market analysis for rectifying and inverter technologies for hvdc system
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Market Analysis for Rectifying and Inverter Systems for HVDC TechnologyHaseeb Ahmad1/2/2012Högskolan på GotlandTurbine Efficiency and Grid IntegrationTRANSCRIPT
Market Analysis for Rectifying and Inverter Systems for HVDC Technology
Haseeb Ahmad
1/2/2012
Submitted to
Dr. Bahri Uzunoglu
Högskolan på Gotland
Turbine Efficiency and Grid Integration
P r o j e c t S u m m a r y
Project Title Market analysis for rectifying and inverter systems for HVDC technology
Investigator Haseeb Ahmad
The goal of this work is to focus on the emerging transmission technology, the HVDC. In
today’s competitive world, the biggest challenge, for the power companies, is efficiency of
electrical transmission. HVDC is known for low losses in long distance bulk power transmission
however for short distances, losses have become quite negligible in densely populated areas.
HVDC comprises of number of components, among which, converters are the most important
part of an HVDC system. They perform rectification and inversion operation. In this report,
advantages of an HVDC system along with its process, configurations and components are
discussed. In the second half, cost analysis and market players for rectification and inversion
systems are discussed. Moreover, another objective of this document is to provide basic
understanding about HVDC systems.
Key words: High Voltage Direct Current, Rectification, Inversion, Electricity Transmission,
Turbine Efficiency and Grid Integration
Table of Contents
1. Introduction ........................................................................................................................................... 1
1.1. Advantages and features of HVDC ............................................................................................... 2
1.1.1. Long distance bulk power transmission ................................................................................ 2
1.1.2. Interconnections .................................................................................................................... 3
1.1.3. Multi-Terminal Systems in HVDC ....................................................................................... 4
1.1.4. Support for AC System ......................................................................................................... 4
1.1.5. Limitation of Faults ............................................................................................................... 4
1.1.6. Limitation of Short Circuit Level .......................................................................................... 4
1.1.7. Control of Power Flow .......................................................................................................... 5
1.1.8. Voltage Control ..................................................................................................................... 5
1.1.9. Environmental Benefits ......................................................................................................... 5
1.2. HVDC and Wind Power ............................................................................................................... 6
2. HDVC Process ...................................................................................................................................... 7
2.1. Natural or Line Commutated Converters ...................................................................................... 7
2.2. Capacitor Commutated Converters ............................................................................................... 9
2.3. Forced Commutated Converters ................................................................................................. 10
3. Configurations of HVDC .................................................................................................................... 10
3.1. Mono-polar HVDC System ........................................................................................................ 10
3.2. Bipolar HVDC System ............................................................................................................... 11
3.3. Homo-polar HVDC System ........................................................................................................ 11
3.4. Back-to-back HVDC System ...................................................................................................... 11
3.5. Multi-terminal HVDC System .................................................................................................... 11
4. Components of HVDC System ........................................................................................................... 13
4.1. Classic-HVDC System ................................................................................................................ 13
4.1.1. Converters ........................................................................................................................... 13
4.1.2. Transformers ....................................................................................................................... 14
4.1.3. AC Harmonic Filters ........................................................................................................... 15
4.1.4. DC Filters ............................................................................................................................ 15
Turbine Efficiency and Grid Integration
4.1.5. HVDC Cables or Overhead Lines ....................................................................................... 15
4.2. VSC-HVDC System ................................................................................................................... 15
4.2.1. Converters ........................................................................................................................... 15
4.2.2. Transformers ....................................................................................................................... 16
4.2.3. Phase Reactors .................................................................................................................... 16
4.2.4. AC Filters ............................................................................................................................ 16
4.2.5. DC Capacitors ..................................................................................................................... 16
4.2.6. DC Cables ........................................................................................................................... 16
5. Cost Analysis of HVDC System ......................................................................................................... 16
6. HVDC Market ..................................................................................................................................... 19
6.1. ABB Ltd. ..................................................................................................................................... 19
6.2. Siemens AG ................................................................................................................................ 20
6.3. Alstom ......................................................................................................................................... 21
7. Conclusion .......................................................................................................................................... 22
Bibliography ................................................................................................................................................ 23
Turbine Efficiency and Grid Integration
List of Figures
Figure 1. Tower Configuration for AC and DC Transmission (HVDC Power Transmission) ..................... 3
Figure 2. Offshore HVDC Development (Appleyard, David, 2011) ............................................................ 7
Figure 3. Graphical symbols for valves or rectifier (IEC, 2011) .................................................................. 8
Figure 4. Arrangement of Capacitor Commutated Converter (ABB, 2011) ............................................... 10
Figure 5. HVDC configurations (DU, 2007) (Pualinder, 2003) ................................................................. 13
Figure 6. Six Pulse Valve Bridge for HVDC .............................................................................................. 14
Figure 7. Twelve Pulse Valve Converter Bridge with star/delta Arrangement .......................................... 14
Figure 8. Cost structure for converter stations ............................................................................................ 17
Figure 9. Transmission distance and investment costs for AC and DC power transmission lines ............. 18
Table 1. Comparison between HVDC Classic and HVDC Light®…………………………………………………………20
Table 2. Comparison between HVDC Classic and HVDC Plus®………………………………………….……21
Turbine Efficiency and Grid Integration
Nomenclature and Abbreviations
AC Alternating current
DC Direct Current
HVDC High voltage direct current
MW Mega Watts
HVAC High voltage alternating current
VSC Voltage source converter
IGBT Insulated gate bipolar transistor
TSO Transmission system operator
UHVDC Ultra high voltage direct current
LCC Line commutated converter
CCC Capacitor commutated converter
SCR Silicon controlled rectifier
SCR Short circuit ration
GTO Gate turn-off
PWM Pulse width modulation
CSC Current source converter
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1. Introduction
The High Voltage Direct Current Technology (HVDC) is used for economical power
transmission over very long distances and it’s an adequate way to connect asynchronous grids or
grids having different frequency. In 1954, the first HVDC (10MW) transmission system was
commissioned in Gotland, Sweden. (Hingorani, Apr 1996) Currently, the longest HVDC link
(2071 Km) is between Xiangjiaba and Shanghai the transmission capacity is 6400 MW and it
connects Xiangjiaba dam to Shanghai in China. (Xiangjiaba-Shanghai UHVDC Transmission
project, 2011) In 2012, the longest HVDC link will be between Amazonas and São Paulo
Brazil with length more than 2500 Km. (Rio Madeira, 2011)
HVDC transmission systems, when installed, play a very vital role in electric power
transmission system. The critical feature of HDVC is the high reliability with long useful life.
