Generator Stator Ground Fault Protection

Author: Charles Mozina, Consultant, Beckwith Electric Co., Inc.

This paper discusses the application of three different 100% stator ground fault protection schemes. Two third-harmonic methods and a low frequency-injection method are discussed. Third-harmonic schemes have been widely applied on generators within the U.S. to provide stator ground fault protection over the entire stator winding. In a number of cases, however, these schemes have been found not to be applicable. In many cases, these shortcomings were discovered during commissioning or when they operated improperly resulting in a false tripping of the generator. This paper discusses situations in which third-harmonic schemes will work and outlines the limitations of such schemes. It also discusses the use of low frequency injection sub-harmonic schemes which are gaining acceptance in the U.S.

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