Curve
471108
Fig. 7 Typical Trip Phase Angle Curve for the CN33P Relay- Low Current Values.
zero, the moving contact will move away from the
stationary CLOSE contact and take up a position
somewhere between it and the stationary TRIP
contact.
When the feeder breaker opens, the transformer
will be magnetized from the network.
This flow of
exciting current from the network to the transformer
bank will cause enough current to flow in the current
coils in the type CN33P relay to produce a tripping
torque sufficient to close the TRIP contacts. This
energizes the BN relay which will trip the breaker
when its timer contacts have closed.
The timer
contacts are shunted by the instantaneous triP con
tacts which close when the reverse current exceeds
their setting.
Fig. 4 shows the phasing coils short circuited
through a resistance by
52ri
contacts when the
breaker is closed.
This circuit is used to obtain
the desired tripping phase angle characteristic.
8
CHARACTER ISTICS
Figures 5, 6 & 7 show the operating character
istics of the type CN33P primary network relay for
balanced 3 phase conditions.
Fig.
5
shows the
closing characteristics of the relay. For curve #lA,
lines drawn from the origin at various angles with
respect to the· network voltage represent both in
magnitude and phase position the transformer voltage
which will produce a torque in the relay just suffi
cient to cause its CLOSE contacts to make.
Any
transformer voltage which does not terminate on or
above the closing curve will produce a relay torque
in the tripping direction. Curve #lA in fig.
5
shows
a small section of the closing curve plotted to a
much larger scale so as to show the characteristics
of the relay for the values of phasing voltage at which
it normally operates.
Lines drawn from the origin
to this curve represent in magnitude and phase po
sition the phasing voltage (the voltage across the
open breaker) necessary to produce a torque in the
relay just sufficient to make its CLOSE contacts.
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