TYP E CA-6 R ELAY
Curve
246086
F ig.
5.
Typical B urden Characteristics of The Operating Coil C irc uit
low , corresponding to high percentage unbalance , at
high current s.
The relay is made sensitive at low
currents in order that it wil
l detect light internal
faults on the bus being protected. At the same time,
however, its reduc ed s ensitivity at the higher cur
rent s allows the various current transformers in
volved to depart from their true ratio to a large ex
tent without causing false tripping of the relay for
external fault s .
The
variable percentage characteristics are
particularly advantageous when s evere saturation of
current transformers is caused by the d-e component
of asymmetrical short circuits . In the case of buses
located clos e
to
generating stations where the d-e
component decays slowly, the breakdown in ratio of
the current transformers will b e much greater than
would ever be e xpected from a consideration of the
usual ratio curves of the current transformers
involved.
CHARACT E RISTICS
There are no taps inside the relay cas e . Tap s
4
controlling the s ensitivity of the relay are incorpor
ated in the external
current transformer (see Fig. 1 1
for internal wiring. ) The tap markings are: 1 2 - 19 -
25 - 38. These tap values , as indicated on the tor
que curves of Fig. 3 , are the 60 cycle amperes re
quired in the operating coil circuit to close the relay
contacts against a 70 cmg. restraint torque.
Time of operation of the relay is shown in Fig.
4. To us e this curve, determine the total restraining
torque and substract this from the operating coil
torque to determine the net torque in cmg.
This
curve applies only for a contact spacing of 1/4 inch.
TRANSFORMER RELAY
The type CA-6 transformer differential relay in
cludes an indicating instantaneous trip unit (See
Fig. 2) and has a stronger spiral control spring than
the bus differential relay.
The instantaneous trip
unit is int ended to operate on internal faults on the
order of 100 amps. ( secondary) or higher because,
when a transformer bank is connected to a high
www
. ElectricalPartManuals
. com
