointed out, that the values are roughly estimated. In real applications the costs may be outside the given range depending on the individual conditions.

Technology electric capacity

capital costs EUR/kWe

total costs EUR/ kWhe

Wind Turbine (off-shore) 100 MWe 1500 - 2000 0.050 - 0.120 CHP 40 MWe 550 - 850 0.040 - 0.057 Wind turbine (on-shore) 15 MWe 900 - 1300 0.040 - 0.090 Hydro Power (low-head) 5 MWe 900 - 1000 0.020 - 0.030 Turbine 5 MWe 800 - 850 0.053 - 0.057 Reciprocating engine 5 MWe 500 - 750 0.030 - 0.045 PV 5 MWe 6000 - 10000 0.750 - 1.000 Fuel Cells 5 MWe 1100 - 1600 0.080 - 0.100 Reciprocating engine 50 kWE 600 - 1500 0.070 - 0.150 Turbine 50 kWE ~ 300 0.030 - 0.050 Fuel cells 50 kWE ~ 900 0.090 - 0.150

Table 2.7 Capital and energy costs for DG-facilities Source: CIGRE WG 37-23 [1] There are different ways to classify DG technologies. The functions normally specified for the traditional generator are here used as classifiers: • Power generation • Overload capacity • Short term reserve (spinning or standing) • Performance of frequency control • Provision of voltage control (reactive power) • Generation of a fault current • "Fault-Ride-Through" capability • "Black Start" capability (after a system outage) By proper design, many of the services the traditional generator has offered for a century can also be offered by the "new" generators, this is briefly indicated in table 2.8, where an overview of the main characteristics for the following generators, can be found: • Traditional generator • Combined heat and power plant • Reciprocating engine • Wind turbine 1,2 and 3 • Hydro turbine • Photovoltaic • Micro-Turbine • Fuel cell • Stirling Engine


CG CHP Diesel WEC1

WEC2

WEC3

HP PV Micro Turbine

Fuel Cells

Stirling

Power Generation Flexibility

Yes

Yes

Yes

No

Yes

Yes

Yes

No

Yes

No

Yes

Overload Capacity

Yes

Yes

No

No

No

No

No

No

No

Reserve Yes Yes Yes No Yes Yes Yes No Yes Frequency Control

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

Voltage Control

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

Generation of Fault Current

Yes

Yes

Yes

Yes

No

No

Yes

No

"Fault-Ride-Through" Capability

Yes

Yes

Yes

No

No

No

Yes

"Black Start Capability"

Yes

Yes

Yes

Heat load dependent

Yes/ No

Yes/ No

Yes/ No

No

No

No

No

No

Yes/ No

Yes/ No

Yes/ No

Weather dependent

No

No

No

Yes

Yes

Yes

Yes/No

Yes

No

No

No

Table 2.8 Overview over DG characteristics References [1] Cigre Working Group 37.23, "Impact of increasing contribution of dispersed generation

on the power system", September 1998, www.cigre.org


3. INTERACTION WITH THE ELECTRICITY NETWORK 3.1 Introduction The modern electricity supply network is a complex system. The somewhat vague term „power quality“ is used to describe the interaction between producers operating fossil fired, nuclear, hydro, wind or photovoltaic power plants and consumers. The latter may be large (heavy in-dustry - metal melting) or small (private homes) consumers. In the following, a short introduction is given to each of the electrical parameters which taken together are used to characterise power quality - or more correct, voltage quality - in a given point in the electricity supply system. 3.2 Power quality 3.2.1 Steady state voltages Voltage variations are the main problem associated with power generation. Normal static tol-erances on voltage levels are -10 to +6%. However, fast small variations become a nuisance at penetrations as low as 0.3% and in weak grids. These variations are often found in remote areas where for example the wind conditions are best. This can be the limiting factor on for example the amount of wind turbines which can be installed in these areas. Table 3.1 gives an overview over static voltage levels in EU network connection rules [17].

steady state volt-age

EN 50160

[9]

B D [12] DK E GB NL [13]

Max. ∆ult in LV ±10% 3% - - Max. ∆ult in MV ±10% 2% 1%

(5%) 5%

(2%) -

Max. ∆ult in HV - - - 5% (2%)

-

Max. Ult in LV 110% 106% 106% - 106% Max. Ult in MV 110% - - - 105% Max. Ult in HV - - - - 110% min. Ult in LV 90% 90% 90% - 90% min. Ult in MV 90% - - - 95% min. Ult in HV - - - - 90%

Table 3.1 Overview of steady state voltage levels in EU network connection rules Source: CIGRE WG 37-23 [17] 3.2.2 Short Circuit power level The short circuit power level in a given point in the electrical network is a measure of its strength and while not directly a parameter in the voltage quality it has a heavy influence. The ability of the grid to absorb disturbances is directly related to the short circuit power level of the point in question. Any point (p) in the network can be modelled as an equivalent circuit as shown in Figure 3.1. Far away from the point the voltage can be taken as constant i.e. not influenced by the conditions in p.


Figure 3.1 Equivalent circuit The voltage in this remote point is designated USC and the short circuit power level SSC in MVA can be found as USC

2 / ZSC where ZSC is the line impedance. Variations in the load (or produc-tion) in p causes current variations in the line and these in turn a varying voltage drop (dU) over the line impedance ZSC. The voltage in p (UL) is the difference between USC and dU and this resulting voltage is seen by - and possibly disturbing - other consumers connected to p. Strong and/or weak grids are terms often used in connection with DG installations. It is obvious from figure 3.1, that if the impedance ZSC is small then the voltage variations in p will be small (the grid is strong) and consequently, if ZSC is large, then the voltage variations will be large. Strong or weak are relative terms. For any given DG installation, of installed capacity P [MW], the ratio RSC = SSC / P is a measure of the strength. The grid is strong with respect to the installation if RSC is above 20 to 25 times and weak for RSC below 8 to 10 times. Depending on the type of electrical equipment in the DG generator they can sometimes be operated successfully under weak conditions. Care should always be taken, for single or few DG generators in particular, as they tend to be relatively more disturb-ing than installations with many units. 3.2.3 Voltage variations and flicker Voltage variations caused by fluctuating loads and/or production is the most common cause of complaints over the voltage quality. Very large disturbances may be caused by melters, arc-welding machines and frequent starting of (large) motors. Slow voltage variations within the normal -10+6% tolerance band are not disturbing and neither are infrequent (a few times per day) step changes of up to 3%, though visible to the naked eye. Fast and small variations are called flicker. Flicker evaluation is based on IEC 1000-3-7 which gives guidelines for emission limits for fluctuating loads in medium voltage (MV, i.e. voltages between 1 and 36 kV) and high voltage (HV, i.e. voltages between 36 and 230 kV) networks. The basis for the evaluation is a measured curve giving the threshold of visibility for rectangu-lar voltage changes applied to an incandescent lamp. Disturbances just visible are said to have a flicker severity factor of Pst = 1 (Pst for P short term). Furthermore, a long term flicker severity factor Plt is defined. Where Pst is measured over 10 minutes and Plt is valid for two hour periods. IEC 1000-3-7 gives both planning levels, that is total flicker levels which are not supposed to be exceeded and emission level, that is the contributions from an individual installation which must not be exceeded.

Zsc I

Usc dU = Zsc * I UL

p


Flicker Severity factor

Planning level

Planning level

Emission level

MV HV MV and HV Pst 0.9 0.8 0.35 Plt 0.7 0.6 0.25

Table 3.2 Flicker planning and mission levels for medium (MV) and high voltage (HV) The recommended values are given in table 3.2 Determination of flicker emission is always based on measurement. IEC 61000-4-15 specifies a flicker-meter which can be used to meas-ure flicker directly. As flicker in the general situation is the result of flicker allready present on the grid and the emissions to be measured, a direct measurement requires an undisturbed constant impedance power supply and this is not feasible for DG generators due to their size. Instead the flicker measurement is based on measurements of three instantaneous phase voltages and currents followed by an analytical determination of Pst for different grid impedance angles by means of a „flicker algorithm“ - a programme simulating the IEC flicker-meter. Table 3.3 gives an overview of voltage variations and flicker in EU network connection rules.

voltage quality EN 50160

B D DK E GB NL [14]

Flicker ∆U≤1% Max. Plt in LV 1 0.464 1.0 - - Max. Pst in LV - - - - - Max. Plt in MV 1 0.464 - 0.9 - Max. Pst in MV - - 1.0 0.9 - Max. Plt in HV - - - 0.6 - Max. Pst in HV - - - 0.8 - amplitude of fast voltage fluctuations

∆U≤1%

Max. ∆ust in LV 4-6% 3% - - - Max. ∆ust in MV 5-10% 2% - - - Max. ∆ust in HV - - - - - Harmonics - Limited limited limited -

