Transcript
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Transient stability of differentelectrical concepts for wind farms

UpWind Work Package 9, D 9.4.1

E.J. WiggelinkhuizenJ.T.G. Pierik

(ECN Wind Energy)

ECN-E–11-003

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Transient stability of different electrical concepts for wind farms

AbstractAs part of the EU 6th FP project "UpWind" Work Package 9.3 the dynamic response of twodifferent electrical designs of offshore wind farms has been evaluated for symmetrical onshoregrid faults. The first design uses an HVDC connection to shore based on voltage source con-verters and the second design uses an HVAC connection. In both cases the wind farm consistsof variable speed turbines, with a with permanent magnet generator and full-power voltagesource converter, grouped into 5 feeders connected to a single bus.

The simulation results of a 3-phase fault in the onshore grid have been compared for differentminimum grid voltages, for different wind speeds and for different power ratings of the dc-linkbraking resistor.In all cases the HVAC and HVDC connected wind farms did ride-through the 3-phase onshoregrid fault while supporting the grid voltage during and shortly after the fault. The appliedmethod of active power reduction of the HVDC connected wind farm by a fast decrease of theac-voltage in the wind farm showed to be effective. Also it does not rely on communicationlinks and also the WT converter control is very similar as in the HVAC connected wind farm.On the WT generator side the response to the applied faults was similar in the HVAC connectedWF and in the HVDC connected WF as the full-rated WT convertereffectivly decoupled thegenerator side from the WF grid side. The drive-train oscillations only depend a little on theoperating conditions and are rapidly damped during the rampup of the active power.

Due to the limitations of the models several aspects are not or not accurately represented inthe results, such as harmonics, unbalance and the response of protection systems. However,the models can be used to design evaulate wind farm configurations and wind farm controlsystems. In order to make the models better applicable they should be validated with measure-ments or with other models.

Acknowledgement Project funded by the European Commission under the 6th (EC) RTDFramework Programme (2002- 2006) within the framework of the specific research and tech-nological development programme "Integrating and strengthening the European Research Area"and by the Ministry of Economic Affairs of the Netherlands as ECN Programmafinanciering.

Contract No.: 019945 (SES6)Project title: UpWindWork Package: WP9: Electrical gridTask: 3: Reliability and electrical and control concept of wind farmDeliverable: D9.4.1ECN project number: 7.9466

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Contents

1 Introduction 51.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 Choice of configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51.3 Structure of the report . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 6

2 Model description 72.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Wind farm with HVDC-VSC connection . . . . . . . . . . . . . . . . . . .. . 72.3 Wind farm with HVAC connection . . . . . . . . . . . . . . . . . . . . . .. . 82.4 Wind turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.5 Wind turbine control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 82.6 HVDC control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.7 Model parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

3 Simulations 153.1 Case descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 153.2 Results for HVDC connected wind farm . . . . . . . . . . . . . . . . .. . . . 153.3 Results for HVAC connected wind farm . . . . . . . . . . . . . . . . .. . . . 223.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Conclusions and recommendations for future work 25

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1 Introduction

In the last five years offshore wind energy has taken off, withthe installed capacity reaching2GW in 2009, 3.5GW under construction and a target of 40GW in 2020 [9]. With this in-creasing number and scale of offshore wind farms the influenceon the power balance becomesmore significant, especially in case of failures, grid abnormalities or extreme wind events.Grid abnormalities normally affect all the wind turbines ina large wind farm simultaneouslyand may therefore lead to large power drops or even damage of components in the wind farm.The consequences of grid abnormalities such as voltage dips,outages and unbalance, dependon the control capabilities of the wind turbines and of the electrical transmission system of thewind farm to "ride-through" these disturbances. Also the wind farm control is important as itshould provide adequate grid support during the recovery phase.Therefore most wind-specific connection requirements, or grid codes, include so-called "FaultRide Through" (FRT) requirements for the behaviour of wind turbines during grid faults [11].These requirements specify typical profiles of voltage dips for which the wind turbines shouldstay connected for a certain period of time and support the grid voltage. After the fault hasbeen cleared the wind turbines should support the voltage restoration and in case of trippingthe conditions for reconnection are specified, for example a certain ramp-up rate for the ac-tive power feed-in. Compliance of wind farms should be measured at the Point of CommonCoupling (PCC) where the functional performance requirements apply, supplemented by in-formation gained in certifcation process for single wind turbines.

1.1 Objectives

The objective of this study, which is part of the EU 6th FP project"UpWind" Work Package9.3, is to investigate the design requirements of electrical systems of wind turbines and windfarms due to the need for reliability and controllability ofwind farms in power systems duringgrid transients.The approach is chosen to compare simulations of different electrical designs of offshore windfarms for onshore grid faults, also in relation to results available through literature. As oneof the evaluation criteria compliance with Fault-Ride-Through requirements in grid codes isused, for example the E.ON and REE grid codes [8], [23]. Anothercriterion is to comparethe levels of electrical and mechanical stress resulting from grid transients. Other aspects forstudying are the sensitivity for parameter variations and disturbances or other capabilities andlimitations of the technology.A number of simplifications are made in the models to reduce thecomputation time so thatmore complex wind farms can be simulated. The converter models, for example, are non-switching controlled voltage sources and a very simple gridmodel has been used. Thereforea detailed analysis of harmonics, protection systems and component losses is out of the scopeof this study.The chosen electrical parameters in this study do not refer topractical systems and also thesimulation results have not been validated with measurements. Finally, only a limited numberof designs can be evaluated in detail, therefore only two basic designs have been chosen whichare representative for offshore wind farms and which are notfully investigated yet.

