A. N. Arvindan

Department of Electrical &.Electronics Engineering, SSN College of Engineering, Anna University (Chennai),

Chennai, India – 603 110

E-mail: lkana0@yahoo.com

Anirudh Guha

Department of Electrical &.Computer Engineering,

University of Texas,

Austin, TX 78712-0240, USA,

E-mail: anuguha87@mail.utexas.edu

Abstract—Two 24-pulse rect

i f

i

er topolog

i

es based on phase

sh ft ng by convent onal magnet cs s proposed. Four 3-phase systems are obtai ned from a si ngle 3-phase source usi ng novel interconnection of conventional single- and 3-phase transformers. Phase shi fts of 15º and 30º are made usi ng phasor addi ti on of relevant li ne voltages wi th a combi nati on of si ngle-phase and

three-phase transformers respect

i vely. The four three-phase

systems are mutually di splaced from each other by 15º. Each three-phase system feeds a 6-pulse di ode recti fi er and the four d ode rect f ers are ser es cascaded to prov de a 24-pulse dc

output voltage. PSCAD based s

i mulat

i

on and exper

i

mental

results that confirm the design efficacy are presented.

Keywords-Total harmonic distortion, Multipulse converter, Multipulse rectifier, Power quality, Pulse number

I.I NTRODUCTION

The 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 to address power quality issues 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],[5] 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 magnetic for the realization of a 24-pulse rectifier involving the transformation of a single 3-phase system to four 3-phase systems using novel interconnections of conventional 3-phase and single-phase transformers. A 12-pulse rectifier is realized 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.M ULTIPULSE C ONVERTERS

The number of pulses in the dc output voltage within one time period of the ac source voltage is the pulse number. 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 [6], [7] 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 [8], and an injection transformer, reduces TH D to input ac currents and ripples in

the output dc voltage. The cost and weight of input transformers can be reduced by using autotransformers [9]-[11]

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

V ARIATION O F H ARMONICS A ND R IPPLE W ITH P ULSE N UMBER

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 0.003

24 23,25,47,… 24fs 0.0022

108 978-1-61284-379-7/11$26.00c 2011IEEE

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

Figure 2. Phasor representation of four three-phase systems of topology I.

injection employing interphase reactors. The rating, size, cost, and weight of different components of these converters are reduced using novel concepts in autotransformer configurations [12], [13] 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 [13], [14] and injection transformers [15] 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. III.

R EALIZATION O F 24-P ULSE R ECTIFIER T OPOLOGIES

In this paper two topologies of 24-pulse rectifier are proposed that involve novel interconnections of single- and three-phase conventional transformers for phase shifting. A.24-Pulse Rectifier: Topology I

Fig. 1 shows the proposed first 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 [16]

Phase shift = 60ƕ/Number of six-pulse converters .

The phasor representation of the four 3-phase systems – a 0b 0c 0, a 15b 15c 15, a 30b 30c 30, and a 45b 45c 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 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 number implies elimination of lower order current

20111st International Conference on Electrical Energy Systems 109

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

Figure 4. Phasor representation of four three-phase systems of topology II.

harmonics and presence of higher order ones on the ac side, lower ripple content on the dc voltage and higher ripple frequency.

B.24-Pulse Rectifier: Topology II

Fig. 3 shows the proposed second topology of the 24-pulse

rectifier. The phasor representation of the four 3-phase systems – a 0b 0c 0, a 15b 15c 15, a 30b 30c 30, and a 45b 45c 45 shown in Fig. 3 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. 4. IV.

T RANSFORMATION :O NE 3-P HASE S YSTEM T O F OUR 3-P HASE S YSTEMS

The transformation of a single three-phase system to four three-phase systems is achieved by two different interconnections of single- and three-phase transformers that provide two topologies.

A. Transformation in Topology I

As shown in Fig. 1, the source represented by lines A 0B 0C 0feeds 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 30º are obtained from the secondary wye (y0) and delta (d11) windings respectively. The line voltages V a0b0,V b0c0, V c0a0and V a30b30, V b30c30, V c30a30are shown in Figs. 5 and 6 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 single-

a - 30

a 0 a -15

-45

a 0c

c - 30

c - 45 b - 30 b - 45

b -15b 0

11020111st International Conference on Electrical Energy Systems

Figure 5. Input line voltages V a0b0, V b0c0 and V c0a0 at diode bridge I.Figure 6. Input line voltages V a30b30, V b30c30 and V c30a30 at diode bridge II.phase transformers with appropriate turns ratio. The secondary voltages of the single-phase transformers corresponding to V a0b0and 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 15º. This 15º phase shift is obtained by phasor addition of appropriate line voltages.

The line voltage V a0b0 leads the phase voltage V a0 by 30º and the line voltage V a30b30 leads the phase voltage V a30 by 30º,however,since V a30 also leads V a0 by 30º it is obvious V a0b0 is in phase with V a30. This implies that V a30b30 leads V a0b0 by 30º.The phasor addition of these two line voltages that are equal in magnitude gives the resultant V a15b15 as follows:

V a15b15 = (V 2a0b0 + V 2a30b30+ 2V a0b0 V a30b30 Cos30º)1/2

(1) Since the magnitudes of V a0b0 and V a30b30 are equal, the resultant V a15b15 bisects the 30º angle between V a0b0 and V a30b30. Thus the line voltage V a15b15 leads V a0b0by 15º. 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 c30a30respectively. The voltages V a15b15, V b15c15 and V c15a15 are equal in magnitude and are 120º 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. 7. 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 -30º 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 30º the voltages V a45b45, V b45c45 and V c45a45

Figure 7. Input line voltages V a45b45, V b45c45 and V c45a45

at diode bridge III.

