Laljibhai Chaturbhai Institute of Technology,Bhandu-384120,13-14 Apirl,2012 1

Abstract— Three phase multipulse converters are developed for power quality improvement. They reduce harmonics in ac mains and ripples in dc output. This paper deals with different transformer winding connections. Calculation of electrical parameters such as voltage, current and KVA for various winding types derived mathematically. The design of a specified phase-shifting angle and calculation of voltage and current distribution in polygon winding are proposed with resistive load. Low voltage high current converter topology for twelve pulse is simulated for special applications.

Index Terms—A Polyphase transformer, Polygon winding, Power quality, low voltage high current converter

I. INTRODUCTION

iterature on multipulse converter topology has been existed for several years. Multipulse converter is

commonly used in, ac drives, DC arc furnaces, Plasma Torch and HVDC system. It is a clean power converter. Any number of n 6-pulse rectifier is connected in series or in parallel, where n should be greater than one. Two main advantage of multipulse converter are reduction in ac input line current harmonics and in dc output voltage ripple[1].

L

Harm=

Mag=

k=any positive integern=number of six pulse converter

In order to prevent the harmonics from affecting the utility lines negatively, an IEEE Standard 519 has been establish in 1981 as the “recommended Practice for Harmonic Control in electrical Power System” giving limit on voltage distortion. It has been reissued in 1992 giving clear limit for voltage and current distortions. Several methods based on increasing number of pulses in clean power converters are there. These methods use two or more 6 pulse converters, where harmonic generated by one converter is cancelled by other converter, by proper phase shift. Multipulse converter are controlled and uncontrolled with 12, 18, 24, 48 pulses. Increasing the number of rectifier bridges raise the number of steps in the primary current and produces a sinusoidal shaped supply

current. Multiphase transformer is one that has several phase shifted secondary windings. In case of 12 pulse converter, the transformer has two secondary windings those are connected in delta and star respectively and primary in delta, such as shown in fig.1. It provides two phase shifted output voltage, phase shifted by 0º and 30º from primary voltages and these are achieved popularly by employing delta/delta-star or star/star-delta transformer connection. Large current converter in 12 pulse configurations is commonly used in several processes like Dc arc furnaces etc [1]. The multi-pulse converter topology generally acts as an interface between the power electronics load and the utility supply. Some of the most common applications for multipulse converter systems include motor drives; interruptible power supplies (UPS) systems, aircraft variable speed constant frequency (VSCF) systems, and frequency changer systems.Most common converter in practical application is 12 pulses and 18 pulses fed from multiphase transformer to meet IEE 519 standard for input line current is still high and may not qualify as clean power at high loads. The multi-pulse converter topology generally acts as an interface between the power electronics load and the utility supply. Some of the most common applications for multipulse converter systems include motor drives; interruptible power supplies (UPS) systems, aircraft variable speed constant frequency (VSCF) systems, and frequency changer systems.

In this paper, a transformer has several secondary windings are connected in polygon is proposed. This transformer can also be extended to autotransformer connection if isolation is not required. By using such transformer kva rating is reduced.

Fig.1 Twelve pulse converter with isolated two transformer

TRANSFORMER CONNECTION FOR MULTIPULSE CONVERTER APLLICATIONS

Parmar Poonam N., M.E. [Electrical] Student Department Of Electrical Engineering,Sarvajanik College Of Engineering And Technology, Surat, Gujarat

Urvi Jariwala, Assistance Professor(Electrical),Sarvajanik College of Engineering And Technology, Surat, Gujarat

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II.POWER QUALITY CONSIDERATION

The major objective of multipulse converter is to improve power quality at input ac mains and output dc loaf in wide applications. The main drawbacks of conventional ac-dc converters have been harmonic injection into ac mains which results in poor power factor, poor utilization of distribution system, EMI, RFI. Similarly ripples in dc output voltage causes derating of loading equipment etc. Multipulse converter are quite effective to improve power quality at ac mains and output dc loads[3].

III. TRANSFORMER WINDING CONNECTION

Primary and secondary windings of transformers could be any combination of delta and wye connections. The polygon, zigzag and extended delta windings are special winding connection. Primary winding is connected in series, so current sharing will be same. They offer new features but still keep certain characteristics from wye or delta windings. A zigzag winding has each electrical phase linked equally by two phases. It is also used to provide phase-shifting in rectifier transformers. Polygon winding and extended delta winding are mainly used in phase shifting transformers for harmonic mitigation purpose [3]. The winding connection and voltage vector diagrams are shown in figures.

Fig. 2 (a) Star connection (b) Delta connection

Fig. 3 Zigzag connection & voltage vector dia.

Fig. 4 Polygon connection & voltage vector dia.

IV. KVA CALCULATION FOR DIFFERENT TRANSFORMER WINDINGS

1. Polygon windings These connections are used in phase shifting transformers, and it is characterized by hexagon relationship between voltage vectors, shown in fig.5.

Fig. 5 Voltage vector of polygon winding

Polygon has two windings small and long. It is same as delta winding connection but asymmetric.

