IEEE TENCON '93 / BcijfnR

SIMULATION OF A PWM REACTIVE POWER COMPENSATOR USING PSPICE

A. H. B. M. Yatini MIEEE and M. A. S. Miah Faculty of Electrical Engineering

Universiti Teknologi Malaysia Jalan Semarak

54100 Kuala Lumpur Malaysia

ABSlRACT

The simula~ion of a PWM Reactive Power Compensator (RPC) using PSPICE is presented i n this paper. The RPC uses a three phase solid-state voltage source inverter having a fixed dc input voltage iind the output terminals Fonnected to the ac mains through t'ilter iiiductors. T h e output vol tage is controlled through the PWhl control of the inverter switches and thc RPC causes leading or lagging V A R , l o the ac system thus the load a s vvell as the transmission system is conipeiisated. This compensatnr is suitable for a balanced three-phase load.

W O D U C T ION

Conventional RPCs which includes switched shunt c a p a c i t o r l r c a c t o r b a n k s a n d l o r synctironous condensers are still being used i n the industry for power factor correction due to non-linear and power electronic loads. But they have some limitations such as slow action, mechanical failure, almost ineffective under f a s t t r ans i en t l o a d , h a r m o n i c resonances caused by switching surges and breaker strike, high cost and large size. Therefore, the convcnlional system have been replaced by new methods cmploying solid- state devices to ove rcome the above I i m i ta ti o n s.

This paper descr ibes the pr inciple o f operation[i-2] and the design of the RPC whereby the PWM inverter can be controlled to produce the required induc t ive or capacitive VAR for power factor correction without the use of large storage components. The main advantages of this scheme arc that I t has a fast response time which allows almost instantaneous reactive power control and low harmonic distortion in the l ine currents. The performance of the RPC i s analysed and evaluated using t h e PSPICE simulat ion

package .

PRINCIPLE OF OPERATION

The basic schematiC diagram of the proposed RPC is shown i n Fig. 1. The amplitude of the output voltage of the stmulared RPC is controlled by the gatt ing control of the inverter switches and the output current is kept in such a way that it leads or lags the corresponding l ine to neutral voltage of the IC mains by 90 degrees so illat the RPC can provide the required leading or lagging rcact ivc power for conipensation of thc system. Due to this 90 degree phase difference Uic real power taken by thc RPC from the ac supply is zero. The small amount of power dissipated in the RPC conies from the dc input voltage source of the inverter. For this type of operation the output voltage of tlic RPC is kept in the same phase as the ac mains voltage.

The operating principle of the proposed compensator can be explained by considering a controllable voltage source connected to the ac mains with a pure reactor as shown in Fig. 2[2]. When Vi is greater than Vs, reactive power .f lows from the a c system to the inverter and vice versa when Vs is greater

p- m a i n s

Wg. 1 Scbematic diagram of Ihe RPC systcm

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111a11 vi. Fig. 3 SlloWs thc actual cquivalent circuit considering the rcsistancc of thc reactor. Therefore Ihe vector diagram in Fig.

3 explains why in practice the output voltage of the inverter Vi should not be kept at exactly in phase with the nc mains voltage Vs to compensate for the effect of R.

If Vi is kept in the same phasc as Vs. the reactive current and power can be calculated as follows.

Reactivc current I= (Vs-Vi)/X Rcactive power Q=(I)(Vs) This reactive power is provided by mcnns of the circulating currciit withiil tlie inverter as the power flow hctwcen the ac mains ;iiid RI'C ;I[ any instant is zcro.

I I X

h \ Illa

1 m c t 2 nlf t 4 2 m p f 1 2nirt 5

[y r' zia) Z ( d

Fig. 2 (a) Single phase equivalent circuit diagnm of the voltage source force commutated RPC at Uie fundamental frequency (b)vector diagrun for inductive RPC (c) vector d i a g m for capacitive RPC

1 .o 0.4 0.612 0.245 0.195 0.037 0.01 1 0.111 0.227 0.020

Fig. 3(a) Single phase equivalent circuit diagram of the voltage source force commutated RPC at the fundamental frequency cosidering a small resistor with filler inductor (%) vector diagrani for inductive RPC (c) vector diagram for capacitive RPC

P\VM SIGNAL GEhZRATOR

To generate PWM pulses. t h e frequency niodulation index is choseii to be an odd multiple of 3[3],, to eliminate the even and the niost dominant harmonics in the l ine to l ine voltage. I n this case, the frequency spectra of the line to line voltage is given in Table 1.

Table 1 Fundamental component of the invcrter output line to line voltage expressed as per unit of the input voltage at different h and ma, in;: aiiiplitude modulation ratio, b: barnionic number, mf': frequency modulation ratio

DFSIGN OFTHE SIMULATED CIRCUIT

For the three phase inverrcr[4] RPC, it is assumrd that Rntcd outpui power = 100 KVA Rated phase load voltage = 240 V (rms) Now considering P single phase equivalent circuit for tlie fundalnenral liated output power = 100 KVAI3 = 33.333 KVA

There fo re . Rated current = (33.333 KVA)/240 V = 138.85 A Rated inipedance = 240 V/38,88A = 1.728 ohm AC supply frequency = 50 CIS Tliercfore, I'iltcr rcactor inductance = 5.5 m H

Rated Illlasc load vclllagc = 240 v

ANALYSIS EXAMPLE AND SIhlULATlON RESULTS

To verify thc design cxrrniplc and to establish the cffzciivcness. thc proposed RPC is siinuIated[5-6] by PSPICE clectrical circuit simulator. on a 33 MHz PC of 4 h4B RAM running on 486 micrpprocessor. I n this siinulation, the switch niacro model is used at the converter level. PWM pulses are generated by coiiipariiig a triangular carrier waveforill with three sinusoidal reference waveforms[7] whose phases have a 120 degree difference with each other. This is done by using tlie PSPICE analog behavioral modeling feature.

