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Zabc

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Key point: generators only produce positive sequence voltages ; therefore only the positive sequence has a voltage source. 14

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� The positive and negative sequence diagrams for transformers are similar to those for transmission lines.

� The zero sequence network depends upon both how the transformer is grounded and its type of connection. The easiest to understand is a double grounded wye-wye

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� The first step in the analysis of unbalanced faults is to assemble the three sequence networks.

� To do further analysis we first need to calculate the thevenin equivalents as seen from the fault location.

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With a SLG fault only one phase has non-zero fault current we’ll assume it is phase a.

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With the sequence networks in series we can solve for the fault currents

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Example : In the following power system the value of fault impedance is j0.03 pu. Generators are running at no-load at rated voltage and in phase. All network data is expressed on a 100 MVA base. Find the fault current for SLG.

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Positive Sequence Network

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Negative Sequence Network

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Zero Sequence Network

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