TYP E CA-6 R ELAY
70
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l::i
TAPS
1 2
1 9
25
38
�50
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i
CHANGE SCALE
If
�40
�Q30
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. . ,
20
10
00
:
'
:
t
-
71 1
90
2
6
8
10 30
70
AtJI'ERES I N ffi
I
MARY OF AUX I Ll ARY TRANSFaMR
Curve 246086
Fig.
5.
Typical B urden Characteristics of The Operating Coil C ircuit
low , corresponding to high percentage unbalance , at
high current s.
The relay is made sensitive at low
currents in order that it will 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 rel ay for
external faults.
The
variable percentage characteristics are
particularly advantageous when severe saturation of
current transformers is caused by the
d-e
component
of asymmetrical short circuits. In the case of buse s
loc ated clos e to generating stations where the d-e
component decays slowly, the breakdown in ratio of
the current transformers will be 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 case. Taps
4
controlling the sensitivity of the relay are incorpor
ated in the external current transformer ( see Fig.
1 1
for internal wiring . ) The tap markings are :
12
-
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 amp s . ( secondary) or higher because ,
when a transformer bank is connected t o a high
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