HVDC Offshore Wind Farm

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HVDC Connected Offshore Wind Farm

DIgSILENTPowerFactory

Abstract

This paper discusses the modelling of anHVDC link for a 400 MW offshore wind farmconsisting of DFIG wind turbines. Voltagesource converters (VSC) are used for theHVDC connection. The paper covers loadflow calculation and time-domain simulation(RMS and EMT).

1 Content

This document presents a model of a highvoltage direct current (HVDC) system. Simu-lations are performed and the results are dis-cussed. The simulations show the behaviourof the HVDC system in the case of onshoreand offshore faults.

2 Base Model

In the Base Case of the HVDC OffshoreWind Farm example, a model of an off-shore wind farm is represented. The windfarm consists of 80 wind turbines connectedvia a voltage source converter high voltagedirect current system (VSC HVDC, Figure5) to a 380 kV onshore transmission net-work. Each wind turbine has a nominalactive power of 5 MW. The machines areequipped with doubly-fed induction genera-tors (DFIG, wind turbine type 3). Only 10of the 80 wind turbines are modelled as in-dividual machines (these are represented in

DIgSILENT GmbH, Heinrich-Hertz-Str. 9, 72810Gomaringen, Germany, www.digsilent.de

the Detailed Feeder single line diagram).The remaining 70 turbines are aggregatedinto four DFIG models: three models repre-senting 20 parallel machines and one rep-resenting 10 parallel machines. This ap-proach offers a compromise where detailedstudies of the explicitly modelled feeder canbe carried out but where the overall the sizeof the system is reduced. The wind farmmodel was built using a Template of a windturbine model fromPowerFactorys global li-brary. The template includes models of themain elements of the wind turbine (Gener-ator and Transformer), as well as the addi-tional dynamic models which represent the

controllers, PLL etc. during dynamic RMSand EMT simulations. The asynchronousgenerator model used for the DFIG containsthe simulation equations for the frequencyconverter used on the rotor. Hence only oneconnection is needed to connect the gener-ator to the network (the frequency converteris connected internally). The HVDC link wasmodelled using the VSC element in Power-Factory. The control structure is of a generictype. A description of the control structureused is given in chapter4. A short descrip-tion of the usage of the HVDC system inother projects is given in Section 5. TheBase Case has been configured for a loadflow calculation. For all other kinds of calcu-lation or simulation, please refer to the otherStudy Cases (see section3).

3 Calculations and Simula-

tions

The following sections will describe the re-sults which can be obtained using the dif-

DIgSILENTPowerFactory, r1337 1


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ferent study cases of the project HVDC Off-shore Wind Farm.

To run time-domain simulations as described

in Section 3.2, 3.3 and 3.4 the RMS/EMTSimulation toolbox should be selected asshown in Figure1.A simulation has to be ini-tialised first. The required dialogue is acces-sible via the button Calculate Initial Condi-tions ( ). For the study cases described inthe following subsections, all settings in theproject are already correctly configured. Bypressing Execute the simulation initialises.The simulation can then be started by press-ing theStart Simulation button ( ) and exe-cuting the command. The results are visible

after the simulation has finished.

3.1 Short-Circuit Calculation

In the study case 02 Short Circuit Calcu-lation of the HVDC Offshore Wind Farmexample the short-circuit calculation is pre-sented. Please press the Calculate Short-Circuit button ( ) to execute the short-circuit calculation.

Since the IEC 60909 standard does not re-flect the fault behaviour of modern wind tur-bines, the DIgSILENT Complete Method isused to calculate their short-circuit contribu-tions. The quantities Ik , Ib, Ith etc. are

calculated, which express the stress on thegrid elements. In this study case, settingshave been selected for the Complete Methodshort-circuit calculation which are similar tothose of IEC 60909 (c = 1.1 for max. short-circuit currents; loads, capacitances of lines,magnetizing currents of transformers, shunt

elements and filters are ignored in the pos-itive sequence). In addition, settings havebeen applied which take the short-circuitcontribution of the wind turbines into accountaccurately. Accuracy is achieved by usingcurrent iteration to calculate the transientshort-circuit current Ik. In this current itera-

tion the additional reactive currents providedby the wind turbines are taken into accountprecisely, based on the k factor setting andthe max. current of each turbine/converter.Ib and Ith are calculated on the basis of

Ik and I

k. Please note that the option

Current Iteration has been activated on theAdvanced Options page of the short-circuit

command dialogue!

