Qualification test results on multi-turn coils of large hydrogenerators

Multi-turn coil windings are widely applied in large hydrogenerators up to 150 MVA power output and voltages as high as 18 kV. A well manufactured, properly installed and maintained winding is subject to slow degradation processes resulting from electrical, thermal, mechanical and chemical stresses which are acting at various rates and locations on the entire winding. Several IEEE test standards are in place to determine the effectiveness of the insulation system of multi-turn coils to withstand these stressors and assess the quality of the insulation in the winding. This paper outlines the evaluation process for the groundwall or main insulation for 13.8 kV multi-turn coils for a hydrogenerator. The program covers diagnostic results consisting of dissipation factor, dissipation factor tip-up, partial discharge derived from thermal cycling and voltage endurance tests. The diagnostic results along with the dissection of the tested multi-turn coils are used to determine the degree and rate of degradation of insulation properties due to thermal and electrical aging.