Ramped voltage testing of transformer oil

It has been shown that, compared with dimethyl siloxane, transformer oil has similar reproducible current characteristics as a function of ramped voltage applied up to a high prebreakdown electric stress, where it obeys the equation I∝V^s . Moreover, at a very high electric stress, the displacement current component apparently reverses, which was not previously observed in silicone oil. This abnormal phenomenon is very dependent upon the conditions. From an earlier study in silicone oil, the average current response to the rising and falling voltage appears on a log-log scale with two linear portions which are sharply divided. Transformer oil has similar characteristics under similar testing conditions, but with increasing voltage amplitude of the applied ramp the slopes of the linear portions are changed. These two lines may become one if the maximum voltage and ramp rate are very high. This is true for copper and aluminum electrodes at differing gap separations