Answers for energy.

Special reprint from BWK – Das Energie-Fachmagazin Volume 60 (2008), No. 11, pages 6 – 13

Authors: Matthias Claus, Karl Uecker, Dietmar Retzmann

Power Electronics for Enhancementof Grid Efficiency

Authors

Power Electronics for Enhancement of Grid Efficiency

The security of power supply in terms of reliability and blackout prevention has the utmost priority when planning and extending power grids. The availability of electric power is the crucial prerequisite for the survivability of a modern society and power grids are virtually its lifelines. The aspect of sustainability is gradually gaining in importance in view of such challenges as the global climate protection and economi-cal use of power resources running short. It is, however, not a means to an end to do without electric power in order to reduce CO2 emissions. A more appropriate way is to integrate renewable energy resour-ces to a greater extent in the future (energy mix) and, in addition to this, to increase the efficiency of con-ventional power generation as well as power transmission and distribution without loss of system security.

future for both security and sustainabi-lity of power supply [1 to 4]. With the help of power electronics the power sys-tem can be given dynamic support, but not only; the efficiency of power trans-mission at various voltage levels can al-so be increased. Power electronics is ea-sily controllable which makes the grid more flexible and, due to this, it can rea-dily include availability-dependent rege-nerative and distributed energy sources.

Regenerative power generation on the basis of availability-depen-dent energy resources – particu-

larly wind power – can hardly follow the load profile which leads to significant congestions in the grid. That is, the re-quirements of wind power to flexibility and loading capacity of the grids are ex-tremely high. In view of these require-ments, power electronics will play an in-creasingly more important role in the

Matthias Claus, Graduate Engineer, born

in 1968; studied Electrical Engineering at

the University of Erlangen-Nuremberg,

since 1996 Consultant in the field of Power

System Planning (real-time simulation of

HVDC/FACTS and system protection); sin-

ce 2001 in the field of Basic Design for

FACTS/Reactive Power Compensation; sin-

ce 2005 Senior Sales and Marketing Mana-

ger at Siemens Energy, Power Transmission

Solutions, Erlangen.

Karl Uecker, born in 1962, since 1982

Specialist in the field of Commissioning of

Power Plants and High-Voltage Installati-

The figure gives a brief insight into the

transmission of electric power based

on hydro resources by means of HVDC.

ons at Siemens; since 1991 Sales Manager

for Power Quality in the region of Asia, lo-

cation Singapore and Kuala Lumpur; since

2000 Senior Sales Manager for HVDC and

FACTS; since 2006 Vice President HVDC/

FACTS Sales & Marketing, Siemens Energy,

Power Transmission Solutions, Erlangen.

Dietmar Retzmann, Prof. Dr.-Ing., born in

1947, studied Electrical Engineering at the

Technical University of Darmstadt, recei-

ved a Doctorate at the University of Erlan-

gen-Nuremberg; from 1974 till 1975

Commissioning Engineer at former Brown

Boveri & Cie AG, Mannheim; since 1976

Scientific Assistant at the chair of Electric

Power Supply at the University of Erlan-

gen-Nuremberg and since 1982 at Sie-

mens AG, Erlangen. 1998 he was ap-

pointed Visiting Professor at the Univer-

sity of Tsinghua, Beijing, China, and

2002 at the University of Zhejiang,

Hangzhou, China; since 2004 Lecturer

on HVDC and FACTS at the University of

Karlsruhe. Siemens Top Innovator, Tech-

nical Director Sales & Marketing and In-

novations HVDC/FACTS, Siemens Ener-

gy Sector, Power Transmission Solutions,

Erlangen.

i dietmar.retzmann@siemens.com

A flexible grid of this kind is also termed “Smart Grid” [3].

Power electronics is used in high-vol-tage systems for flexible AC transmis-sion – FACTS 1) as well as for high-volta-ge direct-current transmission systems (HVDC). HVDC helps prevent bottlen-ecks and overloads in power grids by means of systematic power-flow control.

1) FACTS: Flexible AC Transmission Systems

2

ITC: International Transmission Company PTDF: Power Transfer Distribution Factor

25 %

Max % PTDF

5 %2 %

“An Outlook“ – National Transmission Grid Study, U.S. DOE, May 2002

Grid enhancementis essential!

