PULSE MULTIPLICATION IN FORCED- COMMUTATED CURRENT SOURCE CONVERTERS BY DC RIPPLE REINJECTION

Transcription

1 Queensland University of Technology From the SelectedWorks of Lasantha Bernard Perera Spring September 26, 2004 PULSE MULTIPLICATION IN FORCED- COMMUTATED CURRENT SOURCE CONVERTERS BY DC RIPPLE REINJECTION J. Arrillaga, university of canterbury N. R Watson, university of canterbury Lasantha B Perera, university of canterbury Y. H Liu, university of canterbury Available at:

2 Australasian Universities Power Engineering Conference (AUPEC 2004) September 2004, Brisbane, Australia PULSE MULTIPLICATION IN FORCED-COMMUTATED CURRENT SOURCE CONVERTERS BY DC RIPPLE REINJECTION Abstract J. Arrillaga*, N.R. Watson*, L.B. Perera* and Y.H. Liu* * Dept. of Electrical & Computer Engineering, University of Canterbury, New Zealand A dc-ripple reinjection scheme is described that doubles the number of pulses of the forcecommutated current source converter. The reinjection circuit includes a feedback converter in series with the dc output which provides automatic adjustment of the reinjected current as the dc side current changes. The reinjection concept is also generalised to produce pulse multiplication. 1. INTRODUCTION A third harmonic injection via the converter transformer neutral and returning through the conducting rectifier switches was proposed [1] to modify the rectifier current waveform and, thus, reduce the harmonic content. The main shortcomings were the need of an external tripleharmonic current generator, the difficulty of adjusting such source under varying operating conditions and poor efficiency. These shortcomings were avoided with the use of the dc-ripple reinjection scheme [2], a solution developed for the line commutated converter. Despite its relative simplicity, the dc ripple reinjection alternative has so far not been found cost competitive with the use of filters. This is probably due to the need to provide substantial reactive power compensation, for the line commutated conversion process. With the development of forced-commutated conversion the problem of reactive power compensation does not arise and the reinjection concept becomes attractive. This paper describes the application of the dcripple concept to the GTO- based converter bridge and shows, theoretically and experimentally that the modified six-pulse converter can be made to operate effectively as the twelve-pulse configuration. Moreover, the concept can be extended to produce pulse multiplication. 2. PULSE DOUBLING STRUCTURE AND ANALYSIS The description of the reinjection principle is made with reference to the idealised circuit of Figure 1, where: represents the secondary winding of the converter transformer, T 2 the feedback (or reinjection) transformer, C a blocking capacitor for dc, L the smoothing reactor, S 1 S 2 & S 3 the common anode switches of the main converter and S 4 S 5 the feedback converter switches. The feedback converter acts as a current source with an alternating (triple frequency) current proportional to the dc current, the proportionality being determined entirely by the associated feedback transformer ratio. Thus for the ideal case of a perfectly smooth direct current the injected current waveform is rectangular, as shown by (i) in Figure 2(a). The problem of injected current phase adjustment is solved by using fully controllable feedback switches. The firing angle control of the feedback converter is thus locked to the main converter control, e.g. if the feedback converter switches are fired 30 0 after the corresponding main converter switches then the waveforms illustrated in Figure 2(b) result. Fig. 1 Proposed harmonic re-injection 2.1. Optimal harmonic reduction The current waveform of Fig. 2 (iii) is an odd function with half-wave symmetry and the general Fourier term is given by the expression 4 π 2 an = f( x)sin( nx) dx π 0

3 where π f ( x) = 0 x= 0 6 π π f ( x) = 1 FB x = 6 3 π π f ( x) = 1+ FB x = 3 2 and FB is the ratio of feedback current to the direct current (see Fig. 3) a n ( FB) π 3 1 sin( nx) dx 4 π 6 = π 2 π + ( 1+ FB) sin( nx) dx π 3 ( 1 FB ) [ cos( nx) ] π 3 4 π 6 n = π ( 1+ FB) π 2 + [ cos( nx) ] π 3 n where n=1,3,5,7,.. Take n= a1 = ( 1 FB) ( 1 FB) 0 π π 2 Take n= a5 = ( 1 FB) ( 1 FB) 0 5π π 2 In general it can be shown that for n=1,11,13,23, an nπ 2 In general it can be shown that for n=5,7,17,19, an nπ 2 These are put to zero when 3 3 FB 1 + = 2 i.e. 3 FB = = The percentage increase of fundamental and 12-pulse harmonics is: ( 1 32) 32 FB 100 = 7.18% The a.c. input power is therefore increased by 7.18 % for rectifier operation. It can be shown that the a.c. output power is correspondingly reduced for inverter operation Experimental verification Fig. 2 Proposed synthesis of 12-pulse current (inductive d.c. load) (i) Triple-frequency injected waveform (ii) Rectifier current before modification (iii) Modified phase current, rectifier winding (iv) Second phase displaced 120 (v) Resultant phase current on delta primary Fig. 3 Current waveform in the main diodes dashed line unmodified current equal to 1 per unit solid line modified current waveform

