APPLICATION

BASIC PRINCIPLES

Differential relaying is the most selective form of fault

protection that may be applied to the individual

elements, or zones, of ac power systems. Various

types of differential relays and relaying systems have

evolved to take advantage of the differential principle.

The BE1-87G is a single or three-phase solid-state

variable percentage differential relay designed to

provide selective, high-speed differential protection

for generators, motors, and shunt reactors.

The selectivity of differential relaying is based on the

ability of the relay to distinguish between an internal

fault (within the protected zone) and an external fault.

Under normal operating conditions the current into

the protected zone equals the current out of the

protected zone with a net operating current equal to

zero. Internal faults upset this balance and result in a

difference between the input and output currents.

External faults have relatively little effect on the

balance, because the current in still equals the

current out of the protected zone. Therefore, by

comparing the currents on both sides of the protected

element or zone and detecting when these currents

are not equal, a differential relay acts to isolate the

element or zone from the system.

The BE1-87G typically trips a lockout relay (86), which

in turn trips the generator breaker and, when present,

the field and/or neutral breakers.