Problems in synchronouslyinterconnected systems only

t

“The Blackout“ – ITC, August 2003

Congestion, overloads and loop-flows

0

0

00

X XX

X

XXX

4.8 GW

3.7 GW

2.2 GW

0.2 GW

Generator DownTransmission Line DownDirection of Power Flow

0X

The function of the HVDC which is deci-sive for system security is that of an au-tomatic firewall. This firewall function can prevent the spread of a disturbance, which occurs in the system, at all times; as soon as the disturbance has been cleared, power transmission can imme-diately be resumed. Moreover, the HVDC technology allows for grid access of ge-neration facilities on the basis of availa-bility-dependent regenerative energy sources, including large offshore wind farms, and, compared with the conven-tional AC transmission, it boasts a sig-nificantly lower level of transmission losses on the way to the loads.

FACTS technology was originally crea-ted to support weak AC grids and to sta-bilize AC transmission over very long distances. FACTS technology encompas-ses systems for both parallel and series compensation. It rests upon the princi-ple of reactive power elements, control-led by means of power electronics, which can reduce the transmission an-gle of long AC lines or stabilize the volta-ge of selected grid nodes. Due to a high utilization degree of AC power grids, the application of FACTS technology will be-come an increasingly more interesting issue also in the case of meshed power systems, e.g. in Europe. FACTS and HVDC applications will consequently play an important role in the future de-velopment of power systems. This will result in efficient, low-loss AC/DC hybrid grids which will ensure better controlla-bility of power flow and, in doing so, do their part in preventing “domino effects” in case of disturbances and blackouts.

More Security and Flexibility due to Power Electronics

From the point of view of the design concept, AC grids are seldom configured as wide-area bulk power transmission systems. By way of example, the Wes-tern European Power Union (UCTE) at a transmission voltage of 400 kV was ori-ginally based on the concept of a system which provides power generation near the loads and has additional links to support the adjacent grids in the case of disturbances or planned outages of indi-vidual generation units. In the course of deregulation and privatization of Euro-pean power markets the idea of an All-European interconnected system came up, and in view of climate change, the is-sue of bulk power transmission of envi-ronmentally compatible energy comple-ted the picture [1 to 4]. However, prior to implementing this vision to the full ex-tent, the grid concept must be adapted to these modified conditions. To describe this, Fig. 1 shows a very clear picture from the USA, where a large-scale study on the transmission grid in the year 2002 detected a number of bottlenecks

related to local overloads and loop-flows, which indeed lead to an enor-mous blackout in that particular area one year later [5]. The figure depicts se-parate lines in load-dependent colours; the red colour marks a significant over-load, and the green one reflects a situa-tion in which even more current can ea-sily flow through. For the sake of a con-sistent load flow, the ideal solution would be to furnish the grid, which is entirely “open” for power trading, with yellow lines, which helps do away with the less loaded grey ones. It is needless to say that in the context of a complex, largely meshed grid without any additio-nal measures to boost its efficiency, an optimal load-flow control such as this is not possible. The rough idea is given in Fig. 1: the fact is that in 2003, the grid problems affected only the synchro-nously interconnected eastern areas of the US and Canada, whereas the Hydro-Québec grid in the East of Canada which is connected by means of HVDC remai-ned unaffected. The HVDC systems in-stalled there prevented further spread of the blackout fully automatically without any human interference thanks to their Firewall function. Moreover, Québec was in a position to help restore the affected adjacent grids in a very effective man-ner by means of power injection from its HVDC systems [5].

This example proves that even large AC grids can be enhanced by means of power electronics. Electronically con-trolled converter systems can namely control active and reactive power and, subsequently, the grid voltage at a requi-red response time in a far more flexible and effective way compared with power plants or phase-shifting transformers 2) distributed in the grid.

