Major factors influencing static electrification in an aged transformer

Increase in static electrification was experienced during the service operation of aged power transformers. The increase was noted a few years after on-site replace of the bushings and conservator. Analysis of the dissolved gas in the transformer oil suggested that slow intrusion of air, which could have led to the increase in static electrification, took place after the on-site work. However, the electrical characteristics of the oil did not demonstrate increase in static electrification. The investigation on the aged oil by ECT (electrostatic charging tendency) measurement suggested the possibility that some deteriorated compound in oil was adsorbed to the pressboard and increased the static electrification. However, the ECT measurement with the aged pressboard did not demonstrate enough increase, which suggested the necessity of the evaluation of the accumulated charge on the pressboard for assessing static electrification in transformers. At the static electrification tests with the model transformer using the aged pressboard and oil, strong acceleration effect of oxygen on the increase of static electrification was demonstrated. The rapid increase of the static electrification was succeeded by gradual decrease later. This suggested the possibility that yet unknown transient products by oxygen injection played strong role on the static electrification.