A Transformer Connection For Multipulse Rectifier Applications

Syafri Martinius, Burhanuddin Halimi, Pekik Argo Dahono

Abstract--In this paper, a polygon-connected transformer for multipulse rectifier applications is proposed. By using the polygon form on the secondary windings, the windings can be designed to have the same voltage and impedance. The proposed transformer connection can also be extended to autotransformer connection if a galvanic isolation is not required. By using the proposed autotransformer, the required kVA rating can be much smaller compared to the rated load.. The design and analysis of the proposed transformer connection are detailed. Several experiment?l results are included to show the effectiveness of the proposed method.

Indexing Terms - Multipulse rectifiers, polyphase transformers.

I . INTRODUCTION A multipulse rectifier is commonly used in high-power dc power supplies, ac drives, and HVDC system. At present, a multipulse rectifier is usually constructed by using several three-phase rectifiers fed by a multiphase transformer. The most popular multiphase transformer is the one that has several phase-shifted secondary windings. In the case of 12 pulse rectifier, for example, the transformer has two secondary windings those are connected in wye and delta, respectively [l], [3], such as shown in Fig. 1. There are several problems with this conventional transformer connections. The first problem is very difficult to built wye and delta connected windings with equivalent electrical characteristics (the same voltage and impedance). The second problem is the flexibility. Under emergency condition, for example, a 12 pulse rectifier sometimes is operated as a 6 pulse rectifier. Under this condition, the wye and delta connected windings can not be paralleled. Such flexibility is also important during initial phase of a project.

In this paper, a transformer that has several secondary windings those are connected in polygon form is proposed, such as shown in Fig. 2. This proposed concept uses separate transformers in which the primary windings are connected in series and the secondary converter are in parallel. Due to the

Syafri Martinius, Burhanuddin Halimi, and Pekik Argo Dahaono are with the Department of Electrical Engineering, Bandung Institute of Technology, J1. Ganesa No. 10, Bandung 40 132 (Email:syafri@onversi.ee.itb.ac.id, burhan@konversi.ee.itb.ac.id, pekik@konversi.ee.itb.ac.id)

primary windings are connected in series, it can be ensure that the current sharing can work well. So, it do not need a interphase reactor. In this case, the secondary circuit are forced to share current by reason of the balance maintained by the transformer ampere turns [2], [3]. Because the secondary windings are connected in a polygon form, the windings can be designed to have the same voltage and impedance. Moreover, the secondary windings can be connected in parallel if required. Thus, the proposed transformer connection is suitable for multipulse rectifier applications. The proposed transformer connection can also be extended to autotransformer connection if a galvanic isolation is not required. By using the proposed autotransformer, the required kVA rating can be much smaller compared to the rated load.

Interphase n Reactor

Fig. 1. Conventional 12-pulse converter (kVA = 1-.03 Po)

N

Rectifier I

!cn

-U Rectifier II

Fig. 2. Proposed transformer connection (kVA = 2 x 0.58 Po)

0-7803-7459-2/02/$1,7.00 0 2002 IEEE - 1021 -

11. PROPOSED TRANSFORMER CONNECTION The basic idea of the proposed transformer connection is how to design a transformer for multipulse rectifier applications that is flexible in application, can overcome the current sharing problem and have the same voltage and impedance. The proposed concept uses separate transformers in which the primary windings are connected in series and the secondary converter are in parallel. Due to the primary windings are connected in series, it can be ensure that the current sharing can work well. So, it do not need a interphase reactor. Because the secondary windings are connected in a polygon form, the windings can be designed to have the same voltage and impedance. Fig. 3 shows the proposed transformer connection for analysis. From the vector diagram, it can be obtained

(1) V u - VNL - VNS

sin120~ sin@ sin1P

By using (l), the long winding voltage is

VNL =[$]vu

and the short yinding voltage is

VNS = [ 7 I v r S h-1 (3)

In 1 : 1 voltage ratio outputhnput case, the ratio of windings can be written as

TQ - 1

‘a I

‘3 +

(a) Transformer connection for 6-pulse rectifier

VNL (b) Secondary winding vector of (a)

P (c) Transformer connection for 12-pulse rectifier

Fig. 3 Proposed transformer connection for analysis. (4)

In Fig. 3(a), the winding current can be written as In proposed connection for 12 pulse rectifier (Fig. l), by using (6) it is obtained the secondary winding of each il -i2

3 ‘Im =- transformers as

i2 - i3 i2m =-

3 . i3 -il 1 3 ~ =-

3

1 6 (5) Isec .winding-1 2p = - &Io (7)

The kVA rating of transformer can be calculated in the following manner [3] :

The rms value of each input currents of 6 pulse rectifier (il,

winding rms current as iz, and is) is J2/31dc . Thus, it can be obtained the secondary kvA = o.5xcv,indingxIwinding (8)

By substituting (2), (3),and (7) to (8), for 12 pulse rectifier configuration in which the output dc output (Yo) is 1.35 VI,,,. ,,e , kVA rating of each transformers in the proposed connection is 0.58 Pa. Though the it is larger than the

(6) 1 3 Isec .winding-6p = - f i I d c

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conventional transformer, it have some advantages those the conventional one do not have, i.e. the flexibility in applications, do not need a interphase reactor in the multipulse rectifier application, and have the same voltage and impedance.