Renewable Energy Sources: Challenges to the Grids

Sustainability of power supply stands for a number of measures for efficiency enhancement – with regard to power ge-

neration, it means the increase in effi-ciency ratio during energy conversion at a power plant, the reduction in trans-mission losses in the grid and, last but not least, efficiency enhancement at the load. The decisive role in terms of sus-tainability is played by the renewable energy sources, particularly those capa-ble of producing entirely CO2-free power, such as hydro, solar and wind energy. As far as Europe is concerned, wind power constitutes a cornerstone of its future energy supply; hydro power resources are relatively small with the exception of Nordel grid and solar energy is availa-ble virtually only in the South of Europe. In the course of the last years, Germany could boast extremely high increase in the amount of wind power plants. The aggregated installed rated power of on-shore installations increased from around 12 GW in the year 2003 to over 22 GW in the year 2007. In terms of futu-re use of offshore wind energy in the German parts of the North and the Bal-tic Sea, a long-term feasibility of 30 to 50 GW of installed capacity was deter-mined in numerous studies. This is an impressive value when compared with today's national German installed gene-ration capacity of around 120 GW.

Fig. 2 depicts power infeed from a group of onshore power plants as well as the load pattern of this control area. Ac-cording to it, the balance between gene-ration and consumption does not match at all in this case; the unbalance requi-res large amounts of reserve capacity from the rest of the grid which must be at hand. In the case of thermal power plants, this kind of reserve capacity is comparatively expensive (peak power).

Fig. 1

Extreme increase in

the load flow results

in a blackout.

2) PST: Phase-Shifting Transformer

3

Lo

ad

in

GW

Po

wer

infe

ed

in

GW

2

10

12

14

18

0

Mo Tu We Th Fr Sa Mo Tu We Th Fr SaSu Su

8

16

6

4

Time period between November, 17 and 30, 2003

0.5

2.5

3.0

3.5

4.5

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1.0

Th Th

0.5

2.5

3.0

3.5

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2.0

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Source: E.ON Netz

Maximal load

Wind power plants

Problems with wind power generation – in Germany and in most other countries:

• Due to severe fluctuations, wind power can hardly follow load patterns.• Additional reserve capacity is required.

Fig. 3 depicts how this problem was solved in Australia, namely by means of energy mix. A combination of flexibly applied hydro power plants and availa-bility-dependent that is not permanent-ly available (“fuzzy”) wind power plants plus the HVDC technology as a highway for power trade and back-up reserve in both directions is a glorious example of how feasible environmentally compati-ble power supply can be. For the sake of grid security, a thermal power plant both in Tasmania and on the Australian con-tinent would have had to be constructed at higher cost alternatively to an HVDC system.

Security and Sustainability due to Power Electronics

Europe carries a high potential for im-plementation of renewables as well. In [4], an example from Denmark is given, where a Static Var Compensator 3) is used to stabilize the voltage of offshore wind farms in the weak grid of the Lol-

land island. Moreover, the combination of wind and hydro power has already been intensively used in different coun-tries; the electric power provided by hy-dro power plants situated in the Nordel area is basically delivered to the UCTE system through the sea by means of HVDC systems. Item [2] gives an exam-ple of how significantly the capacity and stability of the Baltic Cable HVDC system could be boosted by means of the Static Var Compensator Siems on the side of the weak 110 kV grid in the North of Ger-many. The initial objective at the plan-ning stage was to connect the HVDC sys-tem directly to the stronger 400 kV grid which, however, could not be implemen-ted due to the right-of-way problems in that region.

Now, the question is how renewable energies, wind power in particular, influ-ence the grid in the event of an outage. The prime example here is the massive outage experienced in the European grid on November, 4, 2006. The events started in the evening around 9:30 pm, and were triggered by the deliberate disconnecti-on of two 400 kV lines over the Ems river in order to let a large vessel pass. Due to this, a number of lines were overloaded which resulted in a domino effect typi-cal of massive outages of this kind and ended up in the splitting of the UCTE

system into three areas at different fre-quencies. It was the over-frequency area which, in addition to the congestion pro-voked by the failed lines, suffered from an excessive electric power infeed from wind farms, which was exactly what an over-frequency area required the least at that period of time. This scenario is de-picted in Fig. 4.

