Chapter 4. Fault Current Calculations

Chapter 4. Fault Current Calculations - 4. FAULT CURRENT...

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4. FAULT CURRENT CALCULATIONS Summary Fault currents at the terminals and in the windings are calculated for transformers subjected to the standard faults: 3-phase line to ground, single phase line to ground, line to line, and double line to ground. Since these faults result in unbalanced currents in the 3 phase system, except for 3-phase faults, the method of symmetrical components is introduced and used in the fault analysis. In this method, the unbalanced voltages and currents are replaced by balanced systems of positive, negative, and zero sequence quantities. The circuits associated with each of these sequences can differ. After solving the sequence circuit equations, the unbalanced quantities are obtained by a reverse transformation. A 2-terminal per phase transformer is modeled using a single leakage impedance and a 3-terminal per phase unit is modeled with 3 leakage impedances (T-equivalent circuit). The leakage impedances are the same for the positive and negative sequence circuits but can differ for the zero sequence circuit. The systems attached to the external terminals of the transformer are treated simply as a voltage source in series with an impedance. An asymmetry factor is included to account for an initial transient surge which usually accompanies a fault. 4.1 INTRODUCTION It is necessary to design transformers to withstand various possible faults, such as a short to ground of one or more phases. The high currents accompanying these faults, approximately 10 to 30 times normal, produce high forces and stresses in the windings and support structure. Also, depending on the fault duration, significant amounts of heat may be generated inside the unit. The design must accommodate the worst case fault which can occur from both the mechanical and thermal standpoints. The first step in designing to withstand faults is to determine the fault currents in all the windings, which is the subject of this report. Since this is an electrical problem, it requires a circuit model which includes leakage impedances of the transformer and also relevant system impedances. The system is typically represented by a voltage source in © 2002 by CRC Press
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FAULT CURRENT CALCULATIONS 120 series with an impedance, since we are not interested here in detailed fault currents within the system external to the transformer. The transformer circuit model considered here is that of a 2 or 3 terminal per phase unit with all pairs of terminal leakage reactances given either from calculations or measurement (from these the T-equivalent reactances can be obtained). We ignore core excitation since, for modern power transformers, its effects on the fault currents are negligible. The transformers dealt with here are 3-phase units and the fault types
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This note was uploaded on 10/19/2010 for the course ENGINEERIN ELEC121 taught by Professor Tang during the Spring '10 term at University of Liverpool.

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Chapter 4. Fault Current Calculations - 4. FAULT CURRENT...

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