Power conversion (rectification and inversion) are the most important systems in HVDC
technology. The conversion system is used as an interface to the AC transmission system, and
converts AC to DC and vice versa.
Applications of HVDC
Following are some important applications of HVDC:
HVDC systems can be used for bulk energy transfer through long distance overhead lines
The bulk energy transfer submarine cables can be done by HVDC.
It can be used to link systems having different frequencies by using an asynchronous
Back-to-Back. The HVDC link has no constraints with respect to frequency or phase
angle between the two AC systems.
HVDC helps to create a positive damping of electromagnetic oscillations and enhance the
stability of the network by modulation of the transmission power using a Back-to-Back.
(Chan-Ki Kim et al. 2009) Consequently, it allows the fast and precise control of the flow
of energy.
HVDC systems can be extremely useful from renewable energy source’s perspective
when the consumer and production units are located far away.
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Multi-terminal DC link is helpful to supply power in the regions or countries through
which transmission lines pass.
Since reactive power does not get transmitted over a DC link, two AC systems can be
connected through a HVDC link without increasing the short circuit power, this
technique can be useful in generator connections.
The VSC based HVDC technology is gaining more attention. This new technology has
become possible as a result of important advances in the development of insulated gate
bipolar transistors (IGBT). In this system, pulse-width Modulation can be used for the
VSC as opposed to the thyristor based conventional HVDC. This technology is well
suited for wind power connection to the grid. (Chan-Ki Kim et al. 2009)
1.1. Advantages and features of HVDC
Apart from the main advantage of HVDC system to transmit bulk power over long distances
with low cost and less losses, another significant feature of HVDC is that there is no stability
limit related to the amount of power or transmission distance.
1.1.1. Long distance bulk power transmission
The transmission network is one of the most critical parts of power systems. It is very
important to design it efficiently to transmit power keeping in mind the economic factors,
network safety and redundancy. The transmission network consists mainly of power lines,
cables, circuit breakers switches and transformers. The transmission network is usually
administered on a regional basis by an entity such as transmission system operator (TSO).
HVDC is a good option to transmit power over long distances because the capacitance of an
AC high-voltage cable gives rise to charging current. This charging current effectively reduces
the amount of power a cable is able to transport. The charging current is proportional to the
length of the cable and thus the longer the cable, the lesser power it is able to transport. An AC
high-voltage cable has a maximum length of approximately 60 to 100 kilometer (Fu, 2010).
While for DC, the capacitive reactance of the cable is infinite so no charging currents exist. This
makes the transmission distance virtually unlimited. From the figure below, the difference
between tower configuration of AC and DC is shown. The DC towers are significantly smaller
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than AC tower because they carry two conductors or power lines to transmit DC while three
phase AC current is carried through three conductors or power lines.
Figure 1. Tower Configuration for AC and DC Transmission (HVDC Power Transmission)
1.1.2. Interconnections
If we compare the interconnection between two or more independent systems for an AC and
DC, the AC link poses certain issues like security, reliability, frequency control, voltage control,
primary and secondary control of reserve capacity. In most of the cases more than one AC link is
required for reliability and stability of the system. While interconnecting the systems with DC
eliminates any limitation concerning stability problems and control strategies however it does not
eliminate the issues related with primary and secondary control of reserve capacity.
As far as the submarine interconnections are concerned, the voltage variation and instability
increases with power flows due to charging current in AC connections while the installation of
intermediate reactive compensation units become impractical. The maximum practical length of
submarine cable is approximately 80 Kilometers, beyond this length HVDC is the only viable
solution to such problems however slightly higher cost is the only concern. (Zaccone, 2010)
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1.1.3. Multi-Terminal Systems in HVDC
Multi-Terminal system in HVDC provides an opportunity to transmit power to the countries
or regions within one country through that power transmission lines pass. It is economically and
politically very important to offer connections to the potential partners
A multi-terminal HVDC transmission system consists of two or more conversion stations.
This kind of transmission is little complex than point to point transmission. In particular, the
control system is more elaborate and the telecommunication requirements between the stations
become larger.
The first large scale (2000MW) multi-terminal system “Hydro Québec - New England
transmission” was built between 1987 and 1992 by ABB. Now it is the plan to build ±800 kV
UHVDC transmission link in the North East - Agra HVDC in India. The power transmission
system will have the possibility to convert 8,000 MW hence it will be the largest HVDC
transmission ever. (HVDC multi-terminal system, 2011)
1.1.4. Support for AC System
HVDC systems can also be used to provide support and stability to AC systems if the
disturbances are caused by frequency change. The frequency changes can either be caused by the
difference of power generation and demand or by the difference of voltages in different parts of
the network. HVDC can feed (or extract) the active power into the disturbed system
instantaneously. It is due to the damping torque which is an inherent and valuable feature of
HVDC link.
