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24-PULSE RECTIFIER REALIZATION BY 3-PHASE TO FOUR 3-PHASE TRANSFORMATION USING CONVENTIONAL TRANSFORMERS

A. N. Arvindan and P. Pushpakarthick S.S.N. College of Engineering, Anna University (Chennai), Tamilnadu

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 single-phase and three-phase transformers respectively. PSCAD based simulation and experimental results that confirm the design efficacy are presented.

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.

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.

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],

TABLE I

VARIATION OF HARMONICS AND RIPPLE WITH PULSE NUMBER

Pulse Number AC Harmonics Ripple

Frequency Ripple Factor

1 1,2,3,… fs 1.21

2 1,3,5,…. 2fs 0.482

3 2,4,5,…. 3fs 0.182

6 5,7,11,…. 6fs 0.042

12 11,13,23,… 12fs 0.01

18 17,19,35,… 18fs 0.00643

24 23,25,47,… 24fs 0.0022

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Figure 1 24-pulse rectifier realized by transforming a single 3-phase system to four 3-phase systems using conventional single- and three-phase transformers 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.

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 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 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

Figure 2 Phasor representation of four three-phase systems.

a- 45

a- 30 a0 a-15

c0

c-15

c- 30

c- 45

b- 30

b- 45

b-15

b0

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Figure 3 Input line voltages Va0b0, Vb0c0 and Vc0a0 at diode bridge I

Figure 4 Input line voltages Va30b30, Vb30c30 and Vc30a30 at diode bridge II efficiency, high power factor, low THD at input ac mains, and ripple-free dc output of high quality.

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 – a0b0c0, a15b15c15, a30b30c30 and a45b45c45 shown in Fig. 1 feeding 3-phase diode bridges (four 6-pulse rectifiers) DBI, DBII, DBIII and DBIV respectively, with successive systems displaced by 15º 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

Figure 5 Input line voltages Va15b15, Vb15c15 and Vc15a15 at diode bridge III

Figure 6 Input line voltages Va45b45, Vb45c45 and Vc45a45 at diode bridge IV 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.

Transformation: One 3-phase system to four 3-phase systems As shown in Fig. 1, the source represented by lines A0B0C0 feeds the Yy0d1 vector configured, 3-phase, 3-winding, step down transformer and two 3-phase systems, one (represented as a0, b0 and c0) with line voltages (Va0b0, Vb0c0, Vc0a0) in phase with the source line voltages and the other (represented as a-30, b-30 and c-30) with line voltages (Va-30b-30, Vb-30c-30, Vc-30a-30) lagging the source line voltages by 30º are obtained from the secondary wye (y0) and delta (d1) windings respectively. The line voltages Va0b0, Vb0c0, Vc0a0 and Va-30b-30, Vb-30c-30, Vc-30a-30 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 Va0b0, Vb0c0, Vc0a0, Va-30b-30, Vb-30c-30, Vc-30a-30 are isolated using 6 single-phase transformers with appropriate turns ratio. The secondary voltages of the single-phase transformers corresponding to Va0b0 and Va-30b-30 are connected in

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series in order to yield Va-15b-15, a voltage equal in magnitude to the six line voltages but lagging Va0b0 by 15º. This 15º phase shift is obtained by phasor addition of appropriate line voltages.

The line voltage Va0b0 leads the phase voltage Va0 by 30º and the line voltage Va-30b-30 leads the phase voltage Va-30 by 30º, however, since Va0 also leads Va-30 by 30º it is obvious Va-30b-30 is in phase with Va0. This implies that Va0b0 leads Va-30b-30 by 30º. The phasor addition of these two line voltages that are equal in magnitude gives the resultant Va-15b-15 as follows: Va-15b-15 = (V2

a0b0 + V2a-30b-30 + 2Va0b0 Va-30b-30 Cos30º)1/2

...(1)

Since the magnitudes of Va0b0 and Va-30b-30 are equal, the resultant Va-15b-15 bisects the 30º angle between Va0b0 and Va-30b-30. Thus the line voltage Va-15b-15 lags Va0b0 by 15º. Similarly, the line voltages Vb-15c-15 and Vc-15a-15 are obtained by the phasor additions via the secondary windings of the single-phase transformers corresponding to the line voltages Vb0c0 and Vb-30c-30; and Vc0a0 and Vc-30a-30 respectively. The line voltages Va-15b-15, Vb-15c-15 and Vc-15a-15 are equal in magnitude and are 120º apart and, therefore; the windings with these voltages are connected in delta to form a balanced 3-phase system. Fig. 5 shows the voltages Va-15b-15, Vb-15c-15 and Vc-15a-15. Hence, in Fig. 1 the (phasors) lines a-15, b-15 and c-15 are obtained and are fed to a 3-phase transformer of the Yd1 configuration which provides a phase shift of -30º i.e. 30º laggingº and hence, yields the (phasors) lines a-45, b-45 and c-45 respectively. The corresponding line voltages Va-45b-45, Vb-45c-45 and Vc-45a-45 that lag by 30º the voltages Va-15b-15, Vb-15c-15 and Vc-15a-15 respectively are shown in Fig. 6. Thus, four 3-phase systems with successive 3-phase systems displaced by 15º are realized.

