Fault Ride-Through of DFIG-based Wind Farms connected to the Grid throughVSC-based HVDC Link

Christian Feltes Holger Wrede Friedrich KochUniversity of Duisburg-Essen E.ON Engineering GmbH REpower Systems AG

Duisburg, Germany Gelsenkirchen, Germany Rendsburg, Germanychristian.feltes@uni-due.de holger.wrede@eon-engineering.com friedrich.koch@repower.de

Istvan ErlichUniversity of Duisburg-Essen

Duisburg, Germanyistvan.erlich@uni-due.de

Abstract - This paper describes two new control ap-proaches for DFIG-based offshore wind farms connectedthrough voltage source converter (VSC) based HVDC linkwith extended fault ride-through (FRT) capability. Thesecontrol strategies establish a coordination between theHVDC terminals and the wind turbines, allow safe operationof the wind farm during grid faults and also fulfill the actualgrid code requirements. Both introduced FRT methods uti-lize different means for a fast and reliable power reductionin the wind farm to protect the HVDC transmission againstovervoltages, which may occur as a result of a power imbal-ance between sending and receiving end during grid faults.This way the costs for a high rated DC chopper can be saved.For evaluation of these approaches detailed simulation mod-els of the HVDC and the wind turbines are presented. Thesimulations are carried out in MATLAB/Simulink with Sim-PowerSystems Toolbox. The simulation results show an ex-ample of a severe three-phase grid fault, which proves thegood performance of the proposed control strategies.

Keywords - Offshore Wind Energy, Doubly FedInduction Generator, VSC-based HVDC, Fault Ride-Through, Coordinated control

1 Introduction

THE wind energy sector experiences a huge growth allover the world. While in the early 90’s only sin-

gle turbines with power ratings of less than a hundredkW were installed, today offshore wind farms (OWF) areplanned with capacities of more than 1000 MW. TheseOWF may be located far away from shore and an HVDClink (Figure 1) should be considered for their grid con-nection, because AC transmission length and capacity islimited due to the large charging currents of the cables.

~

~

~

~HV Grid

Wind Farm

VSC-HVDC Transmission

REC SEC

Figure 1: Wind Farm connected through VSC-based HVDC Transmis-sion System

Since large OWF have capacities comparable to con-ventional power plants, grid operators require them to par-ticipate in grid voltage support in steady state and also dur-ing faults. The latter requirement means that OWF have toride through grid faults and supply reactive currents to thegrid as stated in the grid codes (e.g. the German E.ONgrid code [1, 2]). This grid voltage support is the task ofthe grid side HVDC converter. Since the converters areusing self-commutated IGBT valves, the decoupled con-trol of active and reactive current is simple (section 3.1).But if the wind farm is operating close to rated powerwhen the fault occurs, the current limitation of the gridside HVDC converter with reactive current priority maylead to a reduction of its active current. Together withthe reduced grid voltage, the resulting active power flowto the grid may become very small. Without a coordi-nated control of the HVDC link and the wind turbines thiswould lead to a significant unbalance between sending andreceiving end power of the HVDC. As a result, the DCtransmission voltage would increase rapidly, which is un-acceptable. One way to limit the HVDC voltage is theuse of a huge DC chopper to dissipate the excess power.But this solution requires a chopper rated for the full WFpower, leading to higher investment costs. With the pro-posed control the active power output of the DFIG windturbines can be reduced very fast and the DC transmissionvoltage can be maintained at an acceptable level withoutthe need for a full-rated DC chopper.Section 2 gives a short introduction to some requirementsof the German E.ON grid code. Section 3 describes theHVDC system and its control. In section 4 the DFIG sys-tem is explained. In section 5 two methods for FRT of thesystem are introduced. Section 6 shows simulaton resultsand section 7 finally gives some conclusions.

