16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, VARIATION OF HARMONICS AND RIPPLE WITH PULSE NUMBER Pulse Number

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1 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Novel 24-Pulse Rectifier Topology based on Single 3-Phase to Four 3-Phase Transformation using Conventional Transformers for Phase Shifting A. N. Arvindan 1 and Anirudh Guha 2 Electrical and Electronics Engineering Department, S.S.N. College of Engineering, Anna University (Chennai), India Electrical & Computer Engineering, The University of Texas, Austin, TX , USA Abstract-A 24-pulse rectifier has been designed for high voltage, low current applications. Four 3-phase systems are obtained from a single 3-phase source using novel interconnection of conventional single- and 3-phase transformers. From two 30º displaced 3-phase systems feeding two 6-pulse rectifiers that are series connected, a 12-pulse rectifier topology is obtained. Thus, from the four 3-phase systems that are displaced by 15º two 12- pulse rectifiers are obtained that are cascaded to realize a 24- pulse rectifier. Phase shifts of 15º and 30º are made using phasor addition of relevant line voltages with a combination of singlephase and three-phase transformers respectively. PSCAD based simulation and experimental results that confirm the design efficacy are presented. I. INTRODUCTION Conventional ac-dc converters are developed using diodes and thyristors to provide controlled and uncontrolled unidirectional and bidirectional dc power, however, these converters have problems of poor power quality in terms of injected current harmonics, resultant voltage distortion and slowly varying rippled dc output at load end, low efficiency, and large size of ac and dc filters. To overcome these drawbacks and meet contemporary power quality standards [1]-[3] it has become imperative that research in power converters has to address to power quality aspects like reducing harmonic currents, higher power factor, lower EMI/RFI at input ac mains and well-regulated dc output. Increased awareness of power quality has led to the development of a new breed of ac-dc converters referred to as improved power quality ac-dc converters (IPQCs) [4] that have been classified as switch-mode rectifiers, power-factor correctors, pulse width modulation rectifiers, multipulse rectifiers, etc. Multipulse rectifiers are unidirectional multipulse converters that are used for high power applications which involve high voltage and low current. This paper is about the design of magnetics for the realization of a 24-pulse rectifier involving the transformation of a single 3-phase system to four 3-phase systems using novel interconnection of conventional three-phase and single phase transformers. A 12- pulse rectifier is implemented by cascading two 6-pulse rectifiers fed from two 3-phase systems displaced by 30º. The 24-pulse rectifier topology is obtained by cascading two 12- pulse rectifier systems which translates to cascading of four 6- pulse rectifiers fed from four 3-phase systems displaced by 15º. II. MULTIPULSE CONVERTERS Pulse number is defined as the number of pulses in the dc output voltage within one time period of the ac source voltage. In high-power applications, ac dc converters based on the concept of multipulse, namely, 12, 18, 24, 30, 36, 48 pulses are used to reduce the harmonics in ac supply currents. These are named as multipulse converters. They use either a diode bridge or thyristor bridge and a special arrangement of magnetics through transformers and tapped inductors. The variation of harmonics in the input current and the ripple frequency on the dc side for different pulse numbers are shown in Table I. A. Bidirectional Multipulse Converters These converters normally use thyristors and harmonics reduction is made effective with pulse multiplication [5], [6] using magnetics. The use of fully controlled thyristor bridge converters offers bidirectional power flow and adjustable output dc voltage. The use of a higher number of phases through an input multiple winding transformer and pulse multiplication