Table 3.3 Overview of voltage variations and flicker in EU network connection rules Source: CIGRE WG 37-23 [17] 3.2.4 Harmonics Harmonics are a phenomenon associated with the distortion of the fundamental sine-wave of the grid voltages, which is purely sinusodial in the ideal situation. The concept stems back to the French matematician Josef Fourier who in the early 1800 found that any periodical function can be expressed as a sum of sinusodial curves with different frequencies ranging from the fundamental frequency - the first harmonic - and integer multiples thereof where the integer designates the harmonic number. Harmonic disturbances are produced by many types of electrical equipment. Depending on their harmonic order they may cause different types of damage to different types of electrical equipment. All harmonics causes increased currents and possible destructive overheating in capacitors as the impedance of a capacitor goes down in proportion to the increase in frequency. As har-


monics with order 3 and odd higher multiples of 3 are in phase in a three phase balanced net-work, they cannot cancel out between the phases and cause circulating currents in the delta windings of transformers, again with possible overheating as the result. The higher harmonics may further give rise to increased noise in analogue telephone circuits. Highly distorting loads are older unfiltered frequency converters based on thyristor technology and similar types of equipment. The characteristic for this type is that its switches one time in each half period and it may generate large amounts of the lower harmonic orders. Newer transistor based designs are for example used in most variable speed wind turbines today. The method is referred to as Pulse Width Modulation (PWM). It switches many times in each period and typically starts producing harmonics where the older types stop, that is around 2 kHz. Their magnitude is smaller and they are easier to remove by filtering than the harmon-ics of lower order. IEC 1000-3-6 put forward guidelines on compatibility and planning levels for MV and HV net-works and presents methods for assessing the contribution from individual installations to the overall disturbance level. The distortion is expressed as Total Harmonic Distorsion (THD) and the recommended com-patibility level in a MV system is 8 % whereas the indicative Planning levels for a MV system is 6.5 % and 3 % in a HV system. 3.2.5 Frequency The electrical supply and distribution systems used worldwide today are based on alternating voltages and currents (AC systems). That is, the voltage constantly changes between positive and negative polarity and the current its direction. The number of changes per second is des-ignated the frequency of the system with the unit Hz. In Europe the frequency is 50 Hz whereas it is 60 Hz in many other places in the world. The frequency of the system is proportional to the rotating speed of the synchronous genera-tors operating in the system and they are - apart from an integer even factor depending on machine design - essentially running at the same speed: They are synchronised. Increasing the electrical load in the system tends to slow down the generators and the frequency falls. The frequency control of the system then increases the torque on some of the generators until equilibrium is restored and the frequency is 50 Hz again. The requirements to frequency con-trol in the West European grid are laid down in the UCTE rules. The area is divided in a number of control zones each with its own primary and secondary control. The primary control acts on fast frequency deviations, with the purpose of keeping equilibrium between instantaneous power consumption and production for the whole area. The secondary control aims at keeping the balance between production and demand within the individual zones and keeping up the agreed exchange of power with other zones. The power required for primary control is 3000 MW distributed throughout the control zones whereas the frequency control related to keeping the time for electric grid controlled watches is accomplished by operating the system at slightly deviating frequencies in a diurnal pattern so that the frequency on an average is 50 Hz. In the Scandinavian grid a similar scheme is oper-ated in the NORDEL system. 3.2.6 Reactive Power Reactive power is a concept associated with oscillating exchange of energy stored in capaci-tive and inductive components in a power system. Reactive power is produced in capacitive components (e.g. capacitors, cables) and consumed in inductive components (e.g. transform-ers, motors, flourecent tubes).


The synchronous generator is special in this context as it can either produce reactive power (the normal situation) when over-magnetised, or consume reactive power when under-magnetised. Voltage control is affected by controlling the magnetising level of the generator, i.e. a high magnetising level results in high voltage and production of reactive power. As the current associated with the flow of reactive power is perpendicular (or 90 deg. out of phase) to the current associated with active power and to the voltage on the terminals of the equipment the only energy lost in the process is the resistive losses in lines and components. The losses are proportional to the total current squared. Since the active and reactive currents are perpendicular to each other, the total resulting current is the root of the squared sum of the two currents and the reactive currents hence contribute as much to the system losses as do the active currents. To minimise the losses it is necessary to keep the reactive currents as low as possible and this is accomplished by compensating reactive consumption by installing capacitors at or close to the consuming inductive loads. Furthermore, large reactive currents flowing to inductive loads, is one of the major causes of voltage instability in the network due to the associated voltage drops in the transmission lines. Locally installed capacitor banks, mitigates this tendency and increases the voltage stability in area. Many wind turbines are equipped with induction generators. The induction generator is basi-cally an induction motor, and as such a consumer of reactive power, in contrast to the syn-chronous generator which can produce reactive power. At no load (idling), the consumption is in the order of 35-40% of the rated active power increasing to around 60% at rated power. In any given local area with wind turbines, the total reactive power demand will be the sum of the demand of the loads in and the demand of wind turbines. To minimise losses and to in-crease voltage stability, the wind turbines are compensated to a level between their idling re-active demand and their full load demand, depending on the requirements of the local distribu-tion network operator. Table 3.4 gives an overview of var-compensation in EU network connection rules.

Var-compensation EN 50160

B D DK E GB NL [13]

Control required yes yes yes Max. Var-consumption

48% P 10% 60%

Max. Var-injection 75% P 40% 60% Table 3.4 Overview of var-compensation in EU network connection rules Source: CIGRE WG 37-23 [17] 3.3 Protection The extent and type of electrical protective functions in a DG generator is governed by two lines of consideration. One is the need to protect the generator, the other to secure safe opera-tion of the network under all circumstances. The faults associated with first line are short circuits in the generator, overproduction causing thermal overload and faults resulting in high, possibly dangerous, over-voltages, that is earth-faults and neutral voltage displacement.


The second line can be described as the utility view. Here the objective is to disconnect the generator when there is a risk to other consumers or to operating personnel. The faults asso-ciated with this line are situations with unacceptable deviations in voltage and/or frequency and loss of one or more phases in the utility supply network. The required functions are: • Over frequency • Under frequency • Over voltage • Under voltage • Loss of mains • High over-currents (short circuit) • Thermal overload • Earth fault • Neutral voltage displacement Depending on the generator design, that is if it can operate as an autonomous unit, a Rate Of Change Of Frequency (ROCOF) relay may be needed to detect a step change in frequency indicating that the generator is operating in an isolated part of the network due for example to tripping of a remote line supplying the area. Table 3.5 gives an overview of protective relaying in MV in EU network connection rules.

protective relaying in MV

EN 50160

B D DK E GB NL [13]

limitation of SCC Yes yes - yes Yes over current - yes coordi-

nated -

over voltage / % 0-15 6-10 10 10 under voltage / % 0-30 10-30 15 10 70 over frequency / Hz 0-2 1-3 1 1 1 under frequency / Hz 0-2 2.5-3 1 6 2 Other - - yes -

Table 3.5 Overview of protective relaying in MV in EU network connection rules Source: CIGRE WG 37-23 [17] 3.4 Network stability The problem of network stability has been touched upon briefly above. Three issues are cen-tral in the discussion and all are largely associated with different types of faults in the network such as tripping of transmission lines (e.g. overload), loss of production capacity (e.g. any fault in boiler or turbine in a power plant) and short circuits. Permanent tripping of transmissions lines due to overload or component failure disrupts the balance of power (active and reactive) flow to the adjacent areas. Though the capacity of the operating generators is adequate large voltage drops may occur suddenly. The reactive power following new paths in a highly loaded transmission grid may force the voltage operating point of the network in the area beyond the border of stability. A period of low voltage (brownout) possibly followed by complete loss of power is often the result. Loss of production capacity obviously results in a large power unbalance momentarily and unless the remaining operating power plants have enough so called „spinning reserve“, that is generators not loaded to their maximum capacity, to replace the loss within very short time a large frequency and voltage drop will occur followed by complete loss of power. A way of rem-edy in this situation is to disconnect the supply to an entire area or some large consumers with


the purpose of restoring the power balance and limit the number of consumers affected by the fault. Short circuits take on a variety of forms in a network and are by far the most common. In se-verity they range from the one phase earth fault caused by trees growing up into an overhead transmission line, over a two phase fault to the three phase short circuit with low impedance in the short circuit itself. Many of these faults are cleared by the relay protection of the transmis-sion system either by disconnection and fast reclosing, or by disconnection of the equipment in question after a few hundred milliseconds. In all the situations the result is a short period with low or no voltage followed by a period where the voltage returns. A large - off shore - wind farm in the vicinity will see this event and disconnect from the grid immediately if only equipped with the protection described above. This is equivalent to the situation „ loss of production capacity“, and disconnection of the wind farm will further aggravate the situation. Up to now, not many transmission network operators has put forward requirement to dynamic stability of wind turbines during grid faults. The situation in Denmark (Eltra) and Germany (E-ON) today, and the visions for the future, has changed the situation for wind farms connected to the transmission grid, that is at voltages above 100 kV. In these countries this is now required. 3.5 Synchronising, switching operations and soft starting Influence of single events on voltage quality, of which synchronising of DG units with synchro-nous or asynchronous generators is the most important. A synchronous generator should not be switched parallel to the network, unless the following constraints are fulfilled:

∆∆∆

U 10 %f 0.5 Hz

10

<<< °ϕ

Connection and - to a smaller degree - disconnection of electrical equipment in general and induction generators/ motors especially, gives rise to so called transients, that is short duration very high inrush currents causing both disturbances to the grid and high torque spikes in the drive train of for example a wind turbine with a directly connected induction generator. In this context, wind turbines fall into two classes, one featuring power electronics with a rated capacity corresponding to the generator size, in the main circuit, and one with zero or low rat-ing power electronics in a secondary circuit - typically the rotor circuit of an induction genera-tor. The power electronics in the first class can control the inrush current continuously from zero to rated current. Its disturbances to the grid during switching operations are minimal. Unless spe-cial precautions are taken, the other class will allow inrush currents up to 5-7 times the rated current of the generator after the first very short period (below 100ms) where the peak are considerably higher, up to 18 times the normal rated current. A transient like this disturbs the grid and to limit it to an acceptable value all wind turbines of this class are equipped with a current limiter or soft starter based on thyristor technology which typically limits the highest RMS value of the inrush current to a level below two times the rated current of the generator. The soft starter has a limited thermal capacity and is short circuited by contactor able to carry the full load current when connection to the grid has been completed. In addition to reducing the impact on the grid, the soft starter also effectively dampens the torque peaks in the air gap of the generator associated with the peak currents and hence reduces the loads on the gear-box.