1.2 Choice of configurations

While the size of wind farms is steadily increasing as well asthe distance to shore, HVACtechnology will reach its technical and economical limits and connection through HVDC willbecome more attractive. Therefore a wind farm with HVDC connection is studied and com-pared to an HVAC connected system. Two basic HVDC technologies exist, namely HVDCwith Line Commutated Converters and with Voltage Source Converters. HVDC-LCC hasbeen applied for many years already, but is not well suitablefor offshore wind energy, becauseof its large footprint and its limited controllability [4],[1], [26]. Therefore HVDC-VSC, whichis better suitable because of its good controllability and small footprint, has been selected forthis study. Several strategies for FRT of HVDC-VSC connected wind farms are explained in[13]. Recently several studies simulating HVDC connected wind farms have been performed

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Transient stability of different electrical concepts for wind farms

or have just started, cf. [10, 24, 28]. Most studies apply more detailed converter modelsincluding switching with less detailed wind turbine models, cf. [5].The type of wind turbine that is modelled is a variable speed pitch turbine with full-ratedconverter, as recently the share of this turbine type with its superior control capabilities isincreasing [14], [6]. However it would be relevant to also study different types of wind turbinesin future. For instance, by applying simple and robust wind turbines with directly coupledinduction generators in combination with HVDC-VSC connection, the major difficulties withgrid connection of this type of wind turbines can be overcome, e.g. [3].The grid is represented as a single synchronous generator with series impedance and the faultis represented as a symmetrical 3-phase shunt resistance. Response to unsymmetrical faults,cf. [16], [2] as well as to dc-faults and other events leadingto transients, such as tripping ofwind turbines or lines and tap changer activation, has not been considered. The model doesnot include negative and zero sequence control and protective equipment such as groundingtransformers and circuit breakers.For the wind farm model the following configurations are studied and compared:

• Size: 60 wind turbines of 6MW

• Turbine electrical system: Variable Speed collectine Pitch

• Turbine electrical system: Permanent Magnet Generator (PMG) with full-scale back-to-back VSC

• Internal grid: 5 feeders connected to single collector bus, 34kV AC

• Distance to shore: 75km

• Connection to shore: HVAC at 150kV vs. VSC-HVDC 400MW at +/-200 kV

• Grid voltage: 230kV / 50Hz., short-circuit power 2000MVA

The following aspects have been studied:

• Generator torque control

• DC-voltage control of HVDC link and in WT converter

• AC voltage control of onshore grid at PCC and of internal WF grid

• Startup and normal operation

• Response to symmetrical onshore grid faults

1.3 Structure of the report

Section 2 presents the model description of the wind farm withHVDC-VSC connection andof the wind farm with HVAC connection, including the controls and the main parameters.In section 3 the simulation results are presented and briefly discussed and conclusions andrecommendations can be found in section 4.

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2 Model description

2.1 Scope

A number of simplifications have been carried through in the models to reduce the computationtime when simulating wind farms with a large number of turbines and converters and alsobecause the required information, such as parameter settings, is not available. The modelshave been implemented in Simulink and are based on earlier work in which dynamic windfarm models have been developed and demonstrated, cf. [21, 19, 20] and also partly verified,cf. [17, 18].The models only consider symmetrical AC-voltages and currents without harmonics. Devicesfor protection, balancing and harmonics filtering are not included. Nonlinear magnetic effects,such as transformer core saturation and hysteresis losses,are not modelled either. This meansthat the influence of inrush currents on the voltage recovery after severe dips is not accuratelyrepresented in the models. The cable models consist of lumpedfrequency-dependent networkelements. The HVAC cable to shore has been subdivided into 3 sections in cascade for amore accurate modelling of the voltage profile. Converters are modelled as non-switching andlossless, only a low-pass filter is included to represent the switching delay. Further, low-passfilters are included in the converters to represent the behaviour of the PLL for voltage phasorestimation. At the DC-side voltage ripples from converter switching and voltage unbalanceare neglected.

2.2 Wind farm with HVDC-VSC connection

The wind farm configuration with HVDC-VSC connection consists of five feeders, each con-sisting of a 6MW turbine scaled to represent a string of twelve 6MW turbines, connected to abus through a single cable, cf. figure 1. The farm is connected tothe onshore grid through anHVDC link consisting of two identical voltage-source converters, with the wind farm converter(WF-VSC) operating as rectifier and the Grid-side converter (GS-VSC) as inverter, connectedthrough a DC-cable pair. The DC-voltage of 400 kV (+/-200kV) is chosen as the highest thatis currently available, to minimize the losses over longer distances. Two transformers adaptthe voltage levels of the converters to the wind farm voltageand the onshore grid voltage. Achopper-controlled braking resistor is included to limit the DC-voltage rise in case of a suddendrop of the active power to the grid, e.g. because of a grid fault. The power rating of thebraking resistor is kept relatively small because of the high costs, so the WF-VSC should beable to restore the power balance in order to prevent tripping of the wind farm.

