Percentage Restraint

A primary concern in differential relay applications is security against high current levels caused by faults

outside the protected zone. Inevitable differences in the saturation characteristics between current

transformers require a compensating decrease in relay sensitivity. It is also necessary to be able to adjust

the sensitivity to compensate for transformer voltage taps or CT mismatches. This is accomplished by

providing a restraint factor proportional to the current flowing through the protected zone (through

current).

BE1-87T relays maintain sensitivity at a specified ratio of trip current to through current. This ratio,

generally referred to as slope, is front-panel adjustable in 5 % increments from 15 to 60 %.

Fifth-Harmonic Restraint

Power transformer overexcitation causes additional exciting current to flow into one set of terminals. This

presents an apparent differential (or operating) current not attributable to an internal fault. Although

potentially damaging, overexcitation is not an internal fault and, therefore, is not an appropriate condition

for transformer differential relay operation. One of the principal components in the complex waveform

produced during overexcitation is the fifth harmonic. BE1-87T relays use fifth-harmonic restraint to inhibit

the differential relay operation.