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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