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Transformer PRV Operated with Differential
A 38MVA 20/11/11 KV transformer's PRV operated along with differential protection. But fault happened outside transformer. How PRV can operate for an earth fault outside transformer?

A 38MVA 20/11/11 KV transformer's PRV operated along with differential protection. It was obvious that an internal short circuit has happened. But during investigation, it was found that NSPB (non-segregated phase bus), which was connected to one of LV winding of transformer and falls within differential zone, was grounded through its heating cable. How PRV can operate for an earth fault outside transformer?

Though technically the PRV shall operate for internal faults within the transformer, it operates on the principle of expansion. hence how fast was the fault isolated, and the magnitude of these fault currents. Did it happen after an overhaul or major maintenance?