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 bridge = 110/4 = 27.5V.

The dc output voltage of a 3-phase diode bridge is given by,

mdc V 33 Vπ

= ..…... (2)

where, Vm = peak value of phase voltage feeding the bridge. Substituting Vdc = 27.5V, the peak value of the 3-phase line voltage feeding each bridge is obtained from (2) as,

28.791V V 3 m = ...…... (3)

and the corresponding rms value is given by,

20.37V V 23

m= ..…… (4)

Turns ratio of various transformers The main transformer connected to the three- phase utility is a three-phase, three-winding one with Yyod1 vector configuration. The primary wye winding is connected to the 3-phase 415V utility.

Yyod1 main transformer

As evident from Fig. 1 diode bridges DBI and DBII are fed from the y0 and d1 secondary windings respectively, therefore, the line voltages pertaining to these have to be in conformity with eq. (4). The turns ratio for the Yyo winding is obtained as follows:

Y-primary Vline(rms) = 415V yo secondary desired line voltage = 20.37V i.e. N2/N1 = 20.37 / 415 = 0.0491

and that for the Yd1 winding is obtained as follows:

Y-primary Vline(rms) = 415V d1 secondary desired line voltage = 20.37V N2/N1 = (20.37 x √3) / 415 = 0.0850

Single-phase transformers

There are six single-phase transformers whose primary windings are used to isolate two sets of line voltages, Va0b0, Vb0c0, Vc0a0 and Va-30b-30, Vb-30c-30, Vc-30a-30 pertaining to the wye (yo) and delta (d1) 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 Va0b0 and Va-30b-30, Vb0c0 and Vb-30c-30, and Vc0a0 and Vc-30a-30 that are synthesized by series cascade connection to obtain the line voltages Va-15b-15, Vb-15c-15 and

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Vc-15a-15 respectively. The synthesis of the voltage combinations is as per eq (1). The synthesis of Va0b0 and Va-30b-30 as per eq (1) yields Va-15 b-15 as follows:

Va0b0 = 20.37/30º V Va-30b-30 = 20.37/0º V

i.e. Va-15 b-15 V3037.2037.20237.2037.20 22 oCos×××++=

i.e. Va-15 b-15 = 39.35/15º V

Similarly, the line voltages Vb-15c-15 and Vc-15a-15 are obtained by the synthesis of the relevant voltage combinations. The synthesis yields magnitudes as follows:

│Va-15b-15│ = │Vb-15c-15│ = │Vc-15a-15│ = 39.35V

The three secondary pairs are connected in delta to form a 3-phase winding with Va-15b-15, Vb-15c-15 and Vc-15a-15 as the line voltages. The delta winding feeds the diode bridge DBIII and therefore, as per eq (4) the desired magnitudes the line voltages have to be as follows:

│Va-15b-15│ = │Vb-15c-15│ = │Vc-15a-15│ = 20.37V

The turns ratio of each single-phase transformer is thus:

N2/N1 = 20.37 / 39.35 = 0.5176 ≈ 0.52

Yd1 transformer

The voltages Va-15b-15, Vb-15c-15 and Vc-15a-15 are fed to a Yd1 transformer to obtain line voltages Va-45b-

45, Vb-45c-45 and Vc-45a-45 that lag the input wye voltages by 30º and feed diode bridge DBIV. The magnitudes of the line voltages on wye and delta sides must be equal i.e. │Va-15b-15│ = │Vb-15c-15│ = │Vc-15a-15│=│Va-45b-45│ = │Vb-45c-45│ = │Vc-45a-45│= 20.37V. Thus, the turns is given by

N2/N1 = 1.7321 3 =

RESULTS AND DISCUSSION The topology of the 24-pulse rectifier has been simulated using the PSCAD software educational version 4.2.1. The simulation assumes a balanced 3-phase source and neglects saturation in the transformers.

Simulation results Figs. 7 and 8 show line currents in phase ‘a’ of the y0 and d1 windings respectively of the Yy0d1 transformer in Fig. 1. The line currents in the secondary wye (y0) and the delta (d1) windings, shown in Figs. 7 and 8 respectively, of the Yy0d1 main transformer comprise currents drawn by a

Figure 7 Line current in phase a of y0 winding of Yy0d1 main transformer

Figure 8 Line current in phase a of d1 winding of Yy0d1 main transformer diode bridge DBI (y0) / DBII (d1) 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 that outputs of DBI and DBII are displaced by 30º. This is because the line voltages at the input of these bridges as shown in Figs. 3 and 4 are displaced by 30º. 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

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Figure 9 Six-pulse dc output voltage of diode bridge, DBI

Figure 10 DC 6-pulse output voltage of diode bridge, DBII

Figure 11 DC 12-pulse output voltage by cascading diode bridges I and II 12-pulse dc outputs in Figs. 11 and 14 are displaced by 15º.

