Risk in surge testing of turn insulation in windings of rotating machines

In normal operation, the thin turn (or interturn) insulation in windings of rotating machines experiences relatively small electrical stress. However, under certain conditions, e.g., switching operations, the turn insulation may experience very large electrical stress. The turn insulation, if weak, may fail under such conditions. Many of the groundwall failures in machines are suspected to have initiated from a turn insulation failure. Hipot tests are used to test the groundwall insulation. This often leads to suggestions that surges (with sub-microsecond rise time) should be used to test the turn insulation in machine windings. However, the detection of a turn insulation failure in a single coil in a tested complete winding is difficult. The consequent risks involved in surge testing of the turn insulation in a complete machine winding are emphasized and reiterated in this paper. Tests using both single surge generators and repetitive surge generators are illustrated.