WT

grid

UDC GSC

34/180kV WF-VSC +/- 200kV GS-VSC 180/230kV PCC 10km 50km 230kV 430MVA 75km 430MVA line line 50Hz

Rfault

P,QWF P,QWFC P,QGSC

Vdq_gridVdq_WF

fgridf

WF grid

WT 3-ph.faultRBR chopperP :72MW

VSP PMGfull-size conv60 x 6MW

C C

UDC WFC

MAX

Figure 1:Wind farm and HVDC schematic overview

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Transient stability of different electrical concepts for wind farms

2.3 Wind farm with HVAC connection

WT

grid

34/150kV 90MVA 150kV 90MVA 150/230kV PCC 10km 50km 230kV 430MVA 75km 430MVA line line 50Hz

Rfault

P,QWF P,Qgrid

Vdq_gridVdq_WF

fgridf

WF

WT 3-ph.fault

VSP PMGfull-size conv60 x 6MW

Figure 2:Wind farm and HVAC schematic overview

As a reference, the same wind farm has been modelled with HVACconnection to shore. The150kV cable has reactive power compensators at both ends.In this wind farm model the same wind turbines models have been applied as in the HVDCcase, only with a different control of the reactive power. Inthe HVAC model the wind turbinescontribute to the AC voltage control, both under normal operating conditions and during volt-age dips, while in the HVDC model the WF-VSC controls the voltage of the wind farm whilethe wind turbines operate at unity power factor.

2.4 Wind turbine

The wind turbine type is variable speed pitch with a permanentmagnet generator and full-ratedconverter. The wind turbine model includes a 2-mass drive-train model and a 1-mass flexibletower. The aerodynamic torque and thrust coefficients are calculated as a function of tip-speedratio and pitch angle, based on a BEM rotor model. The wind modelincludes turbulence,rotational sampling, tower shadow and wind shear. It also includes dynamic inflow and pitchactuator models. The wind turbine strings are fed with different wind speed realisations. Thewind turbine power and rotor speed are controlled by collective blade pitching and generatortorque control [27]. In the generator from [19], parametersfrom a 2MW multi-pole generatorfrom a wind turbine have been used, cf. [25]. The upscaling is performed by multiplyingthe current and the electrical torque with a factor of three and inserting an ideal gearbox tocompensate for the difference in rated rotational speed. Theconverter controls the generatorelectrical torque according to the setpoint from the wind turbine controller and also controlsthe generator reactive power to zero to minimize losses. At the grid-side the converter controlsthe dc-link voltage through the active power. The reactive power to the grid is either controlledto follow a reactive power reference or to control the terminal voltage or the power factor.

gearbox PMG rect inv 4/34kV J J 7.5MVA

θ1,2,3

ωg Udc P,QWT

Vw

X+RL

P,QRectωsvdq s

CiWTvWT

Tmωr

gr

Figure 3:Wind turbine schematic overview

2.5 Wind turbine control

Figure 4 shows schematics of the wind turbine control and the generator-side converter control(upper figure) and of the grid-side converter control (lower figure). Under normal operatingconditions the rectifier setpointTe set sm equals the electrical torque setpointTe set from the

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2 MODEL DESCRIPTION

wind turbine controller, see upper figure. The inner control loop controls the currentid, whichis aligned with the stator flux and directly linked to the reactive power, to zero, whileTe iscontrolled through the currentiq, cf. [22]. The series filter inductance between the converterand the generator is neglected, but does not seriously influence the control response. The lim-itation of the converter voltageuc is derived from the maximum possible fundamental voltagethat equals

3/2 Udc/2.

In case of a high DC-link voltage the generator torque setpoint is decreased gradually, whichis illustrated in the upper left box. The high-pass filter circuit of the DC-link voltage em-phasizes a fast rise in the DC-link voltage in order to detectthe DC-voltage rise at an earlystage. During the reduction of the generator torque a low-pass filter is enabled in order tosmoothen the oscillations in the torque setpoint from the turbine controller. The speed of thetorque reduction is chosen askred/τred for a DC-voltage rate of decrease of1/τred. After thepower balance has been restored and the grid-side inverter starts to ramp-up active power, thedc-voltage decreases below a thesholdUdc raised and the generator torque is slowly ramped-up.

The lower scheme shows the wind turbine inverter control. The output current is controlledin a similar way as for the generator-side converter. The current setpointsid ref andiq ref arecalculated from the active and reactive power setpoints andthe measured grid voltage. Pleasenote that the power-invariant dq-transform is used here. Thebandwidth of the current controlloop, which is limited by the switching delay, is much higherthan the bandwidth of the outerloops controlling active and reactive power.