1.1.5. Limitation of Faults
HVDC restricts the effect of certain critical faults on AC system. Faults like voltage
depression on power swing do not transmit across DC barrier. They may appear on the other side
of a DC link as a reduction in power, but voltage will not be affected. (Chan-Ki Kim et al. 2009)
1.1.6. Limitation of Short Circuit Level
The short-circuit level of the system increases with the addition of new AC lines. It is an
unavoidable problem which has to be resolved through expansive modification in switchgear
equipment. In case of DC, no reactive power exist that means active power can be increased
without increasing the short circuit level since impedance and resistance are fixed.
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1.1.7. Control of Power Flow
HVDC link can operate at any condition of voltage and frequency of the two AC systems. An
independent control is therefore available to transmit power, leaving each system’s existing load
frequency control to act normally. This control feature uses reserves to keep the voltage and
frequency within allowable limits.
1.1.8. Voltage Control
In the conversion stations, converters absorb reactive power. This reactive power can be used
to control the voltages. It is important to realize that the normal constant power regime of a DC
link can destabilize an AC network under distress. A normal feature of the DC link is the
voltage-dependent current limit where DC power is limited when voltage drops below the
normal range, so that the reactive power is made available to the AC system. Under disturbed
conditions, it is a good principle to look after the AC voltage first, and then order the power flow
accordingly. There are substantial AC filters at the converter stations, which can be used to
bolster AC voltage if stability is threatened. The DC control drops DC power, so that the
converters absorb less reactive power and the reactive capacity of the filters is available to the
network. Though the loss of power flow is unwelcome, the boost to AC voltage maybe more
valuable. Self-commutated VSCs can provide independent control of active and reactive power.
Reactive power generation or absorption is possible, within converter ratings, at any DC power
transfer rate. (J. Arrillaga et.al 2009)
1.1.9. Environmental Benefits
It is however difficult to compare the environmental benefits of AC and DC but a qualitative
comparison is presented below
The DC line has less visual impact compared with the AC line of the same power. It is an
advantage.
The small right of way width of DC line, compared with AC line, provides suitable routes
in densely populated areas and regions having complex terrain.
All high tension electrical lines generate crackling and humming sound this phenomena is
called corona. (Corona Effect, 2011) It is a power loss and in some cases it can damage
system’s components. DC lines produce fewer coronas compared with AC lines.
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Radio interference is generated by from Corona discharges; it generates high frequency
currents in the conductors producing electromagnetic radiation, in the vicinity of the
lines. Radio noise from a DC line is considerably lower than from AC lines of similar
capacity. (Crane, 2010)
The DC line contains an unchanging electric field that means it doesn’t exert any
magnetic field on the surroundings.
1.2. HVDC and Wind Power
In the past few years, a rapid increase in wind turbine connection to distribution and
transmission networks is observed and the increased penetration makes the power network more
dependent on, and susceptible to, the wind energy production. Large scale wind generation
facilities have become a very obvious component of the interconnected power grid in many
countries especially in Europe. One of the major challenges faced by the electricity industry is
how to effectively integrate significant amount of wind power into the electricity system. For a
successful integration the electricity industry has to deal with challenges arising from market
liberalization, electricity networks renewal and innovation, the limited predictability of wind and
the frequency and voltage capabilities. (European Commission. Directorate-General for
Research., 2006)
AC links are good option if some hundred megawatts are to be transferred through few tenths
of kilometers but if both power and distance increased, DC connections would be more
competitive option. In the present scenario, HVDC with voltage source converters (VSC) seems
to be best suited for such power transmissions. A more detailed introduction of VSC will be
presented later in this report.
There are a couple of projects which have been constructed keeping in view the suitability of
VSC HVDC in conjunction with wind power. One project is at Gotland, Sweden where 60 MW
(80kV) is transmitted through 70 km by DC connection. The other project is in Tjaereborg
Denmark where 7 MW (10kV) is transmitted through 4 km by a DC connection. (Asplund) A
latest and remarkable advancement in the application of HVDC in wind industry is the
acquisition of around $ 1 billion order by ABB from the Dutch-German transmission system
operator Tenne T to supply the Dolwin2 HVDC transmission line connecting offshore North Sea
wind farms to the German mainland grid. By 2015, 900 MW HVDC converter and cable system,
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operated at ±320 kV, is to be installed. This project follows the 800 MW Dolwin1 link scheduled
for 2013 and the 400 MW Borwin1 which was installed in 2009. (Greiner, 2011)
Figure 2. Offshore HVDC Development (Appleyard, David, 2011)
Siemens is also installing multiple transmission links off the German coast, notably the 864
MW SylWin link scheduled for 2014, the 800 MW BorWin2 and the 576 MW HelWin1 links
both scheduled for 2013. The offshore platforms SylWin alpha and DolWin alpha are both being
certified by Det Norske Veritas (DNV). (Greiner, 2011)
2. HDVC Process
The fundamental process in HVDC system is the conversion of AC to DC (rectification) at
the transmitting end and from DC to AC (inversion) at the receiving end. The conversion can be
achieved by following three ways
Natural or line Commutated Converters (LCC)
Capacitor Commutated Converters (CCC)
Forced Commutated Converters
2.1. Natural or Line Commutated Converters
Natural or line commutated converters are most used in HVDC systems. LCC method uses
thyristor, a controllable semiconductor which can carry currents up to 4000 A and block voltages
up to 10 kV, for the conversion process. These thyristors are arranged in series to make a
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thyristor valve which can operate at several hundred voltages. The thyristor valve operates at
certain frequency i.e. 50 or 60 Hz and by means of control angle it is possible to change the DC
level of the bridge. In this way transmitted power is controlled rapidly and efficiently. The
convertor which is used to convert single-phase or three-phase AC voltages into DC voltages is
called rectifier. The rectifier can either be controllable or uncontrollable.