3.6 Reserve Table 3.6 gives an overview of contribution to reserve in EU network connection rules.

contribution to re-serve

EN 50160

B D DK E GB NL [13]

Primary - Yes - 3% Secondary - - - - Tertiary - - - -

Table 3.6 Overview of contribution to reserve in EU network connection rules Source: CIGRE WG 37-23 [17] 3.7 Connection of DG generators to distribution network Connection of some DG generator schemes is compromised by the affect is has on the local distribution system. This can lead to a reduction in allowable connectable generation capacity. There are a number of issues that can limit the installed capacity of DG generators. These are often voltage related and the most common is steady-state voltage rise. A number of tech-niques can be applied to limit steady-state voltage rise, some of which are static in time (e.g. network reinforcement) and some dynamic (.e.g. power factor control) A general characteristic of non-dispatchable DG is a fluctuating power output which is usually not directly correlated with the electrical load. The resulting network voltage fluctuations super-impose themselves on existing fluctuations caused by changes in load and may lead to a wid-ening of voltage bands. This widening of the voltage bands uses up network reserve which are then unavailable for additional customers. The acceptable capacity of a DG generator is usually established by deterministic methods, by taken the "worst case scenario" of simultaneously: • maximum primary substation bus-bar voltage • minimum consumer loading • maximum generator output (in case of for example wind turbines) If the voltage is found to be high, it is reduced until an acceptable level is reached and this marks the limit of allowed installed capacity. The example of Figure 3.2 shows that after deducting the max. expected voltage drop in the low voltage network (∼ 5%), the local network transformers (∼ 2,5%) and taking into considera-tion the stepping tolerance of the voltage regulator on the HV/MV-transformer (∼ 2%) a restric-tion of the voltage bands in the medium voltage network to approx. 6,5% is necessary. A lower voltage drop in LV increases the capacity in MV and vice versa. Additional components for voltage regulation installed in dispersed locations in such networks may increase the transmis-sion capacity from this point of view. To enable full utilization of a given site a number of methods may be employed to limit the steady-state voltage rise. These fall into three categories: • reduction of line impedance • increase of generator reactive power import • reduction of generator power output Reduction of line impedance means network reinforcement.


6,5 %

MV-network LV-networkHV-network∆UU

2,5 % 5 %

deviation voltage drop

230 V+6%

-10%16 % variation admissible

2 %

Figure 3.2: Example for the calculation of admissible voltage drop in a MV-network Increase of generator reactive import can to a certain degree increase the voltage bands. This may also be desirable from the point of view of optimising the reactive power flow in higher voltage levels. Thus, the power factor of a DG generator should not generally be set to a fixed value (often 1) but should be regarded as a degree of freedom of the connection which is set in every individual case in order to meet the local requirements. The idea of reducing the generator power output comes from the fact, that the power produc-tion from for example wind turbines mostly occur below the rated capacity and this of course reflects the variable nature of the wind. The load factor for a normal wind turbine is between 30% to 40%. The probability of an over-voltage relies on simultaneous occurrence of high gen-erator output, high sub-station bus-bar voltage and low consumer loading, and can often be very low. Using only "worst case scenarios" to determine installed capacity can be very limiting. Some authors have examined the benefits of using a probabilistic approach to determine over-voltage rather than the deterministic approach [45]. The connection of DG will - if not by admissible voltage bands - be limited by the current carry-ing capacity of the equipment which is determined by the thermal load it will bear. Nowadays it is normally assumed that the thermal current limit can be exceeded for short periods - e.g. for the rapid restoration of supply after a failure. On the other hand the generation characteristics of many types of DG with longer periods of rated power output don’t allow the same maximum current on the equipment as for typical MV-loads with its fluctuations during the day 3.8 Costs of grid Connection The costs for grid connection can be split up in two. The costs for the local electrical installa-tion, and the costs for connecting the generator to the electrical grid. The local electrical installation comprises the medium voltage grid in the generator up to a common point and the necessary medium voltage switchgear at that point. Cited total costs for this item ranges from 3 to 10 % of the total costs of the complete generator. It depends on local equipment prices, technical requirements, soil conditions, the distance between the tur-bines, the size of the generator and hence the voltage levels for the line to the connecting point the existing grid. If the generator is large and the distance to the grid long there may be a need for a common transformer stepping up the medium voltage in the generator to the local high voltage transmission level. The costs for connection to the electrical grid ranges, from almost 0% for a small farm con-nected to an adjacent medium voltage line and upwards. For a 150 MW off-shore wind farm a figure of 25 % has been given for this item. Compared to onshore wind farms there is a number of additional costs and uncertainties to take into account when assessing the production costs from large offshore wind farms. The relationship between the different costs items usually specified is quite different from the rela-tionship found for onshore wind farms.


The following Table 3.7 indicates a probable distribution between the different items for a 150 MW offshore wind farm situated approximately 20 km from the shore and with a further 30 km to the nearest high voltage substation where it can be connected to the existing grid. The table further gives the absolute costs in Mill. € (Euro) and - for comparison - shows the distribution between comparable items for a typical onshore wind farm.

Item Off-shore On-shore Costs in Mill. € % %

Foundations 36 16 5,5 Wind turbines 113 51 71,0 Internal electrical grid 11 5 7,5 Off-shore transformer station 4,5 2 Grid connection 40 18 7,5 O&M facilities 4,5 2 Engineering and project administration

8,9 4 2,5

Miscellaneous 4,5 2 7,0 Total 1650 100 100,0 Table 3.7 Costs of a 150 MW wind farm 3.9 Comment to network connection rules 3.9.1 Denmark In Denmark there is a set of recommendations which describe the technical demands to grid connected wind turbines. These specifications ensure that there is no unacceptable distur-bance in the distribution grid caused by wind turbines. For instance there are requirements for continuous production, compensation by capacities, flicker and the protection system for the wind turbine. As guidance to the distribution company there are recommendations of how to solve problems with regard to design of the grid, rising voltage, apparent power and transformer sizes. Until now there are approx. 5,000 wind turbines connected to the distribution networks respect-ing these specifications Eltra [www.eltra.dk] has developed a set of regulations regarding connections of large wind farms to the transmission network. The regulations apply both to onshore and offshore wind farms. The small-scale production has prioritised access to the network so that distribution companies are obliged to connect them. Costs are only paid to the nearest 10 kV connection point even though there is a need for larger reinforcement or for another connection point. 3.9.2 The Netherlands The technical requirements are written down in an official publication of EnergieNed [www.energiened.nl] (Association of Energy Distribution Companies). These have been devel-oped in mutual co-operation between the responsible utilities. The main differences compared to the Tennet [www.tennet.nl] requirements for production units are: • DG units are seen as negative loads; so no unit commitment • primary regulation is performed only in case of severe frequency deviations (> 150 mHz) • no secondary response • voltage support only in agreement with local utility


• design margins for voltage and frequency deviations are more narrow. • For some of the bigger industrial plants with a relative high capacity operational agree-

ments have been made with Tennet. 3.9.3 Germany There is no central transmission system operator, the facilities remain in the hands of a num-ber of companies. No regulator is implemented, the general idea is to find voluntary agree-ments in questions under discussion. The technical criteria for access to the system have to be published, which is already done for the transmission system [12]. 3.9.4 United Kingdom Since the privatisation of the electricity supply industry in 1990, the industry has been organ-ised as three primary businesses. The generation of electricity is a totally open and competi-tive market. There are currently some 30 generating companies in the UK. The bulk transmis-sion of electricity (400 kV and 275 kV) is carried our by the National Grid Company (NGC) operating as a regulated monopoly. The distribution of electricity is again a regulated monop-oly business carried out in England and Wales by twelve Regional Electricity Companies (RECs). DG will, almost without exception, be connected to a REC distribution system. The REC oper-ates under a Public Electricity Supplier licence. This licence places many obligations on the REC including the obligation to offer to connect DG to its system. The UK RECs have a common code of practise, the Distribution Code that sets out the way in which the distribution systems are planned and operated. This code is supported by a number of engineering recommendations and standards. These cover a range of issues including sup-ply security, voltage control and harmonics. There are two engineering recommendations that apply directly to DG. The first, G59-1 [10], applies to DGs of less than 5 MWe being connected at 20 kV or below. The second, G75 [11], applies to DGs larger than this or connected to higher voltages. These two engineering recommendations set out the technical requirements for DGs. A poten-tial DG developer will have to make a detailed application to the REC clearly showing that the proposed connection meets the required standard particularly in terms of the protection scheme. 3.10 Conclusion Today, there are two principle approaches concerning the planning procedure for the network connection of DG. They both have the same background: the constraints of customer quality, e.g. according to the European Norm EN 50160 [9]. The different approaches can be ex-plained by two examples: • Only the customer requirements are relevant for the decision whether or not a DG may be

connected or how the design has to be. The network operator checks a possible interfer-ence in every single case. This procedure is applied e.g. in the UK, the respective quality standards are defined in engineering recommendations [10, 11].

• In order to make the handling of a large number of DG-connections easier special connec-

tion rules are set up: e.g. rules that are easier to handle in practical use. The German net-work connection rules [12] for instance are derived from the respective standards on net-work operation and customer requirements, assuming typical MV-networks with average loads and typical type and length of lines.