θ1,2,3 θset_1,2,3

Te_set

WTctrl

Udc raised

dc limdc set

2

Te_set_red

dc_det2

Treduction

e_setUdc

Te_set_red

i = 0d set

+ -

k +k /spq iq

|u | = f(U )C DC

+ -

k +k /spd id +

- udq sωg

Te Tm

Te_set

LPF

τ

smooth switch: 0.5s

U

UU

= 0.2s

Te_set_sm

Te

id udq decHPF

Udc max Udc_det ωref

Wind turbine and rectifier control

Wind turbine inverter control

Te_set_sm

dc limdc set2 2

τ

τ = 20ms

red

red

e_set_smred

redrate = K /

K = T *2(U - U )/C

2 2

iWTuCk +k /spc ic

i priority Plim

X+RL+ -

-+

-

idq_refi = Pu /|u| + Qu /|u|

i = Pu /|u| + Qu /|u|

2 2 d d q 2 2 q q d

uG

|u | = f(U )C DC GSC

P P = u i + u i Q = u i - u i

DC WTC d d q q

WTC q d d q

uWT`

+- k +k /s

pDC iDC

uWT

u2

u2Udc ref

Q =0 ref ext

dq decuiWT

PQ ref

UDC

1 1 + s

0...1.2 p.u.

-0.9..0.9 p.u.

τP

1 1 + sτP

uWT ref ÷ kv

Q ref

Q/V-ctrl. mode

uWT~

Q/V control

current controller

DC-link

Udc control

P ref

PQ control

|u|LPF

priority P

1/sC ÷

++

k pFF

UDC

P P = T *Rect PMG e g≈ ω

Figure 4:Wind turbine and converter control

In the model a common reference frame is used for all voltagesin the model. The dynamicresponse of the PLL that is commonly used in converters for voltage angle estimation is mod-elled using a low-pass filter, withu denoting the estimated voltage. The filter time constantτp

is chosen as 20ms. The active and reactive power setpoint are filtered similarly. These filtersalso prevent high-frequency oscillations of the grid voltage that can occur because small gridvoltage variations are amplified by the controller that maintains the active power.

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Transient stability of different electrical concepts for wind farms

In case when the WT is connected to an HVDC converter controlling the ac-voltage to areference value aligned with the q-axisuq ref , alternative current references were applied inorder to obtain a stable voltage, based on:

iW T ref = i

W T q ref = PW T ref/u

W T q (1)

Additional damping of the grid voltage is implemented in themodel using the reactive currentcontrol of the wind turbines with a small feedback gainkp u:

iW T d ref = −kp u u

W T d (2)

The active power setpoint is maintainted by adjusting the current setpoint as follows:

iW T q ref = (P

W T ref + kp u u2

W T d)/uW T q (3)

The DC-link voltage control, that provides the active power setpoint includes a feedforwardterm of generator power to increase the bandwidth and to reduce the dc-voltage deviationscaused by steep changes in the generator power.

In case of a voltage drop at the WT terminals the WT inverter current increases, which maylead to current limit operation. During a voltage drop the WTcontroller usually prioritizes thereactive current for maximum voltage support. In case of a reduction of the terminal voltagecontrolled by the WF converter in an HVDC connected wind farmother priorities may bechosen, e.g. maximizing active current.

2.6 HVDC control

The figures 5, 6 and 7 show the control schemes of the modelled HVDC components: the WindFarm Converter (WFC), the Grid-Side Converter (GSC) and the controlled braking resistor.Under normal conditions the WFC controls the ac-voltage to a constant reference valueu

W F ref .This outer control loop sets the reference valuesid ref andiq ref of the inner current controlloop. The feedforward termiWFC(Rcu + jXleak) compensates for the voltage difference ofthe transformer andidq dec compensates for the currents in parasitic shunt capacitors. With thefeedforward termP

DC W F C(Rcable/U2

DC) the DC-voltage at the GSC is estimated, so that the

voltage setpoints and limits of both converters are comparable.

iWFCuCk +k /spc ic

i priorityi / i / prop.lim

d q

X+RL+ -

-+

-

idq_ref -+

uWFC

|u | = f(U )C DC WFC

P P = u i + u i Q = u i - u i

DC Rect d d q q

Rect q d d qu WFC

dq decuiWFC

k +k /spv iv

-+

uWFC

uWF ref

|u|WF ref

UDC lim

1.0 p.u.

0.5 p.u.

ratelim.

UDC hys

PDC WFCUDC GSC UDC WFC

+ -

R /U2

DC

ac-volt.control

Udccontrol

|u | = f(U )WF ref DC WFC

UDC cable

cable

∆U

DC GSC

i (R + jX )WFC cu leak

++

dq deci

U U control mode AC DC

DC-link≈

WF voltage controller current controller

WFC DC-link input-output

uWF set

Figure 5:HVDC Wind Farm Converter

The GSC model is similar to the WT inverter model, but with an additional voltage droopcontroller for the reactive power referenceQref . Another difference with the WT inverter isthat during a grid voltage dip the GSC raises DC-voltage setpoint. Based on the measuredDC-link voltage the WFC reduces the WF voltage as a measure to limit the WF active power,until the DC-voltage drops. The GSC may also have an external active power limitation, e.g.for frequency support, but this is not included in the model.

