Design and Analysis of Neutral Grounding Transformer for Hydro Alternators

Abstract

Power Generators of high KVA rating especially that of hydropower plant are very much prone to the ground faults. Stator ground faults are the most common winding failure in generators. During stator ground faults, short circuit currents flow from the damaged phase to ground through the stator core. Experience has shown that stator ground fault damages are proportional to phase-to-ground fault current as well as fault duration. For that reason, Generator Neutral Grounding must be applied in order to; I. Limit phase-to-ground fault current. II. Provide a means of stator ground fault detection. There are various generator grounding classes and types available. In this paper, high-resistance grounding has been chosen. High-resistance generator neutral grounding scheme based on a grounding transformer with a secondary resistor The advantage of the distribution transformer resistor combination is that the resistor used in the secondary is of comparatively low ohmic value and of rugged construction as compared to obtaining the same result by installing a highohmic, low-current resistor directly in the generator neutral. This research introduces some important and applicable practices which came from few years of practical as well as theoretical studies and discussions with some national and international power system experts. The research was made on the hydro power plants installed at Tarbela generating unit. The important parameters concerning the high impedance grounding of the generator were calculated. These results will be a kind of ready references for neutral grounding transformer design calculations and analysis. Keywords— NGT (Neutral Grounding Transformer), KVA Rating (Kilo Volt Amperes), Secondary Resistor, Tarbela Generating Unit.