The series cascading of the 6-pulse outputs of two diode bridges I (or III) and II (or IV) yielding a 12-pulse output is based on the rather obvious premise that the mean value of the individual 6-pulse output of either bridge is the same with only the ripple displaced by 30°. The cascaded 12-pulse output thus comprises a mean value twice that of the single bridge and with a ripple that is twelve times the fundamental frequency.

Figure 12 Six-pulse dc output voltage of diode bridge, DBIII

Figure 13 DC 6-pulse output voltage of diode bridge, DBIV

Figure 14 DC 12-pulse output voltage by cascading diode bridges III and IV The two 12-pulse systems comprising DBI, DBII, and DBIII, DBIV, are cascaded to obtain a 24-pulse dc output with an average value of 110V that is shown in Fig. 15. The phase a input line current of the primary Y-winding of the Yy0d1 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 input line current is shown in Fig. 17 which confirms that the 23rd and 25th harmonics alone are significant lower order harmonics that is typical of the 24-pulse system.

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Figure 15 DC 24-pulse voltage by cascading DBI, DBII, DBIII and DBIV

Figure 16 Line current in phase a of Y winding of Yy0d1 main transformer

Figure 17 Line current in phase a of Y winding of Yy0d1 main transformer Experimental results Typical waveforms of the output 24-pulse dc voltage observed on the oscilloscope are shown in Figs. 18 and 19. The experimental set up is shown in Fig. 20.

Figure 18 Panned view of 24-pulse dc voltage

Figure 19 24-pulse dc voltage

Figure 20 Experimental set up

CONCLUSION A 24-pulse rectifier is realized by conventional transformers that meets the theoretical harmonic and ripple estimates. REFERENCES [1] IEEE Recommended Practices and Requirements for

Harmonics Control in Electric Power Systems, IEEE Std. 519, 1992.

[2] Electromagnetic Compatibility (EMC)—Part 3: Limits-Section 2: Limits for Harmonic Current Emissions (Equipment Input Current (16A per Phase), IEC1000-3-2, Dec., 1995.

[3] Draft-Revision of Publication IEC 555-2: Harmonics, Equipment for Connection to the Public Low Voltage Supply System, IEC SC 77A, 1990.

[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, 641-660.

[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. 131–136, Jan./Feb. 2002.

[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. 342–352, Jun. 2006.

[7] M. Villablanca, J. D. Valle, J. Rojas, and W. Rojas, “A modified back-to-back HVDC system for 36-pulse operation,” IEEE Trans. Power Del., vol. 15, no. 2, pp. 641–645, Apr. 2000.

[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. 1483–1492, Jul. 2006.

[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. 2500–2511, Oct. 2007.

[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. 488–497, Mar. 2007.

[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. 680–690, Sep. 1996.

[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. 531–541, Mar./Apr. 1997.

[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. 790–797, Sep./Oct. 1986.

[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. 430–439, Mar. 2006.

[15] D. A. Paice, Power Electronic Converter Harmonics: Multipulse Methods for Clean Power. New York: IEEE Press, 1996.

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A. Narayanan Arvindan graduated in Electrical and Electronics Engineering from CEG, Guindy, Anna University, in 1988. He obtained the masters degree in Power Systems and the PhD degree in Power Electronics from NIT,Tiruchirapalli, in 1995 and the Faculty of

Engineering and Technology, Jamia Millia Islamia (a Central University), New Delhi, in 2007 respectively. He has over 19 years of professional experience including 10 years in industry and 9 years in academics. His industrial experience includes a stint at ABB, where he designed high current rectifiers in the power electronics division. He has wide field exposure and has completed several projects including electrification of large continuous process plants and overhead transmission. In 1999, he took to academics by joining St. Joseph’s Engineering College, Anna University, as Assistant Professor and was conferred ‘The Best Teacher Award’ for three successive academic years 2000-2001, 2001-2002 and 2002-2003. Currently, he is a professor in the EEE Department of SSN Engineering College, Anna University, Chennai. He is a life member of ISTE, India, and member of the IEEE, USA. He is listed in Marquis Who’s Who in the World, 2008 and 2009. His projects on multipulse converter was recognized as one of the top six Electrical & Electronics Engineering projects in 2008-2009 and received funding by the Tamilnadu State Council For Science and Technology. His fields of interest include power electronics, power quality, ac/dc drives and simulation software.

P. Pushpakarthick graduated in Electrical and Electronics Engineering from Anna University, Chennai, in 2008.He is currently a post graduate scholar in the EEE department of SSN College of Engineering, Anna University, Chennai, pursuing the masters degree in power electronics.