2 Grid Code Requirements

In the past wind turbines were separated from the gridfollowing grid faults. However, as of now, separation ofwind turbines for voltage values below 80% of the nomi-nal voltage would lead to loss of an undesirable portion ofpower generation. Therefore, utilities now require an FRTcapability as specified in Fig. 2. Wind turbine must stayconnected even when the voltage at the point of commoncoupling (PCC) with the grid drops to zero. The 150-ms

delay shown in the figure accounts for the normal operat-ing time of protection relays. The red solid line in Fig. 2marks the lower voltage boundary rather than any charac-teristic voltage behavior.According to the new E.on (one of the major utilities inGermany) grid code of 2006 [3] short term interruption(STI) is allowed under specific circumstances. STI inarea 3 requires resynchronization within 2 s and a powerincrease rate of at least 10% of the nominal power persecond. In area 2 the interruption time allowed is muchless, just a few hundred milliseconds. During fault-ridethrough, reactive power supply by wind turbines is a re-quirement. According to the German grid code wind tur-bines have to provide a mandatory voltage support duringvoltage dips. The corresponding voltage control character-istics are summarized in Fig.3. Accordingly wind turbineshave to supply at least 1.0 p.u. reactive current alreadywhen the voltage falls below 50%. A dead band of 10%is introduced. Actually, for wind farms connected to thehigh voltage (HV) grid continuous voltage control withoutdead band is also under consideration.

, ,

STI: Short Term Interruption

No tripping

PossiblySTI

No tripping4

STIresynchronisationbefore primary control

Stepwise tripping by systemautomatic Safeguard IIafter 1.5 … 2.4s

Figure 2: Fault-ride through requirements

20%-50%

Additional

reactive current

IQ/IN

Voltage U/UN

Dead band around

reference voltage

10%

-100%

Voltage support

(over-excited mode)

-10%

Control characteristics

Reactive_current/voltage gain:

k= IQ/ U 2.0 p.u.

Rise time < 20 ms

Maximum available reactive

current IQ_max = IN

Voltage limitation

(under-excited mode)

Within dead band, e.g.

const. power factor control

Activation of voltage control

by exceeding dead band

Continuation of voltage

control after return into dead

zone at least about 500 ms

20%-50%

Additional

reactive current

IQ/IN

Voltage U/UN

Dead band around

reference voltage

10%

-100%

Voltage support

(over-excited mode)

-10%

Control characteristics

Reactive_current/voltage gain:

k= IQ/ U 2.0 p.u.

Rise time < 20 ms

Maximum available reactive

current IQ_max = IN

Voltage limitation

(under-excited mode)

Within dead band, e.g.

const. power factor control

Activation of voltage control

by exceeding dead band

Continuation of voltage

control after return into dead

zone at least about 500 ms

Figure 3: Characteristic of wind turbine voltage control

3 VSC-based HVDC

VSC-based HVDC transmission is applicable for con-nection of large offshore wind farms over long distances.The manufacturers offer systems with a transfer capabilityof 1100 MW at a DC transmission voltage of±300 kV [4].The transmission lengths are only restricted through the

losses in the cable resistances. The commonly used VSCsystem for HVDC is based on a two-level topology (Fig.4). Each converter consists of 6 semiconductor switchingdevices, commutating inductors, harmonic filters and theDC-circuit capacitors.

L1

L2

L3

AC filter

L

2C

2C

DC

Figure 4: Two-level VSC

This topology is the simplest VSC-configuration as itrequires a simple control, but it has high switching losses,especially at low power transfer. A lot of research has beendone about alternative topologies, e.g. multi-level topolo-gies and special soft-switching strategies. But the two-level topology is chosen because of its simplicity. Thefocus of this study is more on the FRT control methodsused for coordination of the HVDC and the DFIG windfarm during grid faults. Those methods can be applied todifferent VSC topologies.

