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Nl-
TOP
2-D-1487
Fig. 12.
Outline
&
Drilling P lan
ol
the A uxiliary Current TransFormer.
In cases
Ill,
V, VII, and VIII of Table I, at least
one pair of current transformers is paralleled. Al
most complete restraint cancellation will occur on
an external fault, if all but two of the circuits are
out of service and if the remaining circuits have
their current transformers paralleled. In selecting
the circuits to be paired, the following points
should be observed:
1)
Parallel circuits which are not likely to be
left alone on the bus.
2) Where sources are to be paired , parallel the
smaller sources, if there are less than
8
equivalent circuits .
3) Avoid p airing the two largest sources.
4)
If feasible , parallel matched current trans
formers.
In case IV, with two feeder groups one of the
restraint elements must be connected to 2 source
circuits. These 2 source circuits should be selected
so that they are the least likely to be left alone on
the bus and are not the two largest sources.
Where the nature of the bus application is such
that nearly complete restraint c ancellation is very
likely to occur for cases III, V, VII, and VIII of
Table I, two relays per phase should be employed to
insure restraint under all possible external fault con
ditions. Connections for 6 and
8
e quivalent circuit
buse s , using 2 relays per phase, are shown in Figs.
8
and
9,
respectively.
Where there are more than 8 e quivalent circuits,
the connections in Fig.
9
should be employed by
1 1
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