As a matter of principle the second approach does not guarantee the quality of supply e.g. in situations with a high density of DG. On the other hand there are many situations were addi-


tional DG can be connected referring to the customer requirements but the simplified network connection rules don’t allow this. This disadvantage is opposed to the advantage of easy han-dling and the definition of items, which DG have to stick to and which are written in DG-certificates. Table 3.2-3.6 gives an overview of existing network connection rules. In case special connec-tion rules according to the second point above exist (D, DK, NL) they are listed in the table. Otherwise the given values are those which are used as customer requirements according to the first point above (GB). Only those requirements are given which differ from the respective European norm EN 50160, which is given in Tables as a reference. In Germany (D), Denmark (DK), Spain (E) and the Netherlands (NL) the accepted steady state voltage variation is much stricter than in the EN 50160. This results from the assumption, that the total permissible range from the customers' point of view must not be given to one single DG but has to be shared among customers and DG. The same applies for voltage quality in Germany. Var-consumption as a means of reducing the increase of steady state voltage is accepted in a certain range in some countries. In Netherlands Var-injection is not considered to be useful, in other countries this is applied in special cases. In all countries there is no charge for Var-consumption, if cosϕ > 0.9. If cosϕ < 0.9 there are tariffs between 0 and 0.015 EUR/kVAh in Germany and between 0 and 4% of the kWh-price in Spain. In the Netherlands different agreements are applied. Only in Spain there is also a tariff for Var-delivery from DG. The technical aspects synchronising and protective relaying always have to fulfil proven tech-nical standards, the values in Table 3.5 are an indication of useful settings and depend much on the specific protection system. 3.11 References [1] PETRELLA, A. J., "Issues, impacts and strategies for distributed generation challenged

systems", CIGRE, Neptun Conference, 1997 [2] EUNSON, E; BACKMAN, T.; CASAZZA, J; GLENDE, I; MALLET, P; POPPLE, C; RAY,

C; SALVADERI, L; SCHWARZ, J., "Power system planning and open trading", on behalf of CIGRE WG 37-20.

[3] CASTELLEANO, G; SCARPELLINI, P.; VASCELLARI, S., "Dynamic security assess-

ment in competitive environment: analysis tools and security criteria, CIGRE, Neptun conference, 1997

[4] DIN IEC 38, "IEC-Normspannungen", Berlin, Mai, 1987 [5] DIN EN 50160, "Merkmale der spannung in öffentlichen Elektrizitätsversorgungsnet-

zen", Berlin, Oktober, 1995 [6] VDEW, "Grundsätze für die Beurteilung von Netzrückwirkungen", VDEW-Verlag Frank-

furt/M,1992 [7] VDEW, "Technische Richtlinien - Parallelbetrieb von Eigenzeugungsanlagen mit dem

Mittelspannungsnetz des Elektrizitätsversorgungsunternehmens (EVU)", VDEW-Verlag, Frankfurt/M, 1994

[8] ZIEGLER, G., "Protection of Distributed Generation - Current Practice", on behalf of

CIGRE SC34 [9] EUROPEAN STANDARD EN 50 160, Voltage characteristics of electricity supplied by

public distribution systems 1994 and suggested amendment by CENELEC Task Force BTTF 68-6 April 1997


[10] ELECTRICITY ASSOCIATION, "Recommendations for the connection of embedded generating plant to the Regional Electricity Companys Distribution Systems",G59/1, 1989

[11] ELECTRICITY ASSOCIATION, "Recommendation for the connection of embedded

generating plant to public electricity suppliers' distribution systems above 20 kV or with outputs over 5 MW", G75, 1996

[12] VDEW, "Parallelbetrieb von Eigenzeugungsanlagen mit dem Mittelspannungsnetz des

Elektrizitätsversorgungsunternehmens (EVU)", 1. Ausgabe, VDEW-Verlag, Frankfurt/M, 1994

[13] ASSOCIATION OF ENERGY DISTRIBUTION COMPANIES IN THE NETHERLANDS,

"Technical terms of connection To the public network for local production units", Arn-hem, May, 1994

[14] IEC 555/3 [15] WEBER, H; MADSEN, B; ASAL, H.P., "Kennzahlen der Primärregelung im UCPTE-Netz

und künftige Anforderungen", Elektricitätswirtschaft, Jg. 96 (1997), H. 4, S. 132-137 [16] PETRELLA, A.J.; "Issues, Impacts and Strategies for Distributed Generation challenged

Power Systems", Contribution to CIGRE, 1997 [17] CIGRE WORKING GROUP 37.23, "Impact of increasing contribution of dispersed gen-

eration on the power system", September 1998 [18] NORDEL "Operational Performance Specifications for small Thermal Power Units”,

1995 [19] DEFU, “Relay protection for local CHP units”, TR 293, 2nd edition (in Danish) [20] DEFU, “Voltage quality in low-voltage transmission networks”, R16 (in Danish) [21] DEFU, “Nettilslutning af decentrale produktionsanlæg”, KR88, 1998 (in Danish) [22] DEFU, “Grid connection of wind turbines”, Committee Report 111, 1998 (in Danish), [23] DEFU+RISØ, “Power quality and grid connection of wind turbines”, Parts 1, 2 and 3 (in

Danish), Risø-R-853 and DEFU-TR-362 [24] ELTRA, “Power Station Specifications for Plants < 2 MW”, SP91-515h [25] ELTRA, “Power Station Specifications for Plants between 2 and 50 MW”, SP92-017a [26] ELTRA, “Power Station Specifications for Plants > 50 MW”, SP92-230h [27] SENER, "Planvoimaloiden littäminen jakelverkoon",Sener jukaisu, 2001 [28] SLY, "Retningslinier for tilkobling af vindkraftverk", Jukaisusarja 3/90, 1990 [28] SINTEF, "Retningslinier for tilkobling af vindkraftanlæg", TR A5089, 1999 [29] ELFORSK, "AMP - Anslutning av mindre produktionsanlägningar i elnätet", 1999 [30] SVERIGES ELLEVERANDØRER, "Kriterier för spänningsgothet vid leveransspänding

över 1000 V", 1997


[31] VDEW, "Eigenzeugensanlagen am Mittelspannungsnetz. Richtlinie für Ancluss und Pa-rallelbetrieb von Eigenzeugungsanlagen am MittelspannungsNetz", VDEW-Verlag Frankfurt/M, 2. Ausggabe,1998

[32] ELECTRICITY ASSOCIATION, "Planning Limits for Voltage Fluctuations Caused by

Industrial, Commercial and Domestic Equipment in the United Kingdom",P28, 1989 [33] ELECTRICITY ASSOCIATION, "Distributed Generators larger than 5 MWe or con-

nected to higher voltages",G75 [34] ELECTRICITY ASSOCIATION, "Connecting of Inverter-connected Single-phase Photo-

voltaic (PV) Generators up to 5 kVA to Public Distribution Networks",G77, 2000 [35] EESTI ENERGIA AS, "Technical requirements for connecting wind turibine installations

to the power grid", EE 10421629 ST 7-2001, 2001 [36] ELECTRIC TRANSMISSION COUNCIL, "Technical Guideline for interconnection of

Generators to Distribution Systems", Alberta, Canada, 1998 [37] NRECA, "The NRECA Guide to IEEE 1547 (Application Guide for Distributed Generator

Interconnection)", USA, 2001 [38] GE RESEARCH AND DEVELOPMENT CENTER, "DG Power Quality, Protection and

Reliability. Case Studies Report", USA, 2001 [39] CENTRAL RESEARCH INSTITUTE OF POWER ELECTRIC INDUSTRY, "Technical

recommendations for the grid connection of dispersed power generating systems", JEAG 970-1993, JAPAN, 1994

[40] IEEE, "Standard for connecting Distributed Resources with Electric Power Systems",

IEEE P1547 (draft), USA, 2002 [41] IEC, "Wind turbine generator systems - Part21: Measurement and assesment of power

quality characteristic of grid connected wind turbines", IEC 61400-21 (draft), [42] DTe, the office for Energy Regulation, "GridCode", "System Code", "Measuring Code"

www.nma-dte.nl [43] EnergiNED, the Association of Energy Distribution Companies, www.energiened.nl [44] Tennet, the Dutch Electricity Generating Board, www.tennet.nl [45] N. D. Hatziargyriou, T.S. Karakatsanis, and M. Papadoupoulos, "Probabilistic load flow

in distribution systems containing dispersed wind power generation", IEEE Trans. Power Syst., vol. 8, pp.159-165, Feb. 1993


4. LARGE SCALE INTEGRATION OF DISTRIBUTED GENERATORS 4.1 Introduction To establish a basis for our discussion of the adaptation of network operation to large scale integration of distributed generators, we will use an example of a restructured electricity sector. An unprecedented high share of bound electricity production from renewable energy sources (RES) and combined heat and power (CHP) has developed in the western part of Denmark. Therefore, the Transmission System Operator here (Eltra) must take special precautions due to the unpredictability of wind power, due to the deteriorated power regulation and due to the localisation of generation in local networks. Since the early 1980s the following capacity has been added to the system:

• 1,500 MW of local CHP • 1,900 MW of wind power

Figure 4.1 Local CHP and wind turbines in the western part of Denmark 4.2 The Eltra system The electricity transport system includes a network for transmission and a network for distribu-tion. Eltra is co-operating with regional transmission operators and distribution network opera-tors to provide transport services. The rules for co-operation are defined by law. Figure 4.2 The Eltra system

610 MW

950 MW1000 MW

1200 MW

800 MW

Primary: 3330 MWLocal CHP: 1556 MWWind: 1929 MW

Min. load: 1150 MWMax. load: 3800 MW

The Eltra area representsapproximately 60 %of total Danishconsumption. Germany

NorwaySweden

580 MW


A variety of distributed generators are connected to the Eltra system: - wind turbines ranging from 11 kW to 2 MW - gas-fired CHP units ranging from 7 kW to 99 MW - straw-fired CHP units ranging from 2 MW to 19 MW - coal-fired units ranging from 18 MW to 44 MW - waste-fired units ranging from 90 kW to 26 MW A special wind production facility under construction is the Horns Rev offshore wind farm. This farm will consist of 80 turbines with a unit size of 2.0 MW, in total 160 MW. The wind farm is situated 54 km from the transmission network and is planned to go into operation later in 2002. At the transmission level, there are radial tie lines to three neighbouring countries: Sweden, Norway and Germany.

