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5 Power electronics for integration and control of wind turbines in power systems 5.f. Dedicated HVDC for wind power transmission New HVDC-Concept for power transmission from offshore wind farms University (U) T. V6lker, C. Mehler, H. Raffel, B. Orlik University of Bremen Institute for Electrical Drives, Power Electronics and Devices Otto-Hahn-Allee, NW1 28359 Bremen, Germany Tel.: +49/ 421 - 218 3285 Fax: +49/ 421 - 218 4318 E-Mail: VoelkergIALB.Uni-Bremen.de URL: http://www.IALB.Uni-Bremen.de Keywords HVDC, power transmission, thyristor, IGBT, VSC, offshore, wind farm, wind power, transmission line, wind park, distribution of electrical energy, power quality, Voltage Source Converter (VSC) Abstract The future for wind power generation in Germany is offshore wind energy. The preferred concept for power transmission over long distances from the offshore wind farm to the mainland is the High Voltage Direct Current (HVDC) power transmission. In this paper a new transmission concept based on HVDC is presented. Introduction Wind power is the world's fastest growing electricity generation technology, because wind power is a renewable energy resource of a high technical standard. The aim should be to replace conventional power stations supplied fossil fuel with power plants based on renewable energy in the future. The increase of onshore wind power generation in Germany has reached the maximum because there are only a few areas left to build wind power plants on the mainland. The offshore wind power generation is the future in the area of renewable power generation. For the installation of the first German offshore wind farms already for a long time the permission is present, however all technical questions are not solved yet. One important task that has still to be solved is the power transmission from the wind farm to the mainland. The preferred concept for power transmission from the offshore wind farm is the High Voltage Direct Current (HVDC) transmission, which is the best solution for long distances of power transmission. The losses of the HVDC transmission line are quoted as about 3 % per 1000 km. The losses of the converter station are about 1.5 % of the nominal transmission power [5]. Furthermore the HVDC allows the entire offshore wind farm to operate at a variable frequency, which can give some benefit in energy storage [2]. Different HVDC concepts Nowadays there are several HVDC transmission lines operating throughout the world. The main function for UVDC-systems is -the transmission of energy over long distances and -the connection of two a. c. grids with different frequencies. If the transmission line is realized by submarine or underground cable, like it must be offshore, the breakeven distance is much less than overhead transmission. It is not practical to consider a. c. cable systems for distances longer than 80 km but d. c. cable transmission systems are projected which

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Page 1: [IEEE 2008 Wind Power to the Grid - EPE Wind Energy Chapter - 1st Seminar (EPE-WECS) - Delft, Netherlands (2008.03.27-2008.03.28)] 2008 Wind Power to the Grid - EPE Wind Energy Chapter

5 Power electronics for integration and control of wind turbines in power systems5.f. Dedicated HVDC for wind power transmission

New HVDC-Concept for power transmission from offshore wind farmsUniversity (U)

T. V6lker, C. Mehler, H. Raffel, B. OrlikUniversity of Bremen

Institute for Electrical Drives, Power Electronics and DevicesOtto-Hahn-Allee, NW128359 Bremen, GermanyTel.: +49/ 421 - 218 3285Fax: +49/ 421 - 218 4318

E-Mail: VoelkergIALB.Uni-Bremen.deURL: http://www.IALB.Uni-Bremen.de

KeywordsHVDC, power transmission, thyristor, IGBT, VSC, offshore, wind farm, wind power, transmission line,wind park, distribution of electrical energy, power quality, Voltage Source Converter (VSC)

Abstract

The future for wind power generation in Germany is offshore wind energy. The preferred concept forpower transmission over long distances from the offshore wind farm to the mainland is the HighVoltage Direct Current (HVDC) power transmission. In this paper a new transmission concept basedon HVDC is presented.

Introduction

Wind power is the world's fastest growing electricity generation technology, because wind power is arenewable energy resource of a high technical standard. The aim should be to replace conventionalpower stations supplied fossil fuel with power plants based on renewable energy in the future.The increase of onshore wind power generation in Germany has reached the maximum because thereare only a few areas left to build wind power plants on the mainland. The offshore wind powergeneration is the future in the area of renewable power generation. For the installation of the firstGerman offshore wind farms already for a long time the permission is present, however all technicalquestions are not solved yet. One important task that has still to be solved is the power transmissionfrom the wind farm to the mainland.The preferred concept for power transmission from the offshore wind farm is the High Voltage DirectCurrent (HVDC) transmission, which is the best solution for long distances of power transmission.The losses of the HVDC transmission line are quoted as about 3 % per 1000 km. The losses of theconverter station are about 1.5 % of the nominal transmission power [5]. Furthermore the HVDCallows the entire offshore wind farm to operate at a variable frequency, which can give some benefit inenergy storage [2].

