A rapid ECT tester for transformer oil studies

A method for the measurement of the electrostatic charging tendency (ECT) of transformer oils using a sintered metal filter as the charge inducer has been developed. The apparatus is a modification of the previously described recirculating charge tester and gives the charge density in μC/m³ induced in the oil after passage at 100 ml/min through a 60 μm sintered metal filter. The new method uses fresh transformer oil as a reference, switched to flow through the filter before and after the introduction of each test-oil. The signal from the test-oil appears as a pulse on the background of the low level produced by the reference oil. Another significant change is that the test oils are taken directly from their sample bottles, thus ensuring rapid change-over time between samples. This feature has been adopted in naming the apparatus the rapid charge tester (RCT). The RCT was used to compare new and service-aged oils from large generator and transmission transformers on the NSW power system (Australia). The apparatus is described and the range of ECT values measured is presented with some discussion of their significance