Failure of a circuit breaker to interrupt fault current
Circuit breakers are strategically located in power systems to connect circuits and electrical apparatus. Circuit breakers are commanded to open and close by protection and control systems that monitor conditions on the power system.
Protection systems are coordinated so that the circuit breaker(s) nearest the fault are opened to interrupt, or clear, the fault, with minimum impact to the remainder of the power system. This critical operation requires that the circuit breaker interrupt, or clear, fault current.
While infrequent, circuit breakers occasionally fail to trip, or fail to clear a fault. Depending on the power system network topology other circuit breakers must then be called upon to trip and isolate the sources contributing to the fault. Referring to Figure 1, assume a fault exists between breakers 3 and 4. Protective relays associated with breakers 3 and 4, designed to detect faults on the line between these breakers, operate and command breakers 3 and 4 to trip.
In this example, Breaker 3 fails to interrupt the fault current. Therefore, all sources that continue to supply fault current through Breaker 3 must be interrupted. Assuming sources at stations A and C, locally, breakers 2, 5, and 7 must be opened, or remotely, breakers 1, 6, and 8 must be opened.
To implement remote breaker failure backup protection for Breaker 3, the protective relays at breaker 1, 6, and 8 must have overreaching elements that sense faults anywhere on the line between breaker 3 and 4, and operate after a time delay, typically about 0.5 seconds. This time delay is required to allow time for the local line protection on breaker 3 to operate, and for the breaker to successfully clear the fault, recognizing that the local protective relay scheme on breaker 3 may include time delayed tripping to coordinate with other protective relays.
Local breaker failure protection, on the other hand, receives a signal directly from the line protection relays at the same station as the impaired breaker, indicating when the trip command is sent to the breaker. The local breaker failure protection only needs to wait for the breaker to successfully clear the fault. If local breaker failure protection for Breaker 3 detects a breaker failure, it commands breakers 2, 5, and 7 to trip to clear the fault.
Remote back up therefore has several disadvantages: First, all tapped loads between breakers, 1-2, 5-6, and 7-8 are dropped causing widespread customer outages. Second, the lengthy back up clearing time will cause excessive system voltage dip duration, additional damage to faulted equipment, possible damage to unfaulted equipment, and may lead to system instability. Third, due to possible infeed effects from the other lines, it may be difficult to set the relay at 1 to detect faults on the entire length of the adjacent lines. Fourth, the settings required at 1 to provide sufficient reach to detect faults out to the remote ends of the adjacent lines, may be so sensitive that the line is susceptible to tripping under heavy load during extreme disturbances, which could initiate or exacerbate a wide-area cascading outage. The advantage of remote breaker failure protection is that it is completely independent of protective relays, control systems, and battery supplies at the station with the failed breaker.
An alternative to remote back up is local backup, and its subset breaker failure relaying. Local breaker failure protection eliminates the disadvantages of remote back up protection.
Using the example in Figure 1 again, when the protective relay on Breaker 3 senses a fault on line 3-4 and issues a trip to Breaker 3, local breaker failure protection starts a timer. If the timer times out, and the fault is not cleared by Breaker 3, then the local breaker failure scheme sends trip signals to adjacent breakers 2, 5, and 7. The local breaker failure scheme has several benefits over the remote backup scheme. If there are sources at buses A and C, tapped loads between breakers 1-2, 5-6, and 7-8 can still be served. The total clearing time for the fault is reduced substantially compared to the remote back up method. The timer setting for the local breaker failure scheme is composed of breaker interrupting time plus some margin. Margins have been used from less than a cycle to 3 cycles as dictated by the critical fault clearing time.
Protective relay time is not included in this time delay. This timer setting is typically less than 12 cycles, compared to 30 cycles for remote back up. This time difference could mean the difference between a stable and unstable system for some critical faults and may substantially reduce the extent of damage at the fault.
The primary disadvantage of local breaker failure protection is that it may suffer from common mode failure. Station battery failure, for example, that may be the original cause of the breaker failure condition, may also disable the local breaker failure protection.
There are many things that may cause the failure of a circuit breaker to interrupt fault current. If a defective trip coil or trip circuit is the cause for the failure, slow clearing of the fault will not cause further damage to the breaker. However, some common causes for the failure to interrupt are that the breaker mechanism travel is incomplete or the mechanism is slow; components needed for the interruption: (resistors or capacitors), are faulty; or the dielectric material in the interrupter is out of specification( low pressure, low temperature) or contaminated. If these are the causes for the failure to interrupt, the breaker needs protection to prevent further damage.
By the time the remote backup protection has operated, the arcing inside the interrupter will likely cause a phase to ground fault internal to the breaker. These internal faults may lead to explosions and fires. As a result of the slow clearing of the original fault, what could have been a minor breaker repair project, if the faulty breaker had been isolated in a timely manner, now may require the replacement of the breaker and possibly other equipment in close proximity to the faulty breaker.
RESOURCE: Draft 7 Guide for Breaker Failure Protection of Power Circuit Breakers – IEEE