3.1 Receiving End Converter

The function of the receiving end converter (REC) isto feed in the active power transmitted by the sending endconverter (SEC) while maintaining the DC voltage at thedesired level. The reactive power channel is used to sup-port the grid voltage during faults and also in steady-state[5, 6]. The control structure for the REC is shown in Fig.5.

sendp

ACu

ugrefdi _I

PsT

K1

1

DCu-

-

I

PsT

K1

1

-

ACu

ugrefqi _

magnitudelimitation

slow setpoint tracing

reactive current characteristic

refDCu_

refACu _

Figure 5: DC and AC voltage control of receiving end VSC

The PI-controller maintains DC voltage through activeconverter current under consideration of a feed-forwardterm representing the power transfer through the DC link.AC voltage control is performed by two controllers. ThePI controller in the upper branch is slow and only respon-sible for set-point tracing in steady-state operation. Thecontroller in the lower branch is a fast P controller withdeadband. It is activated when the voltage error exceeds0.1 p.u. and it is responsible for grid voltage support dur-ing faults. The magnitude of the current outputs is limited.In steady-state operation the DC voltage control and thus

the d-component of the REC current has higher priority. Incase of grid fault the priority is switched to reactive cur-rent to fulfill the grid code requirements concerning volt-age support. The decoupled control of active and reactivecurrent is achieved by a feed-forward current control witha very good dynamic response [7]. This control is basedon a vector control approach with its rotating referenceframe aligned to the grid voltage. Due to voltage orienta-tion, active power can be controlled through d- and reac-tive power through q-component of the converter current.The utilized current control structure makes use of stan-dard PI controllers (Fig. 6). The magnitude of the outputvoltages is limited by maximum modulation degree andDC voltage.

ugrefdu _

ugrefqu _

-

I

PsT

K1

1

I

PsT

K1

1

-

L

L

-

-

-

magnitudelimitation

ugrefqi _

gdu

ug

refdi _

ug

di

ugqi

gqu

Figure 6: Feed-forward Decoupled Current Control

3.2 Sending End Converter

The SEC is responsible for transmitting the activepower produced by the wind farm, while maintaining theAC voltage in the wind farm grid. Furthermore, it can beused for frequency control which in turn controls the slipof the DFIGs connected to the wind turbines. This maybe used for reduction of active power transfer through thefractional rated converter in the DFIG rotor circuit [8]. Asthe power control is performed by the wind turbines, asimple voltage magnitude controller can be used for theSEC, thus fulfilling the aforementioned requirements. Thefrequency can be directly regulated without the need for aclosed loop structure. Figure 7 shows the control structureof the SEC.

ugjeug

refgqu _

fu

I

PsT

K1

1refACu _

ugrefgdu _

0

ACu

0

_ refgu

s

2 ugreff

minu

maxu

-

Figure 7: Sending end VSC control

Since no current control is used, a current limitationcan only be achieved with an indirect method or by block-ing the IGBTs during a severe fault in the wind farm grid.The voltage control capability of the SEC can be used toinitiate a controlled voltage drop to reduce the WF powerduring FRT in HV grid.

4 DFIG-based wind turbines

The most commonly used generator type in modernwind turbines is the DFIG. A typical layout of a DFIG sys-tem is shown in Fig. 8. The back-to-back frequency con-verter in combination with pitch control of the rotor bladesenable variable speed operation, leading to higher energyyields compared to fixed speed wind turbines. Since theIGBT-converter is located in the rotor circuit, it only hasto be rated to a small portion of the total generator power(typically 20-30%, depending on the desired speed range).A rotor crowbar is used to protect the rotor side converteragainst over-currents and the DC capacitors against over-voltages [9]. A line inductor and an AC filter are used atthe grid side converter to improve the power quality.

ASG

3~

Back-to-Back PWM Converter

GeneratorGearbox

L

Crowbar

Line

Figure 8: DFIG based wind turbine system

In a DFIG system the function of the grid side con-verter is to maintain the DC voltage and to support thegrid with reactive power during a fault. Especially whenthe machine rotor is short circuited through the crowbarresistors, the generator consumes reactive power. This re-active power has to be compensated by the grid side con-verter. The rotor side converter controls active and reactivepower of the DFIG and follows a tracking characteristic toadjust the generator speed for optimal power generationdepending on wind speed. Both converter controls are us-ing a feed-forward decoupled current control [10] similarto that used for the REC of the VSC-based HVDC. Whenthe wind speed exceeds its nominal value, the pitch controlreduces the mechanical torque by pitching the rotor bladesto recover the generator rated speed. A detailed descrip-tion of the complete DFIG control system can be found in[11].

