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41-337.3L
Figure 9:
External Schematic of the Type CA-16 Relays
for Protection of a Six Circuit Bus with Three
Feeder Groups.
4. CHARACTERISTICS
4.1.
CA-16 Bus Relay
This relay has variable percentage characteristics
which means that the operating coil current required
to close the relay contacts, expressed in percent of
the total restraint current, varies with the magnitude
of the restraint current.The relay sensitivity is high,
corresponding to a low percentage ratio, at light cur-
rents, and its sensitivity is low, corresponding to high
percentage unbalance, at high currents. The relay is
made sensitive at low currents in order to detect light
internal faults on the bus being protected. At the
same time, however, its reduced sensitivity at the
higher currents allows the various current transform-
ers involved to depart from their true ratio to a large
extent without causing false tripping of the relay for
external faults.
The variable percentage characteristics are particu-
larly advantageous when severe saturation of current
6
Courtesy of NationalSwitchgear.com
transformers is caused by the dc component of
asymmetrical short circuits. In the case of buses
located close to generating stations where the dc
components decay slowly, the breakdown in ratio of
the current transformers will be much greater than
would ever be expected from a consideration of the
usual ratio curves of the current transformers
involved.
The time of operation of the relays is shown in Figure
6.
4.2.
CA-26 Transformer Relay
The type CA-26 transformer differential relay
includes an indicating instantaneous trip unit (see
Figure 4), which operates on internal faults. The
instantaneous unit should have a setting equal to the
maximum rms symmetrical external fault current.
Such a setting will prevent operation of the instanta-
Sub 5
neous unit when a current transformer is severely
187A425
saturated by the dc component of an asymmetrical
external fault current.
4.3.
Trip Circuit
The main contacts will safely close 30 amperes at
250 volts dc and the seal-in contacts of the indicating
contactor switch will safely carry this current long
enough to trip a circuit breaker.
The indicating instantaneous trip contacts will safely
close 30 amperes at 250 volts dc and will carry this
current long enough to trip breaker.
The indicating contactor switch has two taps that pro-
vide a pick-up setting of 0.2 or 2 amperes. To change
taps requires connecting the lead located in front of
the tap block to the desired setting by means of a
screw connection.
4.4.
Trip Circuit Constants
Indicating Contactor Switch (ICS)
0.2 ampere tap
2.0 ampere tap
6.5 ohms dc resistance
0.15 ohms dc resistance
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