Page 96 - Practical Power System and Protective Relays Commissioning

94 Practical Power System and Protective Relays Commissioning

FIGURE 7.9 Closing circuit of circuit breaker.

Also, it can be seen that the following conditions should be correct in
order to complete the closing order:
1. SF 6 pressure is correct.
2. Oil pressure for mechanism is correct.
3. There is no antipumping condition during the closing condition.
4. Breaker failure relay is not operated.
Refer Fig. 7.10 for the trip circuit of a CB.
As shown in the circuit in Fig. 7.10, the tripping can be done through
protection relay contact, or from the NCC, through pole discrepancy circuit,
or by a manual trip by the operator.
The trip should be done when the oil pressure of the hydraulic mecha-
nism is correct and the pressure of the insulating medium SF 6 is correct also.

7.6 CIRCUIT BREAKER TESTING

7.6.1 Contact Resistance Testing
This test is to confirm the resistance of the main contacts.
A 100 A DC current is injected through the main contact by keeping the
CB closed. The voltage drop across the contact is measured and the