Experience and Development in Non-destructive D-C Testing for Maintenance of High-Voltage Stators [includes discussion]

By the previously published technique1 of measuring insulation resistance at increasing direct voltages, the serviceability of the major stator insulation of 25 high-voltage rotating machines of various sizes, types, and ages has been appraised for maintenance purposes. In some cases, adverse conditions have been detected, always nondestructively. So far there has been no service failure of insulation shown by the test to have adequate strength. Thanks to the collaboration of several utilities, further information has been obtained from tests on condemned windings. Developments described relate to reduction of testing time, and improvement of instru-mentation, technique, and interpretation. Conditions favorable and unfavorable to reliable measurements are exemplified. The significance of electric strength as a criterion of serviceability is discussed.