using tapped reactor [7], and an injection transformer, reduces THD to input ac currents and ripples in the output dc voltage. These converters are used in high rating dc motor drives, HVDC transmission systems, and in some typical power supplies. The cost and weight of input transformers can be reduced by using autotransformers [8]-[10] in low- and medium-voltage applications. B. Unidirectional Multipulse Converters Normally, diode bridges are used with a higher number of pulses for reducing harmonics in ac mains and reducing the value of ripple voltage in the dc output. These are developed in 12-, 18-, 24-, 30-, 36-, 48-pulse converters, through input multipulse auto / isolation transformers and ripple current TABLE I VARIATION OF HARMONICS AND RIPPLE WITH PULSE NUMBER Pulse Number AC Harmonics Ripple Frequency Ripple Factor 1 1,2,3, fs ,3,5,. 2fs ,4,5,. 3fs ,7,11,. 6fs ,13,23, 12fs ,19,35, 18fs ,25,47, 24fs

2 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Figure pulse rectifier realized by transforming a single 3-phase system to four 3-phase systems using conventional single- and three-phase transformers. transformers and pulse multiplication through input tapped reactors, interphase [12], [13] and injection transformers [14] at the dc link are vital for these converters. Normally, these converters employ only slow converter grade diodes, thus resulting in negligible switching losses, high efficiency, high power factor, low THD at input ac mains, and ripple-free dc output of high quality. Figure 2. Phasor representation of four three-phase systems. injection employing interphase reactors. The rating, size, cost, and weight of different components of these converters are reduced using novel concepts in autotransformer configurations [11], [12] to achieve a higher number of phases from input three-phase AC mains through phase splitting at different angles. The concepts of phase shift through input III. REALIZATION OF 24-PULSE RECTIFIER TOPOLOGY Fig. 1 shows the proposed topology of the 24-pulse rectifier. It is clear from Fig. 1 that the realization of the 24-pulse rectifier involves obtaining four 3-phase systems with a defined phase shift between them from a single 3-phase system using interconnection of three-phase and single-phase transformers. For harmonic elimination, the required minimum defined phase shift is given by [15] Phase shift = 60 /Number of six-pulse converters. The phasor representation of the four 3-phase systems a 0 b 0 c 0, a 15 b 15 c 15, a 30 b 30 c 30, and a 45 b 45 c 45 shown in Fig. 1 feeding 3-phase diode bridges (four 6-pulse rectifiers) DBI, DBIV, DBII and DBIII respectively, with successive systems displaced by 15 º is depicted in Fig. 2. For an n pulse rectifier the characteristic harmonics are of the order nk ± 1 where k = 1, 2, 3,. From the data in Table I also it is clear that a higher pulse number implies elimination of lower order current harmonics and presence of higher order ones on the ac side, lower ripple content on the dc voltage and higher ripple frequency.

3 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Figure 3. Input line voltages V a0b0, V b0c0 and V c0a0 at diode bridge I. Figure 5. Input line voltages V a45b45, V b45c45 and V c45a45 at diode bridge III. Figure 4. Input line voltages V a30b30, V b30c30 and V c30a30 at diode bridge II. A. Transformation: One 3-Phase System To Four 3-Phase Systems As shown in Fig. 1, the source represented by lines A 0 B 0 C 0 feeds the Yy0d11 vector configured, 3-phase, 3-winding, step down transformer and two 3-phase systems, one (represented as a 0, b 0 and c 0 ) with line voltages (V a0b0, V b0c0, V c0a0 ) in phase with the source line voltages and the other (represented as a 30, b 30 and c 30 ) with line voltages (V a30b30, V b30c30, V c30a30 ) leading the source line voltages by 30 º are obtained from the secondary wye (y0) and delta (d11) windings respectively. The line voltages V a0b0, V b0c0, V c0a0 and V a30b30, V b30c30, V c30a30 are shown in Figs. 3 and 4 respectively. It is noteworthy that the six line voltages of both the 3-phase systems are balanced and