Electrical Substations Control and Auxiliary Systems for Beginners by Boris Shvartsberg

Author:Boris Shvartsberg [Shvartsberg, Boris]
Language: eng
Format: azw3
Published: 2020-05-28T16:00:00+00:00

For better understanding, a Fig. 7 shows as an example a logic diagram for a breaker 2 BFPS operation during the fault SC-2 on line A1-B1.

Fig. 7. Logic Diagram of Breaker Failure Protection Operation in System Shown in Fig. 6.

1.7. Reclosing

Faults in power systems may be transient (temporary) or permanent. The example of temporary fault is a tree branch falling on the overhead wires and after that either going on fire or falling on the ground. In any case, the reason causing the fault disappears, but line remains de-energized, because line relay protection reacted on original fault and tripped breakers on both ends of the line. To avoid lengthy customer outages, we certainly would like to restore service to the customers and close the breakers if fault passed, but the problem is that we never know if fault is temporary or permanent. All we can do is to close breakers back after relay protection tripped them, hoping that fault is indeed temporary and breakers will not trip again. This attempt to close breakers after the fault is performed by a reclosing scheme, whose mission is:

• Expeditious return of a faulted power line back to service by reclosing of previously tripped circuit breakers (time delay 6-24 cycles, depending on the voltage level of the line)

There are the following kinds of reclosing:

• Successful: Fault is transient, and breakers remain closed

• Unsuccessful: Fault is permanent, and breakers trip again and stay open

Because faults on underground cables are almost always permanent, reclosing is applied only to overhead lines, where a chance of a temporary fault based on statistics is about 70%.

There are the following types of reclosing:

• Single shot – one reclosing attempt is made, usually on transmission lines

• Multi-shot – several reclosing attempts are made, usually on distribution lines

• Dead line – reclosing on the de-energized line

• Synchro-check – reclosing on the energized line when voltages on both sides of the open breaker are in phase

Let’s discuss the sequence in restoration of service for an overhead line with two breakers shown in Fig. 8.

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