Effects of time and frequency on insulation test of transformers

Permanently grounded transformers must be given the insulation lest 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 of the form
$Kv. = A(a + {1-a \over \root {1} \of T})$
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. The breakdown by creepage over solid insulation with the electrodes either on the same side or on opposite sides of the sample (arranged in such a manner that the solid insulation is under considerable stress) is not affected by time nearly so much as is the puncture voltage of solid insulation. The behavior of oil without barriers is so erratic that no very definite relation can be obtained between time and dielectric strength. In general time decreases the strength quite rapidly for the first few seconds after which the effect decreases and probably disappears entirely after two or three minutes. The momentary strength ranges from 25 to 30 per cent higher than the one minute strength. The effect of time on the strength of solid insulation and oil in series is about the same as for solid insulation alone until the oil distance exceeds the solid insulation thickness after which it begins to be the same as for oil without barriers. The strength-time curves for solid insulation are of approximately the same shape for all frequencies from 60 to 420 cycles, although the strength decreases with an increase in frequency F approximately in accordance with the formula kv. = 1.75/ F
^0·137
Failure by creepage over the surface of solid insulation which is under no stress (i. e., with the electrodes on the same side of the barrier) takes place at approximately the same voltage for all frequencies from 60 to 420 cycles; but if the electrodes are so arranged (on opposite sides) that the insulation is under considerable stress the failure voltage decreases with an increase in frequency in about the same order as the puncture voltage of solid insulation does. The rupture voltage of oil is the same for both 60 and 420 cycles. The effect of frequency on the puncture voltage of solid insulation and oil in series is the same as for solid insulation until the oil distance exceeds the thickness of solid insulation, after which the effect decreases and as the oil distance increases the effect approaches that for oil without barriers. By considering the effects of both time and frequency on dielectric strength it is shown how to determine the proper length of time to make the voltage strain at higher frequencies equal to the strain at 60 cycles for one minute. 8. ACKNOWLEDGEMENTS It is desired to acknowledge the valuable assistance rendered in the Testing Department by Messrs. N. M. Albert, H. L. Garver, C. F. Green, L. D. Martin and W. F. Weikel in carrying on the long and tedious tests, covering a period of three to four months.