equal in magnitude and differ only in phase angle. The six line voltages V a0b0, V b0c0, V c0a0, V a30b30, V b30c30, V c30a30 are isolated using 6 singlephase transformers with appropriate turns ratio. The secondary voltages of the single-phase transformers corresponding to V a0b0 and V a30b30 are connected in series in order to yield V a15b15, a voltage equal in magnitude to the six line voltages but leading V a0b0 by 15 º. This 15 º phase shift is obtained by phasor addition of appropriate line voltages. The line voltage V a0b0 leads the phase voltage V a0 by 30 º and the line voltage V a30b30 leads the phase voltage V a30 by 30 º, however, since V a30 also leads V a0 by 30 º it is obvious V a0b0 is in phase with V a30. This implies that V a30b30 leads V a0b0 by 30 º. The phasor addition of these two line voltages that are equal in magnitude gives the resultant V a15b15 as follows: V a15b15 = (V 2 a0b0 + V 2 a-30b V a0b0 V a-30b-30 Cos30 º ) 1/2 (1) Figure 6. Input line voltages V a15b15, V b15c15 and V c15a15 at diode bridge IV. Since the magnitudes of V a0b0 and V a30b30 are equal, the resultant V a15b15 bisects the 30 º angle between V a0b0 and V a30b30. Thus the line voltage V a15b15 leads V a0b0 by 15 º. Similarly, the line voltages V b15c15 and V c15a15 are obtained by the phasor additions via the secondary windings of the single-phase transformers corresponding to the line voltages V b0c0 and V b30c30 ; and V c0a0 and V c30a30 respectively. The voltages V a15b15, V b15c15 and V c15a15 are equal in magnitude and are 120 º apart and, therefore; the windings with these voltages are connected in star to form a balanced 3-phase system, with V a45b45, V b45c45 and V c45a45 as the line voltages that are shown in Fig. 5. Hence, in Fig. 1 the (phasors) lines a 45, b 45 and c 45 are obtained and are fed to a 3-phase transformer of the Yd1 configuration which provides a phase shift of -30 º and hence, yields the (phasors) lines a 15, b 15 and c 15 respectively. The corresponding line voltages V a15b15, V b15c15 and V c15a15 that lag by 30 º the voltages V a45b45, V b45c45 and V c45a45 respectively are shown in Fig. 6. Thus, four 3-phase systems with successive systems displaced by 15 º are realized. B. Design Aspects Of The Rectifier Topology The 24-pulse rectifier topology as shown in Fig. 1 clearly involves cascading of four 6-pulse producing 3-phase diode bridges, DBI, DBII, DBIII and DBIV. The design parameters of the rectifier topology including those of the ratings of the devices pertaining to the individual bridges and the transformers are dependent on the output of the rectifier. The rectifier topology is designed for a dc output voltage of 110V. The four series cascaded diode bridges produce an output voltage of 110V, therefore, the dc voltage provided by each

4 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, bridge = 110/4 = 27.5V. The dc output voltage of a 3-phase diode bridge is given by, 3 3V V m dc = (2) π where, V m = peak value of phase voltage feeding the bridge. Substituting V dc = 27.5V, the peak value of the 3-phase line voltage feeding each bridge is obtained from (2) as, 3 Vm = V (3) and the corresponding rms value is given by, 3 2 Vm = 20.37V (4) C. Turns Ratio Of Various Transformers The main transformer connected to the three-phase utility is a three-phase, three-winding one with Yy0d11 vector configuration. The primary wye winding is connected to the 3- phase 415V utility. 1) Yy0d11 main transformer: As evident from Fig. 1 diode bridges DBI and DBII are fed from the y0 and d11 secondary windings respectively, therefore, the line voltages pertaining to these have to be in conformity with eq. (4). The turns ratio for the Yy0 winding is obtained as follows: Y-primary V line(rms) = 415V y0 secondary desired line voltage = 20.37V i.e. N 2 /N 1 = / 415 = and that for the Yd11 winding is obtained as follows: Y-primary V line(rms) = 415V d11 secondary desired line voltage = 20.37V N 2 /N 1 = (20.37 x 3 ) / 415 = ) Single-phase transformers: There are six single-phase transformers whose primary windings are used to isolate two sets of line