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2 MODEL DESCRIPTION

iGuCk +k /spc ic

i priority:P / Q / proplim

X+RL+ -

-+

-

idq_refi = Pu /|u| + Qu /|u|

i = Pu /|u| + Qu /|u|

2 2 d d q 2 2 q q d

uG

|u | = f(U )C DC GSC

P P = u i + u i Q = u i - u i

DC GSC d d q q

GSC q d d q

uG`

+ -

k +k /spDC iDC

uG

u2

u2Udc ref

Q ref ext

dq decuiWT

PQ ref

UDC GSC

1 1 + s

0...1.2 p.u.

Grid Side Converter (GS-VSC)

-0.9..0.9 p.u.

τP

DC-link

1 1 + sτP

1.05

1.00 normal operation

voltage dip detection

U[p.u.]

dc refug ref ÷ kv

Q ref

Q/V-ctrl. mode

rate lim.

uG~

Q/V control

current controller

GSC DC-link input-output

Udc control

P ref

|u |g|u |g lim|u |g hys

Udc ref

PDC GSC

PQ control

|u|LPF

voltage dip

PQ-priority

Figure 6:HVDC Grid Side Converter

A chopper-controller braking resistor is modelled to limitfast HVDC voltage rise during grid-side faults. A high-pass filter circuit is included to emphasize a steep increase of the DC-voltage for early activation of the dump load. The maximum peak power is relatively small,because of the high costs of the chopper and also the continuous power of the load is limitedusing a simple thermal model.

DC-link with chopper

UDC WFCHPF max

UDC det

U U U 2 2

DC off DC on DC det

Pbrakeset

0

+ -

1/C1 + sτ

Pbrake

therm

Pbrake set

Pred k p red

0≥

0Tmin

T

chopper control

+-

P DC WFC

÷

UDC WFC

IDC WFC

X+R

addC/2+C

÷

UDC GSC

IDC GSCIDC WFC IDC GSC P DC GSC

DC-link

est

thermal limit

Figure 7:HVDC DC-link with chopper

2.7 Model parameters

The parameters of the wind farm model with HVDC connection aresummarized in Table 1.The wind turbine generator model has been upscaled with a factor of three with an ideal gear-box inserted to match the rated angular speed of the rotor.The wind turbine converter DC-link capacitance is chosen rather large to reduce the slope ofthe DC voltage rise in case of grid loss. Another choice couldhave been to apply a smallerDC capacitance in combination with a braking resistor to dissipate the excess power from thegenerator during the reduction of the generator power.For the string cables a single cable with an averaged length has been chosen, however this isnot very accurate because the of the unequal currrents in thedifferent cable sections.For this simulation the series impedances of the HVDC converters have been chosen to limitthe current ripple well below 0.1 p.u., however in general a more careful optimization of thecomplete converter system, including filtering, should be made.Several values the braking resistor rated power have been applied in the simulation. The powerramp rate is related to the chopper switching frequency.The control parameters are summarized in Table 2.The bandwidth of the inner loops controlling the current of the modelled converters is set to500 rad/s, while the bandwidths of the DC-voltage conrtol loops are set to 150 rad/s. Theresponse of the DC-link control in the WT converter for changes in the generator power willbe faster because of the feedforward termkpF F

PP MG

.The filter time constantτp used in the calculation of the current references is set to 20ms forthe converters that are directly connected to the grid and to100ms for the WT converters thatare connected to the grid via the HVDC link. This slower response is to prevent that the WTconverter interferes with the ac-voltage control of the WF-VSC. The rate of change of theac-voltage setpoint, which is controlled by the WF-VSC, is limited to±0.025[1/s] so that theconnected WT controllers can follow the voltage change.

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Transient stability of different electrical concepts for wind farms

Parameter value [unit]Rotor

Aerodynamic control collective pitch to vaneRated angular speed 1.24 [rad/s]Inertia 38.6 106 [kgm2]Shaft stiffness 87.5 [kNm]

Generator (PMG 30 pole pairs)Inertia 35 103 [kgm2]Rated angular speed 2.85 [rad/s]Rated voltage 4 [kV ]Rated power 2 [MW ]Power Factor 0.92 [−]

Converter (back-to-back VSC)Rated power 6 [MV A]DC-link voltage 8 [kV ]DC-link capacitance 2 [mF ]Switching delay 1 [ms]Series impedance 0.0125 + j 0.6281 [Ω]

Unit transformerRated power 7.5 [MV A]Leakage reactance 0.074 [p.u.]Copper resistance 0.003 [p.u.]

WT string cableLength (averaged) 6.3 [km]Series impedance 63 + j 100 [mΩ/km]Susceptance j 9.27 10−5 [S/km]

WF- and GS-transformerRated power 430 [MV A]Leakage reactance 0.13 [p.u.]Copper resistance 0.0023 [p.u.]