111. AUTOTRANSFORMER ARRANGEMENT OF THE PROPOSED CONNECTION

The proposed transformer connection can also be extended to autotransformer connection if a galvanic isolation is not required. One form of conventional autotransformer connections is differential delta connection, such as shown in Fig. 4. In this connection the kVA rating of autotransformer is 0.18 Po [ 11. By using extension of the proposed transformer connection, such as shown in Fig. 5, the kVA rating of autotransformer is 0.101 Po [4]. It means that by using the proposed autotransformer, the required kVA rating can be much smaller compared to the rated load.

Interphase Reactor

Load

Fig. 4 Differential delta connection (kVA = 0.18 Po)

Interphase Reactor

Fig. 5 Extension of the proposed transformer connection (kVA = 0,101 Po)

Fig. 6 shows the detail winding connection and vector diagram of the extension of the proposed transformer connection. By using similar manner in the previous section, it can be obtained

In this case, the ratio of winding can be written as

For a 6-pulse rectifier, the rms value of input current of each rectifiers is

Substituting (9),(10), and (12) to (8), it can be obtained that the kVA rating of transformer is 0.101 Po. It means that the kVA rating the extension of the proposed transformer connection is much smaller compared to the conventional differential delta-connected transformer.

(a) Winding configuration

V

(b) Vector diagram

Fig. 6 Extension of proposed transformer connection for analysis

Iv. EXPERIMENTAL RESULT

To verify that the proposed configuration can overcome the current sharing problem and robust to unbalanced condition, a experimental verification is carried out on a small system.

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The experimental system has two identical 380 V, lOkVA transformers, two 20A, 1200V three phase diode rectifiers module, and a load (consists of 20 ohm resistor and a 29 mH inductor). Configurations used in the experiment are referred to Fig. 2. Fig 7(a) shows utility line current under balanced condition. In Fig. 7(b), it can be seen that the dominant harmonic in utility line current is the 12-pulse characteristic harmonic (h=12&1, k = 1,2,3 .... ). The 11” and 1 3 ~ are 1.24375 A and 1.025 A, respectively. THD of the utility line current is 11.3 %. The output current of each rectifier and load current are shown in Fig. 8. Fig. 8 shows that the output current of each rectifiers is differ 30” in phase each other and its rms value is equal (= 10 A).

The experimental result under unbalanced condition is shown in Fig. 9. Unbalanced condition experiment is set up by connecting in series a 1 ohm resistor on each of all phases of one of the rectifiers input side. Fig. 9 shows that the output current of each rectifiers is differ 30” in phase each other and its rms value is equal (= 10 A). It verify that the proposed configuration can ensure that the current sharing can work well .

. ~. - ~ ~~ . . . . . . . . . . . . . . . . . . . . . .

, , , , . , . . . . . . . . . . . . . . .

(a) Utility line current[ 5A/Div, 7.5ms/Div]

I S In

Harmonic Number

(b) Spectrum of utility line current Fig. 7 Proposed transformer connection under balanced

condition 1 ...........,Lo&. .... ....; .:.. ;. ..+ ...;. .. j . . . . _ . . . . . . . . . . I . . . . - . . . . . . . . - . . . . . . . . - . . .

t .... : .... : .... :... ,: .... I. ... :.. ..:.. ..: .... :.. . . i

. . . . . . . ........... ...................................... . . . .

. . . . - . . . .

. . . . . . . . . . . . . . . . . . . . . . .

Fig. 9 Output current waveform of proposed transformer connection under unbalanced condition [5A/Div, 2.5ms/Div]

V. CONCLUSION In this paper, a polygon-connected transformer for multipulse rectifier applications was proposed. The proposed connection has some advantages .i.e. it is flexible in application, can overcome the current sharing problem and has the same voltage and impedance, so it can be connected in parallel if required. The proposed transformer connection can also be extended to autotransformer connection if a galvanic isolation is not required. By using the proposed autotransformer, the required kVA rating can be much smaller compared to the rated load i.e. just 0,101 Po .

VI. REFERENCES

S. Choi, P. N. Enjeti, and I.J. Pitel, ‘‘ Polyphase Transformer Arrangements with Reduced kVA Capcities *

for Harmonic Current Reduction in Rectifier-Type Utility Interface”, IEEE Power Electronics, Vol. 11, Number 5, September 1996. April, G. E., and G. Oliver, “ A Novel Type of 12 Pulse Converter,” Conference Record, IAS, IEEE-IAS-1982, Annual Meeting, pp 913-922. Derek A. Paice, ”Power Electronic Converter Harmonics,” New York: IEEE Press, 1995. Syafrudin, P. A. Dahono, and T. M. Soelaiman, “A 12- Pulse Three-phase Rectifier with A Small kVA Rating Transformer (In Indonesia : Rectifier Tiga Fasa 12-Pulsa dengan Rating kVA Transformer Rendah),” Proceedings of Indonesia Industrial Electronics Seminar, 1999

Fig. 8 Output current waveform of proposed transformer & n a t i o n under balanced condition [SAiDiv, 2.5ms/Div]

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