Effects of Time and Frequency on Insulation Test of Transformers

Permanently grounded transformers mnust be given the insulation test by inducing the necessary voltage across the windings. The A. I. E. E. Standards specify that the time of induced voltage tests be the same as for high potential test. In certain cases where the transformers are of very large capacity the induced voltage must be made at a frequency several times higher than normal. Since the dielectric strength of most insulating materials decreases with an increase in the frequency, an investigation has been made to determine the proper and fair length of time to make induced voltage test when the frequency is higher than normal. Following are the main points brought out in the investigation, Above a certain voltage, time of voltage application as well as voltage causes failure of insulation. The dielectric strength can be expressed as a function of both time and voltage by an equation on of the form Kv. = A(a + 1-a/??T in which A is the kilovolts necessary to cause failure in one minute ``a´´ is a constant representing the ratio of strength for infinite time to the one minute strength and T is time in minutes. The value of ``a´´ varies for different materials apparently depending mostly on the dielectric loss.