voltages, V a0b0, V b0c0, V c0a0 and V a30b30, V b30c30, V c30a30 pertaining to the wye (y0) and delta (d11) secondary windings respectively of the main transformer. The six secondary windings of the single-phase transformers are segregated into three pairs, with each pair comprising two relevant secondary windings corresponding to the voltage combinations V a0b0 and V a30b30, V b0c0 and V b30c30, and V c0a0 and V c30a30 that are synthesized by series cascade connection to obtain the voltages V a15b15, V b15c15 and V c15a15 respectively. The synthesis of the voltage combinations is as per eq (1). The synthesis of V a0b0 and V a30b30 as per eq (1) yields V a15 b15 as follows (with V a0 as reference): i.e. Va 15 b15 V a0b0 = 20.37/30 º V V a30b30 = 20.37/60 º V 2 2 o = Cos 30 V i.e. V a15 b15 = 39.35/45 º V Similarly, the line voltages V b15c15 and V c15a15 are obtained by the synthesis of the relevant voltage combinations. The synthesis yields magnitudes as follows: V a15b15 = V b15c15 = V c15a15 = 39.35V The three secondary pairs are connected in star to form a 3- phase winding with V a15b15, V b15c15 and V c15a15 as the phase voltages and V a45b45, V b45c45 and V c45a45 as the line voltages. Thus V a45b45 = 3 x 39.35/75 º etc. The star winding feeds the diode bridge DBIII and, therefore, as per eq (4) the desired magnitudes of the line voltages have to be as follows: V a45b45 = V b45c45 = V c45a45 = 20.37V The turns ratio of each single-phase transformer is thus: N 2 /N 1 = /( 3 x 39.35) = ) Yd1 transformer: The voltages V a45b45, V b45c45 and V c45a45 are fed to a Yd1 transformer to obtain line voltages V a15b15, V b15c15 and V c15a15 that lag the input wye voltages by 30 º and feed diode bridge DBIV. The magnitudes of the line voltages on wye and delta sides must be equal i.e. V a45b45 = V b45c45 = V c45a45 = V a15b15 = V b15c15 = V c15a15 = 20.37V. Thus, the turns ratio is given by N 2 /N 1 = 3 = IV. RESULTS AND DISCUSSION The topology of the 24-pulse rectifier has been simulated using the PSCAD software educational version The simulation assumes a balanced 3-phase source and neglects saturation in the transformers. A. Simulation Results Figs. 7 and 8 show line currents in phase a of the y0 and d1 windings respectively of the Yy0d11 transformer in Fig. 1. The Figure 7. Line current in phase a of y0 winding of Yy0d11 main transformer. Figure 8. Line current in phase a of d11 winding of Yy0d11 main transformer.

5 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Figure 9. Six-pulse dc output voltage of diode bridge, DBI. Figure 12. Six-pulse dc output voltage of diode bridge, DBIII. Figure 10. DC 6-pulse output voltage of diode bridge, DBII. Figure 13. DC 6-pulse output voltage of diode bridge, DBIV. Figure 11. DC 12-pulse output voltage by cascading diode bridges I and II. line currents in the secondary wye (y0) and the delta (d11) windings, shown in Figs. 7 and 8 respectively, of the Yy0d11 main transformer comprise currents drawn by a diode bridge DBI (y0) / DBII (d11) and three single-phase transformers. Fig. 9 depicts the dc output voltage of diode bridge DBI. Between two half pulses, each of width 1.666ms at either end of the 20ms time period corresponding to the 50Hz fundamental frequency of the ac utility, there are five pulses each of 3.333ms thus accounting for the six-pulse dc voltage output. Fig. 10 shows the output voltage of diode bridge DBII that comprises six complete pulses for the 20ms time period corresponding to the 50Hz fundamental frequency. It is clear Figure 14. DC 12-pulse output voltage by cascading diode bridges III and IV. that outputs of DBI and DBII are displaced by 30 º. This is because the line voltages at the input of these bridges as shown in Figs. 3 and 4 are displaced by 30 º. Fig. 11 shows the 12- pulse dc voltage obtained when output voltages of bridges DBI and DBII are series cascaded. Figs. 12 and 13 show the 6-pulse outputs, which are in conformity with the corresponding input line voltages shown in Figs. 5 and 6, of diode bridges DBIII and DBIV respectively. The outputs of bridges DBIII and DBIV are series cascaded to provide a 12-pulse dc voltage that is shown in Fig. 14. It is clear that because of the relevant phase shifts the two 12-pulse dc outputs in Figs. 11 and 14 are displaced by 15 º. The two 12-pulse systems comprising DBI,