Should even far higher input from off-shore wind farms into the northern Ger-man grid come into play in the future, as the figure suggests, the HVDC technolo-gy provides the best possibility to for-ward the power surplus from the low-lo-ad North directly to the southern load centres of Germany or to the adjacent countries with higher power demand. This idea rests upon a well-known expe-rience with hybrid grids in other coun-tries, according to which the DC point-to-point connection carries out an easy power transfer over large distances and the adjacent AC grid is additionally sup-ported by means of FACTS [5]. The most devoted user of this hybrid concept is China. In the South of China, Siemens together with its Chinese partners is cur-rently implementing an 800 kV HVDC project Yunnan-Guang at a transfer capa-city of 5 GW (Fig. 5). Further 800 kV pro-jects at a transmission capacity of up to 7.2 GW are being currently planned. A total of 20 “Bulk Power” HVDC projects are planned in China for the time period between 2008 and 2019. The total rating amounts to 104 GW or higher (as cur-rently planned).

At a DC voltage of 800 kV the line los-ses drop by approx. 60 % compared with the present standard of 500 kV at the sa-me power rating. A great number of the-se projects in China is meant for power transmission from hydro power plants situated in the middle of the country to the distant load centres.

The project Yunnan-Guang helps save around 33 Mt CO2 in comparison with local power generation, which, in view of the current energy mix in China, would be connected with a relatively high car-bon amount.

The 2nd 800 kV HVDC project Xiangjia-ba-Shanghai, which also involves Sie-mens as well as ABB and Chinese part-ners, boasts significantly higher yearly CO2 savings of over 40 Mt thanks to even higher hydro power transmission capa-city of 6.4 GW. When comparing trans-mission losses of AC and DC, it becomes apparent that the latter typically has 30 to 40 % less losses. For instance, in the

Fig. 2

Example of actual power generated

by an onshore wind farm during a

week of maximum load.

Benefits of HVDC• Clean power• CO2 reduction• Cost reduction

“Flexible“

“Fuzzy“

Hydro power for baseload and energy storage

Plus wind power

For base load and peak-load demand 2005

Fig. 3

The Basslink HVDC project in Australia

enables the utilization of renewable

energy sources from Tasmania.

3) SVC: Static Var Compensator

4

case of the 500 kV HVDC project Ballia-Bhiwadi in India, the transmission rating of 2.5 GW helps saving approximately 0.7 Mt CO2 due to the transmission los-ses which are 37 % lower than those of the 400 kV AC double-circuit system, ty-pical of this country. The converter los-ses (i.e. those of both converter stations, incl. transformers, valve cooling and ot-her equipment) amount to 1.4 % of the rated power only.

HVDC and FACTS – an Insight into Converter Technology and Station Equipment

HVDC: “Bulk Power“example at 800 kV

The HVDC systems at 800 kV require the most state-of-the-art converter technology. The separate components of this kind of installations boast impressi-ve design and dimensions owing to the required insulation clearance distances. Fig. 6 depicts one of the all in all 48 transformers of the 5 GW HVDC system Yunnan-Guang. All the type tests of this world-wide first 800 kV HVDC were suc-cessfully completed in September 2008, which constitutes a milestone in the field of the ultra-high voltage DC power transmission. Additionally, the figure shows huge DC-disconnectors, DC-vol-tage dividers and one of the converter valve towers. For reasons of transporta-tion, the 800 kV DC voltage is generated by two converter halls at 400 kV each which allows for smaller dimensions of the transformers, i.e. critical elements from the point of view of transportation, for, with the exception of wall bushings, they cannot be taken apart.

China requires this HVDC technology to construct a high-power DC system, superimposed to the AC grid, in order to transmit electric power from huge hydro power plants in the centre of the coun-try to the load centres located as far as 2,000 to 3,000 km away with as little los-ses as possible. An HVDC system at a DC voltage of 500 kV is depicted at the front page of this issue.

