Testing of harmonic restraint relays with single frequency sources

Transformer differential relays with harmonic restraint have been traditionally tested using a diode half-wave rectifier. Electronic test sets which can provide a single frequency output are being used to test these relays using second harmonic current. Manufacturer´s instructions for testing these relays does not usually provide procedures or recommendations for testing with single-frequency inputs. An examination of the relay operating principles indicates that essentially the same results are obtained with either method. This paper discusses the type of relay which uses a fundamental frequency pass filter on the operate coil and a fundamental frequency reject filter on the restraint coil