A new approach to the study of surface discharge on the oil-pressboard interface

The materials used as the dielectric media of choice in large transformers continue to be a combination of pressboard and mineral oil. These materials offer a cost effective solution and are backed by a historical knowledge base of performance data and lifetime characteristics. A proven method to prevent inter-phase flashover is to use successive layers of pressboard and oil gaps. However, a number of failures in the field have been attributed to surface discharge or creeping discharge over the pressboard surface. This paper reports on an experiment to study surface discharge on pressboard in oil. A new technique for the generation of repeatable surface discharge is outlined. The paper explains the differences between surface tracking, and surface flash-over. The results indicate that the tracking is the result of surface discharge and that surface discharge and surface flash-over are two distinct phenomena.