Modeling Of A Zig-zag Load Transformer: Etap Knowledge Base

ETAP Knowledge Base

Modeling of a Zig-Zag Load Transformer Description: How to model a zig-zag load transformer in ETAP Category:

Transformer, Zig-Zag Load Transformer

Type:

Modeling

Keywords:

Zig-Zag Load Transformer

This article explains how zig-zag load transformers may be modeled in ETAP. The model is developed for the zig-zag transformer scheme shown in below figure:

Figure 1: Zig-Zag Load Transformer Model Concept The operation of zig-zag load transformers, under balanced conditions is similar to the operation of any other load transformer. When there are equal third harmonic or zero sequence currents flowing into the terminals A4, B4, C4 in Figure 1 they produce no net magnetizing effect, since they flow in opposite directions in the two windings on each core; that is the third-harmonic or zero sequence component of the phase A current flows in the direction A4-A3, while an equal and in-phase third-harmonic or zero sequence component of the phase B current flows in the opposite direction B1-B2, and, since windings A4-A3 and B1-B2 are on the same core, the net third-harmonic magneto-motive force acting on the core is zero. This essentially means that there will not be any zero-sequence or third harmonic current flowing on the star connected secondary winding. For example, when there is a single line to ground fault on the left of the terminals A4, B4 or C4 zero sequence currents will flow in the windings A1-A4, B1-B4, C1-C4, however there will be no zero sequence current flowing in the windings a1-a2, b1-b2, c1-c2.

This document is confidential and proprietary to Operation Technology, Inc. and may not be reproduced, published or disclosed to others without the written authorization of Operation Technology, Inc., 17 Goodyear, Suite 100, Irvine, CA 92618, USA. © 2010 by Operation Technology, Inc. All Rights Reserved.

© Operation Technology, Inc.

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Modeling of a Zig-Zag Load Transformer Similarly, when there is a single line to ground fault on the right of the terminals a2, b2 or c2 zero sequence currents will flow in the windings a1-a2, b1-b2, c1-c2 which will induce zero sequence voltages in corresponding windings on the zig-zag side. However for reasons explained above there will be the net zero sequence voltage on each phase will be zero, which means there will be no zero sequence current flowing in the zig-zag windings. The zero sequence diagram of such a transformer is shown below:

Zg is negligible, 3Zg ≈ 0 The above diagram shows that for a single line to ground fault on the Star-grounded side, there is no fault current flowing to the Zig-zag side. This is similar to the case of Star-ground-Delta transformer. Hence in ETAP, the equivalent model is shown in the following section.

Equivalent Model Assuming that the ratings for zig-zag transformer T1 are: Primary Voltage Secondary Voltage kVA Rating Pos. Seq. Impedance (%) Zero Seq. Impedance (%) X Over R Connection

: 33 kV : 415 V : 150 kVA : Z1 : Z0 : X/R : ZNyn1

This document is confidential and proprietary to Operation Technology, Inc. and may not be reproduced, published or disclosed to others without the written authorization of Operation Technology, Inc., 17 Goodyear, Suite 100, Irvine, CA 92618, USA. © 2010 by Operation Technology, Inc. All Rights Reserved.

© Operation Technology, Inc.

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Modeling of a Zig-Zag Load Transformer

Figure 2: ETAP Model of System in Figure 1

Transformer ID Primary kV Secondary kV Rated kVA % Positive sequence Impedance % Zero sequence Impedance X Over R Primary Connection Secondary Connection Phase Shift

T1 33 0.415 150 Z1 Z0 X/R Star – Solid Grounded Delta Special (-30 deg)

This document is confidential and proprietary to Operation Technology, Inc. and may not be reproduced, published or disclosed to others without the written authorization of Operation Technology, Inc., 17 Goodyear, Suite 100, Irvine, CA 92618, USA. © 2010 by Operation Technology, Inc. All Rights Reserved.

© Operation Technology, Inc.

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