HVDC Converters (WF-VSC and GS-VSC)Rated power 360 [MV A]DC-link voltage 400 [kV ]DC-link capacitance 0.15 [mF ]Switching delay 1 [ms]Series impedance 0.0125 + j 6.28 [Ω]

HVDC LinkLength 75 [km]Series impedance 19.5 + j 59.9 [mΩ/km]Susceptance j 8.63 10−5 [S/km]

Braking resistorRated power 0, 0.1, 0.2 [p.u.]Power ramp rate ±200 [1/s]

HVAC LinkLength 75 [km]Series impedance 10.4 + j 50.6 [mΩ/km]Susceptance j 1.44 10−4 [S/km]Shunt reactances 90 (each side) [MV Ar]

GridNominal voltage 230 [kV ]Nominal frequency 50 [Hz.]Short circuit power 2000 [MV A]Impedance angle 85 [deg.]

Table 1: Wind farm parameters

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2 MODEL DESCRIPTION

Parameter description symbol value [unit]Generator control

torque P-ctrl kdq 1 [V/kNm]torque I-ctrl kiq 10 [V/kNms]id P-ctrl kpd 10−6 [V/A]id I-ctrl kid 10−5 [V/As]Udc limit Udc lim 1.20 [p.u.]Udc raised setpoint Udc raised 1.05 [p.u.]Gain of fastUdc rise 2.5 [−]Te ref LPF time constant τTe 5 [ms]Te ref LPF switch rise/fall time tTe sw 0.5 [s]Te ramp-down time constant τred 5 [ms]Te ramp-up time constant τ 200 [ms]

WT ConverterUdc P-ctrl kpDC 0.2 [A/V ]Udc I-ctrl kiDC 0 [A/V s]PPMG feedforward kpFF 1.0 [−]Q/V control mode voltage (HVAC), const. ref. (HVDC) [−]Qref External ref. Qref ext 0 [−]Qref P-ctrl kpQ 0.5 [−]Qref I-ctrl kiQ 2.5 [1/s]uWT andPQref LPF time constant τp 20 (HVAC), 100 (HVDC) [ms]ud FB gain kp u 0 (HVAC), 0.002 (HVDC) [A/V ]idq P-ctrl kpc 1.0 [V/A]idq I-ctrl kic 3.12 [V/As]current limit imax 1.2 [p.u.]

HVDC WF Converterudq P-ctrl kpv 2500 [A/V ]udq I-ctrl kiv 100 [A/V s]idq P-ctrl kpc 1.5 [V/A]idq I-ctrl kic 0.25 [V/As]Udc limit Udc lim 1.20 [p.u.]Udc hysteresis Udc hys 0.05 [p.u.]u

W F ref control rate limiter ±40 [1/s]HVDC GS Converter

Udc P-ctrl kpDC 0.0094 [A/V ]Udc I-ctrl kiDC 0 [A/V s]Qref P-ctrl kpQ 0.5 [−]Qref I-ctrl kiQ 2.5 [1/s]uWT andPQref LPF time constant τp 20 [ms]idq P-ctrl kpc 10−6 [V/A]idq I-ctrl kic 10−5 [V/As]current limit imax 1.1 [p.u.]ug P-gain kv 1 [V A]

Braking resistorGain of fastUdc rise 2.5 [−]Activation voltage UDC on 1.20 [p.u.]De-activation voltage UDC off 1.10 [p.u.]Power limitation FB-gain kp/Ctherm 5 [−]

Table 2: Control parameters

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14 ECN-E–11-003

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3 Simulations

3.1 Case descriptions

The following simulation cases have been elaborated:

• response to 3-phase onshore grid faults with 0.1 and 0.2 p.u. remaining voltage at PCCand 200ms and 500ms duration, both for the HVDC- and HVAC-connected wind farms;

• response to a voltage dip for different average wind speeds, from 0.7Vrated to 1.5Vrated

for the HVDC-connected wind farm;

• response to a voltage dip for different choices of the ratedpower of the braking resistorof the HVDC-connected wind farm: 0, 0.1, 0.2 and 0.5 p.u.

3.2 Results for HVDC connected wind farm

Start-up and normal operationThe start-up sequence, which is shown in figure 8, starts with ramping up the DC-voltage ofthe HVDC link through the GS-VSC in rectifier mode. In the model the charging power hasbeen limited by a resistor. The ac-voltage at the wind farm, which is limited by this DC-linkvoltage, is ramped up by the WF-VSC. The grid side ac-voltages inthe model are aligned withthe negative q-axis.In the WT converters the DC-link is charged and as soon as the nominal dc-link voltage isreached, after about 3 sec., the generator torque control isactivated. Until about 12 sec. thetorque setpoint is zero while the rotor accelerates. From 12 sec. to 23 sec. the turbine oper-ates in partial load in which the rotor speed is controlled through the generator torque to keepoptimal tip speed ratio. As the average wind speed of the monitored WT is around rated, therotor speed reaches the rated speed around 22 sec. and then the turbine starts pitching to limitthe rotor speed while keeping the output power constant. The total WF output power is around0.8 p.u. as the average wind speed for 2 WT strings is chosen as0.7Vrated.The ac-voltage control setpoint of the WF-VSC is around 1.2 p.u.and varies slightly withthe WF output power, which results in a stable voltage of around 1.1 p.u. at the wind farmcollector bus. As an alternative a direct setting of the WF-VSCvoltageuc to the setpoint valueu

W F setwas tested, which showed no significant differences, but did reduce simulation time.The dc-link voltages in the WT converters and in the WF-VSC increase to about 1.1. p.u. atnominal power, as no integral feedback is included in the model.The WT converter current along the d-axisi

W T d ref , is controlled according to (Eq. 2) to pre-vent voltage oscillations. As the feedback gain is kept verysmall, this current is practicallyzero and the WT reactive power, which is not controlled, slightly varies with the active power.The WF-VSC reactive power control keeps the grid voltage at the converter terminal constant.The voltage drop over the dc-cable at rated power is about 0.5%.

