Westinghouse KC-4, Installation, Operation & Maintenance Instructions Manual

Page: 70 / 178

T Y P E K C -4 R E LA Y
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BREAKER .. 2 FA I L U R E
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86BF2
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L I N E I
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T R I P CKT
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L I N E I
B A C K U P
T R I P C I( T
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B F T I M E R
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B K R 2
T R I P C K T
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B A C K U P
T R I P C K T
1459 Cl0
Sub 2
Fig. 2. External Connections for the KC-4 Relay - Breaker Failure Protection of a Single Bus/Single Breaker A
- One Timer per Bus.
rrangement
has been initiated. They energize the TD-5
breaker-failure timer (62) through the KC-4 (50)
contacts, which have already closed in response
to the same fault which operated the line relays .
I f the breaker interrupts fault current as expected,
the KC-4 drops out and the T D-5 timer is de­
energized before it times out; no further action
is taken. But if current flow continues beyond the
expected time limit, the T D-5 delay expires and
the output (T R) contact of the T D-5 energizes
the 62A auxiliary relay for contact multiplication.
62A contact energizes the 868 lockout auxiliary
to strip the bus and block reclosing.
N ote that, in figure 2, each breaker has its own
line protection relays, 8FI auxiliary and KC-4
current detector; but that all of these funnel into a
single T D-5 timer. This is done for ecomony since
the same tripping device, 868, is energized regard­
less of which breaker fails. Targeting of the parti­
cular failed breaker is accomplished by the ICS
in the associated KC-4 in conjunction with the 62X
and 62Y B F I contacts for that line. The zener
blocking diode inside each KC-4, connected to
4
terminal 50-2, isolates the m ultiple trip circuits
of the KC-4's from one another so that only the
target of the failed breaker will drop.
After the TD-5 has operated and picked up
62A, the 62A contact also energizes the 86BF
auxiliary relay. Refer to the primary breaker
tripping circuit. If a fault occurs and the breaker
operating mechanism is stuck (the most frequent
type of breaker failure), line relays energize the
trip coil with no effect. The breaker failure scheme
will operate the initiate backup tripping, elimin­
ating threats to power system integrity. H oweve r,
de current will continue to flow in the primary
relay ICS and in the breaker trip coil, uninter­
rupted by the stuck 52a contact. The trip coil, and
other devices in the tripping path, are only inter­
mittently rated and may disintegrate or ignite
before operators or repair crews can intervene.
For this reason, a normally-closed 868F contact
is shown in series with the trip circuit, to interrupt
de current flow.
After the T D-5 has operated, a sneak path may
be created through the supervising lamp for the
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