Fig. 4 (capacitive RPC) and Fig. 5 (inductive RPC) show the PWM gatting pulses. output l ine to line voltage of t h e RPC, output phase voltage Vi and the respective phase voltage of t h e ac mains Vs. T h e fundamen ta l component(8] of the RPC output current I is shil'ted by 90 degrees compared to the ac mains voltage Vs as shown i n Figs 4(d) aiid S/d). Fig. 4(e) shows that Vi is greater than Vs as in the case of the capacitive RPC and vice versa for Fig. 5(e). Table 2 shows the VAR c o m pens a t i o n amp 1 i tu d e modulation ratio ma.

DISCUSSION

w i th d i f feren t

The analysis and design a re done by assuming the inverter switches to be ideal and the 'ac system and load voltages are balanced.

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Fig. 4. Simulated waveforms lor CapacLive RPC (a) PVv1.4 gatling pulsr?r (S) oulpul line lo line Mllage of Ihe RPC (c) output phase vonage 01 Ihe RPC and the iespeclhe phase voltage of Ihe ac mains v (d) ac mains voltage v and t h e inveller oulput current i (e) vs. i. and the funZdamental component of t<e inverter oulput phase vollage vi (0 inverlsr input wrrenl (g) inverier dc vollage.

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Table 2 : VAR cnmpcnsricd 11 di:frrmt m, I-. l i l t e r inductor

Tile l<pC uses J conveitlic~iial P W M vollagt: source inverter whose input terminals are connected i o a d c voltage s w r c c . For this rcasoii tlie V A R needed by the system is chaiiged by conrrol l i i ig t h e ampl i tude inodulation ratio ina. For an inductive RPC, the inverter o u ~ p u t voltage is iiiucli less than that o f the capacitive RPC and the amplitude niodularion index needs 10 be reduced greatly A s a result the switching pulses become narrow and cause much switching sircss. To overcome this problem, a variable d e supply can be used at tlie input of the inverter[9] with a e I i 111 i n n t i o 11 technique[ IO].

CONCLUSION

A solid state PWM reactive power compensator is analyzed and simulated in this paper. Thc results obtained by simulation agree closely with the theory. The proposed RPC shows high performance in compensating a' load which can either be .leading or lagging. This is done by varying the ; m p l i t u d e modulation ratio ma.

p rog r alii 111 cd ha r nio n i c

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Fig. 5. Simulnled waveforms for induniw RPC (a) F'WJ galling pulses (b) oulpul line lo lim voltage of lhe RPC (c) oulput phase voltage of Ihe RPC and the respecthe phase voltage of Ihe IIC mains vs (d) BC mains vonage v and ihe invefter oulpul currenl i (e) vs. i. and the fundmental component of I t e inverter oulpul phase voltage v, (0 inverter input Curtent (9) inver:er dc vollage.

Thc RPC ciiiploys tlic u b c ot' power scniicoiiducior devices wittiout [lie use of large storage coiiiponeiits ;incl rcquire small filter coinpoiwits as 1112 n i i i ~ u ~ i t of disioriion iri the line currents is very low. Therefore, thc usc of' tlic proposed RPC w i l l eiisurr: et'l'icien t a i id ccononiic oper nt ion.

REFERENCES

L. Gyugyl. 'Reoctive Power Generolicn ond Conlrd b y Thyrislor Circuits.' IEEE Trons. Ind. Appl.. Vol. IA-15. NO. 5.

Y . Sum1 el 01.. "New Slotlc VAR Control Usng Force- Commutoted Inveilers,' GEE Trons. Ind. Appl., V d . PAS-100. NO. 9 Sept. 1981, pp. 4216-4224. J. W. A. Wilson and J. A. Yeomons. 'Inlrinsc Harmonics of ldeollred Inverter PWM Syslems." 1976 IEEEilAS Annuol Meeting. pp. 967-979. P. D. Zlogos et al., 'A Compuler-Aided Anolyss ond Oesgn Approoch for Slotlc Voiloge Souice Inveilers.' IEEE 110m. Ind. Appl.. Vol. IA-21. NO. 5. Sepl./OCl. 1985, PP.1234-1241. L. solozcr et 01.. 'Slmple !dodels for SPICE A d r t Pcwer Elect tonics Circuil Simulotion on PO's,' Confelsnce Record of the 1988 IAS Annuol Meeling, Vol. 1. pp. 1053-1068. L. Solozof and G. Joos, "PSPICE Simulcflon of Three-Phose Inveilers by Meons 01 Switchlng Functions," IECON '90. 16th Annual conference. Vd. 2, pp. 982-989. MlCrOSlm Corporation. "Circuil Arroiys's Usel's Gude.' 20 Folrbonks. Irvine, California, 1991. P. W. Tuinengo, "SPICE A Gulde lo Circuit Simulation ond Analysis Using PSplce.' Prentice HoII. Englewood Clilfs. New Jersey, 1988. L. T. Moron et 01.. 'Anolyss ond Desgn of a 1hree.Phose Synchronous Solid-State VAR Cornpensotor.' IEEE Trons. Ind. APPl.. Vol. 25, No. 4. July/Augt. 1989, pp. 598-608. H. S. Paiei ond R. G. Holt, 'Generalized Technique of Hormonlc Ellmlnolion and Voltage Conlrol In Thyristor lnverlerr Pori I-Hormonlc Elimlnatlon.' IEEE Trons. Ind. Appl.. Vol. IA-9, No. 3. Moy/June 1973.

SeFf /Oct. 1979. FP. 521 -532.

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