Pressure wave propagation technique to measure the space charge distribution in pressboard impregnated with aged transformer oil

The pressure wave propagation (PWP) technique makes it possible to quantitatively investigate the relationship between the space charge and the conduction process of liquid impregnated pressboard. The space charge and the electric field distributions in an pressboard impregnated with oxidized oil are investigated using the PWP technique under dc voltages at room temperature. A special test cell was designed to hold the sample which was dried in vacuum and thereafter impregnated with the oxidized oil. DC electric fields of ±15.0 kV/mm, ±30.0 kV/mm and ±45.0 kV/mm were applied to the sample to study the dependence of accumulation of space charge on the amplitude and the polarity of applied voltage. Negative space charge is always observed in the vicinity of the negative electrode. At low fields negative space charge is observed also at the positive electrode. Above a certain “critical value” however, both positive and negative charges are observed