3.2 Offshore Fault RMS

In the study case 03 Offshore Fault RMSa simulation of a short-circuit in the offshorenetwork is presented. The RMS simula-tion method is used. The fault location isthe node Term b located in the detailedfeeder (first node). The fault is defined viaa short-circuit event. The fault starts at 0.0 sand is cleared after 150 ms by opening thebreaker which connects the detailed feederto the Sub A/A1 terminal. The results are

presented in the two virtual instrument (VI)panels Windpark res and HVDC res. Thefirst VI panel contains results of the wind tur-bines and the fault current. The second VIpanel shows results of the two HVDC termi-nals.

The results show that the short-circuit canbe handled without tripping the HVDC con-nection. This is possible because the fault isin the 33 kV medium voltage network. Thevoltage drop at the HVDC AC terminal isnot below 0.8 p.u. due to the impedance of

the three winding transformer. The DFIGwind turbines in the faulted feeder supply acontrolled fault current even after the feederis disconnected (after 150 ms). Finally theprotection scheme disconnects the wind tur-bines (see Figure2).

3.3 Onshore Fault RMS

In the study case 04 Onshore Fault RMSa simulation of a short-circuit in the onshorenetwork is presented. The RMS simulationmethod is used for this example. The fault lo-cation is the node Onshore Slack. The faultis defined via a short-circuit event. The faultstarts at 0.1 s and is cleared after 150 msby a second event which removes the fault.The results are presented on the two virtualinstruments (VI) panels Windpark res andHVDC res. The first VI panel contains re-sults of the wind turbines and the fault cur-rent. The second VI panel shows results ofthe two HVDC terminals.

The results show that the offshore wind farmis not affected by the fault. This is possible



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because the wind farm is decoupled from theonshore AC network via the DC circuit. Thepower which cannot be transmitted to the on-shore network is consumed by the chopper

resistors (see Figure3).

The onshore converter switches to a reactivecurrent priority mode during the fault. Thisallows the combined voltage/reactive powerdroop control to increase the reactive currentto the maximum value. The active current islimited during this time via the current limita-tion (see Section4for further controller de-scription).

3.4 Offshore Fault EMT

In the study case 05 Offshore Fault EMT asimulation of a short-circuit in the offshorenetwork is presented. The EMT simulationmethod is used for this example. The faultlocation is the node Term b located in thedetailed feeder (first node). The fault is de-fined via a short-circuit event. The faultstarts at 0.0 s and is cleared after 150 ms byopening the circuit breaker which connectsthe detailed feeder to the Sub A/A1 termi-

nal. The results are presented on the twovirtual instrument (VI) panels Windpark resand HVDC res. The first VI panel containsresults of the wind turbines and the fault cur-rent. The second VI panel shows results ofthe two HVDC terminals.

The results show that the short-circuit canbe handled without tripping the HVDC con-nection. This is possible because the fault isin the 33 kV medium voltage network. Thevoltage drop at the HVDC AC terminal isnot below 0.8 p.u. due to the impedance ofthe three winding transformer. The DFIGwind turbines in the faulted feeder supply acontrolled fault current even after the feederis disconnected (after 150 ms). Finally theprotection scheme disconnects the wind tur-bines (see Figure4).

The difference between RMS and EMT sim-ulation is that the EMT simulation takes theinductive and capacitive behaviour of linesand transformers into account via differen-tial equations (electro-magnetic transients).

This leads to more precise results. The re-sults include the DC component of the short-circuit current. This phenomenon cannot be

simulated with the RMS simulation.

4 HVDC Control

The HVDC control is based on a genericstructure. It consists of two separate com-posite models: one controls the onshoreconverter with its chopper resistors, the otherone controls the offshore converter.

4.1 Onshore Converter Control

The composite model of the onshore con-verter control is based on the frame definitionFrame onshore VSC which is displayed inFigure6. Each block represents an elementin PowerFactory. The chopper control withthe two DC valves is marked in red. Besidesthe chopper control, there are two more con-trollers in the composite model. These arethe Main Controller and the Current Con-troller. All controllers will be described in thefollowing sections.

4.1.1 Main Controller

The onshore converter is a voltage sourceconverter. It can control the active currentas well as the reactive current. The con-trol objective of the active current is the DCvoltage. The control objective of the reactivecurrent can be selected via an input parame-ter. It can be set to control either the voltage(with a PI controller), the voltage via a droopor the reactive power (via a PI controller).This can be changed via the input parameterMODE. The block definition of the onshorecontroller is shown in Figure 7. The activecurrent control path is marked in red, the re-active current control path is marked in greenand the current magnitude limitation block ishighlighted in blue. During normal operationthe current limiter prioritises the active cur-rent (id) and limits the reactive current (iq).But during and up to 500 ms after faults (i.e.u < (u0U t)) the reactive current is priori-tised and the active current is limited.