Figure 4.3 Production capacities at each voltage level within the Eltra area. In the Eltra system, contracts take up to 80% of the total energy to be supplied. The remaining energy requirements will be met by real-time control functions of the control centre. The basic operating function of Eltra, is load following control while preserving service reliability and se-curity. The objective is met through ancillary services and real-time control. In the Eltra area, the production balance responsible players are dispatching their generators themselves, with no TSO intervention except for ancillary services. Eltra will not interfere with the competitive mechanism of the free market, except for the re-quirements of system reliability and security. Unit maintenance schedules for example are subject to review and approval by the TSO. Schedules will be verified for their impact on static and dynamic security and modified in case security problems will or are likely to be encountered. Schedules will be subject to modification on an hour-ahead basis.


The electricity market will reach another important milestone on January 1, 2003, where even households will be permitted to choose their electricity supplier. Already at the opening of the wholesale market in 1999 the network operators were faced with a new distribution of tasks. Models existed in Norway and Sweden, where electricity markets had been established in 1993 and 1995. Producers and traders are the commercial market players. Market players are supposed to assume a balance responsibility, which may include production, consumption or trade. The balance responsibility is a financial responsibility for the fulfilment of the schedules given to the TSO. This means that a fee must be paid in case of deviations from the schedule. 4.3 Priority Production Electricity from wind power and local CHP is defined as environmentally friendly. This produc-tion amounts to nearly half the electricity consumption in Jutland and on Funen. Environmen-tally friendly electricity is prioritised. This means that the owners of the power plants concerned assume no balance responsibility. It has been assumed that maximum production from these plants also means maximum environmental benefit.

Even the large power stations have a considerable heat load causing a constrained electricity production. So far, this production has not been prioritised, but is sold on normal market condi-tions. For some time it has been obvious that the total amount of constrained electricity production increasingly exceeds the demand for electricity. The problems have been analysed by a com-mission under the Danish Energy Authority [1], [2]. The amount of constrained production will exceed the electricity demand in Jutland and on Funen in about 3,250 hours per year by 2005. During the winter months there will be an elec-tricity surplus in about 60 per cent of the time. 4.4 "Smooting" the green electricity The distribution network operators and the TSO are legally obliged to purchase the environmentally friendly electricity and spread it evenly across all consumers. The TSO is responsible for balancing this production. Wind power plants and local CHP plants are connected to the network at 60 kV and below. Thus, distribution network operators purchase most of the environmentally friendly electricity. However, as production does not follow demand, the TSO must perform a “smoothing proc-ess” by purchasing the entire amount as produced. After smoothing, the same amount is dis-tributed via the distribution network operators to the consumers.

0

1000

2000

3000

4000

5000

00:00 04:00 08:00 12:00 16:00 20:00 00:00

MW Compulsory production on February 11, 2002© Eltra amba

Wind

Small scale CHP

Consumption Compulsory Central CHP

Figure 4.4 Bound production on a winter day


The smoothing process solves two problems. Firstly, there is a difference between the profile of electricity consumption and the day-ahead schedule for wind power and local CHP. Sec-ondly, there is a difference between scheduled and actual production. The latter is the real imbalance. Eltra performs the first phase of the smoothing process by trading in the spot market. In the second phase, Eltra sets off the difference between scheduled and actual production by trad-ing in the real-time market. 4.5 System imbalances Imbalances in a traditional system System imbalances arise when production ± foreign trade deviates from load. In an isolated electric system of moderate size even a small imbalance will change the frequency. The Jut-land-Funen power system is connected to the continental European network, and modest im-balances in Jutland and on Funen will as a main rule not affect the frequency measurably. Imbalances are caused by unscheduled variations in consumption or production. Such imbal-ances are normal and they occur in any electrical network in the world. Large variations may be caused by sudden outages of production units. International agreements on mutual support have been made for relieving large imbalances on condition that each area is able to restore its normal balance shortly. Unreliable wind forecast The wind power imbalance is the difference between the wind power schedule used in the day-ahead market and the measured wind power production. The wind power schedule is mainly based on wind forecasts provided by meteorologists. For the western part of Denmark an average error in the magnitude of 35 per cent of installed wind power capacity has been experienced. The maximum error is about 60 per cent of installed wind power capacity. It adds to the problem that wind power forecast errors often have a very long duration. Before the wind power boom, imbalances up to 400 MW occurred for short periods and at long intervals. Now, imbalances up to 1,100 MW lasting several hours frequently occur. Insufficient resources for system regulation Due to wind power the system needs much more regulating capacity than before. However, the regulating capacity has been reduced, because half of the installed capacity now is beyond system control. Consequently, the Eltra system operation is very often short of resources for regulating up or down.

0

3 0 0

6 0 0

9 0 0

1 2 0 0

1 5 0 0

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0H o u rs p e r y e a r

M W

Figure 4.5 Real-time surplus of power Generally, resources for regulating down are missing during periods with abundant wind power, while resources for regulating up are missing during periods with scarcity of wind power. For the time being imbalances that cannot be eliminated within the area are counterbalanced by foreign neighbours. In some cases it has not been technically possible to exchange the entire imbalance across the borders. A contingency plan has been prepared for such cases [3]


describing procedures for stop of power stations and wind power. The extent of missing inter-nal resources for regulation has been simulated. Figures 4.6 and 4.7 show results for a year with Nordic spot prices below average. The deficiencies shown exceed by far the level assumed when the international agreement mentioned in 4.3 was made.

0

3 0 0

6 0 0

9 0 0

1 2 0 0

1 5 0 0

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0H o u r s p e r y e a r

M W

Figure 4.6 Real-time shortage of power 4.6 Increased risk of interruption The risk of widespread system disturbances has increased. Among the decisive factors are:

• Distributed generation units have not been designed to support system stability during

power system failures • The transmission system must often be loaded to its capacity limits in order to serve the

market players There is nothing wrong in loading the system to the capacity limits provided that the limits are known with reasonable accuracy, but it has become much more difficult to measure and fore-cast local system loads because the measurements now show the local totals of loads and productions. This means that the daily system analyses are less precise, and it leaves the Eltra system operation with the choice between reduced utilisation of the network and an increased risk of exceeding a critical capacity limit. One of the most important protective measures against spreading a power system failure is the curtailment of load or generation in order to equalise the total load with the resources available. However, the mixture of loads and production units in the local networks makes it very difficult to control a curtailment to restore the balance between load and production. Thus, an impor-tant protective measure against spreading of system failures is weakened. Correspondingly, the recovery after a serious failure has become far more complicated, be-cause the network must be disintegrated into small parts, so it is well-defined if a load or a production unit is being connected to the network. This may cause much longer restoration procedures. 4.7 Mobilising local resources Optimum production allocation The environmentally friendly electricity production varies on a monthly basis between 30 and 60 per cent of the electricity consumption in the western part of Denmark. This production is controlled by wind and heat demand independently of electricity demand. The remaining elec-tricity production depends on market conditions.


Whether prioritised or not, all electricity production depends on the same electrical network, but the two parties cannot have equal network access under the present rules.

When there is a simultaneous demand for electricity and heating, producing both in a CHP plant is highly beneficial in terms of energy and environment. When only one of the two prod-ucts is demanded, it is cheaper and less environmentally harmful to produce only the product demanded. The price signals of the electricity market can be used to decide whether electricity should be generated at a certain local power plant or not. Environmental and other externalities can be included in the calculations. If the prioritising of environmentally friendly electricity production is cancelled, it will be possible to allocate the system resources optimally according to the market signals in the day-ahead schedules. In case of a cancellation the owners of the power plants concerned must be indemnified com-pared to the present conditions. Participation in system regulation As shown in section 4.3, the resources available for regulation are insufficient to eliminate the imbalances caused by poor wind forecasts. The local CHP plants could contribute significantly to the necessary regulation, but new tech-nical solutions and several new rules will be required. The small CHP plants could participate in the regulation by a dynamic adjustment of the times for start and stop during the day. The heat accumulators will add to the flexibility together with the possibility of producing heat directly on heat boilers. In the long run, even the possibility of using electrical heat boilers and cooling with air should be considered. It is assumed that small CHP plants should still be operated automatically, but that the use of each plant for regulation should be coordinated for a cluster of plants by a balance responsible market player. Local voltage control The voltage in each node of the network must be kept within certain limits in order to maintain the quality of supply and the system stability. The voltage is controlled by the power stations by regulating the production of reactive power and by switching certain static components (condensers and reactors). So far, the voltage control is performed at the upper voltage levels. The increasing and oscillat-ing local electricity production has caused an increasing transport of reactive power. This transport increases network losses and reduces the capacity for transport of active power.

