Overvoltages in Medium Power Transformers

Some suspected cases of transformer short circuit failure have been found to be due to overvoltages. Voltages up to 150 per cent of normal can exist during double line to ground faults. Fuses cannot protect against such an occurrence. A case is cited showing damage which occurred during a double line to ground fault on a transformer protected by fuses. Opening of supply lines-may also give rise to transformer overvoltage and overexcitation. A computer program for calculating such overvoltages is developed. Combinations of length of line and transformer MVA which result in damaging overvoltages are defined.