Redundancy considerations for protective relaying systems

Abstract

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. What is RedundancyRedundancy is the existence of more than one means for performing a given function. Redundancy plays an important role for reliability.Reliability is a combination of dependability and security. Note that reliability denotes certainty of correct operation (dependability) together with assurance against incorrect operation (security) from all extraneous causes. Purpose of redundancyRedundancy is required for several reasons including governmental and regulatory requirements, reliability, maintain customer satisfaction, increase system stability, and for maintenance purposes.Because protective relaying provides no profit and is only required for infrequent and random abnormal operation of the power system, it can be described as insurance that prevents damage to the main grid equipment while minimizing outage time. Like all insurances the economics of the risks versus benefits are analyzed by utility managers and engineers. Larger utilities with abundant financial and technical resources research different protection schemes to determine the optimal balance between robustness and performance. However, there are different types of economic just...

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