Controllable Rectifier is one which can be forced to turn on by control signals i.e. thyristors are
also called Silicon Controlled Rectifiers (SCRs). The output quantities can be adjusted using
controllable rectifiers. The controlled rectifier can also be used to convert energy from DC
voltage to a single-phase or three-phase ac. It will be the inverting mode of rectifier. (Doncker,
2011) Uncontrollable Rectifier is one which contains all diodes as electric valves. An
uncontrolled converter provides a fixed output voltage for a given ac supply.
Rectification The controlled converter, as well as the uncontrolled converter, consists of diodes.
The uncontrollable converters prevent the output voltage from going negative. Such converters
only allow power flow from the supply to the load. This is called rectification of current, and
such converters are also called unidirectional converters. (Ned Mohan, 2003)
Inversion The controlled converters allow an adjustable output voltage by controlling the phase
angle at which the forward biased thyristors are turned on. The polarity of the load voltage of a
fully controlled converter can reverse, allowing power flow into the supply, it is called inversion
of current, and such converters can also be described as bidirectional converters because they
allow power flow in both directions. (Ned Mohan, 2003)
Figure 3. Graphical symbols for valves or rectifier (IEC, 2011)
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2.2. Capacitor Commutated Converters
The strength of and AC network can be measured by short circuit ratio (SCR) which is
defined as:
SCR = Short Circuit Level of AC Bus/ DC Power
A system is said to be weak if SCR is less than 3. A weak system is more sensitive to voltage
fluctuations which cause problems in the HVDC network and special control methods are
required to partially eliminate this problem. (Mazumder, 2002)
The LCC based HVDC system becomes unreliable when it operates with weak AC systems. The
reason behind the unreliability is commutation failure due to small disturbances in AC system.
These commutation failures eventually initiate other changes such as voltage and frequency
instability in AC networks. Furthermore, LCC stations consume large amount of the rated DC
power which leads to the requirement of adding large capacitor banks and the filters.
(Mazumder, 2002) Such additions increase the cost of the system and may cause operational
problems such as development of low frequency resonance with AC networks.
The chances of commutation failure can be greatly reduced by adding capacitors between the
converter transformers and valve side. This arrangement is called Capacitor Commutated
Converters (CCC)
The (CCC) improve system’s performance and give following advantages (Capacitor
Commutated Converter, 2011)
They are good for reactive power compensation
They give stability to HVDC transmission at low SCR
They provide good control properties at long cable transmission
They are very sensitive to commutation failure.
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Figure 4. Arrangement of Capacitor Commutated Converter (ABB, 2011)
2.3. Forced Commutated Converters
The force commutated converters are also called self-commutated converters utilize the
valves made up of semiconductors with the ability not only to turn-on but also to turn-off.
Another name of such these converters is voltage source converters (VSC). Two types of
semiconductors are normally used in VSC, named Gate Turn off Thyristors (GTO) and Insulated
Gate Bipolar Transistors (IGBT). Both of them have been in use since eighties. The VSC
commutates with high frequency. The operation of the converter is achieved by Pulse Width
Modulation (PWM). With PWM it is possible to create any phase angle and/or amplitude (up to
a certain limit) by changing the PWM pattern, which can be done almost instantaneously. Thus,
PWM offers the possibility to control both active and reactive power independently. This makes
the PWM Voltage Source Converter a close to ideal component in the transmission network.
From a transmission network perspective, it acts as a motor or generator without mass that can
control active and reactive power almost instantaneously. (DU, 2007)
3. Configurations of HVDC
HVDC converter bridges together with lines or cables can be arranged as the following
configurations.
3.1. Mono-polar HVDC System
In the mono-polar system, a single pole line connects the two converters, and positive or
negative DC voltage is used. Only one insulated transmission conductor is installed in mono-
polar systems while ground or sea provides path for the return current. However, a metallic
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conductor may be used as the return path. In 1965, Konti-Skan project and in 1967, Sardinia
Italy project used mono-polar HVDC system. (Arrillaga, 1998)
3.2. Bipolar HVDC System
In the bipolar system, two mono-polar systems are combined; one is run with the positive
polarity voltage and the other with negative polarity. The two poles can be operated
independently if both neutrals are grounded. The bipolar configuration increases the power
transfer capacity. Under normal operation, the currents flowing in both poles are identical and
there is no ground current. In case of failure of one pole, power transmission can continue in the
other pole which increases the reliability. Most overhead line HVDC transmission systems use
the bipolar configuration and it is the most commonly used configuration for HVDC
transmission systems. (Arrillaga, 1998)
3.3. Homo-polar HVDC System
In the homo-polar configuration, two or more conductors have the negative polarity and can
be operated with ground or a metallic return. With two poles operated in parallel, the homo-polar
configuration reduces the insulation costs. However, the large earth return current is the major
disadvantage. (Sood, 2004)
3.4. Back-to-back HVDC System
In the back-to-back HVDC configuration, two converters at the same site are used. They are
connected to each other without any transmission line in between. This option is used when the
aim of the HVDC link is to connect two power systems with different frequencies i.e. 50 and 60
Hz. These configurations are mostly found in Japan and South America. (Biledt et al. 2000)
3.5. Multi-terminal HVDC System
In the multi-terminal configuration, three or more HVDC converter stations are
interconnected through transmission lines and cables, these converters are geographically apart,
and these can either be connected in parallel or in series. In the parallel arrangement, all
converter stations are connected to the same voltage while in series arrangement; one or more
converter stations are connected in series in one or both poles. A hybrid multi-terminal system
contains a combination of both the parallel and series arrangement. Sardinia-Corsica-Italy
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(SACOI) connection, the Pacific Intertie in USA and the Hydro Quebec - New England Hydro
from Canada to USA employed multi-terminal HVDC system. (Hausler, March 1999)
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Figure 5. HVDC configurations (DU, 2007) (Pualinder, 2003)
4. Components of HVDC System
There are two main components of HVDC system: the convertor station at the transmission
and receiving end and the transmission medium. Rectification and inversion use essentially the
same machinery. However, for better understanding, the components of classic-HVDC and VSC-
HVDC will be discussed separately in the following section.
