Analysis of ground protection of unit connected generators using third harmonics

On unit connected generators, ordinary zero sequence voltage relays used for stator winding protection against single phase ground fault can´t cover 100% of the stator winding, i.e. there is a dead zone at or near the generator neutral. With the increase of generator capacity, especially the application of inner water cooling technology, ground protection for 100% stator winding is required. Protection based on response to third harmonic voltage can reliably cover the dead zone of zero sequence voltage protection and together with it constitutes the stator´s 100% ground protection. Third harmonic ground protection can be realized in different ways. The authors deal with a protection scheme with additional compensation circuitry. It differs from the Kiskachi scheme in that amplitude and phase compensation circuitry are used so that the protection sensitivity is favorably improved. A comprehensive analysis of the scheme is made