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Current differential relays are now widely used for the protection of transmission feeders. Simplicity of operation and the ability to clearly discriminate internal and external faults ensures their popularity, particularly when digital communication systems are available. Increasing availability of digital communication systems creates tremendous opportunities for protection but the move to switched hierarchical communication networks has created problems for phasor synchronization. The solution is the global positioning system (GPS). GPS provides time synchronization to an accuracy of plusmn1 mus, this relates to a 50-Hz differential phasor error of 0.04deg. Although the normal phasor error is insignificant, concerns remain about the temporary loss of the GPS time reference signal. The performance of the differential relay is investigated on the EMTDC simulator for the loss of the GPS time reference signal. This paper presents a technique to cope with such a scenario using symmetrical component analysis and discusses how an adaptive bias can be used to maintain system stability.
Date of Publication: Oct. 2008