A-C and D-C Methods ror the Evaluation and Maintenance Testing or High-Voltage Insulation in Electric Machines

Before high-voltage insulation is put into practical service, numerous tests are carried out to determine its mechanical, electrical, and thermal properties. Of prime importance is the dielectric strength. This should remain constant for a maximum period of time and is determined by voltage-endurance tests carried out until a breakdown takes place.1-3 Other important characteristics such as air content, corona discharge, and degree of hardening of the insulation can be determined by non-destructive tests with alternating current. These test methods are therefore not only suitable for the evaluation of new insulation systems, but also for supervision during manufacture and, together with d-c tests, for checking the insulation of windings during service. 45 The descriptions given herein are based on practical experience.