The dielectric strength and life of impregnated-paper insulation — IV

STUDIES of the causes and mechanism of the failure of impregnated-paper insulation are commonly obscured by the burning due to the final complete breakdown. In a number of years of study of the more accessible properties of impregnated paper and their relation to stability and breakdown, constant attention has been given to the restriction of this burning and a nearer view of the beginnings of failure. Simple fuses and even rapid circuit breakers in the primary circuit of the testing transformer are of no avail. Some improvement is obtained by the use of a thyratron in the grounded lead of the sample with suitable connection to a rapid circuit breaker in the primary. But by far the best results, and indeed very complete success, have been reached by a method for detecting the first occurrence of gaseous ionization within the test specimen. This method is a development of the pioneer work of Paine¹ and the subsequent applications of Arman and Starr and of Whitehead and Shaw,² using resonance methods for following the oscillations which arise in the ionization of gas spaces in the insulation wall. New features which have been added are increased sensitivity, improved balancing of the Schering bridge for the frequency of maximum oscillations, the use of greater amplification, and a sensitive recording instrument for registering the first occurrence of internal ionization. The complete setup, as described in this paper, is called the “ionization recorder.” With this instrument it is possible to detect the beginnings of failure in a completely impregnated specimen initially free of gas spaces or voids, often to within one layer of the impregnated structure, and also to follow its growth, with interruption at any desired stage on the way to complete breakdown. So far, the instrument has been applied only to the study of the influence of paper density on dielectric strength and stability, and the comparative behavior of two types of paper and two types of oil. From the results it will be seen that a very intimate picture of breakdown and its causes is presented.