ABB CA-26 Instructions Manual - Page 6

Browse online or download pdf Instructions Manual for Relays ABB CA-26. ABB CA-26 16 pages. Percentage differential relay for bus protection class 1e applications
Also for ABB CA-26: Instruction Leaflet (16 pages)

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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

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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

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