A complex power station may have a large number of rights-of-way (ROW) with multi circuit power lines on each ROW. These circuits may be operated at different voltage levels. A fault current study for an L-G fault at each transformer voltage level should be produced. Each fault current study should be examined as follows:

a) If the vectorial sum of all zero-sequence fault current contributions to the transformer bus fault from all transmission and distribution lines entering the station under study is greater than the sum of all current contributions from all grounded sources at that station (including generators, grounded transformers, shunt capacitors, etc.), then at the voltage level for which the fault current study is presently being examined, the bus fault will usually produce a worse GPR than the line fault.

b) If the reverse is true, that is, the vectorial sum of the line contributions is smaller than the local ground source current sum, the line fault will produce a greater GPR.

This is because the local ground current will return partially, in the case of the line fault, through the station ground impedance, adding to the GPR caused previously by the line current contribution.

In the bus fault case, the current merely circulates through the faulted transformer winding, the station ground bus, and the fault impedance.

Having determined the worst-fault location (bus versus out on the-line), to select that fault current study with the highest fault current is not appropriate.

Variances between grounding networks of lines with the various voltage levels may, for instance, cause the study showing lower zero-sequence fault currents to result in a GPR greater than that caused by the higher currents. Instead, all faults should be investigated for fault locations as determined above.

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