ageing_3 - IEEE Transactions on Electrical Insulation Vol....

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IEEE Transactions on Electrical Insulation Vol. EI-20 No.2. April 1985 303 WEIBULL STATISTICAL ANALYSIS OF DIELECTRIC BREAKDOWN IN N-HEXANE C. J. Frei Section de Physique Universit6e de Gene've Geneve, Switzerland ABSTRACT A much-refined experimental technique and analysis of measurements of dielectric break- down in hydrocarbon liquids, in particular: n-hexane, has shown that the time lag distri- bution of an unperturbed liquid is governed by a Weibull probability function. Among di- electric liquids stressed at a constant elec- tric field this is the first account of the applicability of the statistics of extreme values. This result and the field dependence of the breakdown probability are to be con- sidered as strong arguments in favor of the recently postulated theory of electrical breakdown based on fluctuations of the elec- tric field in solid and liquid dielectric materials. INTRODUCTION of the discharge current. The underlying Gaussian statistics is convoluted with the statistics of the During our previous experimental investigations of basic process. The novelty of our observation was the the breakdown process in liquids we have been subject- occurrence of a maximum at much longer time scale and ing the test sample to a constant voltage and then its dependence on whether the dielectric liquid was have measured the time intervals during which the moving or stationary. At a later stage, we noticed specimen is able to hold the applied field. These that the decisive feature of the experiment was not time intervals vary stochastically. Their distribu- the liquid being in a state of motion, but the fact tion, in a general manner, followed an exponentially that the dielectric liquid has been perturbed since the decreasing law. Deviations from this behavior, how- preceding voltage application [2]. It then became ever, could occur in several ways: on a logarithmic obvious that the simple idea of explaining breakdown plot where a decreasing exponential function is dis- by assuming a Poisson process based on a constant played as a straight line with a negative slope the probability of initiating a discharge had to give way measurements occasionally showed up as a curve which to a more complicated description. was bending upwards or downwards. An upward bending can easily result form the superposition of several We interpreted the new findings by assuming that the exponentials with different characteristic times. A liquid dielectric must go through an activation phase vna lsryingdifferent characteristic timewouldap as. A cin order that the probability of breakdown raises to varying characteristic time would appear as a conse- thcurnvaes Aprubtinotekndm- quence of slight instabilities of the measuring system the current values. A perturbation
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ageing_3 - IEEE Transactions on Electrical Insulation Vol....

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