Predicting Insulation Failures with Direct Voltage

A scientific study is presented of the characteristic effects of direct and alternating voltage stresses on composite insulation systems designed for alternating voltage service. It shows some of the basic differences between these two kinds of stresses, deals with a few of the limitations of the direct-voltage method of testing composite insulation designed for alternating voltage service, and describes a new mechanism of ionization, probable damage, and breakdown of such insulation under these stresses. Tests were carried out on a large number of 12 different kinds of sheet insulation samples designed either to simulate rotating machine insulation or to yield pertinent information. Equipment used in these tests was specially designed and it comprised an assembly of ten pairs of circular brass electrodes and a lightproof box containing one electrode pair and a scintillation probe for ionization detection. The latter was used as a new method for studying ionization initiation, severity, location, and spread inside the samples.