Method to observe the change of charges in an electric double layer between pressboard and the insulating oil in a power transformer

Space charge is one of the important causes of electrical breakdown. It has been necessary to sample the oil in an oil-cooling transformer to establish how charged it is. A method to detect the change in accumulated charges on a pressboard in an oil transformer is suggested. The change in the level of charge relates to the charges in the oil, so the charges in oil and on a pressboard can be observed without sampling the oil by using this method. An electrometer, a high input impedance voltage meter, and an oil circulation system were used to detect the change. When the oil went through the pressboard pipe, it was charged positively due to the streaming electrification. The voltage change on the surface of the pressboard was measured by a high input impedance voltmeter and the current from the surface by a micro-ammeter. From these data, the change in static charge is known.