Va- small winding voltage VA- long winding voltage - Line-to-line voltage

α- phase shifting angle

One bridge converter generates 6 pulses so for 12, 18, 24 pulses 2, 3, 4 converters are required respectively. Hence for 12 pulse converter 30˚shifting, 18 pulse 20˚shifting, 24 pulse 15˚shifting from primary voltage is needed. So phase shifting

angle calculated form fig.5

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According to cosine law in voltage distribution can be

determined as follows:

= (1)

VA = (2)

Substituting Va , VA in

(

3)

(4)

Hence turns ratio is

= (5)

Current distribution in two windings are

Fig. 6 Current vector of polygon winding

(6)

(7)

(8)

(9)

(10) By using Eq. (6),(7),(10) current in winding A is

Output Kva of transformer is

(11)

As example for 12 pulse converter VL_L of transformer is 11000volts and line current is 1200 amps so phase shifting angle for 12pulse is , the voltage winding of A will be:

Fow winding A:

volt

For winding a:

So transformer Kva in polygon winding is Kva_total = 3*3457 =10372 Kva

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=22864 Kva

Comparison indicates that polygon winding is economical and have low Kva.

2. Zigzag windings

Fig. 7 zig. voltage vector

A zigzag connection is a special purpose transformer with zigzag or ‘interconnected star’ winding connection. The most common zigzag transformer application is for the derivation of a neutral connection from an ungrounded 3-phase system and the grounding of that neutral to an earth reference point. Zigzag transformers are also used to control of triplen (3 rd , 9th, 11th , 15th etc)  harmonic currents, to supply 3-phase power as an autotransformer (serving as the primary and secondary with no isolated circuits)[4]. Series connection of two windings voltage is 60° out of phase. For required phase shift angle voltage magnitudes of two windings can change.VA is vector sum of two voltages on same leg.

(12)

Similar to star connection

Hence zigzag winding supplies 86.6% Kva of star connection to the output circuit.

V: PROPOSED POLYGON WINDING CONNECTION

Polygon windings are used in phase shifting transformers.

is phase shifting angle. Proposed transformer connection

for analysis is shown in fig. 8 Two polygons are design for 12 pulse 30º phase shift is needed. One polygon is design for -15º shifting of voltage from primary voltage and Second polygon is for +15º shifting [6]. Turn ratio for phase shifting angle is calculated by eq.(5)

Turn ratio is T1 (for α = -15º)

T2 (for α = +15º)

Fig. 8 Polygon connection for 12 pulse converter

VI: SIMULATION

The 12 pulse isolated and non isolated converters are modeled and simulated in MATLAB. These are fed from 440V, 50Hz AC supply. The load is resistive. The simulations of 12 pulse delta/delta-star and delta/double polygon are carried out for same supply and load conditions to compare their performance. A special design of low voltage high is design for application like DC arc furnace, chemical electrolysis etc, which need high current at low voltage. Fig.9 shows interconnection of polygon 12 pulse converter. The interconnection of polyphase transformer is shown below .The simulink model is shown where multi winding 1,2,3 are for upper polygon secondary connection for - 15º shifting of voltage which fed to one bridge rectifier, so connection 4,5,6 are connected to bridge. Connection 1,2,3 are supply to primary of transformer. Another multi winding 4,5,6 for +15° shifting and fed to second rectifier bridge. Turn ratio for multi winding is T1 and T2 for upper and lower polygon respectively which are 2.73 and 0.36.

Fig. 9 Model of delta/double polygon transformer for 12 pulse converter

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Fig. 60 Low output voltage and high output current of 12 pulse converter

Converter provides continuous load current 12 pulses per cycle of the supply frequency. At any instant, load current (

) to be shared by each bridge rectifier through four diodes

simultaneously, two in the top and two in the bottom bridge. Secondary currents are trapezoidal wave with 30˚ phase shift as shown in fig. 11.

Fig. 11 source current and secondary side voltage

Low voltage high current is accomplished by this topology of multipulse converter. The line current THD in source side is low with polygon transformer connection as compare to delta/delta-star transformer connection locations.

Fig. 12 Low voltage high current output of 18 pulse converter

VII: CONCLUSION

A comprehensive review of multipulse converter has been carried out for two configuration, isolation transformer and non isolated transformer. Multipulse converter is considered as better alternative because of simple construction, low cost, low THD in source current and low ripple in output voltage. The proposed transformer is well suited for low voltage high current (LVHC) application.

REFRENCESx

[1] Derek A. Paice, Power Electronic Converter Harmonics:Multipulse Methods For Clean Power. New York: IEEE PRESS MARKETING, 1996.

[2] P. N. Enjeti,I.J. Pitel S. Choi, "Polypahse Transformer Arrangements with Reduced kva Capacities For Harmonic Current Reduction in Rectifier Type Utility Interface," in IEEE Power Electronic, vol. 5, september 1996, p. 4.

[3] Sanjay Gairola,Brij N. Sighn,Ambrish Chandra,Kamal Al-Haddad Bhim Singh, "Multipulse Ac-Dc Converters For Improving Power Quality: A Review," Transactions, vol. 23, pp. 260-280, JANUARY 2008.

[4] William Jackson,Ryan Laughy Xiaodong Liang, "Transformer Winding Connection For Practical Industrial Application," in IEEE, Canada, 2007, p. 9.

[5] Sanjay Gairola Bhim Sigh, "Zigzag Autotransformer Based Full Wave AC-DC Converters," Asian Power Electronic Journal, vol. 2, pp. 23-29, April 2008.

[6] Burhanuddin Halimi, Pekik Argo Dahomo Syafri Martinious, "A Transformer Connection For multipulse Rectifier Apllications," IEEE, 2002.

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