Testing for low breakdown probability with special reference to liquid insulation

Several techniques for the determination of withstand voltage of electrical insulation are investigated. As a basis for comparison, impulse tests are carried out on a uniform field gap in transformer oil. Measurements of breakdown time lags suggest that different breakdown initiating mechanisms may be simultaneously active, and contribute to the change found in the parameters of the breakdown probability curve between the low and high probability regions. Digital simulation of the dielectric test serves as a useful tool to rationalize the experimental procedure, as well as to identify the risk of breakdown corresponding to the measured withstand voltage.