Figure 4.7 Priority production by month


So far, the local CHP plants have participated in the local voltage control only by following a firm pattern. Thus, the production of reactive power is changed only at certain times of the day instead of being adjusted in accordance with actual needs. A revised operational pattern is being considered aiming at participation by the local CHP plants on an equal footing with the central power plants in the voltage control. It must be pos-sible to use local criteria for the control of local voltages and reactive power. Both automatic and manual local voltage control must be established. Restoration after power failures When the local CHP plants are relieved from the present operational patterns, they can be prepared for the participation in restarting the system after a total blackout. A number of power plants distributed all over the Jutland-Funen area could be starting units for their own regions prior to a reconnection of the regions. This strategy might reduce the time of interrupted supply after a total system collapse. Also this function requires a common contact to the local CHP plants organised in order to facilitate communication in critical operational conditions. 4.8 A framework for new control structure The unutilised potential Considerable improvements of the operation of the power system in Jutland and on Funen are possible by appropriate use of existing resources. If local CHP plants are given the same conditions as the conventional power plants they will be able to provide important contributions to the day-ahead market, to the real-time market, to voltage control and during emergency operation. Even new wind power plants will be able to contribute to controlling system balance and volt-age. In Figure 4.8 it is demonstrates how the system imbalances from figures 4.5 and 4.6 can be reduced to a more decent level after participation of local CHP plants.

0

300

600

900

1200

1500

0 100 200 300 400 500Hours per year

MW

0

300

600

900

1200

1500

0 100 200 300 400 500Hours per year

MW

Fig. 4.8 Real time surplus and shortage of power, without and with active local CHP plant participation.


4.9 Conclusion A new strategy After the mixture of production and consumption in the same local networks the operational tasks have become far more complicated, particularly under emergency conditions. The present control structure is based on a division of the electrical network into two parts: a distribution network connecting end-users to the electricity supply system and a transmission network connecting power plants and cross-border lines to the network. So far, little opera-tional co-ordination has been required between the two networks, both under normal condi-tions and in emergency situations. Therefore, the most important basis for a new strategy is the recognition that the distribution networks no longer can be considered as passive appendages to the transmission network, but that the entire network must be operated as a closely integrated unit. Organising coopera-tion on this task is a major challenge. Furthermore, a number of technical improvements have to be developed and implemented. Conditions for central and local electricity production must be equalised bringing all power plants to contribute to system stability and flexibility. Electricity production and consumption must be measured and switched separately. The qual-ity of the current system analyses must be improved, and manual system control necessary during emergency operations must be organised in cooperation with the distribution network operators. New principles A systematic elimination of the weaknesses demonstrated above is possible, but considerable time and some new equipment will be required for supervision, measurements, analyses and control. Several international studies [5] have presented ideas for the integration of distributed electric-ity production. Some principles for use in the Eltra area have been identified as the basis for a long-term solution: • A control hierarchy consists of a central control centre (at Eltra) and 3-6 regional control

centres. Each region consists of a number of local areas. Each local area is connected to the transmission system via one 150/60 kV substation. An unambiguous operational re-sponsibility must be defined for each local area.

• The prioritising of electricity from local CHP plants must be cancelled, so these power

plants can be operated in the same way as conventional power plants in accordance with price signals from the day-ahead market and the real-time market. This principle offers network access on equal terms for all producers and opens up for a better utilisation of the network.

• The balance of reactive power within each local area must be kept within certain limits to

be defined in a new set of rules. There must be a local responsibility for observing these rules and the control of local reactive resources (including condensers and local CHP plants) must be local as well.

• New rules for measuring must provide all necessary data for the regional control centres

and to the extent necessary to Eltra. Reliable information on the state of the system and data for accurate system analyses must be available at any time.

• During emergency operation it must be possible to switch loads and production units sepa-

rately. The principle applies to both automatic load shedding by frequency relays and to the manual restoration after serious power failures.


• During normal operation only Eltra’s control centre will be manned 24 hours per day. In other system states (from alert to emergency) it must be possible to man the regional and local control centres concerned. Restoration after a complete system collapse will be the ultimate challenge. Procedures for this situation must be trained, but hopefully never used.

Cooperation required A number of tasks must be solved together with the regional transmission operators and the distribution network operators in preparation for the implementation of these principles. Mapping of production and consumption From the very beginning a mapping of the distribution of production and consumption for all 60/10 kV terminals and of the equipment for measuring will be required. Rules for Measurements, Generators and Communication Furthermore, new rules must be prepared for a number of areas, including measurements, reactive power, local power plants and communication. Procedures for Reactive power balance, curtailment of load and restoration Finally, new procedures for local control of reactive balance, manual curtailment of load and production and for the restoration after system failures must be developed. 4.10 References [1] "Handling of Electricity Overflow and Electricity Shortage within the Danish Electricity

System", Eltra and Elkraft System, Doc. No. 104280 [2] "Report from the Task Group on CHP and RE Electricity", English Summary, October

2001, The Danish Energy Authority, www.ens.dk [3] "Status of the Preparation of a Contingency Plan for Handling of Electricity Overflow

during the winter of 2001/2002", Eltra, Doc. No. 119926 [4] "System Plan 2002 – Summary and Conclusions", Eltra, Doc. No. 137578 [5] "Impact of increasing contribution of dispersed generation on the power system", Cigre

Working Group 37.23, September 1998, www.cigre.org [6] www.eltra.dk [7] www.cigre.org [8] www.ens.dk


5. A REDESIGN OF THE GRID 5.1 Introduction In the previous chapter we recognised that the most important basis for a new strategy is the recognition that the distribution network no longer can be considered as passive appendage to the transmission network, but that the entire network must be designed and operated as a closely integrated unit. In this chapter we will therefore present and review ideas that can lead us towards this goal. Firstly we will present the ideas of Ivo Bouwmans from the Netherlands. He has compared the Internet with the electricity Grid. From this comparison comes some structural thinking for a future Grid. Then we will present the ideas of Frank van Overbeeke from The Netherlands and Vaughan Roberts from the UK. They have a vision for "active networks", as facilitators for embedded generation. They foresee that passive distribution networks as we know them have to evolve, gradually into actively managed networks. From their viewpoint this is both technically and economically, the best way to facilitate DG in a deregulated electricity market. This vision is the result of research that has been conducted in the Strategic Technology Pro-gramme, a cooperative research programme for electricity distribution companies managed by EA Technology, UK. And at last, the ideas of EPRI USA, regarding the concept of micro-grid will be presented. All these initiatives come from different backgrounds, but are actually are going in the same direction. 5.2 The Grid and the Net In reference [1] Ivo Bouwmans from the Netherlands has compared the Internet with the elec-tricity grid. As we have also seen in this report new development on the Grid lead to • a larger number of power input nodes, • bidirectional energy flows and • new technologies enable direct routing of electricity (FACTS). These aspects already characterises the flow of information on the Internet. The Internet uses the concept of distributed control, where each node acts autonomously. Ivo Bouwmans ex-plores the Internet concept with a view to its possible application in the electricity grid. In the rest of this section the electricity infrastructure will be referred to as 'the Grid', while the Internet will be called 'the Net'. The Net Reliability was the fundamental idea behind the Net. In the 1960s, the US military worried about communication in an event of a nuclear attack. The Rand Corporation came up with a new type of communication network. In this network the nodes containing information are in-terconnected by an interconnected network. If part of the structure would be damaged, com-munication would still be possible through the remaining part of the network. As in the case of the Grid, the Net originated at local level. Unlike the Grid, which mainly grew by joining existing networks, the Net, spread from a single network at UCLA by the addition of new nodes that themselves had not necessarily functioned in a network before. Increase in the number of controlled nodes As we have seen from the previous chapter the advent of DG generation units changes the demands on the Grid. These DG generators call for a new approach to networks. The Grid is


changing because the energy flow is no longer in one direction only, and a considerable in-crease in the number of nodes is expected. This implies that the Grid is going to be more like the Net, where information flows to and from a large number of nodes. Part of the new approach may therefore well be inspired by the Net. In the light of this suggestion, we will now look at the direction of transport in the Grid and the increasing number of nodes to be controlled. Uni-directional Grid The Grid was originally designed for uni-directional transport only. Its task was to deliver the electricity generated by the power stations in the central nodes of the grid to the customers at the end of the branches. Even the process of connecting the local distribution networks did not change this basic idea. Although additional transport became possible between the local grids, the currents within these grids remained uni-directional. Bi-directional Grid New technologies enable wide-spread generation of electricity on a much smaller scale. These technologies deliver electricity locally, but at times of limited power demand they may sell the surplus of energy to the distribution network. This development therefore requires a bidirec-tional grid. Control of the routing of electricity Is the present Grid there is hardly any way to control the flow of electricity other than simple application of Ohm's law. Given the voltages at the nodes, the currents will follow from the resistance and impedance in the connections. The only way to change the current for a given demand configuration is to change the distribution of the power input at the supply nodes. During the last decade, however, power electronic systems have emerged that offer ways to control the routing of the electricity. FACTS - Flexible AC transmission Systems - can act in much the same way that the routers in the Net do. They can control the power flow. This tech-nology will resemble the Net more closely. After all, the Net was intended for bidirectional transport of information in the first place. In case of a nuclear attack: The distributed informa-tion would have to be available at any node of the Net, no matter where the attack would strike The information is transported between any two nodes anytime a user needs it. Distributed control The second major change that distributed generation bring about is a substantial increase in the number of Grid nodes where power input should be managed. Each supply node, ranging from a large scale power plant to a single solar panel, needs to be controlled. As the number of supply nodes grows, so does the combined control tasks. The application of FACTS in the Grid will make control even more complicated. Each switching node has to be monitored and controlled, because the task of the FACTS will be to adjust the electric currents according to the actual nodes in the Grid. They play an active role in the over-all management of the Grid, which thus depends on their performance as much as on the dis-tributed generators. In the past, control shifted from local via regional to national and even international level as the Grid was growing. However, centralised control of the thousands of distributed input nodes and switching nodes will become increasingly difficult. The Net, in contrast, operates under 'distributed control'. Each node acts autonomously: it receives information from outside and responds accordingly by replying to request or by pass-ing on information that is required elsewhere. The Protocol The Net is characterized as an 'open-architechture network environment', where the technol-ogy of any local network is not dedicated by a particular network architecture, but can be se-lected freely. A meta-level protocol takes care of the trouble-free operation of the Net as a


