Basler Electric BE1-87G - Manual (Page 11)

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 flowing
into the protected zone equals the current flowing output 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 relays typically trip a lockout (86) relay which in turn trips the generator breaker and, when
present, the field and/or neutral breakers.
APPLICATION
When used with current transformers (CTs) of an accuracy class of C20, T20, or better, BE1-87G relays
are recommended for the following specific applications:
•
Generators:
o Any terminal voltage and a rating of 1,000 kVA and above
o Any kVA rating and a terminal voltage of 5 kV and above
o A terminal voltage of 2,200 V or higher and a rating of more than 500 kVA
• Motors rated at 1,500 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 illustrated in Figures 1-1 and 1-2.
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 applied to terminals 7 and 8, 13 and 14, and 17 and 18 of three-
phase relays. Exercise caution when grounding or testing current transformer
circuits connected to these terminals.
9170800990 Rev N BE1-87G General Information 1-1