5 FRT methods

To secure fault ride-through of the described system ithas to be ensured that the generated wind power can bedissipated through the HVDC. Otherwise the HVDC volt-age would increase above the acceptable limits. This canbe avoided by use of a DC chopper rated for the full WFpower, but this means higher investment costs. Anotherway, which is favoured here, is a coordinated control ofREC, SEC and the DFIG WTs with a WF power reduc-tion after fault detection.When a fault occurs on the HV grid side, the receivingend HVDC converter can only deliver a small portion ofthe generated power to the grid, according to the followingrelationship:

pRC = |uRC |ia,RC (1)

pRC : Active power at REC in p.u.uRC : AC voltage at REC in p.u.ia,RC : Active current at REC in p.u.

During fault the current limitation of the REC switchesto reactive current priority as a result of required voltagesupport, i.e., the active current is limited to:

ia,RC,lim =√

i2RC,lim − i2r,RC (2)

ia,RC,lim: Active current limit at REC in p.u.iRC,lim: Current magnitude limit at REC in p.u.ir,RC : Reactive current at REC in p.u.

with reactive current according to the grid supportcharacteristic:

ir,RC =

{

2(uRC − uref ), if |uRC − uref | ≦ 0.5p.u.−1p.u., if |uRC − uref | > 0.5p.u.

(3)uref : Pre-fault value of AC voltage at REC in p.u.

the maximum transferable powerpmax,RC (in p.u.) is:

pmax,RC = |uRC |√

i2RC,lim − i2r,RC (4)

To avoid a continous increase of the HVDC voltage, apower reduction in the WF has to be implemented. Thereare several ways for the realiziation, of which two will bediscussed here:

• Reduction of WT output power by changing the ac-tive current setpoint in the machine side convertercontrol.

• Controlled voltage drop in the wind farm grid withindirect power reduction.

5.1 Active current reduction in the WT generator control

This FRT approach requires full communication be-tween the HVDC converters and the wind turbines. Afterfault detection at the REC, the active power of the WTgenerators can be reduced according to the following rela-tionship:

pref,WTi =

{

pref0,WTipmax,RC

p0,W F, if pmax,RC ≦ p0,WF

pref0,WTi, if pmax,RC > p0,WF

(5)pref,WTi: Active power setpoint of WT i in p.u.pref0,WTi: Pre-fault active power setpoint of WT i in p.u.p0,WF : Pre-fault total active power of the WF in p.u.

In this setpoint calculation the transmission losses areneglected, bringing along a safety margin to avoid a powerexcess on the WF side HVDC converter. The resulting ref-erence value is passed to the DFIG converter, which con-trols the rotor current of the generator to reach this value.Using this approach, the WF voltage is kept at a constantlevel. A communication delay between the two HVDCterminals and the WTs has to be considered.

5.2 Controlled voltage drop in the WF grid

This FRT method only requires communication be-tween the two HVDC terminals. After fault detection theWF voltage is reduced to effect an indirect power lmita-tion of the WTs. The voltage support feature of the WTsshould be disabled in this case. To enable a power balancebetween SEC and REC, the WF voltage has to be reducedto:

uref,WF =

{ pmax,RC

ia,W F, if pmax,RC ≦ p0,WF

uref0,WF , if pmax,RC > p0,WF

(6)

Since no communication between HVDC and WTs andthus no direct power setpoint reduction is assumed here,the WTs will react to the voltage drop by increasing theiractive currents inside the limits. This has to be correctedby the HVDC voltage control through a further reductionof the WF voltage. The active current limits of the windturbine should be set to a value close to rated current forthis application anyway.The controlled voltage drop in the WF grid should be doneas fast as possible to maintain the HVDC voltage at anacceptable level. But if the voltage drop is too steep, itis like a short circuit for the generators, leading to DFIGshort circuit currents [12] including high peaks and DCcomponents. This also means mechanical stress for thegenerator and also for the DFIG converter. In worst casethis may lead to a crowbar ignition at the DFIGs, whichbrings along a strongly restricted controlability and shouldbe avoided urgently.To get a fast voltage drop without the DC components inthe generator currents a new method based on directed de-magnetization of the machines is used here. For a simpleexplanation of this approach a virtual converter flux is in-troduced equivalent to the stator fluxes of the WT genera-tors:

ψvirt

(t) =

∫

uconv(t)dt+ ψvirt0

(7)

ψvirt

: Space vector of virtual flux in p.u.uconv: Space vector of converter voltage in p.u.ψ

virt0: Space vector of initial virtual flux in p.u.

