Study on increase of electrostatic charging tendency of insulating oil and the diagnosis on streaming electrification

The investigation of electrostatic charging tendency (ECT) of new oil has been studied since the 1970's, but that of aged oil has never been studied. We studied the following three factors on ECT and methods to prevent the increase of ECT; (1) Effect of the increasing of ECT of new oil by aging. (2) The influence on new oil mixed with oxidized oil or aged oil. (3) The influence on aged oil by exposure to air. It was found that the ECT of new oil increased in the case of containing small amounts of oxygen and that the ECT of new oil mixed with aged oil increased. The ECT of aged oil exposed to air changed variously depending on the condition of oil. It is confirmed that benzotriazole (BTA) is effective to prevent the increasing ECT of aged oil. Streaming electrification of the transformers in service can be diagnosed by four measurement indices; (1) ECT, (2) leakage current from transformer windings, (3) partial discharge, and (2) supporting properties relating to ECT such as dissolved copper in oil. ECT is the most practical diagnosis method amongst the above four methods.