4.1. Classic-HVDC System
The Classic-HVDC consists mainly of converters, converter transformers, AC side harmonic
filters, DC filters and HVDC cables or overhead lines. (R. Rudervall, March 2000)
4.1.1. Converters
Converters are the most critical part of an HVDC system. They perform two functions i.e.
conversion from AC to DC (rectification) at the sending end, and from DC to AC (inversion) at
the receiving end. These converters are connected to the AC network through transformers. The
classic HVDC converters are current source converters (CSCs) with line-commutated thyristor
switches. A six pulse valve bridge, shown in Figure 6 is the basic converter unit of classic
HVDC for both rectification and inversion. (Diodes and Rectifiers, 2011) Similarly a twelve
pulse converter bridge can be made by connecting two six pulse bridges. The bridges are then
connected to the AC system through transformers using star/star or star/delta arrangement. In
figure 7, a twelve pulse valve converter bridge with star/delta arrangement is shown. (Diodes and
Rectifiers, 2011) The valves can be cooled by air, oil, water or Freon but cooling by de-ionized
water is the most efficient way. Air is used for insulation.
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Figure 6. Six Pulse Valve Bridge for HVDC
Figure 7. Twelve Pulse Valve Converter Bridge with star/delta Arrangement
4.1.2. Transformers
As described above, transformers connect the AC network to the valve bridges. Transformers
also adjust the suitable AC voltage level for converters. The transformers can be of different
types depending on the power to be transmitted and possible transport requirements. (R.
Rudervall, March 2000) The HVDC transformers are made by the leading electrical companies
like ABB, Siemens and Alstom. The Tianwei group China, Reinhausen Group and CG Power
Systems are also manufacturing HVDC transformers.
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4.1.3. AC Harmonic Filters
The AC filters restrict the harmonic current from entering into AC network connected to
HVDC system. The harmonics are produced by HVDC converters. The filter banks also provide
reactive power which is usually consumed by the converters during conversion process.
However rest of the power is provided by capacitors banks.
4.1.4. DC Filters
The HVDC converters produce ripple on the DC voltage. Ripple voltage is the undesirable
mixing of AC voltages with DC output. (Diodes and Rectifiers, 2011)The voltage ripple
produces interference in the telephone networks near the DC line. Usually DC filters are neither
required for pure cable transmission nor for back to back HVDC stations. They are only required
if overhead lines are used in the transmission system. The filters may be turned filters or active
DC filters. (Active filters in HVDC applications, 2003)
The companies dealing with harmonic filters are Circutor SA, CG Power Systems, Schaffner,
Zhuhai Wanlida Electric Co., Ltd, etc. Chinese companies are mostly dealing with harmonic
filters.
4.1.5. HVDC Cables or Overhead Lines
The HVDC cables are normally required for submarine transmission while overhead cables
are required for the connections over land. For a back-to-back HVDC system no DC cable or
overhead line is needed. No serious length limitation exists for HVDC cables. Now there is a
growing trend to use cables for land also because of environmental effects of overhead lines.
4.2. VSC-HVDC System
The components of VSC-HVDC system are converters, transformers, phase rectors, AC
filters, DC capacitors and DC cables. (G. Asplund, K. Eriksson, H. Jiang, J. Lindberg, R.
P°alsson, and K. Svensson, 1998)
4.2.1. Converters
The converters are voltage source converters (VSC) and they employ IGBT power
semiconductors one operating as a rectifier and other as an inverter. The two converters can be
connected either back to back or through a DC cable depending on the application.
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4.2.2. Transformers
The role of transformers is the same for both VSC and classic-HVDC system. They adjust
the AC voltage for converters.
4.2.3. Phase Reactors
The phase reactors are used for controlling both the active and the reactive power flow by
regulating current through them. The phase reactors also act as AC filters to reduce the high
frequency harmonic ACs which are caused by the switching operation of the VSCs.
4.2.4. AC Filters
The AC filters are required to reduce the harmonics on the AC side however with VSC, there
is no need to compensate reactive power and current harmonics on the AC side are directly
related to PWM frequency. Therefore the amount of filters is less in this case as compared with
line commutated converters.
4.2.5. DC Capacitors
The objective of the DC capacitors is to provide an energy buffer to keep the power balance
during transients and reduce the voltage ripple on the DC side. There are two capacitors stacks
installed on the DC side, and their size depends on the required DC voltages.
4.2.6. DC Cables
The cable used in the VSC-HVDC applications is a new developed type where the insulation
is made of an extruded polymer that is particularly resistant to DC voltage. Polymeric cables are
the preferred choice for HVDC mainly because of their mechanical strength, flexibility and low
weight. (Weimers, December 2000)
5. Cost Analysis of HVDC System
The cost of HVDC systems depends on various factors such as power capacity to be
transmitted, transmission medium, environmental conditions, and other safety, regulatory
requirements etc. HVDC transmission systems often provide a more economical alternative to ac
transmission for long-distance, bulk-power delivery from remote resources such as large scale
wind farms. The concept of “break-even distance” always arises with long distance power
transmission. It comes when the savings in line costs and lower capitalized cost of losses offsets
Turbine Efficiency and Grid Integration
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the higher converter station costs. The break-even distance depends on several factors such as
transmission medium (cable or overhead lines) and different local aspects (permits, cost of local
labor etc).
A typical cost structure for the converter stations is shown in the figure 8 below. (D.M.