3-phase grid faultFigure 9 shows the response to a symmetrical onshore grid fault of 500ms during full loadoperation. At 36 sec. the grid voltageudq grid drops to 0.2 p.u. and the GS-VSC responseis to support the grid voltage by maximizing the reactive current. The reactive power rises toabout 0.3 p.u. within 50 ms while the active power decreases to zero. The power balance isrestored by reducing the WF active power and temporary activation of the braking resistor inthe dc-link. As a result the dc-voltage rises to 1.35 p.u. anddecreases slowly by continuedactivation of the braking resistor. After the release of thefault at 36.5 sec. the grid voltagerecovers and when the it reaches a threshold value of 0.9 p.u.the GS-VSC starts to feed inactive power and restores the dc-voltage.At the WF side the WF-VSC detects the dc-voltage increase abovethe threshold of 1.2 p.u.and decreases the ac-voltage setpoint to 0.5 p.u. (the setpoint value is not critical) to let theconnected WT converters decrease their active power. This steep decrease in active powerfrom the WF, which occurs from 100 ms to 150 ms after the start of the fault, leads to anac-voltage disturbance after which it settles towards the setpoint. The ac-voltage setpoint isincreased as soon as the dc-link voltage drops below the threshold of 1.05 p.u., after which theWT converters start to ramp up active power.

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Transient stability of different electrical concepts for wind farms

The WT converter power balance during the fault is maintainedby a fast reduction of thegenerator torque. Because of the response time of the torquecontrol the dc-link voltage risesto a high value of about 1.7 p.u., so in practice a braking resistor or another solution shouldbe implemented to limit the dc-voltage further. The drive-train oscillation resulting from theabrupt reduction of the generator torque is quickly damped by the WT torque controller whenthe WT power is ramped up. Solutions for damping of these osccilations are explained in [15],[12] and [7]. The average rotor speed has increased from about0.9 to 1.0 p.u. and to slowdown the rotor after the voltage dip the turbine starts pitching. At about 40 sec. the pitch angleis decreased again to increase the power output.Figure 10 shows the responses to symmetrical onshore grid faults with different minimum volt-ages and durations during full load operation. For the voltage dips with a remaining voltageof 20% the maximum dc-voltage is slightly lower than for voltage dips with 10% remainingvoltage. Apart from the different transients on the ac-sideof the WF converter at the end ofthe voltage dip, the responses are similar for all four simulations.As mentioned in the scope, section 2.1, the simulation results cannot accurately representswitching transients and harmonics which may trigger protection systems and/or influence thecontrol response. Provided that no protection is triggered and effective measures are imple-mented to reduce the influence of harmonics the simulations are useful to evaluate the WFcontrol response to transients.

3-phase grid fault: response with different ratings of braking resistorFigure 11 shows the response to a voltage dip of 0.2 p.u. during500ms for increasing powerratings of the braking resistor:Prated: 0, 0.1, 0.2 and 0.5 p.u.As expected the peak level of the dc-link voltage decreases with the increasing power of thebraking resistor. As the turbine active power decreases to zero the dc-link capacitors dechargevia the braking resistor. The chosen time constant in the model related to the power limitationof the braking resistor influences the rate of decrease.

3-phase grid fault: response with different average wind speedsFigure 12 shows the response to a voltage dip of 10% over 500ms with a different averagewind speed for each string of turbines.Before the voltage dip it shows that not only the turbine withbelow-rated wind speed (in blue)operates in partial load operation, but also the trubine with the highest wind speed (in purple),where the pitch angle is large in order to limit the rotor overspeed.During the voltage dip it shows the different maximum levelsof the dc-link voltage, directlyrelated to the generator power just before the dip.After the dip all turbines start pitching to decelerate the rotor speed to the setpoint. The turbinewith below-rated wind speed shows a small pitch angle response, as the rotor speed is allowedto increase. As expected, the required pitch angle change todecelerate the rotor depends onthe initial pitch angle. Around rated wind speed with the pitch angle close to zero (in green)the aerodynamic power is less sensitive for pitch angle changes than for larger pitch angles.This requires a larger pitch angle change which leads to a slower response in the decrease ofthe active power. Also when in ramping up the active power to the pre-fault level the turbinewith the wind speed around rated wind speed shows the slowestresponse, although the pitchangle has already reached its working position around 44 sec.

