The breakdown of transformer oil under impulse voltages

The breakdown of pretreated transformer oil has been studied under controlled conditions with microsecond-duration impulse voltages. The oil was filtered, degassed and dried in a circulatory oil-cleaning system, and 3-gallon samples were tested in a glass test-cell with voltages of up to 500kV. The strength was found to depend, not only on the duration of the voltage, but also upon the manner in which it was applied. Several experiments are reported in which oil of a consistent quality was tested to study the effect of such factors as the size, material and preparation of the electrodes. In all these tests the strength was found to vary as the test proceeded¿an effect known as `conditioning¿. It is shown that one of the main factors which determines the breakdown strength obtained is the presence of microscopic gas bubbles on the surface of the electrodes, and these, together with solid particles and fibres in the oil, may well account for the low strengths obtained by previous workers who used undegassed or unfiltered oil.