Figure 9 shows the effect of a steep voltage drop onthis virtual flux without directed demagnetization.

Converter voltages (p.u.)

-1.5

-0.5

0.5

1.5

Virtual flux (p.u.)

-1.5

-0.5

0.5

1.5

0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24

Figure 9: α,β-components of converter voltage and virtual flux duringhard voltage drop

A DC offset in the flux also means DC components inthe WF currents, which decay slowly with a time constant

depending on grid and generator parameters. Those DCcomponents lead to high peak currents of the WT genera-tors, which are fed to the offshore grid and have to be con-sidered by additional reserve margins in the power elec-tronics layout of the SEC. They also have an impact onthe DFIG converter and may cause a fast increase of theDC link voltage, which triggers the protection circuits ofthe DFIG and may in worst case lead to a crowbar ignition.Additionally mechanical stress for the generators and thewhole WT drive train is caused by this peak currents.To suppress the DC offset in the virtual flux a time win-dow is reserved for a smooth transition of the virtual fluxmagnitude from pre-fault value to the value during fault.The size of the time window depends on the maximumproducible converter voltage. 5 ms are chosen here. Thevirtual fluxes at beginning and end of the time window canbe calculated from converter voltage setpoints before andduring fault (eq.6) by:

ψvirt

=uconv

ω0

e−j π2 (8)

ω0: WF grid angular frequency in p.u.

When a linear flux transition is desired for demagneti-zation of the generators, the transition voltage can be cal-culated by:

utr =dψ

virt

dt=

∆ψvirt

∆t(9)

=uconv(tf + ttr) − uconv(tf )

ω0ttre−j π

2

utr: Transition voltage at the HVDC converter in p.u.tf : Starting time of controlled voltage dropttr: Transition time

Figure 10 shows the effect of a controlled voltage dropwith directed demagnetization on the virtual flux. It con-tains no DC components.

Converter voltages (p.u.)

-1.5

-0.5

0.5

1.5

Virtual flux (p.u.)

-1.5

-0.5

0.5

1.5

0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24

Figure 10: α,β-components of converter voltage and virtual flux duringcontrolled voltage drop

Another way to prevent the DC offsets in the genera-tor fluxes is an independent time-triggered voltage drop inall three phases. In this approach the voltage drop is trig-gered on the zero-crossings of the virtual flux. The resultsare similar to the previous method and not shown here.

6 Simulation results

Simulations have been carried out under MAT-LAB/Simulink with SimPowerSystems Toolbox using in-stantaneous value calculation. For the equivalent windfarm model a fifth-order induction generator model is usedand the IGBT converters of the WT and HVDC are mod-eled as ideal switches with antiparallel diodes. The simu-lated test system consists of a 180 MW wind farm equiv-alent connected to the HV grid through a 200 MVA VSC-based HVDC system with a transmission voltage of± 100kV at a transmission length of 100 km.The first simulation scenario (Fig.11) shows the behaviourof the system, when a three-phase grid fault occurs at a re-mote location in the HV grid leading to a voltage dropto approx. 20% of the rated voltage at the receiving endHVDC terminal. For this case the first FRT method (sec-tion 5.1) with full communication between the WF sideHVDC terminal and the WTs is assumed. After fault de-tection the REC contributes in voltage support through re-active current. The setpoint of the WT active power con-troller is reduced after an estimated communication de-lay of 10 ms to avoid a steady imbalance between HVDCsending end and receiving end power. Due to the inertiaof the WT power control there is a temporary power im-balance that leads to an overshoot of the HVDC voltageto approx. 135%. This is not critical for the cables, but ithas to be considered in the converter design. The genera-tor torque shows a very smooth behaviour and there is nomechanical stress for the wind turbines. After fault clear-ance the wind farm output power is increased with a rampfunction to ensure a smooth power up of the system.The second simulation scenario (Fig.12) shows the sys-tem response to the same fault in the HV grid, but witha different FRT strategy. In this case the controlled volt-age drop (section 5.2) with directed demagnitization in thewind farm grid is started 10 ms after fault detection. Thebehaviour on the HV grid side is nearly the same as in thefirst scenario and characterized again by the grid voltagesupport of the HVDC converter. On the wind farm sidethe behaviour is the opposite to the case before. Voltageis reduced instead of current and the current is kept nearlyconstant. The variations in the current result from errors inthe calculation of the transition voltage and from softwareswitching actions inside the controller. In this case the WFactive power is reduced much faster. I.e. that this FRT ap-proach leads to a smaller overshoot in the HVDC voltagesto approx. 120 %. The generator torque shows some os-cillations that are very small and well damped. After faultclearance the WF voltage is increased with a ramp func-tion to its pre-fault value leading to a smooth power up.