Different HVDC concepts

Nowadays there are several HVDC transmission lines operating throughout the world.The main function for UVDC-systems is

-the transmission of energy over long distances and-the connection of two a. c. grids with different frequencies.

If the transmission line is realized by submarine or underground cable, like it must be offshore, thebreakeven distance is much less than overhead transmission. It is not practical to consider a. c. cablesystems for distances longer than 80 km but d. c. cable transmission systems are projected which

Page 2: [IEEE 2008 Wind Power to the Grid - EPE Wind Energy Chapter - 1st Seminar (EPE-WECS) - Delft, Netherlands (2008.03.27-2008.03.28)] 2008 Wind Power to the Grid - EPE Wind Energy Chapter

length is in the hundreds of kilometres and even distances of 1000 km or greater have been consideredfeasible [3].Another advantage of HVDC is the possibility of stabilization decentralized electric power grids.Some wide spread a. c. power system networks operate at stability limits well below the thermalcapacity of their transmission conductors. HVDC transmission can be used to transfer the energy toregions where the energy is consumed. So load of the existing grids will be reduced.The HVDC power transmission can be realized in two different concepts. Today, the highestfunctional d. c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme inBrazil. At present d. c. transmission is an integral part of the delivery of electricity in many countriesthroughout the world [3].

Thyristor-HVDC

The classic HVDC, like it is used for some decades now, is based on thyristors as power electronicsdevices (Fig. 1). The thyristor-HVDC is a proven technology and available for transmission power upto some gigawatts. But the thyristor-HVDC has some disadvantages. The thyristor-rectifier and thethyristor-inverter need reactive power and filtering for an operation with good power quality. Thegrid-voltages and -currents can be seen later in Fig. 3. It is also a problem to create a voltage-systemoffshore to supply the offshore wind park with energy during the installation, times of no wind andstarting-up operation. The thyristor inverter needs reactive power for the commutation that has to besupplied by a strong supply grid or a reactive power source like a big synchronous generator that isconnected to the offshore grid.

Wind Farm Rectifier Inverter PGbidcbusbar Transmission line ri

HVDC

Fig. 1: Structure in Principle of Thyristor-HVDC

Fig. 2 shows the 6-pulse and 12-pulse thyristor units in detail.

_ _ '~~~~~~~~~~~~~~~A- dl

Id

S_ S_ ~~~Id2

Fig. 2: Thyristor-Unit (6-Pulse left, 12-Pulse right)

The thyristor-HVDC-Transmission is more expensive than a conventional a. c. solution for shortdistances. Nevertheless, UVDC may well be used for many offshore wind projects, because [2]:

- UVDC provides a power transmission with very high capacity over only two d.c. cables.

- UVDC has no restrictions for the transmission distance in principle.

Page 3: [IEEE 2008 Wind Power to the Grid - EPE Wind Energy Chapter - 1st Seminar (EPE-WECS) - Delft, Netherlands (2008.03.27-2008.03.28)] 2008 Wind Power to the Grid - EPE Wind Energy Chapter

In Fig. 3 the simulated voltage and the current of a thyristor converter are shown. The phase shiftbetween the voltage and the current and also the harmonic content of the current can be seen.

4 4

x10 RiO.n 2[ i.........itA

;2|iW.

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 1 1.01 1.02 1.03 1.04 1.05 1.06 1.07tins tins

Fig. 3: Simulation of voltage (in V) and current (in A) of Thyristor-Units (6-Pulse left, 12-Puls right)

IGBT-HVDC

The fast changes in the field of power electronic devices with turn off capability like IGBT benefit thedevelopment of Voltage Source Converters (VSC) for HVDC applications. Fig. 4 shows aVSC-HVDC transmission line, where the offshore IGBT-Unit rectifies the a.c. wind park voltages toget a d.c. voltage. The inverter on the onshore side makes a three phase a.c. voltage to feed the energyinto the public grid. In Fig. 5 you can see a IGBT-Units in detail.

