I am modelling a single phase fault on a transmission line to obtain the GPR on the substation's grounding grid fence, but I have only the total fault current. What should I use for the current on the phases that are not faulted?
The current on a phase that is not faulted can deviate significantly from the value it has under load conditions, usually because of interactions in the transformer. If you had access to the detailed phase contribution information, you could assign it explicitly. Without this information, assigning the entire fault current to a single phase should give you the worst case scenario. (You should make sure to examine separately the case where each of the phase wire acting as the source of fault current, and use the worst of those.)
In this case, the remaining phases only carry currents that are "local" to the grid. These do not contribute much to the conductive part of the fence GPR, and should have a very small effect on the induced voltages. Leaving the load energizations in will introduce a small unbalanced current and will affect the induced voltages a little bit. Since this current doesn't have a path to ground, it will have no effect on the conductive part. Therefore, the net effect of this extra current on a small grid should be extremely small.
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