FACTS: Precise use of reactive power in AC grids

In the case of AC grids, the control of reactive power by means of FACTS is the same as a compressor for an engine –

when applied to a correct degree and at the correct point it makes the AC grid to an energy highway similar to an HVDC route, but without the Firewall protecti-on function mentioned above. FACTS parallel compensation provides voltage support of weak grids and helps avoid critical voltage collapse in case of large system outages. When it comes to sub-stantial line length, FACTS series com-pensation can also reduce the inductive component to such an extent that the li-ne length becomes virtually much shor-ter. This means that even long lines re-main within the stable range, for they are virtually cut by 50 to 70 per cent. Due to this, the 1,000 kV AC lines, plan-ned in China, can bridge the distances of 2,000 km and above in a stable way.

The n–1 redundancy criterion, howe-ver, requires a double-circuit system that is, in the case of 1,000 kV, it is an ex-tremely complex matter owing to the substantial route width. When it comes to a DC system, the n–1 redundancy is already fulfilled, for there are two ± lines of one bipolar system.

As far as 800 kV AC systems are con-cerned, the transmission length of ap-proximately 1,500 km is feasible in com-bination with series compensation, whe-reas the 500 kV AC can manage appro-ximately 1,000 km only. A project of this kind with the line length of 1,000 km is, e.g. the North-South Interconnector in Brazil. In the case in this interconnecti-on, in addition to fixed series compensa-tion, a controlled series compensation 4) is installed at both the beginning and the end of the line in order to stabilize power oscillations. Further examples of FACTS projects are depicted in Fig. 7: > SVC Radsted, Denmark, a Static Var Compensator installation with noise-ab-sorbing indoor construction [4].

Area 1 Under-frequency

Area 2 Over-frequency

Area 3 Under-frequency

An idea for Europe: wind power transmission from the North to the South by means of HVDC

AreaArea 1UndeUnder-frequency

Area 2 Over-frequen

The Problem

ReliefReli to ef Area 2Area and frequency supportfrequency sufreq for Area 1Area and and Area 3

HVDCHVDCHVDC

Fig. 4

The European grid outage

on November, 4, 2008 and

an HVDC solution concept.

Fig. 5

Large HVDC projects in southern China

enable low-loss west-to-east transmission

of as much as 5 GW of hydropower-based

electrical energy produced in the country’s

interior to coastal load centers.

4) TCSC: Thyristor Controlled Series Compensation

5

“Bulk Power“ HVDC800 kV DC

> SVC Bom Jesus da Lapa, Brazil, is a Static Var Compensator installation with val-ves and controls in a container; transfor-mers, voltage-limiting reactors as well as voltage-increasing capacitors are pro-vided in form of an outdoor installation. > SVC Pelham, UK, is a Static Var Com-pensator installation with a classic buil-ding solution for valves and controls; the rest of the components are installed outdoor as well. > The thyristor of the TPSC installation 5) in the West of the USA provides only fast overvoltage protection of the series ca-pacitor in case of faults; these seven projects require no closed-loop control at all.

Power electronics – what is it all about?

The core or the “workhorse” of HVDC and FACTS installations are high-power thyristors, triggered optically by means of laser technology or electrically depen-ding on application. Thyristors can only switch on the current. The switching-off is carried out by the next current zero crossing itself. This is the reason why a thyristor converter is referred to as a li-ne-commutated system. Should no line

voltage be available on one side of an HVDC system or in a FACTS application, the system is no longer functioning. An advantage of thyristor converters is their high loading capacity both during nomi-nal and overload operation as well as in the event of contingency. Consequently, bulk-power systems at high transmis-sion capacities of 5 to 7 GW can be im-plemented with thyristors only. A furt-her benefit consists in comparatively low station losses. The TPSC projects described above use special-purpose thyristors capable of withstanding tran-sient overloading of up to approximately 110 kA.

The “strength”, i.e. short-circuit power of the grid is an important design criteri-on for the application of line-commuta-ted HVDC systems. If the grid is too we-ak, a thyristor-based HVDC system must reduce its power or, under certain condi-tions, shut down completely in order to avoid system collapse resulting from re-petitive commutation failures. In the ca-

se of weak grids, remedy is provided by FACTS for grid support, i.e. a combinati-on of the HVDC and FACTS as in the example of the SVC Siems for the HVDC project Baltic Cable. Additionally, the pro-blem can be tackled by means of “self-commutated” converters. Self-commu-tated converters make use of elements which can be switched off, mostly mo-dular of press-pack high-power transis-tors, all of which, in their turn, consist of a number of separate elements, con-nected in parallel. In this way, a conver-ter turns into an electronic generator. Self-commutated converters are nor-mally furnished with a voltage source. With its help a separate capacitor or a number of them keep the voltage con-stant 6), whereas a conventional thyris-tor-based HVDC system keeps the sour-ce current constant 7) by means of re-actors.

