An Electromagnetic Transient Simulation Model For

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AN ELECTROMAGNETIC TRANSIENT SIMULA TION MOD EL FORVOLTAGE SOURCED CONVERTER BASED HVDC TRANSMISSIONBehzad Qah ram an Ebrahim Rahimi Aniruddha M. GoleStudent Mem ber Student Mem ber [email protected] [email protected] [email protected]

AbstractVoltage Sourced Conver ter based High Vol ta ge Dc

Transmission (VSC-HVDC) is an emerging technologyfo r the eficien t transfer of energy.. Because it uses gate-turn offsem iconductor device s such as the GT O or IGBT,the VSC-HVDC option offers several advantages overconventional HVDC transmission. I t can operate into veryweak a c systems a s well a s provide auxiliary servicessuch as reactive power support to the ac network;although it currently suffers from higher losses and lowerpower ratings. As this is an emerging technology, it isimportant to develo p tools and models fo r the study ofVSC -HV DC in various operating conditions. With this inmind, his paper develops an electromagnetic transientssimulation model of a 300 M,W VSC-HVDC scheme thatcan be u sed fo r investigating its operation. A decoupledcontrol system is developed and the transientperformance and faul t behavior of the system is studiedunder different short circuit ratio (SCR)conditions.Keywords: VSC; HVD C; simulation; PSCAD/EM TDC.

1. INTRODUCTIONVoltage Sourced Converters (VSCs) have severaladvantages over conventional thyristor based converters..

These include:i) independe nt control of active and reactive power,i i) operation into a very weak ac systems, even onepurely passive loads with no danger of commutationfailure,iii) the ability to provide reactive power and voltagesupport ,iv)the use of pulsswidth modulation to l imit lowerorder harmonics resulting in a very limited need forharmonic fi l ters.For this reason, several VSC-HVD C systems havebeen commissioned in recent years, including the 33 0MW Cross Sound Cable PI. In addition to point to pointbulk power transmission, .VSGHVDC has al so been

CCECE 2004 CCGEI 2004, Niagara Falls, May/mai 20040-7803-8253-6/04/$17.00 0 2004 IEEE

proposed for other applications [2] such as the connectionof off-shore wind farms or oil drilling platform s into themainland electrical network and for undergroundtransmission or distribution systems within congestedcities.

It must be pointed out however, that because of l imitedratings of gate-turn of today's semico nductors, theamount of power which could be sent by VSC-HVDC ispresently lower than a conventional one and the losses ar ealso higher. Also in the event of a fault on an overhea d dcline, hecause of the anti-parallel diodes in thesemicon ductor switch, the VSC converters at both ends ofan HVDC link have to be disconnected by opening the accircuit breakers to enable the arc to be extinguished [ret].The advantages of VSGHVDC can only be fully realizedwith a Foperly designed control system, which allowsdecoupled control of the real and reactive powers and thedc voltage.The paper describes a simple model of a VSC-HVDCsystem, schematically shown in Fig.1, which can beuseful for the study of different control strategies. Themodel al lows one to use different strengths for thesending and receiving end ac networks.

Typical operating contingency scenarios are simulatedin order to evaluate transient performance consideringtwo different control methodologies..

2. VSC-HVDC SYSTEM MOD ELThe point to point VSC-HVDC transmission system

model, as shown in Fig. 1 is loosely based on the CrossSound Dc system. The dc converters are rated at 30 0M W , +/-150 kV, which is typical of today's state oftechnology [l].The cable represents an underwater cable,whose length can be varied as desired. The ac systems onthe sending and receiving ends are modeled as series R-Lequivalents, the param eter values being selected to givethe appropriate short circuit ratio. The con verters are eachmodeled as two-level ' converters. The converter-levelfiring system is based on sinusoidal pulse widthmodulation with a carrier frequency of 1.62 kHz 27 x 60Hz), which is again typical of current technology. The dc

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cable is represented as a Tsection, similar to the firstH M C Benchmark model [3].

Fig. 1: VSC-HVDC based transmission system3. CONTROL OF VSC

Operationally, the VSC is similar to a very fastsynchronous machine, in that the active and reactivepower may be individually controlled within the capacityof the c onverter by adjusting the magnitude and phase ofac voltage behind the converter transformer reactance.The PW M scheme generates this ac voltage from the dcvoltage through switching operations when fed thenecessary reference waveform With a solitaty VS Cbridge supplied from a fixed dc voltage source, it ispossible to control the real and reactive powers to anyvalue w ithin the converter's ratings. However, the HVDCtransmission scheme consists of two VSCs connected onthe dc side. This adds the constraint that in the steady-state, the real power entering the VSC rectifier ("sendingend") must equal (neglecting losses) the real powerleaving the VSC inverter ("receiving end"). The controlscheme used here a llows independent specification of thereal and reactive currents on the receiving end and onlythe reactive current on the sending end. The real currentorder at the sending end is set through a proportional-integral controller that regulates the dc side voltage to itsrated value.A straight forward method, referred to as "directcontrol" of achieving the control of real and reactivepower is shown in F i g 2 Here, proportional-integralcontrollen act on the real and reactive power errors togenerate the magnitude and phase respectively of thevoltage references to the inner PWM controller.

