Diagnosis of transformer oils by the measurement of their electrostatic charging tendency

The accumulated data on electrostatic charging tendency (ECT) in power transformer oils is still not sufficiently large to make it clear if it can be used as a diagnostic method of reasonable reliability. In this study, various new and aged transformer oils as well as some aged oil mixtures were diagnosed in terms of their ECT levels by means of the spinning disc apparatus (SDA) and the data was cross-referenced with a filter method using the rapid charge tester (RCT). The SDA was adapted from a CIGRE design. The measured levels of both leakage current (SDA) and charge density (RCT) varied with oil type and condition. They were less distinct for new oils, however, clear differentiation appeared for aged oils. Generally, the leakage current values were correlated with the charge density values. The results showed that the spinning disc apparatus can discriminate ECT levels in transformer oils and that these levels could be used for diagnostic measurements of the oils. The prospect for establishing a correlation of ECT with various oil properties is considered