6 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, Figure pulse dc voltage. Figure 15. DC 24-pulse voltage by cascading DBI, DBII, DBIII and DBIV. Figure 20. Experimental set up. experimental set up is shown in Fig. 20. V. CONCLUSION A 24-pulse rectifier is realized by conventional transformers that meets the theoretical harmonic and ripple estimates. REFERENCES Figure 16. Line current I A of Y winding of Yy0d11 main transformer. Figure 17. Harmonic spectrum of line current I A in Y winding of transformer. DBII outputs, and DBIII, DBIV outputs, are cascaded to obtain a 24-pulse dc output with an average value of 110V shown in Fig. 15. The phase a input line current of the primary Y- winding (I A ) of the Yy0d11 main transformer that is the phasor sum of the currents in Figs. 7 and 8 is shown in Fig. 16. The harmonic spectrum of the current, I A, is shown in Fig. 17 which confirms that the 23 rd and 25 th harmonics alone are significant lower order harmonics that is typical of the 24-pulse system. B. Experimental Results Typical waveforms of the output 24-pulse dc voltage observed on the oscilloscope are shown in Figs. 18 and 19. The Figure 18. Panned view of 24-pulse dc voltage [1] IEEE Recommended Practices and Requirements for Harmonics Control in Electric Power Systems, IEEE Std. 519, [2] Electromagnetic Compatibility (EMC) Part 3: Limits-Section 2: Limits for Harmonic Current Emissions (Equipment Input Current (16A per Phase), IEC , Dec., [3] Draft-Revision of Publication IEC 555-2: Harmonics, Equipment for Connection to the Public Low Voltage Supply System, IEC SC 77A, [4] Bhim Singh, B. N. Singh, A. Chandra, Kamal Al-Haddad, Ashish Pandey, and D. P. Kothari, A Review of Three-Phase Improved Power Quality AC-DC Converters, IEEE Trans. Ind. Electron., vol. 51, No. 3, June 2004, [5] S. Choi, New pulse multiplication technique based on six-pulse thyristor converters for high power applications, IEEE Trans. Ind. Appl., vol. 38, no. 1, pp , Jan./Feb [6] B. Singh, G. Bhuvaneswari, and V. Garg, Pulse multiplication in ac dc converters for harmonic mitigation in vector-controlled induction motor drives, IEEE Trans. Energy Convers., vol. 21, no. 2, pp , Jun [7] M. Villablanca, J. D. Valle, J. Rojas, and W. Rojas, A modified back-toback HVDC system for 36-pulse operation, IEEE Trans. Power Del., vol. 15, no. 2, pp , Apr [8] B. Singh, G. Bhuvaneswari, and V. Garg, Harmonic mitigation using 12-pulse ac dc converter in vector-controlled induction motor drives, IEEE Trans. Power Del., vol. 21, no. 3, pp , Jul [9] B. Singh, V. Garg, and G. Bhuvaneswari, A novel T-connected autotransformer-based 18-pulse ac dc converter for harmonic mitigation in adjustable-speed induction-motor drives, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp , Oct [10] B. Singh, G. Bhuvaneswari, and V. Garg, A novel polygon based 18- pulse ac dc converter for vector-controlled induction motor drives, IEEE Trans. Power Electron., vol. 22, no. 2, pp , Mar [11] S. Choi, P. N. Enjeti, and I. J. Pitel, Polyphase transformer arrangements with reduced KVA capacities for harmonic current reduction in rectifier type utility interphase, IEEE Trans. Power Electron, vol. 11, no. 5, pp , Sep [12] S. Choi, B. S. Lee, and P. N. Enjeti, New 24-pulse diode rectifier systems for utility interface of high-power ac motor drives, IEEE Trans. Ind. Appl., vol. 33, no. 2, pp , Mar./Apr [13] S. Miyairi, S. Iida, K. Nakata, and S. Masukawa, New method for reducing harmonics involved in input and output of rectifier with interphase transformer, IEEE Trans. Ind. Appl., vol. IA-22, no. 5, pp , Sep./Oct [14] F. J. Chivite-Zabalza, A. J. Forsyth, and D. R. Trainer, A simple, passive 24-pulse ac dc converter with inherent load balancing, IEEE Trans. Power Electron., vol. 21, no. 2, pp , Mar [15] D. A. Paice, Power Electronic Converter Harmonics: Multipulse Methods for Clean Power. New York: IEEE Press, 1996.