A detailed description of different VSC solutions is given in, e.g. [6]. A general advantage of the VSC-based HVDC sys-tems consists in the fact that one of the power grids subject to coupling can be completely voltage-free or passive, for, with the help of the intact grid, the other one can be started again similar to a power plant. This black-start capability is particularly interesting for connecting large offshore wind farms off the coast of Germany.

In item [6] an innovative development with what is known as the MMC tech-nology 8) is described, which is applied by Siemens as an “HVDC PLUS” for the HVDC projects and as an “SVC PLUS” for FACTS. This technology stands out due to its compact modular design and a new multilevel converter, which allows to generate an AC system of a virtually ideal sinus waveform from DC voltage in the voltage source by means of a great number of fine steps without any addi-tional filters. The reactive power ele-ments and filters of normally 50% of the active power, required in HVDC “Classic” applications, can be done completely away with in this case, which helps re-duce the footprint of an HVDC station by approx. 40 %. An overview of the first MMC project with a 200 kV XLPE DC ca-ble is given in Fig. 8. The goal of this pro-ject is to eliminate bottlenecks in the overloaded Californian grid: new power plants cannot be constructed in this densely populated area and there is no right-of-way for new lines or land ca-bles. This is the reason why a DC cable

Fig. 6

HVDC components for

the energy highway.

SVC Radsted, Denmark SVC Bom Jesus da Lapa, Brazil

TPSC Installation, West of the USASVC Pelham, UK

Fig. 7

Examples of FACTS projects.

6) VSC: Voltage-Sourced Converter 7) CSC: Current-Sourced Converter 8) MMC: Modular Multilevel Converter

5) TPSC: Thyristor Protected Series Compensation

6

Transmission constraintsbefore TBC project

• Power exchange by means of sea cable• No increase in short-circuit power

400 MW88 km

2010

P = 400 MWQ = ± 170 to 300 MVAr

Dynamic voltagesupport

Eliminiation ofbottlenecks

Transmission constraintsafter TBC project

will be laid through the bay, and the power will flow through it by means of the HVDC technology in an environmen-tally compatible way.

HVDC and FACTS: Comparison of Stability and Transmission Functions

In addition to the abovementioned Fi-rewall function, the HVDC systems pri-marily boast capability of directed power control in terms of sign and mag-nitude. When the operator of an HVDC system gives a value of 1,000 MW, there are exactly these 1,000 MW flowing through the line or link.

In the case of FACTS installations, it is quite different; they operate rather indi-rectly with variable impedances for re-active power control: while supporting the grid, they, however, cannot force the polarity of the load flow. The best way to describe FACTS applications is to com-pare them with traffic lights in a big-city traffic: when the traffic lights are used in the right place at the right time, the traffic runs smoothly. If there are no traffic lights, a traffic jam can develop quickly, which can then be bridged only by means of an HVDC power highway. This is pictured in Fig. 9. In order to pro-vide stable power flow from the left to the right grid with the help of a FACTS application, the voltage on the load side is to be stabilized and, in addition to this, the line must be made virtually shorter by means of series compensati-on both steady-state and dynamically for the case if power oscillations occur. In doing so, the volume of power trans-mission can be changed, whereas the di-rection of power flow is determined by phasing of both grids to each other only. That is, compared with HVDC systems, FACTS installations as they are cannot change the direction of the power flow.

In the case of an HVDC system, the magnitude as well as the sign of the power P can be changed, whereas the grids can be quite different in phase and frequency; the HVDC system forces the power flow in any case. Along with the active power control, the reactive power

control on both sides of the HVDC route can be of advantage to the grids. In the case of classic line-commutated HVDC technology, the reactive power variable range of the system can be extended with additional elements. This, however, requires corresponding space.

