Study on voltage-number characteristics of capacitor insulation under impluse voltages with different waveforms

In power system, capacitor could work as long as several ten years, during which time the inner insulation system made up of polypropylene (PP) film and capacitor oil (oil-impregnated PP film insulation system) suffers from all kinds of impulse voltages continually. Such accumulative application of repeated impulse voltage could lead to the irretrievable damage of insulation dielectrics. According to that, we investigated the accumulative effect of repeated impulses on oil-impregnated PP film and the influence of waveforms. In this work, a repetitive impulse accumulating test platform was set up to simulate the accumulative effect of impulse voltage on oil-impregnated PP film. We determined the relationship between the breakdown voltage of oil-impregnated PP film and the wave front/tail time of applied impulses. The V-N characteristic and the influence of waveform parameters were also identified. The results indicate that the breakdown voltage of oil-impregnated PP film increases with the increase in the wave front time or the decrease in the wave tail time. The V-N characteristics of oil-impregnated PP film is in accord with Weibull distribution and the number of voltage applications N increases with the decrease in the applied impulses V and the increasing speed gradually increases. The mathematical expression of the V-N characteristics of oil-impregnated PP film under standard lightning impulse and standard switching impulse are V=18.78 N^-003371 and V=20.51N^-002743 kV respectively. The waveform parameters have impact on the V-N characteristics. The number of repeated impulses to breakdown increases with the increases in wave front time or decrease in the wave tail time.