The fundamental neutral over-voltage relay 59GN commonly used in generator protection does not respond to grounds in the bottom ∼5% of the stator winding. Recent experience with four such failures to large generators suggests that this lack of protection can be a disastrous deficiency. Each of the four failures discussed in this paper caused massive damage to the generator, and collectively had a total cost, including repair and loss of generation, close to $500,000,000.
This paper describes those four failures, and discusses newer ground relay systems that monitor and protect the entire stator winding for grounds. The objective of this paper is to encourage owner/operators to add one of the newer relay systems.