Larruskain)
Figure 8. Cost structure for converter stations
The cost of the traditional HVDC system is high because it requires for filters, capacitors
and other auxiliary equipment. The traditional HVDC system is designed for the transmission of
large amounts of energy measured in hundred of megawatts. This system is not economical less
for than 20 MW loads. (D.M. Larruskain)
In the following figure, the bipolar HVDC transmission is compared with a double-circuit high
voltage AC transmission. (Chan-Ki Kim et al. 2009)
(1) Represents the initial cost of HVAC power transmission
(2) Represents the initial cost of HVDC power transmission which is higher than HVAC due
to higher valve cost included in HVDC.
(3) Represents the cost of construction for HVAC power transmission
(4) Represent the cost of shunt capacitor installed in HVAC transmission
Turbine Efficiency and Grid Integration
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(5) Represents the cost of construction for HVDC power transmission which seems less than
that of HVAC because shunt capacitors must be installed after every 100 or 200 Km in
HVAC systems to maintain the electrostatic stability.
(6) Represents the losses in HVAC transmission
(7) Represents the losses in HVDC transmission, Initial loss levels are higher in the HVDC
system, but they do not vary with distance. In contrast, loss levels increase with distance
in HVAC system.
(8) Represents the total AC cost
(9) Represents the total DC cost
Figure 9. Transmission distance and investment costs for AC and DC power transmission lines
Turbine Efficiency and Grid Integration
19
6. HVDC Market
Each component, in the HVDC system, plays an important role for the success of the project.
One of the key components in HVDC system is the converter station which performs two vital
processes: rectification and inversion. In this section, companies offering HVDC technology will
be discussed. ABB and Siemens, the world’s leading companies in electrical power transmission,
are racing to overcome the biggest challenge of transmitting power more efficiently. According
to Peter Leupp, ABB’s head of power transmission, “The market potential for HVDC is $10
billion a year in the next few years” and “Circuit breakers could add a lot to that”. (Simonian,
2011).
ABB and Siemens are the two rivals in HVDC technology; they used to leapfrog each other in
terms of higher voltages, power capacities and distances. These two account for 80 percent of the
market shares. (Simonian, 2011)
6.1. ABB Ltd.
ABB is a leader in power and automation technologies that enable utility and industry
customers to improve performance while lowering environmental impact. The ABB Group of
companies operates in around 100 countries and employs about 130,000 people. (Our businesses,
ABB, 2011).
ABB offers HVDC Classic and HVDC Light®
; these are highly efficient alternatives for
transmitting bulk power and for special purpose applications. The HVDC Classic is a traditional
technology which is based on thyristors valves. It is used to transmit electricity by overhead lines
and submarine cables over long distances. Another attribute of the technology is to interconnect
separate power systems where traditional AC systems do not work. (The Classic HVDC
Transmission, 2011)
HVDC Light® is relatively new, IGBT based technology. It was developed in 1997 to
transmit power underground and under water over long distances. It gives enormous benefits like
invisible power lines, neutral electromagnetic fields, oil-free cables and compact converter
stations. HVDC Light® uses extruded polymer insulated cables which make this technology
economical and environment friendly. As far as the costs are concerned, the direct cost of HVDC
Light® including converters, cables and their installation, for a case having power capacity 1700
Turbine Efficiency and Grid Integration
20
MW and distance 400 Km, is approximately $ 275 to $ 420 million. This wide range is due to
differences in installation costs and local market conditions. The direct investment cost for
HVDC Light® is 0.6 to 3.2 times the cost for overhead lines which is much improved than
formerly anticipated figures of 5 to 15 times. (Dag Ravemark, Bo Normark, 2005)The difference
between these technologies is presented: (Dr. Le Tang, Feb 9, 2010)
Features HVDC Classic HVDC Light®
Converter Technology Thyristor Valve IGBT
Connection valve - AC grid Converter transformer Series reactor (+ transformer)
Max. Convertor rating at present 6400 MW, ±800 kV (OH line) 1200 MW, ±320 kV (cable)
2400 MW, ±320 kV (overhead)
Relative Size 4 1
Typical delivery time 36 months 24 months
Reactive power demand Reactive power demand = 50%
power transfer
No reactive power demand
Reactive power compensation &
control
Discontinuous control (Switched
shunt banks)
Continuous control (PWM built
in converter control)
Independent control of active
& reactive power
No YES
Scheduled maintenance Typically < 1% Typically < 0.5%
Typical system losses 2.5 - 4.5 % 4 - 6 %
Multi-terminal configuration Complex, limited to 3 terminals Simple, no limitations
Table 1. Comparison between HVDC Classic and HVDC Light®
6.2. Siemens AG
Siemens AG, a leading company, besides ABB which is dealing with HVDC systems.
Siemens provide HVDC Classic, Ultra HVDC and HVDC Plus®
for economical power
transmission over very long distances and also a trusted method to connect asynchronous grids or
grids of different frequencies. HVDC classic for both Siemens and ABB are the same where
thyristors are used for commutation.
Turbine Efficiency and Grid Integration
21
HVDC Plus® employs IGBT technology like HVDC Light
®. It uses new concept of modular
multilevel voltage-sourced converters, HVDC Plus®
is the preferred solution where shortage of
space is a criterion. It is ideal for connection of remote offshore platforms and wind farms to the
onshore grid as well as for power supply high-density areas such as mega cities. (HVDC PLUS,
2011) The HVDC Plus®
technology is used in “Trans Bay Cable Project” where 53 miles long
HVDC cable and converters are installed between two substations in California USA. The rating
is 400 MW and 200 kV are the transmission voltages. HVDC Plus provided a benefit of
approximately $40 billion which had to be consumed in reactive power compensation. (Trans
Bay Cable Project, 2007) In table 2, a comparison between HVDC classic and HVDC Plus® is
shown (Trans Bay Cable Project, 2007)
Another remarkable achievement in the field of HVDC is Ultra HVDC by Siemens, by this
technology voltage level for power transmission reached up to 800 kV and power capacity up to
7 gigawatts. This is technically and economically feasible for the first time. China Southern
Power Grid Co. in Guangzhou is scheduled to commence commercial service by mid-2010 using
this technology. (Ultra HVDC Transmission System, 2011). It employs two 400 kV systems
which are connected in series. A converter station links the DC transmission line at each end to
the AC grids. For UHVDC application; innovative solutions have been implemented to fully
meet the extended requirements for ultra-high voltage bulk power transmission.
