Ferroresonance in a Transformer Switched with an EHV Line

One transformer of a 1000-MVA 525/241.5-kV 60-Hz Y-connected bank of autotransformers, left connected to a 19-mi (30.5-km) 525-kV line but otherwise disconnected from the power system, was overheated after operating for nine minutes in ferroresonance sustained by capacitive coupling to a parallel, energized, 525-kV line. It was later untanked, tested and inspected, and no damage was found. However, to prevent recurrence of ferroresonance, the bank was modified to permit a set of existing 34.5-kV windings to be connected in delta. In similar situations where a Y-connected bank without tertiary windings is switched together with a 525-kV line paralleled by another 525-kV line, it was decided to disconnect the transformer from the line promptly after each de-energization of the transformer.