TRENDnet BE1-87G - Manual (Page 6)

BE1-87G - General Information 1-1
WARNING
Relays manufactured prior to July 22, 1991 (EIA date code symbol 9129 and previous)
do NOT have case jumpers between terminals 7 and 8 (single phase units). This also
applies to three phase relays terminals 7 and 8, terminals 13 and 14, and terminals 17
and 18. Exercise CAUTION when grounding or testing current transformer circuits
connected to these terminals.
SECTION 1 • GENERAL INFORMATION
DESCRIPTION
BE1-87G Variable Percentage Differential relays are single- or three-phase solid state devices designed to
provide selective, high-speed, differential protection for generators, motors and shunt reactors.
Differential relaying selectivity is based on the ability of a 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 protected zone input current still equals the output current.
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 with
unsurpassed effectiveness.
BE1-87G Variable Percentage Differential relays typically trip a lockout relay (86) which in turn trips the
generator breaker and, when present, the field and/or neutral breakers.
APPLICATION
BE1-87G Variable Percentage Differential relays are recommended for the following specific applications
when used with current transformers (CT) with an accuracy class of either C20 or better or T20 or better.

Generators: any terminal voltage and a rating of 1000 kVA and above.

Generators: any kVA rating and a terminal voltage of 5 kV and above.

Generators: a terminal voltage of 2200 V or higher, and a rating of more than 500 kVA.

Motors: rated 1500 horsepower and above.

As primary protection on shunt reactors for transmission lines.

Generator ground differential
Differential relaying is the most selective form of fault protection which 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.
Typical application schemes are shown in Figures 1-1 and 1-2.