Table 2. Comparison between HVDC Classic and HVDC Plus®
6.3. Alstom
Alstom, the world leader in transport infrastructure, power generation and transmission
provides HVDC technology with the name of HVDC MaxSine®. The company has its setup in
100 countries and has 92700 employees. In the 1990s, Alstom successfully developed a voltage
FACTOR HVDC Classic HVDC Plus®
Height of converter station
building
65 feet
35 feet
Noise Along Illinois St. in San
Francisco
72 dB 48 dB
Lightning Arrestor Posts 85 feet 65 feet
Footprint ~5 acres ~3 acres
AC Filters Required Not required
Transformers --- Smaller than Classic
Turbine Efficiency and Grid Integration
22
source converter for reactive power applications (STATCOM technology) using GTOs. In this
technology IGBTs are used as switching devices.
Alstom Grid has been awarded a contract worth approximately €240 million by the Swedish
utility Svenska Kraftnät for the 1440 MW South-West Link. The project will connect Barkeryd
in central Sweden to Hurva in southern Sweden, using High Voltage Direct Current (HVDC)
technology. The contract is registered in the third quarter of Alstom’s fiscal year 2011/2012. This
transmission project will use Alstom Grid’s HVDC MaxSine® Voltage Source Converter (VSC)
technology. Under the terms of the contract, Alstom will supply HVDC converter stations at both
ends, as well as control and protection, converter transformers, switchyard equipment,
construction and project management. The project will be completed by the end of 2014.
(Alstom Grid , 2012)
7. Conclusion
It is quite conceivable that with changed circumstances in the electricity industry, the
technological developments, and environmental considerations, HVDC would be the preferred
alternative in many more transmission projects. To implement the grid that is required for the
future, collaborative planning is needed using a long term, system perspective. We know where
the wind is, and we know where the loads are. Jointly, we can identify strategic broad-based
interstate system plans to harvest renewable resources before the individual projects develop.
Rather than project by project, piecemeal solutions, we must develop and justify an integrated
system. In order to capture the full scale of benefits that high capacity technologies such as
HVDC provide, the system must be examined on an interregional scale that matches the reach of
those benefits. As concluded from the discussion above, the HVDC transmission has high market
potential. The DC transmission fell into disuse decades ago compared with the more
conventional alternating current. But the thirst for power from fast growing economies, such as
China, India and Brazil, and the vast distances over which electricity often has to be transmitted
there, have sparked a huge revival of HVDC.
Turbine Efficiency and Grid Integration
23
Bibliography ABB. (2011). Retrieved December 17, 2011, from
http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/302fc0a0fbc89556c125721a0032e2f7/$fi
le/ccc%20concept.pdf
Active filters in HVDC applications. (2003, April). CIGR´E WG Technical Brochure No.223 , pp. 14-28.
Alstom Grid . (2012, January 2). Retrieved January 3, 2012, from Alstom wins HVDC contract worth
around €240 million in Sweden: http://www.alstom.com/news-and-events/press-releases/Alstom-wins-
HVDC-contract-worth-around-240-million-in-Sweden/
Appleyard, David. (2011, October 11). HVDC Steals A March in Grid Technology. Retrieved December 14,
2011, from Renewable Energy World International:
http://www.renewableenergyworld.com/rea/news/article/2011/10/hvdc-steals-a-march-in-grid-
technology
Arrillaga, J. (1998). High Voltage Direct Current Transmission. London: The Institution of Electrical
Engineers.
Asplund, G. Sustainable energy systems with HVDC Transmission. ABB Power Technologies, Power
Systems, HVDC .
Biledt, J. G. D. Menzies F. (2000). Electrical system considerations for the Argentina-Brazil 1000MW
interconnection. Paris, France,: CIGR´E conference.
Capacitor Commutated Converter. (2011). Retrieved 12 17, 2011, from ABB:
http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/302fc0a0fbc89556c125721a0032e2f7/$fi
le/ccc%20concept.pdf
Chan-Ki Kim, Vijay K. Sood, Gil-Soo Jang, Seong-Joo Lim, Seok-Jin Lee. (2009). In HVDC transmission:
power conversion applications in power systems. John Wiley and Sons.
Corona Effect. (2011, March 23). Retrieved December 13, 2011, from Electrical Notes & Articals:
http://electricalnotes.wordpress.com/2011/03/23/what-is-corona-effect/
Crane, P. C. (2010). Radio-Frequency Interference (RFI) From Extra-HighVoltage (EHV) Transmission Lines
.
D.M. Larruskain, I. Z. Transmission and Distribution Networks: AC versus DC. Spain: Department of
Electrical Engineering, University of the Basque Country - Bilbao (Spain).
Dag Ravemark, Bo Normark. (2005, april). Light and invisible, Underground transmission with HVDC
Light. Grid Flexibility , pp. 25-29.
Diodes and Rectifiers. (2011). Retrieved December 18, 2011, from All about circuits.
