Power Transformers and Corona Testing

There is an increased interest among transformer users in radio influence voltage measurements during routine commercial testing. This paper describes a procedure for determining the presence of corona under oil for transformers. An oscillographic record taken in conjunction with conventional radio influence voltage tests distinguishes between corona in oil which causes progressive deterioration of insulation and corona in air which does not. The data also shows that for high-voltage transformers, the transformer bushing with its capacitance tap provides a suitable coupling capacitor for radio influence voltage tests.