Using Zigzag Transformers with Phase-shift to …

1 Using Zigzag Transformers with Phase-shift to reduce Harmonics in AC-DC Systems H. K. H idalen, R. Sporild Abstract-- In large industry production facilities, twelve pulse rectifier units are often used for the conversion to DC-current. In order to improve the harmonic signature from the plant versus the grid, several transformer units are internally phase-displaced by means of combined couplings involving Z-arrangement. This paper outlines how such a transformer is modelled in ATP both as a Saturable Transformer component and as a coupled RL-matrix.

2 An actual industrial plant with 5 12-pulse rectifying units in parallel is modelled with 6 phase displacement. The analysis of the harmonic content of the supplying current shows that this arrangement is equivalent to a 60-pulse system only in the case of perfect symmetry in the system. Measurements show that the 3rd, 5th , 7th, 11th, and 13th harmonics easily exceed the theoretical 59th and 61st harmonics. Keywords: Saturable transformer, melting furnace, diode rectifiers, harmonics, Phase-shift , Zigzag , test report, ATPDraw.

3 I. NOMENCLATURE The Zigzag winding is characterized by the following parameters: U1 Voltage across the two winding parts U1z Voltage across the z-part U1y Voltage across the y- part n Ratio between U1y and U1z Phase-shift related to a Y-winding L1z Inductance in the z-part in [H] or [pu] R1z Resistance in the z-part in [ ] or [pu] L1y Inductance in the y-part in [H] or [pu] R1y Resistance in the y-part in [ ] or [pu] II. INTRODUCTION n the power supply to large industrial plants involving AC-DC converters, power quality is a problem.

4 The current on the AC-side contains harmonics of the order k n 1, where k is the number of pulses in the rectifying bridge and n=1, Filters on the high voltage AC side are expensive and alternative solutions are often beneficial. When several AC-DC converters are installed in parallel, power Transformers with different phase- shifts can be used to cancel out the H. K. H idalen is with The Norwegian University of Science and Technology, Dept.

5 Of Electrical Power Engineering, N-7491 Trondheim, Norway (e-mail: R. Sporild is with ABB AS Norway, Automation Technologies Division, 6540 Rodel kka, N-0501 Oslo, Norway (e-mail: Presented at the International Conference on Power Systems Transients (IPST 05) in Montreal, Canada on June 19-23, 2005 Paper No. IPST05 - 44 harmonic currents. In a particular industrial plant in Sunndals ra-Norway five 12-pulse AC-DC converters are installed in parallel with phase shifts -12 , -6 , 0 , +6 , and +12 , which results in an equivalent 60-pulse system.))

6 A Zigzag coupling on the primary side and two secondary sides with 30 degrees internal phase shift supplying a 12-pulse rectifying bridge is investigated in this paper. The modeling of Zigzag Transformers with the saturable transformer components in ATP [1] is reported in [2, 3]. In the present paper the analysis in [2] is extended to the case of an arbitrary phase shift of <-60,0> and <0,60> degrees based on [4]. III. MODELING OF Zigzag WINDING In this section the Zigzag winding is modeled both according to the saturable transformer component in ATP and as a more general coupled RL-matrix formulation.

7 A. Zigzag winding basics Two basic Zigzag couplings are used for negative and positive phase shifts as shown in fig. 1. a) Negative phase shift b) Positive phase shift Fig. 1 Investigated Zigzag couplings. For negative phase shifts the Zigzag winding of phase A consists of one part on leg I and one part on leg III in the opposite direction. The total voltage of phase A, U1 is shown in fig. 2. I Leg Z Y + U1z- - U1y+ I II III A B C Phase Z Y + U1z- - U1y + I II III Phase Leg A B C Fig.

8 2 Voltages of the Zigzag winding of phase A. Negative phase shift. The absolute values of the phase voltages of each winding part are )60sin()sin(11 ==zyUUn (1) (1) )60cos()cos(||||11 +=nUUz (2) )60cos()cos(||||11 + =nnUUy (3) with equal to the absolute value of the phase shift. The voltage vectors are D60111 +=nUUz and DD6016011 + =nnUUy (4) The same equations apply to the positive phase shift case. B.

9 The Saturable Transformer approach This section outlines how to model a Zigzag winding compatible with the saturable transformer model in ATP [1]. The short circuit and magnetizing characteristics are handled along with the zero-sequence behavior. The winding resistance, the leakage inductance, and the magnetizing impedance of the total Zigzag winding are now divided in two parts as illustrated in fig. 3. a) Short circuit b) Open circuit Fig. 3 Distribution of the short-circuit and open-circuit impedances.

10 Positive sequence, negative phase shift. The total short-circuit resistance and inductance as obtainable from measurements are yzRRR111+= and yzLLL111+= respectively as shown in fig. 3a). The winding resistance is proportional to the number of windings turns while the leakage inductance is proportional to the square of this number. Since the winding voltages Uz and Uy are proportional to the number of turns this giveszyRnR11 = and zyLnL121 =.