Wind Farm Inverter Inverter Publicbusbar Transmission line Grid

Fig. 4: Structure in Principle ofIGBT-HVDC

AL ~~~~~~~~~~~~~~~~~,Ud/2

~~~Ud~ ~ ~ ~ ~ ~ ~~~,~LUd/2

Fig. 5: IGBT-Unit, (2-Level left, 3-Level right)

The IGBTs switch fast between fixed voltage levels. The desired a. c. waveforms are reached by apulse width modulation (PWM) and low pass filtering, which can be seen in the simulation in Fig. 6.So it is possible to create a voltage system offshore to supply the wind park with energy duringinstallation and times of no wind. IGBT-HVDC provides independent control of reactive power at theconverter stations, which could be a great benefit in increasing the power quality in the public grid.UVDC provides almost no contribution to fault currents, which in many areas are a major limitationon the connection of a new power station. In addition to full power flow control in both directions, theIGBT-HVDC system can prevent fault propagation, increase low frequency stability, reduce networklosses and increase voltage stability. But on the other hand the power range of the IGBT-HVDC isonly about some hundred megawatts, which is not enough for the big wind farms build in the NorthSea.

Page 4: [IEEE 2008 Wind Power to the Grid - EPE Wind Energy Chapter - 1st Seminar (EPE-WECS) - Delft, Netherlands (2008.03.27-2008.03.28)] 2008 Wind Power to the Grid - EPE Wind Energy Chapter

The IGBT-HVDC-Transmission is more expensive than the thyristor-HVDC relating to the samenominal power. Nevertheless the VSC-HVDC has some advantages for power transmission fromoffshore wind farms [2]:

- VSC-HVDC provides independent control of reactive power at the shore converter station,which could be a great benefit to the network operator, and could allow the networkconnection point to be on a weaker section of network, closer to the landfall.

- VSC-HVDC provides almost no contribution to fault currents, which in many areas are amajor limitation on the connection ofnew generation of any type.

- The power flow can be realized in both directions.- VSC-HVDC provides almost no contribution to fault currents, which in many areas are a

limitation on the connection ofnew generation of any type (no commutation failures) [4].

The sinusoidal voltage and current of the IGBT converter are simulated in Fig. 6. There is no phaseshift between the voltage and the current and the wave forms are sinusoidal.

Voltage in kV Current in kA10 voltage 10

-5 -.-5

Fcurrent -10-10 -10

O.OQ 0.01 0102 0.03 0.04 0.05 0°06 t in s

Fig. 6: Simulation of voltage and current ofIGBT-HVDC

New Solution

There was shown in the previous chapter that the two described HVDC-concepts, the thyristor-HVDCand the IGBT-HVDC, have different advantages and disadvantages. The idea to get a powertransmission system of high power capacity, good power quality and the possibility of energy supplyfor the offshore wind farm is the combined operation of the thyristor- and the IGBT-HVDC like it canbe seen in Fig. 7.

Thyristor- Circuit Breaker Thyristor-Rectivier Ivre

Fig. 7: Parallel operation of thyristor- and IGBT-HVDC

The two different HVDC concepts are connected on the d.c.-side by a circuit breaker to the HVDCtransmission line so they are using the same d.c.-cable. At the a.c.-side the two concepts are connectedby a transformer to the public grid or rather the offshore wind farm grid. The circuit breaker has todisconnect the IGBT-HVDC from the transmission line during maximum power transmission, becausethe d.c.-voltage level of the thyristor-HVDC is usually much higher than the voltage level of theIGBT-HVDC, for UVDC-System you can buy at the market. In this case the IGBT-inverters areoperating as an active filter and are compensating reactive power of the thyristor units, like it is shownin the chapter "Compensation of reactive Power and harmonic content".One point, which is also interesting, is the so-called "black start" characteristics of the offshore windpark. It concerns to take the wind park from the switched off condition to the normal operation. Evenif there is much wind, the wind energy stages need energy for the starting-up, in order to activate theauxiliary equipments such as cooling, oiling system, control etc.. The IGBT-HVDC is able to generatean offshore voltage grid to supply the wind power plant with energy during this time. It is alsonecessary to supply the wind park for heating or rather cooling, control etc. in case of no wind.

