Generator Ground Fault Protection Using Overcurrent, Overvoltage, and Undervoltage Relays

This paper describes the protective relaying schemes employed by Georgia Power Company to protect synchronous generators from single-phase- to-ground faults. Three types of relays are connected in the secondary of a distribution grounding transformer. These include a conventional electro- mechanical overcurrent relay with time overcurrent unit and instantaneous overcurrent unit, a solid- state overvoltage relay (with timing module) tuned to reject frequencies near 180 hertz, and a solid-state undervoltage relay (with timer) tuned to reject frequencies near 60 hertz present at the generator neutral. The proper method of selecting the generator grounding components and protective relays is described, and detailed setting instructions for all relays are also included.