Some issues related to impulse testing of gaseous insulation with synthetic air

Impulse testing procedures are commonly used for various investigation of high- voltage insulation. Especially for introduction of new type of electrical insulation, like ecofriendly gases alternative to SF6, the statistical behavior of breakdown voltage should be evaluated. The paper deals with a comparison of different methods of lightning impulse testing and their suitability for gas-insulated systems using synthetic air. Inhomogeneous field conditions were investigated using point-sphere electrode combinations at various gap distances and gas pressure levels. A most important condition to be met is the statistical independence of the measured impulse voltage values of breakdown events. Different statistic tools for testing independence can lead to different results. Therefore, the validity of independence was checked using five different mathematical procedures. For some combinations of electrode distance and gas pressure a distribution of breakdown voltage values was obtained that is characteristic for discharge events, where at least two different discharge mechanisms are present. For these so-called mixed distributions a practical method is proposed that allows to still obtain a good estimation of the impulse voltage withstand value. It has shown further, that the time-interval between successive impulse voltages, the waiting time, has a significant impact on the statistical independence of the breakdown events. The commonly used value of 300 sec for that is not a sufficient waiting time for every combination of gap distance and gas pressure. Also, in some cases the breakdown path from the high voltage electrode changed unexpectedly from the sphere electrode to the wall of the test object instead. That phenomenon could be explained by the presence of space charge within the gap, generated by the preceding impulses and not compensated during the waiting time. The space charge would change the field distribution within the gap and could lead to the described phenomenon