Dielectric-loss-measuring equipment for field and works testing

Abstract

The paper describes two new direct-reading methods which have been developed for the measurement of dielectric power factor, with particular reference to the field and works testing of high-voltage insulation. The two methods to some extent resemble the Schering bridge in that they incorporate a low-loss condenser of known capacitance and negligible loss angle, as the standard with which the insulator under test is compared, but differ radically from the bridge in the manner in which the comparison is effected.The essential feature of the first method is the production of a series of unidirectional voltage pulses of rectangular waveform and constant amplitude, and of the same frequency as the supply. The circuit is so arranged that the duration of each pulse, and, in consequence, the mean value of the resulting voltage, is directly proportional to the insulator loss angle δ. The final indication of loss angle (or, what is virtually the same thing since δ is small, the power factor sin δ), is either given directly on a calibrated voltmeter, or estimated approximately by visual examination of the pulse waveform on an oscillograph screen.The second method consists essentially in using the difference between two voltages, of equal amplitude but with a mutual phase difference equal to the insulator loss angle, as a measure of the power factor, the latter being indicated directly on a calibrated voltmeter. The particular point of interest about this method lies in the means adopted for the preliminary adjustment, which must be made to ensure that the amplitudes of the two voltages are exactly equal.