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3 SIMULATIONS

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<Idq gridconv [pu]>

Figure 8:Start-up and normal operation: upper left: WT generator, upper right: WT converter,lower left: WF-VSC, lower right: GS-VSC

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Figure 9:Response to voltage dip of 500ms and 0.2 p.u. remaining voltage: upper left: WTgenerator, upper right: WT converter, lower left: WF-VSC, lower right: GS-VSC

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3 SIMULATIONS

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Figure 10: Response to different voltage dips, from left to right: 200ms,10%, 200ms, 20%,500ms,10%, 500ms, 20% and from top to bottom: WT generator torque, WT converter, WF-VSC and GS-VSC

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Transient stability of different electrical concepts for wind farms

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Figure 11:Response to voltage dip of 200ms, 20% with different power ratings of the brakingresistor, from left to right: 0, 0.1, 0.2 and 0.5 p.u. and upper plot: WT-converter, lower:GS-VSC

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3 SIMULATIONS

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Figure 12:Response to voltage dip of 200ms, 10% with different average wind speeds:0.7, 0.9, 1.1, 1.3, 1.5 p.u. (p.u. base: 11.9 m/s)

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Transient stability of different electrical concepts for wind farms

3.3 Results for HVAC connected wind farm

start-up and normal operationFigure 13 shows the normal operation of an HVAC connected WF with an average wind speedof 1.1 p.u. for 3 WT strings and 0.7 p.u. for the other 2 strings. The left figure shows thesignals of an WT converter where the average wind speed is 1.1. p.u. The response of theactive power is similar to that of the HVDC connected WT, but here the reactive power iscontrolled to stabilise the ac-voltage amplitude at the WF collector bus.3-phase grid faultFigure 14 and 15 show the responses of an HVAC connected WF to voltage dips of 500 mswith remaining voltages of 0.1 p.u. and 0.2 p.u. at the PCC. The WT converter response isthat the current hits the limit value in order to maintain thepower setpoint. The reactive poweris prioritized in order to support the terminal voltage. The main difference between the tworesponses is the slower recovery of the voltage and active power for the more severe voltagedip. As in the simulation of the HVDC system, the WT converterdc-link voltage during thedip is high, so in practice a braking resistor or another solution should be implemented to limitthe dc-voltage further. The response of the drive-train is also similar to that of the HVDCconnected WF.

0.99

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Figure 13:Normal operation: left: WT converter, right: HVAC cable

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3 SIMULATIONS

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Figure 14:Response to voltage dip of 500ms, 10%: left: WT converter, right: HVAC cable

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35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40−1.5

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−0.5

0

0.5

1

Time

Idq Wtconv [pu]

−0.5

0

0.5

1

1.5

2PQ_WF (blue, green), PQ_grid (red, orange) [p.u.]

−2

−1.5

−1

−0.5

0

0.5

1u_dq_WF (blue, green), u_dq_grid (red, orange) [p.u.]

35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40−2

−1.5

−1

−0.5

0

0.5

1

1.5

Time

i_dq_WF (blue, green), i_dq_grid (red, orange) [p.u.]

Figure 15:Response to voltage dip of 500ms, 20%: left: WT converter, right: HVAC cable

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Transient stability of different electrical concepts for wind farms

3.4 Discussion

In all simulated cases the HVAC and HVDC connected wind farmsdid ride-through the 3-phase onshore grid fault while supporting the grid voltage during and shortly after the fault.The applied method of active power reduction of the HVDC connected wind farm by a fastdecrease of the ac-voltage in the wind farm showed to be effective. Also it does not rely oncommunication links and the WT converter control is very similar as in the HVAC connectedwind farm.On the WT generator side the response to the applied faults was similar in the HVAC connectedWF and in the HVDC connected WF as the full-rated WT convertereffectivly decoupled thegenerator side from the WF grid side. In the HVDC case the dc-link provides some bufferingin case of an onshore fault and also the WF-VSC keeps providing voltage control in the windfarm, which may reduce the duration of the voltage dip at the WT terminals and help to rampup the active power as fast as possible without excessive voltage overshoot.The drive-train oscillations only depend a little on the operating conditions and are rapidlydamped during the ramp up of the active power.The pitch system increase after the dip to declerate the rotoris much stronger for pitch anglesclose to working position and close to rated wind speed, which leads to a slow response in theactive power reduction and ramp up.

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4 Conclusions and recommendations for future work

The dynamic response of an HVAC connected and an HVDC-VSC connected wind farm, bothwith variable speed wind turbines with permanent magnet generators and full-rated convertershas been demonstrated. The simulation results of a 3-phase fault in the onshore grid have beencompared for different wind speeds and for different power ratings of the dc-link braking re-sistor.

Due to the limitations of the models several aspects are not or not accurately represented inthe results, such as harmonics, unbalance and the response of protection systems. However,the models can be used to design evaulate wind farm configurations and wind farm controlsystems.

In order to make the models better applicable they should be validated with measurementsor with other models.Suggested improvements are to make them applicable for unbalanced faults and faults at otherlocations, e.g. in the WF, and to include simplified models of the protection systems. To fur-ther limit the dc-link voltage in the WT converter a chopper could be included. Other windturbine models and converter models are available and can beincluded to extend this evalua-tion. Also other FRT approaches could be modelled and evaluated.

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