Practical considerations concerning impulse voltage testing of form-wound coils for rotating machines

The authors review the response of the windings of high-voltage AC rotating machines to system-generated transient voltages and also impulse test techniques and withstand levels of form-wound stator coils. Design tests on coils with a vacuum pressure impregnation (VPI) insulation system have confirmed very high turn-to-turn insulation strength well above actual system transient voltages. Because of this and since the amplitude of line-to-ground overvoltages to which windings are exposed in service can be controlled to some extent, there appears to be no need for impulse testing of complete machines. Also, the line-to-ground withstand capability of a winding would be adequately verified by the standard AC applied potential test. If, under special conditions, the actual turn-to-turn insulation strength must be demonstrated experimentally, such tests should be performed on additionally fabricated sacrificial coils which are manufactured with and identical to those in the winding