Page 95 - Practical Power System and Protective Relays Commissioning

Circuit Breakers Theory Testing and Commissioning Chapter | 7 93

initiating a close order (manual close) then a repeated operation of the
trip and close cycle will occur, which can damage the CB, therefore there
is a relay to open the closing circuit during the trip signal only, called the
antipumping relay. This can happen also if the contacts of pushbutton
jammed in closed position.
4. Trip-free CB feature
If the breaker is able to receive a trip signal and to trip before a clos-
ing operation is completed then this breaker is called a trip-free CB.
5. Antislamming circuit
When a CB is in a closed position and receives a close order this can
cause slamming of the CB mechanism. The antislaming circuit open the
d.c of the closing circuit of the CB when the CB is in the closed position.
6. CB pole discrepancy tripping circuit
If for any reason, mechanical or electrical, the three phases of the CB
are not fully closed or all fully open at a given time, then a three-phase
trip is issued for the CB phases.
7. CB low hydraulic oil pressure alarm and lockout circuit
The purpose of this circuit is to give an alarm when the breaker has a
low hydraulic system oil pressure and indicates that it is necessary to
operate the pump to raise the oil pressure. If the oil pressure continues to
decease below a certain level then a block for trip and close is issued by
this circuit to prevent the CB from operation.
8. CB hydraulic pump fault alarm circuit
9. SF 6 CB low SF 6 gas pressure alarm and lockout circuit
If the pressure of the insulating medium gas (SF 6 ) is low then this cir-
cuit issues an alarm. If the pressure continues to decrease below a certain
level then this circuit issues a three-phase trip signal for the CB and locks
out its operation closing and opening, and sends a trip signal to the
remote end of the high-voltage transmission line to trip the remote end
breaker.
Refer to Fig. 7.9 for CB closing circuit.
As shown in Fig. 7.9, the operator can close the breaker from three differ-
ent locations:
1. Local, at the switchgear;
2. Remote, at the control panel in the control room or control desk com-
puter in the control room in new substations;
3. From National Control Center (NCC) (Dispatch Center) to the master
station.
Only one close order can be issued at a time—this order is done through
manual synchronizing or through auto synchronizing relay.
It is shown from the circuit that autoreclosing can also be done but
through auto synchronizing relay.