The control uses a first order lag filter (lowpass) on the measured AC voltage and onthe measured reactive power. The filter time



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constant is fixed to 10 ms for the voltage and30 ms for the reactive power. The parame-ters used in the model are described in Table1.

On the second page (accessed via the but-ton below Cancel or via the Advanced 1Tap forPowerFactoryversion 15.1 and later)of the common model of the onshore con-troller there are additional parameters whichrepresent a characteristic for the current lim-itation. The values of the characteristic arepresented graphically in Figure8.

4.1.2 Current Controller

The current controller takes the referencecurrents from the main controller and calcu-lates the modulation index which is passedto the converter. It would also be possi-ble to use the built-in current controller ofthe ElmVsc. In this case the option Useintegrated current controller has to be acti-vated on the RMS page of the ElmVsc. Thegraphical definition of the current controlleris shown in Figure 9. The control parame-ters are given in Table2.

4.1.3 Chopper Control

The Chopper Control has the positive andnegative DC voltage as input signals. It willignite the DC valves of the chopper resistors

if the voltage exceeds a certain limit and itwill block these valves if the voltage dropsagain below a defined threshold. The chop-per resistors are needed in case of a fault inthe onshore network. In this case, the powerwhich is fed into the DC circuit on the off-shore side cannot be transferred to the on-shore AC network by the onshore converter.As a result the DC circuit will be charged.This can damage the IGBT valves of the con-verters, therefore the DC chopper has to limitthe voltage to an appropriate value. The

graphical definition of the chopper control isshown Figure10. The parameters are givenin Table3.

4.2 Offshore Control

The offshore controller has to build the off-

shore grid voltage. This means the controllerhas to keep the frequency and the voltagemagnitude of the AC offshore network at aconstant value. The resulting active and re-active power which flows through the con-verter cannot be controlled (the converter isthe slack in the offshore system).

4.2.1 Frame of the Offshore ControlScheme

The definition of the frame used for the com-posite model of the offshore controller isshown in Figure 11. There are measure-ment devices used for detecting the offshoreAC voltage, the AC current of the converterand for the DC voltage (positive and neg-ative). There is also a PLL (phase lockedloop) measurement device, but this elementshould only be in service for EMT simula-tions (this is managed in the project via aVariation).

4.2.2 Common Model of the OffshoreController

The offshore controller has to keep the ACvoltage magnitude and the frequency to aconstant value. This is done via the in-put signal Pm (modulation index) and f0(frequency) of the PWM converter (ElmVsc).The modulation index is controlled via twocascaded PI controllers plus a feed forwardpath using the measured DC voltage. Thecontroller structure is shown in Figure 13.The controller parameters are given in Table4.

The first PI controller gets the AC voltage de-viation as input signal, its output is the reac-tive current reference. This reference signalis compared against the measured reactivecurrent. The output of the second PI con-troller is then the reference voltage. In theblock Udc Feedforward the reference volt-age (AC voltage Uac) is divided by the mea-sured DC voltage Udc and multiplied with

a constant which depends on the modula-tion method (see block: Udc Feedforward).The ratio Uac/Udc cannot be increased in-



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definitely as shown in Figure 12. Theblock Udc Feedforward therefore containsa characteristic which limits the modulationfactor Pm to emulate this effect. For more

information, please read the technical refer-ence paper of the PWM converter elementElmVsc[2].

The controller has two additional functions:

1. It has a built-in limitation of the reactivecurrent. This limitation becomes activeonly in case of a fault in the offshorenetwork. The feedback of the reactivecurrent (iq) leads to a reduction of thevoltage magnitude at the converter.

2. The control can raise the frequency inthe offshore network in case of high DCvoltage. This can be used as kind of sig-nal to the wind turbines in the networkto reduce the active power output. Thisfunction is disabled by default by usingvery high DC voltage threshold values(the chopper resistor will keep the DCvoltage below these values).

4.3 DFIG Wind Turbine Model

The wind turbine models used in this projectare generic DFIG (doubly-fed induction gen-erator) models which are provided withPow-erFactory. These models can be found inthe global templates folder.

