Second-Harmonic Restraint

Magnetizing inrush current presents another problem unique to transformer differential relays. Relays

must be capable of detecting the small differences in current caused by the shorting of a limited number

of turns, yet remain secure against the occurrence of magnetizing currents many times the transformer

rating (as seen at one set of terminals).

Although magnetizing inrush is usually associated with the energizing of the transformer, any abrupt

change in the energizing voltage may produce this phenomenon. Common causes are the transients

generated during the onset, evolution and removal of external faults. Desensitizing the relay only during

energization is therefore insufficient.

Magnetizing inrush produces an offset sine wave rich in all harmonics. BE1-87T relays use the second

harmonic to restrain operation because it predominates and because it does not occur in significant

magnitude or duration at other times.

Three-phase BE1-87T relays use second-harmonic sharing. The second-harmonic content of all three

phases is summed together to derive the restraint for each phase. As a result, the second-harmonic

inhibit range and the associated factory setting, is higher than on single-phase relays.