Figure 8. Input line voltages V a15b15, V b15c15 and V c15a15 at diode bridge IV.

respectively are shown in Fig. 8. Thus, four 3-phase systems with successive systems displaced by 15º are realized. B.Transformation in Topology II

As shown in Fig. 3, the source represented by lines A 0B 0C 0feeds the Yy0d1 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 a-30b-30, V b-30c-30, V c-30a-30) lagging the source line voltages by 30º are obtained from the secondary wye (y0) and delta (d1) windings respectively. The line

voltages V a0b0,V b0c0, V c0a0and V a-30b-30, V b-30c-30, V c-30a-30are shown in

Figs. 9 and 10 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 a-30b-30, V b-30c-30, V c-30a-30 are isolated using 6 single-phase transformers with appropriate turns ratio. The secondary voltages of the single-phase transformers

corresponding to V a0b0and V a-30b-30 are connected in series in order to yield V a-15b-15, a voltage equal in magnitude to the six line voltages but lagging V a0b0 by 15º. This 15º

phase shift is

obtained by phasor addition of appropriate line voltages. The line voltage V a0b0 leads the phase voltage V a0 by 30º and the line voltage V a-30b-30 leads the phase voltage V a-30 by 30º

,

however,since V a0 also leads V a-30 by 30º

it is obvious V a-30b-30 is

in phase with V a0. This implies that V a0b0 leads V a-30b-30 by 30º.The phasor addition of these two line voltages that are equal in magnitude gives the resultant V a-15b-15

as follows: 20111st International Conference on Electrical Energy Systems 111

Figure 10. Input line voltages V a30b30, V b30c30 and V c30a30 at diode bridge II.

V a-15b-15 = (V2a0b0 + V2a-30b-30+ 2V a0b0 V a-30b-30 Cos30º)1/2 (2)

Since the magnitudes of V a0b0 and V a-30b-30 are equal, the resultant V a-15b-15 bisects the 30º angle between V a0b0 and V a-30b-30. Thus the line voltage V a-15b-15 lags V a0b0by 15º. Similarly, the line voltages V b-15c-15 and V c-15a-15 are obtained by the phasor additions via the secondary windings of the single-phase transformers corresponding to the line voltages V b0c0 and V b-30c-30 ; and V c0a0 and V c-30a-30 respectively. The line voltages V a-15b-15, V b-15c-15 and V c-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. 11 shows the voltages V a-15b-15, V b-15c-15 and V c-15a-15. H ence, in Fig. 3 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-45respectively. The corresponding line voltages V a-45b-45, V b-45c-45 and V c-45a-45 that lag by 30º the voltages V a-15b-15, V b-15c-15 and V c-15a-15 respectively are shown in Fig. 12. Thus, four 3-phase systems with successive 3-phase systems displaced by 15º are realized.

V.R ESULTS A ND D ISCUSSION

A.Simulation Results

The topologies of the 24-pulse rectifier have 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.

Figure 11. Input line voltages V a15b15, V b15c15 and V c15a15 at diode bridge III.

Figure 12. Input line voltages V a45b45, V b45c45 and V c45a45 at diode bridge IV.

1)Simulation results for topology I:

Figure 13. DC 12-pulse output voltage by cascading diode bridges I and II.

Figure 14. DC 12-pulse output voltage by cascading diode bridges III and IV. 11220111st International Conference on Electrical Energy Systems

Figure 15. DC 24-pulse voltage by cascading DBI, DBII, DBIII and DBIV.Fig. 13 shows the 12-pulse dc voltage obtained when output voltages of bridges DBI and DBII are series cascaded. 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. 13 and 14 are displaced by 15º. 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.

2)Simulation results for topology II:

Figure 16. DC 12-pulse output voltage by cascading diode bridges I and II.

Figure 17. DC 12-pulse output voltage by cascading diode bridges III and IV.

Fig. 16 shows the 12-pulse dc voltage obtained when output voltages of bridges DBI and DBII are series cascaded. The outputs of bridges DBIII and DBIV are series cascaded to provide a 12-pulse dc voltage that is shown in Fig. 17. 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. 18. The design details for topologies I and II have already been reported by the author in [17] and [18] respectively.

Figure 18. DC 24-pulse voltage by cascading DBI, DBII, DBIII and DBIV.

Figure 19. Line current in phase a of Y winding of main transformer.

The harmonic spectrum of the input line current in phase a of the main transformer Yy0d11 and Yy0d1 corresponding to topologies I and II respectively is shown in Fig. 19 which confirms that the 23rd and 25th 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. 20 and 21.

Figure 20. Panned view of 24-pulse dc voltage

Figure 21. 24-pulse dc voltage.

20111st International Conference on Electrical Energy Systems 113

VI.C ONCLUSION

Two 24-pulse rectifier topologies are realized by transformation of a single three-phase voltage source system to four three-phase isolated systems, using novel interconnections of conventional three-phase and single-phase transformers for obtaining the relevant phase shifts. The 24-pulse dc output voltage results from series cascading of four six-pulse diode bridges that are fed by the four isolated three-phase voltage systems. The simulation and experimental results confirm the efficacy of the topology in terms of the theoretical harmonic and ripple estimates.

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