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

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INSTA L L ATION
Westinghouse
I.L. 41-776.1A
•
O PER ATION
•
M A INTENANCE
INSTRUCTIONS
TYPE KC-4 OVERCURRENT RELAY
CAUTION Before putting relays into service, remove
all blocking which may have been inserted for the
purpose of securing the parts during shipment, make
sure that all moving parts operate freely, inspect the
contacts to see that they are clean and close properly,
and operate the relay to check the settings and elec­
trical connections,
APPLICATI ON
The type
KC-4
is a non-directional current or
fault detector which _?perates for all phase and ground
faults to supervise the tripping of other relays. It i s
well suited
to breaker-failure relaying for indicating
the presence or lack of current flow in the circuit
breaker, The relay can be applied where the phase
units are to be operated indefinitely in the picked up
po sition well below full load, Alternatively, where
the relay is to be used as a fault detector (pickup
above full load) the
98o/c
or betterdropout ratio of the
phase units is advantageous.
Breaker-failure relaying offers advantages over
remote back-up protection. It is faster and more sen­
sitive than remote back-up methods, In addition, it i s
selective. whereas remote back-up protection is
frequently non-selective.
Fig, 2 shows some fault
conditions where breaker-failure relaying could im­
prove the quality of back-up protection, Note that the
generating-station high-voltage bus uses a breaker­
and-a-half arrangement. Lines interconnect the station
to systems
81, 82,
and
8
3
.
If there is no malfunctioning, fault L will be
cleared by line relays tripping breakers
5,
6, and
9.
However, assume that the breaker-6 mechanism stick s
so that current flow through breaker 6 is not inter­
rupted, Now back-up protection must function, If re­
mote backup is relied upon , time-delay relays must
trip remote break ers
7
and
8.
In addition, the local
generator feed through breaker 6 must be interrupte d
by tripping breaker
4,
However, if break er-failure
SUPERSEDES I .L 41-776.1
*Denotes change from superseded issue.
protection is installed, the fault is cleared by trip­
ping breaker
3.
Note that this provided selective
tripping, since as much of the system as po ssible
was left intacL If breakers
4, 7,
and
8
must trip, the
local generator is lost and unn ecessary separation
of the generating station from power systems 8 1 and
82 results. Also, the tapped load is interrupted un­
necessarily instead of being left tied to system 8 ,
2
Remote backup, in addition to not being selective,
may not be sensitive enough because of the relatively
small proportion of the total fault current following
in any one line. For e xample, in Fig, 2 there may be
very little current flow in breakers
7
and
8
for fault
L because of the large current contribution by the
local machine s at the generating station, Thus, it
may be diffiC1llt or impossible at breakers
7
and
8
to
detect adj acent line faults without depending upon
sequential tripping, If the generator feed is interrupted
for fault L , such as by tripping breaker
4,
the current
through breakers
7
and
8
may increase sufficiently
for the relays to operate and trip breakers 7 and
8,
However, the system is by now cut to pieces, and
because of the long time delay in clearing the fault,
the remainder of the system may be unstable.
Although break er-failure protection offers many
advantages , remote backup cannot be completely
eliminated. For example , assume that breaker
3 fails
for bus-fault B in Fig, 2. Breaker-failure protection
will promptly trip breaker
2,
but the fault is still fed
by breaker 8. Likewise, if breaker 2
fails with a line
fault at M, a remote breaker must trip to clear the
fault. Breaker-failure protection trip s breaker 3, but
break er
8
still feeds the fault. Although breaker­
failure protection does not complete the job in these
e xamples, it does e xpeditiously trip the local breaker,
making it easier for the remote relays
to detect the
fault,
EFFECTIVE JAN UARY 1965
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