In electrical power transformers, composite insulation consisting of oil-pressboard insulation is widely used. For optimum design of such insulation systems, partial discharge (PD) behaviour is an important design criterion. A computer-aided PD measuring system which permits a time sequential data acquisition and phase resolved pulse height analysis has been developed. Using this system, PD in a representative oil-pressboard system has been measured. The effect of stress level, their duration and the test procedure on the discharges have been studied. The discharge data is analysed using Gumbel and Weibull statistics. An attempt is made to correlate the Weibull and derived PD parameters of each phase with their counter-parts in the cycle.
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