Testing and performance of transformer differential relays

The phenomenon of transformer magnetizing inrush current has been discussed in many papers throughout the years. A brief review of magnetizing inrush is given to provide the basis of need for transient testing. Previous studies have shown that inrush currents contain harmonic components of the fundamental waveform. Conventional philosophy has been to utilize the second harmonic current to provide inhibited operation of differential relays because of its dominance. Manufacturers have used various designs regarding higher order harmonics. The use of second and third, as well as the inclusion of all harmonics, have been seen over the years. The relay considered in this article utilized the second harmonic current to provide restraint on magnetizing inrush. The reasons for this are discussed. The authors discuss relay transient testing, EMTP transient testing, factory transient tests, EMTP testing facility, transformer differential EMTP tests, and EMTP transformer modeling. Several case studies relating to transformer differential relays are then discussed.