DEVELOPMENTS IN STATOR WINDING PROTECTION OF LARGE GENERATORS

Abstract

Large size generators form a sizable percentage of system capacity and therefore their- availability needs to be ensured at all times by reducing outages. This is possible by a comprehensive reassessment of existing protection and monitoring methods necessitated by operating experience and advances in relay technology. One of the objectives of recent developments is to take corrective action before damage. A review of the developments in stator winding protection has been carried out during the course of the present work. The developments include digital relays for differential and interturn Y fault protections, a scheme to safeguard against accidental energization of 1a generator at standstill and an impedence relay as a back-up protection- to external faults. Certain monitoring methods detecting overheating, stator winding deterioration and vibration are also covered which provide adequate warning of faults at the incipient stages. Protection of the entire winding against earth faults is now considered necessary instead of the usual 90-95% coverage by overvoltage or overcurrent relays. Three methods are available based on subharmonic voltage injection and generator third harmonic voltages. These methods and their comparative performances have been examined here. Two digital relays, namely a 00% stator earth fault relay and a differential relay have •been implemented on a 16-bit microprocessor. An improved logic has been adopted for the protection of stator windings upto the neutral end. The relay uses a current monitoring feature to differentiate between internal and external faults and ensures high speed fault clearance (iv) for all internal faults. The differential relay uses digital filters to eli- minate harmonics from the measured current, signals. It has a variable bias which ensures high sensitivity on lightest internal faults and high stability on heavy external faults.