Redundancy considerations for protective relaying systems

The basic concept of redundancy is simple. Instead of relying on a single piece of equipment, there are duplicate or triplicate sets that perform the same function. Consequently, if one piece of equipment fails, the function will still be performed by a redundant device. Redundancy of components plays a major role in elevating the reliability of protection systems. The impact on the power system when a protection device is not functioning when required is much less severe when there is a redundant device that takes over the job. If the redundant devices are of equal performance, there should be no detrimental effect at all on power system operations, and a non-functioning device would just need to be repaired or replaced. While local redundancy is generally applied, it is not the only mitigation that can be used to improve reliability. Remote protection systems may provide adequate protection system reliability in some situations, provided that remote protection can detect faults and provide clearing times that meet performance requirements. Different users have different terminology for referring to the redundant protection systems. They may be called “System 1” and “System 2” or “System A” and “System B” or sometimes “Primary” and “Backup.” This latter terminology, “Primary” and “Backup”, implies, although unintentionally, that one of the two systems serves the main function of protection and the other serves to assist in the case of failure of the first system, analogous to carrying an undersized spare tire in the trunk of a car in case of a flat. In actual practice, the redundant systems are each fully capable, each system is able to detect and clear faults on its own, and each system serves as a backup to the other. Note that this paper is a summary of the full report. The report will be available on www.pes-psrc.org – Published Reports – in May-June of 2010.