Generator third-harmonic protection explained

Because of the nature of stator winding construction, insulation failure typically results in a ground fault. Other failures - such as interturn, interbranch, and series faults - if undetected usually evolve into ground faults. Since these failure mechanisms can occur on any portion of the stator winding, 100 percent ground fault protection of the stator winding is recommended. Various third-harmonic schemes have been used to provide neutral-side stator winding protection for high-impedance grounded generators. Applying such schemes in conjunction with a fundamental neutral overvoltage (59N) element, which protects the top 85-95 percent of the stator winding, provides ground fault protection coverage for 100 percent of the stator winding when sufficient third harmonic is available. This paper compares the performance of different third-harmonic schemes and shows how these schemes can be applied to provide secure and sensitive stator winding coverage.