Insulation Tests of Transformers as Influenced by Time and Frequency

Many transformers are now being designed for service with one end of the high-voltage winding solidly grounded. These transformers require an overpotential test by induced voltage of either 2.73 or 8.46 times their normal line voltage above ground plus 1000 volts. These tests must be made at more than normal frequency, to avoid too high a flux density in the core, and also to reduce the power required for excitation. It has long been known that the breakdown voltage of solid materials was affected by the length of time of application of voltage. Likewise the frequency of the applied voltage is shown to affect the breakdown voltage for solid insulations, increase in the frequency resulting in a decreased breakdown voltage. The voltage required for creepage failure is shown to be relatively unaffected by frequency. From the results of the tests made it is concluded that induced voltage tests on transformers with graded insulation at higher than normal frequencies should not have the test voltage reduced, but should have the duration of the test shortened to make the severity of the test comparable to the test at 60 cycles on normally insulated transformers.