Space charge distribution in composite oil/cellulose insulation

A pressure wave propagation (PWP) technique has been developed to measure the space charge and the electric field distributions in a composite transformer oil/pressboard insulation under DC voltages at room temperature. Dependence of accumulation of space charge on the geometry of sample and the polarities of applied voltage was investigated. Two typical patterns of charge distributions were experimentally observed with a voltage being applied to the sample for the first time and at stationary conditions. In the former case, the electric fields in the sample distribute mainly according to the permittivities of the oil and the pressboard. However, in the latter case, the conduction processes in the oil and the pressboard govern the charge distribution. With the resistivity of pressboard being much larger than that of the oil, the field concentrates mainly in the pressboard at stationary conditions. On the basis of an ion drift model proposed before, the field distribution in the insulation was simulated and compared with the PWP result