whole: the Transmission Control Protocol / Internet protocol (TCP/IP). Apart from this protocol, no central co-ordination is needed for the operation of the Net, because all subnets follow the same rules that control all information transfer. A protocol for the exchange of information about power demand and supply, much like TCP/IP, could make it possible to distribute the control of the Grid to a much smaller scale. Each node or sub-grid in the 'distribution network' would 'listen' to the rest of the network, adjusting the power production - or even consumption - to the state of the Grid at any particular moment. The FACTS at the nodes between the producer and the consumer would relay the power be-tween the nodes, in the same way that e-mails and web documents are passed on from node to node in the Net. The addition of new sub-grids would become easier, because the responsibility for the control is placed at the level of nodes or sub-grids. Provided the new sub-grid is managed according to the protocol, the Grid as a whole will work fine. Implementation The idea presented here of distributed network managed by distributed control is still rather speculative. Of course, there would be many problems to be settled before a start could be made with the implementation. Implementation could start in sub-grids that are protected by 'firewall' of FACTS between them and the rest of the Grid, analogous to the way intranets in companies are shielded from the rest of the Net. Design There is the question of who will design the protocol that is needed. The first implementation of the Internet Protocol, IPv4, was designed in 1981 by a small group of people when the Net was still small. The next generation of the Internet protocol IPv6, was discussed at length be-fore it was accepted and is now gradually being introduced on the Net. The proliferation of stakeholders with an economic as well as an intellectual investment in the Net makes the procedure increasingly complex. It could take quite some time to work out a completely new protocol standard for the Grid, given the already existing infrastructure and the large number of actors involved in production, transport, distribution and trade. Conclusion Wirth the advent of distributed generation, which shifts the responsibility of the electricity pro-duction to a much larger number of actors that generate on a much smaller scale, it could be advantegous to adopt the idea of distributed networks, where the responsibility for the control and the transport of electricity is also shifted to a smaller scale. Each sub-grid would act autonomously, exchange information with the rest of the Grid using a communication protocol like that used on the Net. Both generation and load could be adapted to the actual supply and demand situation on the Grid, relaying the energy from generator to consumer. The major problem The major problem could be the organisation of the design of the new network structure, as this process would have to involve the large number of stakeholders. Of the Net is has been said that "if the Internet stumbles, it will not be because we lack technology, vision, or motiva-tion. It will be because we cannot set a direction and march collectively into the future" [2]. The same could be true for the Grid.


5.3 Active Networks In reference [3] Frank van Overbeeke from the Netherlands and Vaughan Roberts from UK has given their vision for "active networks", as facilitators for embedded generation. They fore-see that passive distribution networks as we know them have to evolve, gradually into actively managed networks. From their viewpoint this is both technically and economically, the best way to facilitate DG in a deregulated electricity market. This vision is the result of research that has been conducted in the Strategic Technology Pro-gramme, a cooperative research programme for electricity distribution companies managed by EA Technology, UK. The basis for their vision is the recognition: the existing technical and regulatory structure of distribution networks is incapable of supporting the evolution of the power system further. The problems that lay ahead of us cannot be solved by local patchwork. They have also recognized that • The network of the future does not supply power, it supplies connectivity • The "infinite network" as customers used to know it, no longer exists The first one is that the network is not, and must no be considered as a power supply system. The network is a highway system that provides connectivity between points of supply and con-sumption. And the second one is that a network interacts with its customers. A network that remains vir-tually unaffected whatever loads or generators are doing, is a notion of the past. If you as a customer require an infinite network, you may have to pay for that service. Therefore a structural solution is proposed, based on the following concepts: • Interconnection • Local control areas ("cells") • System services are specified attributes of a connection The first one is interconnection as opposed to dominantly radial networks. The second one is using automation to support relatively small local control areas. In absence of a standardised nomenclature, we have introduced the concept of “cells” to denote these control areas. The third concept refers to the way in which system services are charged to individual cus-tomers. In this way the network becomes a NETwork • Analogy to internet and telephone networks • There is always more than one path • Active management of "congestion" • Prevent propagation of an overload by isolating the "sick" part of the network Typical engineering caveats: • Fault levels to be managed • Reliability based on quick reconfiguration :

How to prevent frequent short interruptions ? The question why an electricity network is called a network where there are almost no meshes is bound to disappear. Interconnection is going to be the rule on all voltage levels.


The advantages of that approach can be observed in the telecommunications business: Energy transport is never dependent upon a single path, so that the vulnerability to component failures is significantly reduced. If some paths run a risk of being overstressed, power can be re-routed. Please note that this calls for some form of active network management because in purely passive networks Ohm’s law would determine the routing of power. An inherent risk of interconnected networks is the domino-effect: If the system falls over somewhere, the fault could propagate over a very large area. Appropriate protection mecha-nisms, not too much different from what is being used already, can ensure that the fault is cor-rectly isolated and the rest of the system operates normally. Apart from the protection systems needed, network design engineers have to address the issues of fault levels which tent to increase in interconnected networks; and automatic recon-figuration actions may lead to a shift from infrequent but long supply interruptions to more fre-quent but short supply interruptions.

Figure 5.1 Network subdivided into cells acting as 'independent' islands The most revolutionary change that is proposed is the introduction of “cells”, which are in fact local control areas. The cell concept does not have a large impact on the topology of the power network - at least not as a short-term measure. The difference is in the control hierar-chy. In the existing operational control hierarchy of a distribution network we normally see three levels. The lowest level is formed by the protection systems and interlocks. They operate very quickly, completely autonomously, and serve only to protect the network from propagating faults and to protect the public from unsafe situations.


The second level are automatic control systems, which is limited in practice to automatic volt-age control relays operating on transformer tap changers. The third level is network reconfiguration by either local or remote operation of switchgear. In virtually all cases this requires human intervention. Within a cell: • Local voltage and VAr control • 'Negociated' exchange of power with adjacent cells • Suitable actuators control the exchange • Managed operation of connection with adjacent cells • Faults are managed and isolated within a cell, effect will not propagate to other cells • 'Islanded' operation is available as an emergency condition : If a cell is not capable of im-

porting/exporting as much power as needed, local generator/loads will be controlled in or-der to achieve balance

In this vision this is the ultimate form of an "active" network Each cell will eventually have its own power control system, which is essentially computer-based, that manages the flow of power across the cell’s boundaries. This management is en-abled by all sorts of actuators helping the control system achieve its objectives. Typical examples of such actuators are: • Voltage and reactive power controllers • FACTS and UPFC devices • Remotely controlled contactors • Remotely controllable loads and generators • In a more distant future this means that control systems of adjacent cells will negotiate in

real time how much power will be transferred over their mutual interconnection. Software agents for such processes are under development already

• It also means, that if all connections with neighbouring cells are lost, the cell may still be able to remain powered, by simply disconnecting enough load or generation to attain power balance. This could lead to considerable improvements in the reliability of the elec-tricity supply system as a whole

5.3.1 System services The proposed system services are: • Reactive power (supply/absorption) • Voltage control • Phase symmetry • Network impedance within defined range :

• high enough to limit fault level • low enough to supply starting currents,

absorb load variations and se-cure generator stability

• low enough at harmonic frequencies • Capability to handle fault currents • Security of supply / connectivity The most obvious system service is the balance of reactive power, and in many cases it is more cost-effective to ensure that balance locally. This means that also the distribution net-work operator (DNO) provides that as a service.


Ensuring that the system voltage is within agreed limits is also a duty that each network opera-tor needs to fulfil by himself. Symmetry between phases has never received much attention but may become more of an issue when networks are operated at higher impedance levels. This brings us immediately at the issue of network impedance. The value of that impedance, with the consequence of regulation on one side of the spectrum and fault level at the other side, is a system service and needs to be quantified accordingly. The optimal value will be determined by a trade-off between requirements of voltage stability and absorption of flicker phenomena on one hand, and limitations on fault level on the other hand. Accept fault level contributions from customers, in particular generators, is considered a ser-vice as well and customers may have to choose between paying for that service, or limiting the fault contribution on their side of the meter. And finally the security of supply or connectivity is a system service, where the option of being controlled or even disconnected could result in a discount on your network charges. 5.3.2 Price structure for system services As indicated, each system service will have its price and these prices apply to generators as well as to loads. Some examples of this could be: • Reactive power :

• Standing charge per kVAr for availability • Charge per kVArh absorbed/generated

• Phase symmetry • Standing charge per maximum and

average negative-sequence and homopolar currents

• Network impedance • Guaranteed network admittance corresponds to 100*rating of connection in kVA • Higher value can be contracted, if available • Flicker caused by varying load/generation

to be calculated on basis of contracted admittance • Fault level contribution

• Standing charge per kVA of fault level contribution from customer into the network

Reactive power could be charged very similar to active power. There is a standing charge for availability plus a certain charge per unit actually exchanged with the system. In terms of phase symmetry, exceeding certain limits of negative-sequence currents which is now simply forbidden could be allowed, at an appropriate price. As the variable cost of com-pensating these currents will be relatively low, the customer probably pays a standing charge only. When it comes to network impedance, it is proposed that the DNO guarantees a dynamic im-pedance of, for example, 100 times the contracted power. The customer can then exercise the option of lower supply impedance at a cost. In the same manner, fault level contribution can be charged in the form of a standing charge for each kVA contributed. These charges are all based on the cost that the DNO has to make either to reduce his own fault level contribution, or to accept a higher fault level by upgrading part of the network.


