In-service winding failure of newly installed replacement stator

Installing a new or replacement generator requires careful consideration during installation, offline acceptance testing and increased diligence during initial start-up and operation. After replacing the first of four 132 MVA, 15 kV, 85.7 RPM Allis-Chalmers hydrogenerator stators, Chelan County PUD personnel were confronted with an unexpected distribution of generator stator RTD temperatures during the initial unit start-up. A correlation was established between the location of the higher stator winding RTD readings and the region of overheating shown on a continuous infrared thermal imaging display provided by data from a permanently installed array of fast-response, infrared sensors mounted on the rotor. The thermal map pattern provided a classic indication of circulating currents in a phase circuit. This condition is often seen in windings that have cutout coils. The generating unit operated in this condition for fewer than three weeks before a coil-to-coil jumper failed causing a severe arc that finally went to ground, tripping the unit on a ground fault.