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The effect of surge testing on the voltage endurance life of stator coils

Surge tests with short rise times are used to ensure turn insulation integrity in dry vacuum-pressure impregnated (VPI) coils. The use of this test migrated from the evaluation of low voltage random wound stators and fully processed generator bars. The dielectric properties of dry (unimpregnated) VPI coils are not fully developed. Application of a high voltage transient electric field in uncured or partially processed coils may initiate insulation damage. This damage may shorten the life of the insulation system after complete impregnation and cure. This paper presents a series of laboratory tests and finite element simulations examining the applied test voltage and number of applied pulses as factors in determining the risk of using surges as a manufacturing proof test on unimpregnated coils. Potential failure initiation sites can be linked to the test voltage as a more significant factor than the number of pulses applied. The sympathetic voltage response in a single coil as a function of location within a winding is described for a 20-pole stator undergoing green surge testing as part of a quality assurance program. Voltage endurance testing and sample dissection data are presented to show that the voltage endurance test life of the coils after complete processing is shortened by the surge test.