4 To verify the above ideal characteristics a low power converter was used and experimental voltage and current waveforms obtained with and without reinjection. The effect of reinjection for diode and controlled rectification is illustrated in Figure 4, where (a) and (b) are the conventional six-pulse direct voltage waveforms with α= 0 0 and α= 30 0 respectively. The modified direct voltage waveforms (c) and (d), display the twelve-pulse ripple. Similarly Figure 5 illustrates the difference between the phase currents without (a) and with (b) feedback for α= 0 0, which compare well with the theoretical prediction of Figure 2(b). Verification of the primary side current is meaningless due to the unrealistic magnetising current of the experimental converter transformer. 3. GENERALISATION OF THE REINJECTION CONCEPT Figure 6 shows a modified double bridge configuration that includes any number of reinjection converters. These are connected to the dc ripple voltage via separate secondary windings of the reinjection transformer. On the dc side the outputs of the single-phase bridges are adjusted by the transformer turns ratio (N k /N 0 ) and their phase is shifted by firing angle control to increase the number of pulses of the output voltage waveform. On the ac side, the ripple reinjection bridges produce rectangular-wave currents to modify the output current waveforms. The derivation of the modified dc voltage and ac current waveforms is carried out in reference [3]. The combination of two single-phase reinjection bridges and two three-phase main converter bridges multiplies by four the number of pulses of the conventional 12- pulse configuration. This effect is clearly shown by the theoretical current waveforms of Figure 7, where i Z is the reinjection current, i a and i c the phase currents in the secondary windings of the star connected converter transformer, i ' a the current (phase a) in the secondary of the delta connected transformer and I R the combined 48- pulse output current on the primary side of the converter transformer. Fig. 4 Direct voltage waveforms a α= 0 without feedback b α=30 without feedback c α= 0 with feedback d α=30 with feedback

5 ' ' k 1 ' ' Fig 5. Alternating-current waveforms (α=0 ) a Without feedback b With feedback V X M V Y V S1 V Sk T + M I Z power is reinjected into the dc system. A sufficiently high pulse number should completely eliminate the need for ac and dc sides filters. Added to the four-quadrant capability of the force-commutated converter, multipulse conversion is likely to be a competitive alternative for large power applications. V 1 V 1 V Z 2π p N 0 N 1 L N k V S1 V Sk V 2 L N k V 2 N 0 N 1 π p Fig. 6 Multi-pulse Converter Configuration Experimental verification of the theoretical voltage waveforms, carried out in a scaled down physical simulator, is shown in Figure 8. The 48-pulse dc output voltage is clearly shown in waveform V Z. 4. CONCLUSIONS The reinjection technique transforms the conventional force-commutated current source converter waveforms into multi-step ac current and multi-pulse dc voltage waveforms. This applies equally to rectifier and inverter operation and to variable frequency supplies. The power conversion efficiency is high as the rectified harmonic Fig. 7 Current Waveforms of a 48 pulse dc-ripple reinjection double-bridge converter

6 Fig. 8 Experimental verification of the dc-side voltages for α=75 5. ACKNOWLEDGEMENTS The authors wish to acknowledge the early contributions made to this topic by J.F.Baird and M.Villablanca at the University of Canterbury, New Zealand. 6. REFERENCES [1] Baird J.F. and Arrillaga J., Harmonic reduction in dc-ripple reinjection, Proc. IEE, Vol. 127, No. 5, Part C, 1980, pp [2] Liu Y.H., Arrillaga J. and Watson N.R., Multilevel voltage source conversion by voltage reinjection at six times the fundamental frequency, Proc. IEE Power Applications, Vol. 149, No. 3, 2002, [3] Villablanca M., High pulse ac/dc convertors and their application to hvdc transmission, Ph.D. thesis, University of Canterbury, New Zealand, 1992