5 HVDC Template

The project contains a template of the HVDC

system. This template contains all networkelements, all controllers and all types of theHVDC system (see Figure 5). To use theHVDC system in another project the tem-plate HVDCSystem which is located in thefolder \Library\Templates has to be copiedinto the target project (to the Templatesfolder). Then it can be used via the GeneralTemplates button, as shown in Figure 14.Pressing the General Templatesbutton willopen a new window which shows all avail-able templates. The HVDCSystem has to

be marked. After this it can be added tothe project by clicking in the single line di-agram (please note that the window with the

templates has to stay open while adding thetemplate). All type references (line types andcontroller definitions) are linked to the libraryfolder which is located in the template. This

means the template should not be deletedafter the HVDC-System has been added tothe project.

References

[1] S. Weigel, B. Weise, M. Poeller:Control of Offshore Wind Farms withHVDC Grid Connection, 9th Interna-tional Workshop on Large-Scale Inte-

gration of Wind Power into Power Sys-tems as well as on Transmission Net-works for Offshore Wind Power Plants,

18th-19th Oct. 2010, Quebec, Canada

[2] DIgSILENT Technical Documentation:PWM Converter, DIgSILENT GmbH,Gomaringen, Germany, 2008

[3] TransmissionCode - Netz- undSystemregeln der deutschenUbertragungsnetzbetreiber (Net-work and System Rules of the German

Transmission System Operators), VDN,Berlin, 2007



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Figure 1: Activation of the Simulation Toolbox

Figure 2: Results offshore fault, RMS simulation



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Figure 3: Results onshore fault, RMS simulation

Figure 4: Results offshore fault, EMT simulation



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Figure 5: Single line diagram for the HVDC system as modelled in PowerFactory

Figure 6: Frame of the onshore HVDC control system



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Figure 7: Onshore converter control structure

Figure 8: Current limit characteristic of onshore controller



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Figure 9: Onshore converter current control structure

Figure 10: Chopper control structure



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Figure 11: Frame of the offshore HVDC control system

Table 1: Parameters onshore controller

Parameter Value Unit Description

umax 1.15 p.u. Max. internal voltage allowed

x 0.1487 p.u. Series reactor impedance

Trudc 0.001 s DC Voltage Filter Time ConstantDroop 0.05 p.u. Gain for Droop Control

Ku 12 - Gain of PID Droop-Controller (Q and Vac)

Kiu 100 1/s Integral Constant of PID Droop Controller (Q and Vac)

Kdu 0 1/s Derivative Constant of PID Droop Controller (Q and Vac)

fltMODE 1 - 0 = ctrl. acc. MODE, 1 = ctrl. acc. TC 2007[3]

K flt 2 - Reactive current gain in fault mode

Kd 10 - Gain of DC PI Controller

Td 0.1 s Integral Time Constant of DC PI Controller

iqfmax 1 p.u. Max. reactive current in fault case

Ut 0.1 p.u. Voltage Deviation to enable Fault Mode

MODE 1 - 0 = voltg. ctrl., 1 = droop, 2 = Q ctrl.i max 1.1 p.u. Current Limit



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Figure 12: PWM Converter Characteristic[2]

Figure 13: Controller structure of offshore converter



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Figure 14: Button to enter templates

Table 2: Parameters onshore current controller


Kpm 1.0000 p.u. Proportional Gain for d & q axis

Tpm 0.0020 s Integral Time for d & q axis

P min -3.0000 p.u. min limit Pm for d & q axis

P max 3.0000 p.u. max limit Pm for d & q axis

Table 3: Parameters chopper controller


UdcUpper 1.20 p.u. Threshold for Chopper Connection

UdcLower 1.10 p.u. Threshold for Chopper Disconnection

Table 4: Parameters offshore controller


Kuac 2.00 p.u. Proportional Gain for Voltage Controller

Tuac 0.20 s Integrator Time Const. for Volt. Controller

Kiac 2.00 p.u. Proportional Gain for Current Controller

Tiac 0.02 s Integrator Time Const. for Cur. Controller

F ref 1.00 p.u. Reference Frequency

Tfilter U 0.005 s Time constant for PT1-Filter

UdcHigh 99.00 p.u. Upper Udc Limit for Frequ. Lifting (high value disables function)

UdcLow 0.00 p.u. Lower Udc Limit for Frequ. Lifting

plusFrequ 0.03 p.u. Value of Frequ. Lifting

i min -0.70 p.u. Min Limit Value for Current Limitation

Uac min -0.10 p.u. Min Limit for Offshore Voltage

i max 0.70 p.u. Max Limit Value for Current Limitation

Uac max 1.10 p.u. Max Limit for Offshore Voltage

Frequ Grad 1.00 1/s Gradient for Frequ.


HVDC Offshore Wind Farm

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