Space charge behavior in thick oil-impregnated pressboard under HVDC stresses

The space charge accumulation can affect the dielectric performance of oil-impregnated pressboard in convertor transformers. In this paper, the space charge behaviour in 1mm thick impregnated pressboards is investigated by the means of pulsed electroacoustic method. The dried pressboards are separately impregnated in fresh oil and service aged oil as comparison. The space charge distributions are investigated under ±15 kV/mm and ±25 kV/mm at room temperature. The homocharge injection can be observed for all the samples, which is much more significant in the aged oil impregnated samples. The amount of injected charges and the depth of the injected charges accumulation in the pressboard bulk are closely related to the oil properties, amplitude and polarity of the applied voltages. The service aged oil shows the dominant effects on the local electric field distortion, about 48% enhanced in the aged oil impregnated samples while about 15% enhanced in the fresh oil impregnated samples. The estimations of the apparent charge mobility and trap depth also indicate the great impacts caused by the service aged oil.