HV grid voltages (p.u.)

-1.5

-0.5

0.5

1.5

HV grid currents (p.u.)

-2.0

-1.0

0.0

1.0

2.0

Active and reactive power at receiving end (p.u.)

-1.5

-0.5

0.5

1.5

Q

P

Wind farm grid voltages (p.u.)

-1.5

-0.5

0.5

1.5

Wind farm grid currents (p.u.)

-1.5

-0.5

0.5

1.5

Active and reactive power at sending end (p.u.)

-1.5

-0.5

0.5

Q

P

HVDC voltages at receiving end (p.u.)

-2.0

-1.0

0.0

1.0

2.0

HVDC voltages at sending end (p.u.)

-2.0

-1.0

0.0

1.0

2.0

Wind turbine electromagnetic torque (p.u.)

-1.5

-0.5

0.5

0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

time (s)

Figure 11: Simulation results for a three-phase fault in the HV grid withcoordination of HVDC and WF with output power reduction throughcurrent control

HV grid voltages (p.u.)

-1.5

-0.5

0.5

1.5

HV grid currents (p.u.)

-2.0

-1.0

0.0

1.0

2.0

Active and reactive power at receiving end (p.u.)

-1.5

-0.5

0.5

1.5

Q

P

Wind farm grid voltages (p.u.)

-1.5

-0.5

0.5

1.5

Wind farm grid currents (p.u.)

-1.5

-0.5

0.5

1.5

2.5

Active and reactive power at sending end (p.u.)

-2.0

-1.0

0.0

1.0

Q

P

HVDC voltages at receiving end (p.u.)

-2.0

-1.0

0.0

1.0

2.0

HVDC voltages at sending end (p.u.)

-2.0

-1.0

0.0

1.0

2.0

Wind turbine electromagnetic torque (p.u.)

-1.5

-0.5

0.5

0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

time (s)

Figure 12: Simulation results for a three-phase fault in the HV grid withcoordination of HVDC and WF with output power reduction throughvoltage control

7 Conclusion

This paper deals with two solutions for securing FRTof a wind farm connected to the grid through VSC-basedHVDC transmission. The first described method is usingfull communication between HVDC and WF and reduc-ing the output power of the WTs through power setpointadjustment, when a fault occurs in the connected HV grid.The control recovers the power balance between sendingand receiving end of the HVDC and limits the HVDCvoltage to 135% of its nominal value. This short-timevoltage overshoot is acceptable for the HVDC cables, butit has to be considered in the converter design.The second FRT method initiates a voltage drop in theWF grid after fault detection in the HV grid. This methodeven allows a faster reduction of the WF output powerand limits the HVDC voltage to 120%. Through directeddemagnetization of the WT generators this approach pre-vents the typical DFIG short circuit currents with DCcomponents and reduces the mechanical stress to the gen-erator and the drive train. Compared to a real three-phaseshort circuit the impact on the DFIG converter is also re-duced to a minimum. Additionally, this approach doesnot require communication between HVDC terminals andWTs and only small modifications in the control of exist-ing DFIG-based WTs.

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