WEIBULL STATISTICS IN SHORT-TERM DIELECTRIC-BREAKDOWN OF THIN POLYETHYLENE FILMS

A Weibull statistical analysis of breakdown voltages of thin polyethylene-insulated power cable slices is performed on large populations. Computation of confidence intervals implies that the statistically correct description is a three-parameter Weibull distribution, i.e. with a non-zero location parameter. It is shown that a data set described by a two-parameter Weibull distribution contains additional statistical dispersion factors which may or may not yield information on the insulation itself. In other words, a zero-location parameter always results from inhomogeneities in the sampling. These may be due to uncontrolled experimental parameters or to defects of various origins; but aging is also a source of dispersion of the electrical properties of dielectrics. To discriminate between the two, we proceed by comparative testing. When obtained under carefully controlled experimental conditions, the location parameter value can be considered a true quality factor of the system under test. The possible physical meaning of the so-called threshold may be envisaged once the breakdown mechanism at work under the conditions of the test is known. The statistical analysis of data collected in routine breakdown tests provides a very sensitive tool to investigate small changes in electrical insulation when performed on extensive data sets.