Guide for Breaker Failure Protection Published

How do utilities handle backup protection? What are the advantages of using local backup over remote backup? How is breaker failure protection being implemented by utilities? What are the pitfalls in using the breaker auxiliary contact for breaker position? Where do you go to get these answers? Prior to 2005 no guide existed in applying breaker failure protection to answer any of these questions. The only previous article written by the PSRC was in 1982. An IEEE Power System Relaying Committee Working Group wrote a report titled "Summary Update of Practices on Breaker Failure Protection" and published it in the IEEE Transactions on Power Apparatus and Systems. Vol. PAS 101, no.3, pp 555-563. March 1982.

To answer these questions and provide a reference for engineers to for future generations, the IEEE PES PSRC Substation Subcommittee formed a working group to write a guide on this subject. This working group was formed in 2000. 62 industry experts from utilities, manufacturers, academia and government agencies put in many hours of effort in writing and reviewing the guide before it was published in 2005. The result is IEEE STD C37.119 Guide for Breaker Failure Protection of Power Circuit Breakers - 2005. The following are excerpts from the guide Remote versus Local Backup.

If remote backup is employed, then the time delayed overreaching element (Z2) of the relay at a remote substation operates for a fault on line BC when local breaker fails to clear the fault on a line. Operation of remote breaker interrupts the load connected downstream of it. If local backup is used, when a breaker fails, local breakers clear the fault. The load is not interrupted in this case on the line with the remote substation. Local backup provides faster clearing and less loss of load than remote backup:

In the basic scheme for breaker failure protection, as soon as the relay issues a trip to its breaker, a breaker failure timer is started. If the fault still persists after the timer times out, then a breaker failure condition is declared, and the breakers connected to the bus are tripped.

Breaker Failure Initiate (BFI) is the signal coming from the primary protection to trip the breaker. An overcurrent fault detector (50BF) is employed to determine if the fault is still present. 50BF will drop out if the breaker clears the fault. The guide discusses issues related to the setting and drop out of the fault detector (50BF). The timing for the scheme is seen below. (Fig. 1)

In EHV transmission systems, the total fault clearing time needs to be less then the power system critical clearing time plus some margin. The power system critical clearing time is a function of the steady state stability limit for the power system. Since the critical clearing time to maintain system stability is greater for single line to ground faults than three phase faults, some schemes employ dual timers. (Fig. 3) The 62-2 timer can be much longer than 62-1.

For lower voltage systems, the total clearing time is chosen to limit damage to equipment. Some faults such as transformer or reactor faults have such small currents that a fault detector can not be used. In those cases, a breaker 52b contact is used for indication of breaker operation. (Fig. 4) In some cases where there is a known problem with a breaker before it's called to trip, a bypass scheme can be employed. (Fig. 5)

In this scheme, if there is low gas pressure and the primary relay calls for a trip, the breaker failure scheme is bypassed and the surrounding breakers are tripped immediately. Other schemes are also discussed. A section of the guide deals with design considerations for the breaker failure scheme. Several factors need to be considered. Among them:

Scheme operation should only occur when expected and desired.
Scheme operation should be independent of the types of failures detected in the breaker. For example, the failure mode of the breaker trip coil should not effect the schemes ability to detect the failed breaker and to properly isolate it from the power system.
Scheme operation during loss of dc power to the failed breaker.
Sufficient overlapping of protection and isolation switches to allow maintenance and overall testing of the scheme.
Proper application of auxiliary tripping relays.
Selection of properly rated inputs and outputs when the breaker failure is integrated as part of the equipment protection package and when user selectivity in rating is provided.
Proper application of dc circuits and avoidance of mixing supply sources.
Minimizing the impact of dc transients.

Other sections of the Guide deal with factors that influence settings, communications based breaker failure schemes, and end to end testing.

The guide is available through the IEEE Standards Department