Page 5: [IEEE 2008 Wind Power to the Grid - EPE Wind Energy Chapter - 1st Seminar (EPE-WECS) - Delft, Netherlands (2008.03.27-2008.03.28)] 2008 Wind Power to the Grid - EPE Wind Energy Chapter

Operation Strategy of parallel thyristor- and IGBT-HVDC

To optimize the parallel operation of the thyristor- and the IGBT-HVDC it is necessary to definedifferent operation modes. There are shown two operation modes below:

Mode 1: Starting-up operation of the wind park and Power generation with partial load

o The IGBT-HVDC supplies the wind power plants with electrical energy and createsan offshore a.c. voltage grid,

* onshore: d.c. voltage control* offshore: wind park a.c. voltage control (amplitude and frequency)

o The power transmission from the offshore wind park to the mainland is done by theIGBT-HVDC.

* onshore: d.c. voltage control* offshore: wind park a.c. voltage control (amplitude and frequency)

o The Thyristor-HVDC is switched off.

Offshore Onshore

Power flow

Fig. 8: Control mode of the parallel operation of thyristor- and IGBT-HVDC

Mode 2: Power generation under full load (Fig. 11):o The complete power transmission from the offshore wind park to the mainland is done

by the Thyristor-HVDC while the HVDC voltage is set to maximum.* onshore: d.c. voltage control* offshore: d.c. current control depending on the wind offer

o The IGBT-HVDC is disconnected from the d.c.-line working as SVC (static varcompensation).

Offshore Onshore

IGBT IGBT

Fig. 9: Control mode of the parallel operation of thyristor- and IGBT-HVDC

The correct function of the parallel operation of the thyristor- and the IGBT-HVDC could already beconfirmed in simulations.

Simulation ModelThe simulation model of the parallel operation of the thyristor- and the IGBT-HVDC can be seen inFig. 10. The different units operate with the following control strategy:

o Power transmission from the offshore wind park to the mainland is done by theThyristor-HVDC and the IGBT-units are working as SVC (static var compensation).

o The thyristor-offshore-unit controls the d.c. current and the onshore-unit the d.c. voltage.

O The IGBT-HVDCs are working as SVC (static var compensation) and compensate thereactive power and the harmonics of the thyristor-HVDC. The resulting a.c. offshorecurrent can be seen in Fig. 11.

Page 6: [IEEE 2008 Wind Power to the Grid - EPE Wind Energy Chapter - 1st Seminar (EPE-WECS) - Delft, Netherlands (2008.03.27-2008.03.28)] 2008 Wind Power to the Grid - EPE Wind Energy Chapter

Power-Flow=>

|D.C. Current ||D.C. VoltageControl Control

OffshoreThyristor- Thyristor-OnhrOffshore Rectifier ; IF Irter O r

m-~~~~~~~GT ItGBT1 -t

Control of * Control ofReactive Power Reactive Powerand Harmonics and Harmonics

Fig. 10: Simulation Model of Parallel operation of thyristor- and IGBT-HVDC

Compensation of reactive Power and harmonic ContentIn Fig. lI the simulation of the parallel operation of thyristor- and IGBT-HVDC can be seen. TheIGBT-Converter acts like an active filter, so the reactive power is compensated and the harmoniccontent of the thyristor-HVDC is reduced.

4xlO

-1

0.00.01 0.02 0.03 0.04 0.05 0.06 0.07 t in S

Fig. Y1: Simulation of voltage (in V) and current (in A) of parallel operation

ConclusionThe HVDC transmission is a good solution for connecting offshore wind farms to the mainland. In thispaper two common concepts for realisation of a HVDC transmission were presented. The pros andcons were shown and an advanced concept for the realisation of an improved HVDC transmission linewas presented. The operation modes were explained and the simulations were presented. Also somemeasurements on a test stand were shown in the paper.The results in this paper show that there are advantages to use the parallel operation of thyristor andIGBT-HVDC, like the possibility of power supply of the wind park, better voltage quality and a hightransmission capacity with low losses.

References List[1] T. Petter, H. Raffel, B. Orlik: "Compact, Stand-alone renewable Energy System", PCIM Europe, May

2004, Nulrnberg, Germany

[2] Delfi University of Technology: ,,Offshore Wind Energy - Ready to Power a Sustainable Europe",December 2001, Netherlands

[3] D. A. Woodford: ,,HVDC Transmission", March 1998, Canada

[4] J. Hanson, S. Eicher: ,,Selbstgefuhrte Umrichter fur die Hochspannungsgleichstrom-Ubertragung undihr Einsatz im elektrischen Versorgungsnetz", Bauelemente der Leistungselektronik und ihreAnwendungen, 5. ETG Fachtagung, Bad Nauheim 2006, Germany

[5] Siemens: www.energy-portal.siemens.com