• Reactive power: Controllable production/absorption of kVAs. Reward to be based on availability and the amount actually provided

• Phase symmetry: Reward for controllable phase balancing • Network impedance:

Reward for increasing (small-signal) admittance without increasing fault level • Controllable loads and generators:

Facilitates operation of a network in (partially) islanded mode if upstream connections have failed. Reward to be based on availability and the amount actually provided.

And now, because services have received a price, a network operator has a basis of purchas-ing those services from his customers if that is more attractive than producing them within his network. Let us examine some examples again. A generator providing controllable reactive power production, may receive a premium depend-ent upon the degree of controllability. The compensation he receives may be composed of a standing charge for availability and a per unit compensation for the amount actually provided. Generators using power electronic inverters are perfectly positioned to provide balancing be-tween phases. The network operator would contract the availability of a certain amount of con-trollable negative-sequence current. When it comes to network impedance, generators with fast-acting voltage controllers could contribute considerably to the dynamic admittance of the network. Again power electronic con-trollers have an advantage because they can react extremely rapidly without increasing the fault level. Finally any load or generator that can respond to remote control can be contracted by the DNO in order to ensure power balance in case of islanding or restricted availability of external sup-ply. 5.3.3 The economic advantage of this vision There has to be an economic case for introducing al these novelties instead of running distri-bution networks as have always been done. The most obvious advantage is that the changes proposed ask for virtually no physical reinforcement. Those reinforcements are unavoidable if we are to accommodate huge amounts of embedded generation. • Only a few lines:

Basically to provide interconnection between islands • Reinforcement of existing lines:

Applicable mostly to tapered circuits where local voltage is uneconomical • No new transformers:

Interconnection improves security of supply, existing transformers can be operated to a higher percentage of the rated load.

• More switchgear: Increase options for inter- and disconnection; all switches to be remotely operated. Few circuit breakers, many sectionizers

• More control systems and actuators: Limit level of investment by phased introduction

The form of automation that is envisaged allows us to operate transformers much closer to their physical limits and would thereby reduce the need for new transformers considerably. This does not only save money, it also means that we can save ourselves the trouble of ob-taining land and permission to expand substations, and so on. Part of the money saved on the items before will be spent on more switchgear. A flexible net-work requires as many options for reconfiguration as possible. Moreover those switches must


be remotely controllable because it is wanted to exclude human intervention as much as possible. The biggest investment, not necessarily in money but certainly in engineering effort, is in the introduction of automation to support this. Much work has been going on already in estab-lishing what kind of communications technology is available and applicable for various func-tions. A lot of work will have to go in design and implementation of control strategies. But this can be done in a phased manner - the only issue is to have a clear vision of where one wants to arrive in the end. 5.3.4 Issues for further research Nothing in the vision [3] suggests that this is the way they are working today at ENECO. There are many technical and some regulatory barriers to overcome before they can do this. How-ever, they authors claim that they know what they to need now. They only need some bright engineers and economists to process those questions and translate them into applicable solu-tions. • More dynamic reliability model • Efficient planning requires a mature market for DG.

How to bridge the present immature phase? • Tools for voltage control • Algorithms to (re-)configure networks automatically for optimal power flow and losses • Concepts for protection • Cost of system services to be quantified Finally they hope that representatives from regulatory bodies will take their responsibilities in facilitating innovative solutions for tariffs and system service charges.


5.4 Micro-Grids Based Power Systems EPRI in USA is enabling utilities to consider new options in the design and operation of power systems that can provide improved efficiency, the potential for ancillary services, improved reliability, and lower cost of operation. One of these options is the concept of Micro-grids The Micro-grid concept The power industry began as micro-grids, small power systems unconnected to a bulk power system. Today, because of the advent of new distributed resource technologies, better control systems, the need for improved distribution system performance, and various constraints as-sociated with continued expansion of the traditional bulk power system, there is new interest in returning to micro-grid approaches for some applications. The new micro-grids of the 21st century can perform much better than the early 20th century micro-grids and may be competi-tive with traditional power system approaches Micro-grids are small power systems that can operate independently of the bulk power system. They are composed of distributed energy-production and energy-storage resources intercon-nected by a distribution system. They may operate in parallel with the bulk supply system dur-ing some conditions and transition to islanded (stand-alone) operation during abnormal condi-tions such as an outage in the bulk supply or emergency. Micro-grids may also be created without connection to a bulk supply and operate full-time as an independent island. Potential micro-grid designs range in size from a single house operated independently up to large substation-scale systems that serve many feeders where total load may approach 100 MW. Micro-grids offer the potential for improvements in energy-delivery efficiency, reliability, power quality, and cost of operation as compared to traditional power systems. Micro-grids can also help overcome constraints in the development of new transmis-sion capacity that are beginning to impact the power industry. Goals for the project It is the goal for the Micro-grid project to review the potential architectures, system engineering issues, and economic factors associated with the deployment of micro-grids; to identify areas for future studies and development, such as design approaches for new distribution systems and control and protection technologies. The project team has so far investigated various micro-grid configurations and reviewed the positive and negative issues associated with these systems. Key issues investigated include the type of system layout (network versus radial), operating voltage levels, types and capacity of generation required, and system protection and control needs. The team has made recom-mendations for hardware development and future research projects. The project team have discussed several micro-grid systems, including: • A small micro-grid with a fuel cell serving a cluster of six homes. The system includes an

integrated fuel-cell package, protection and control, a bulk-system isolating device, fuel connection, and heat-recovery equipment for heat distribution to the homes. This system was found to offer reliability and efficiency benefits over a traditional distribution service.

• A low-voltage network with numerous distributed generation sources and a "cellular" ap-

proach to islanded operation. The cellular approach enables separation into sub-grids when parts of the grid are damaged as well as consolidation into one large micro-grid.

• A lower-voltage DC micro-grid in which power is distributed at 400 volts DC. The system

employs inverters at each customer site. Strategic use of blocking diodes on the DC sys-tem helps with power quality and protection.

Findings One of the more interesting findings of this study is that the use of uniformly distributed gen-eration on micro-grids facilitates the ability to build distribution systems that do not need any high-voltage elements -- they are entirely low-voltage. This low-voltage approach has potential


for significant cost savings and power quality / reliability improvements and can provide im-proved safety benefits as well. It was determined that special controls and generator protection are required to facilitate proper operating of micro-grids. Control methods currently under de-velopment for conventional interconnection of distributed generation are not suitable for micro-grids. 5.5 References [1] IVO BOUWMANS, "Distributed Networks - The Gird and the Net Compared", Delft

University of technology, The Netherlands, www.inem.tbm.tudelft.nl/ivo [2] LEINER, B.M. & al. , "A brief history of the Internet",

www.isoc.org/internet/history/brief.html [3] FRANK VON OVERBEEKE, Eneco Energie,VAUGHAN ROBERTS, EA Technoloty,

"Active Networks as facilitators for embedded generation", IQPC Conference on Em-bedded Generation within Distribution Networks", London, January, 2002

[4] ALAN CREIGHTON, Yorkshire Electricity, "Evaluation of the issues for distribution

networks as they become more actively managed and the possibilities this creates for developing distribution ancillary services", IQPC Conference on Embedded Genera-tion within Distribution Networks" London, January, 2002


6. ACRONYMS ∆ult Maximum admissible increase of steady state voltage ∆ust Maximum admissible amplitude of fast voltage fluctuations AG Asynchronous (Induction) Generator CHP Combined Heat and Power station CIGRE The International Conference on Large High Voltage Electric Systems CIRED The International Conference on Electricity Distribution Networks DC Direct Current DG Distributed Generation DNO Distribution Network Operator EHV Extra High Voltage: Un > 220 kV EU European Union EUR Euro FACTS Flexible AC Transmission Systems GHG Green House Gases HP Hydro Power station HV High voltage: 50 kV < Un ≤ 220 kV ICT Information and Communication Technology LV Low Voltage Un≤ 1 kV MV Medium Voltage: 1 kV < Un ≤ 60 kV Plt Long term flicker level (average of periods longer than 30 min) Pst Short term flicker level (average of periods shorter than 30 min) REC Regional Electricity Company REG Regulator RES Renewable Energy Resources RPM Rounds Per Minute P Active power PCU Power Conditioning Unit PV Photovoltaic Systems PWM Pulse Width Modulation SCR Short Circuit Ratio SG Synchronous Generator SVC Static Var Compensator THD Total Harmonic Distorsion TSO Transmission System Operator UCTE Union for the Co-ordination of Transmission of Electricity WEC Wind Energy Converter

s u s t e l n e t - ecn - your energy. our passion. · s u s t e l n e t policy and ... sent to the...

Documents