Report on design, testing and commissioning of 100% Stator Ground Fault Protection at Dominion's Bath County Pumped Storage Station

Abstract

Cradled in Virginia's rugged Allegheny Mountains, the Bath County Pumped Storage Station, which went into operation in 1985, is jointly owned by Dominion and the operating companies of the Allegheny Power System, and managed by Dominion Generation. The facility, which includes six rotating machines, each operating as a synchronous generator or synchronous motor, quietly balances the power demands of homes and businesses across six states. The scheduled operation of the units is based on grid demand and changes daily. Typically, during peak demand hours, several machines will be operating as generators. Conversely, during non-peak hours, several machines will be running as motors pumping thousands of gallons of water back to the upper reservoir some 1200 ft above the powerhouse. The machines have a rated voltage of 20.5 kV which is stepped-up to 500 kV for transmission. The machines are high-impedance-grounded through single-phase neutral grounding transformers. Several years ago the plant began upgrading their protection systems for each of the six machines. The upgrade consisted of replacing single-function electromechanical relays with redundant microprocessor-based generator protection relays. 100% Stator Ground Fault (SGF) protection (ANSI # 64S) was one of the primary enhancements made to the protection. This paper discusses in detail the design, testing and commissioning of a 20 Hz signal injection 100% SGF protection system. The advantage this system offers over all others is the ability to detect ground faults in the stator windings and in the Iso-phase bus prior-to and during start-up of the machines. By using the sub-harmonic signal injection, the measurement of the complex ground impedance of the stator windings can be done independently of generator/motor operating modes.