Pref

......-a

Fig. 2: Direct control

The controller works on the principal that the realpower flow is more affected by the phase angle and thatthe reactive power is more sensitive to the voltagemagnitude. A phase locked loop tracks the phase of the acvoltage and provides the phase reference for thegeneration of the three phase reference voltages. Althoughthe control system is simple, it has the drawback that arequest for a change in real power causes a transient in thereactive power and vice versa.An improved method is that ofdeco upled control [4,5],in which the control system includes additional signalpaths which ensure that a request foia change in the real(reactive) power is effected without any transient in thereactive (real) power. The method is shown schematicallyfor the inverter in Fig. 3. The reference for real current isgenerated to control the active power and the reactivecurrent is used to control the terminal voltage. Therectifier side controls are similar except that the realcurrent reference is controlled by the dc capacitor voltageerror, and the reactive current reference is zero for unitypower factor.

Pref

w,ref

Fig.3: Decoupled control

4. CASE STUDIESi)-Direct vs. decoupled con trolFig. 4, compares the performance of the two controloptions discussed above with respect to the real andreactive power transients .for a step change in the realpower order. As is evident the direct control methodmanifests a significant transient in the reactive powerwhen the real power is changed. N o such kansient isobserved for the decoupled control method.ii) Dependency on the Ac System Stre ngt hStudies were conducted using receiving end networkswith two different strengths, the first a relatively strongsystem of short circuit ratio SCR =2L75" and a muchweaker system with SCR=1.1L75'. The results for theactive power, (rms) terminal voltage ar e shown in Fig. 5 .Although the weaker system shows a somewhat higher

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1Fig. 4: Direct VS. decoupled con trol of P nd Q

Fig.5: Effect of different SCRs on system behavior

overshoot, the perform ance in either case is very good. Itshould be noted that a convefit ional H M C system wouldhave significant difficulty in operating at SCR values lessthan 1.5.iii) Effect of Capacitor Size :

Fig. 5, illustrates the systems dynamic behavior fortwo different dc bus capacitor values. The response to astep change in power for two values large: 667 pF (0.1pu-s) and small 66.7 pF (0.01 pu-s). Here the pu valuesindicate the ratio of steadystate stored energy(1/2 cvz and the rated dc power. As can he seen, theperformance with either value is good.-........ ....... -_.. - .. . __Enen d capacnor size.......... .._.i ---.. c = o . I p u s 0I $ 1

Fig. 6 : Effect of capacitor size on system behavioriv) Fault studies:The model can he used to demonstrate the effect of acside faults on the performance of the dc transmissionsystem. Fig. 7 shows the recovery of dc power and Dvoltage following a three phase to ground inverter sidefault. The short circuit ratio of the ac system was 2.0. Thefault duration is taken as 83.3 ms (five fundamentalfrequency cycles). The recovery to 90% power takes 35

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ms, which is much improved in comparison to thatpossible with a conventional HVDC converter feeding acomparable ac network. Specjal controls that are activeduring the fault include a valve current limit (to protectfiom damage to the IGBTs) and a voltage dependentpower order limit that reduces the power order.

IFig. 7: System recovery from a 3 phase to ground fault

5.CONCLUSIONSA transient simulation model for VSC based HVDC

transmission system was presented. Sample studies fo :evaluating performance considering different controllertypes, different ac system -strengths and faults wasinvestigated. The performance was seen to be adequateover all tested operating ranges. The model is capable ofbeing adapted to a wider range of studies than thosepresented here.

ACKNOWLEDGEMNETSThe authors gratefully acknowledge the support fromManitoba Hydro (Research Grants T261iT262); and theNatural Sciences and Engineering Research Council ofCanada (Discovery Grant Program).

REFERENCES1) N . Horle, K. Eriksson, A. Maeland, T. Nestli,Electrical supply for offshore installations m ade possibleby use of VSC technology, CIGRd 2002 Conference,Paris, France, Aug 20022) G . Reed, R. Pape, M. Takeda, Advantages ofVoltage Sourced Converter (VSC) Based DesignConcepts for FACTS and HVDC-Link Applications,Proceedings of IEEE Power Engineering SocietyTransmission and Distri bution Conference, July 2003.3) M. Szechtman, T. Wess, C.V. Thio, First benchmarkmodel for HVDC control studies, CIGRE WG 14.02.El ec t ra No . 135, pp 5 4- 73, April 19914) C. Schauder, H. Mehta, V ector analysis and controlof advanced static VAR compensators, IEEProceedings, Gen eration, Transmission and Distribution,vol. 140, Issue: 4, pp: 299 - 306, July 19935 ) I. Papic, P. Zunko, D. Povh, M. Weinhold, Basiccontrol of unified power flow controller, Power Systems,IEEE Transact ions on , Vol. 12, Issue: 4, pp.1734 - 1739,November 1997.

Appendix: Model DataTransformers: 350 MVA, 17W138 kV, Xc = 15%,Thevenin equivalents: Series R- L wth R 4 . 2 1 0 ,L=8 1mH fo r SCR=2.T-line data (for each box show n in Fig. 1) are as follows:R=7 0, L=18.8 mH, C=51.12 pF fo r 50 h able line(can he scaled for shorter/longer lines).

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An Electromagnetic Transient Simulation Model For

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