Standardized methods for the determination of breakdown voltages of liquid dielectrics

IEC 60156 and IEC 60897 were standardized with the purpose of evaluating the breakdown voltage of liquid dielectrics both under AC and impulse voltages. AC breakdown voltage determination is a simple method but only able to provide information on the presence of contaminants. The limit of this test is the large scatter in the results and, for this reason, this IEC standard has been recently revised. Impulse breakdown voltage determination allows discrimination of different dielectric liquids, also in terms of their chemical composition. This test has only limited application because of the dimensions of the high voltage generator. The present paper shows a comparison among breakdown voltages of various mineral oils and natural esters under both AC and impulse voltages. The paper presents also an effort to describe how these two tests may reveal the electrical performance of the dielectric liquid under test.