Turbine Efficiency and Grid Integration
24
Diodes and Rectifiers. (2011). Retrieved December 17, 2011, from All about circuits:
http://www.allaboutcircuits.com/vol_3/chpt_3/4.html
Doncker, R. W. (2011). ELECTRICAL POWER ENGINEERING LABORATORY I "Line-Commutated
Converters". Retrieved December 16, 2011, from EDUCYPEDIA:
http://educypedia.karadimov.info/library/V_i_7_Eng.pdf
Dr. Le Tang, V. P. (Feb 9, 2010). High Voltage DC Technologies. ARPA-E Power Technology Workshop.
DU, C. (2007). VSC-HVDC for Industrial Power Systems. Goteborg, Sweden: Chalmers University of
Technology.
European Commission. Directorate-General for Research. (2006). European Technology Platform
SmartGrids - Vision and Strategy for Europe’s Electricity Networks of the Future. publications.eu.int.
Fu, D. Y. (2010, October 6). Kiviniria. Retrieved December 10, 2011, from Long Distance Bulk
Transmission:
http://www.kiviniria.net/media/Techniekpromotie/Thema_sKIVINIRIA/Duurzaam_omgaan_met_energi
e/Presentaties/W212_Son_Sahara_Stroom_Stopcontact_-_Fu.pdf
G. Asplund, K. Eriksson, H. Jiang, J. Lindberg, R. P°alsson, and K. Svensson. (1998). DC transmission based
on voltage source converters. Paris, France: CIGR´E 98, vol. 4.
Greiner, C. (2011, November 10). Clustering and Interconnection of offshore wind could save Northern
Europe €27 billion. Retrieved December 14, 2011, from
http://blogs.dnv.com/research/2011/11/clustering-and-interconnection-of-offshore-wind-could-save-
northern-europe-e27-billion/
Hausler, M. (March 1999). Multiterminal HVDC for high power transmission in Europe. CEPEX99
conference. Poznan, Poland.
Hingorani, N. (Apr 1996). High-voltage DC transmission: a power electronics workhorse. Spectrum, IEEE ,
63 - 72.
HVDC multi-terminal system. (2011). Retrieved December 13, 2011, from ABB:
http://www.abb.com/industries/db0003db004333/709a259fab1b7761c1257481004703a4.aspx
HVDC PLUS. (2011). Retrieved December 26, 2011, from Siemens:
http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-plus/#content=Description
HVDC Power Transmission. (n.d.). Retrieved December 12, 2011, from Stanford University:
http://www.stanford.edu/~rhamerly/cgi-bin/Ph240/Ph240-1.php
HVDC Steals A March in Grid Technology. (2011, October 11). Retrieved December 14, 2011, from
Renewable Energy World International :
Turbine Efficiency and Grid Integration
25
http://www.renewableenergyworld.com/rea/news/article/2011/10/hvdc-steals-a-march-in-grid-
technology
IEC. (2011). Terminology for high-voltage direct current transmission IEC Committee SC22F,IEC reference
number 22F/37/CDV .
Jos Arrillaga, Yonghe H. Liu, Neville R. Watson, Nicholas J. Murray. (2009). Non-Linear Control of VSC and
CSC Systems. In Self-Commutating Converters for High Power Applications (pp. 145-146). Wiley.
Mazumder, A. (2002). Capcitor Commutative Converters for HVDC Transmission System. Quebec,
Canada.
(2003). Naturally Commutating. In T. M. Ned Mohan, Power electronics: converters, applications, and
design, Volume 1 (pp. 264-265). John Wiley & Sons.
Our businesses, ABB. (2011). Retrieved December 23, 2011, from ABB:
http://www.abb.com/cawp/abbzh252/a92797a76354298bc1256aea00487bdb.aspx
Pualinder, J. (2003). Operation and control of HVDC links embedded in AC system. Goteberg, Sweden:
Charlmers University of Technology.
R. Rudervall, J. P. (March 2000). High voltage direct current (HVDC) transmission systems technology
review paper. Washington, USA: Energy Week 2000.
Rio Madeira. (2011). Retrieved December 9, 2011, from ABB:
http://www.abb.com/industries/ap/db0003db004333/137155e51dd72f1ec125774b004608ca.aspx
Simonian, H. (2011, November 27). ABB and Siemens in HVDC power race. Retrieved December 22,
2011, from ft: http://www.ft.com/intl/cms/s/0/50721a8a-18f7-11e1-92d8-
00144feabdc0.html#axzz1i7qA7Pqu
Sood, V. K. (2004). HVDC and FACTS Controllers: Applications of Static Converters in Power Systems.
Norwell,Massachusetts USA: Kluwer Academic Publishers.
The Classic HVDC Transmission. (2011). Retrieved December 24, 2011, from ABB:
http://www.abb.com/industries/us/9AAC30300393.aspx
Trans Bay Cable Project. (2007, April 18). Retrieved December 26, 2011, from
http://www.caiso.com/1bbf/1bbfb7221cb80.pdf
Ultra HVDC Transmission System. (2011). Retrieved December 24, 2011, from Siemens:
http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-ultra/
Weimers, L. (December 2000). New markets need new technology. Proc. of 2000 International
Conference on Power System Technology, vol. 2, (pp. 873–877). Perth, Australia.
Turbine Efficiency and Grid Integration
26
Xiangjiaba-Shanghai UHVDC Transmission project. (2011). Retrieved December 9, 2011, from ABB:
http://www.abb.com/industries/ap/db0003db004333/148bff3c00705c5ac125774900517d9d.aspx
Zaccone, E. (2010, March 23). HIGH VOLTAGE DC LAND AND SUBMARINE. Retrieved December 12, 2011,
from Insulated Conductors Committee:
http://www.pesicc.org/iccwebsite/subcommittees/subcom_e/E4/2010/2010Spring-
HVDCLandSubmarineCableSystem-Zaccone.pdf