In a VSC-based HVDC system, an ad-ditional reactive power control system has already been integrated into the converter design (see Fig. 8); in this case, external reactive power elements are no longer required. In the case of an MMC-based HVDC system, even filters can be dispensed with.

Conclusion

The future power grids will have to withstand increasingly more stresses caused by large-scale energy trading and a growing share of fluctuating rege-nerative energy sources, such as wind and solar power. In order to keep genera-tion, transmission and consumption in balance, the grids must become more flexible, i.e. they must be controlled in a better way. State-of-the-art power elect-ronics with HVDC and FACTS technolo-gies provides a wide range of applicati-ons with different solutions, which can be adapted to the respective grid in the best possible manner. DC current trans-

combination of FACTS and classic line-commutated HVDC technology is feasi-ble as well. In the case of state-of-the-art VSC-based HVDC technologies, the FACTS function of reactive power con-trol is already integrated that is, additio-nal FACTS can be done without. Howe-ver, “Bulk Power” transmission up to the GW range remains reserved to classic, li-ne-commutated thyristor-based HVDC systems.

Fig. 8

The “Trans Bay Cable“ project

in the U.S., world’s first HVDC

VSC system with the MMC

technology.

Literature

[1] Luther, M.; Radtke, U.: Betrieb und Planung

von Netzen mit hoher Windenergieeinspeisung.

ETG-Congress, October 23-24, 2001, Nurem-

berg, Germany.

[2] Waldhauer, H.: Grid Reinforcement and SVC for

Full Power Operation of Baltic Cable HVDC Link.

38th Meeting and Colloqium of Cigré Study

Committee B4 on HVDC and Power Electronics,

Technisches Kolloqium, September 25, 2003,

Nuremberg, Germany.

[3] Retzmann, D.; Sörangr, D.; Uecker, K.: Flexibler

und sicherer. Smart Grids für den Strommarkt von

morgen. BWK 58 (2006) No. 11, pp. 10–13.

[4] Andersen, N.; Megos, J.; Retzmann, D.: Stati-

scher Kompensator für den Offshore-Einsatz. Dyna-

mische Spannungsregelung von Offshore-Windturbi-

nen. BWK 59 (2007) No. 9, pp. 48–53.

[5] Beck, G.; Povh, D.; Retzmann, D.; Teltsch, E.:

Global Blackouts – Lessons Learned. Power-Gen

Europe, June 28-30, 2005, Milan, Italy.

[6] Dorn, J.; Retzmann, D.; Uecker, K.: Vorteile von

Multilevel-VSC-Technik in Energieübertragungs-

anwendungen. VDE Congress 2008, November 3-5,

2008, Munich, Germany.

FACTS

HVDC

~

~

±P G ~

G ~

G ~

G ~

P

G ~

Loads

Loads

Loads

Loads

Fig. 9

Comparison of stability and transmission

functions of FACTS and HVDC systems.

mission constitutes the best solution when it comes to loss reduction when transmitting power over long distances. The HVDC technology also helps control the load flow in an optimal way. This is the reason why, along with system inter-connections, the HVDC systems become part of synchronous grids increasingly more often – either in form of a B2B for

load flow control and grid support, or as a DC power highway to relieve heavily-loaded grids. FACTS technolo-gy was originally de-veloped to support systems with long AC transmission lines. FACTS installations are increasingly more often used in meshed grids to eliminate congestion and bott-lenecks. It goes wit-hout saying that a

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Phone: +49 180/524 70 00

Fax: +49 180/524 24 71

(Charges depending on provider)

E-mail: support.energy@siemens.com

Power Transmission Division

Order No. E50001-G610-A106-X-4A00

Printed in Germany

Dispo 30000

TH 263-090034 470224 SD 03090.5

Printed on elementary chlorine-free bleached paper.

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Trademarks mentioned in this document

are the property of Siemens AG, its affiliates,

or their respective owners.

Subject to change without prior notice.

The information in this document contains general

descriptions of the technical options available, which

may not apply in all cases. The required technical

options should therefore be specified in the contract.