Failure evaluation model of oil-paper insulation under AC-DC combined voltages

The insulation of windings connected with converter valves have to withstand AC, DC and repetitive impulse combined voltages in converter transformers. This paper presented the failure evaluation model by experimental research on electrical breakdown characteristics of oil-paper insulation under AC, DC and AC-DC combined voltages. Both short-time tests and constant-stress tests were carried out when the insulation system was subjected to different applied voltages. The breakdown strength and times-to-breakdown of oil-paper specimens have been investigated by tests. The inverse power model was presented by the weibull parameters which were estimated by the times-to-breakdown of specimens. Using the parameters of weibull model and inverse power model, the functions of failure evaluation model were calculated. It could establish the relationship between times, voltages and failure probability of insulation. And final evaluation results showed it was more effective than tradition failure model and weibull model. The DC component behavior in the insulation system has been analyzed and the influence of AC-DC combined voltages on insulation system was discussed.