Measurements of the DC electric field in liquid impregnated pressboard using the pressure wave propagation technique

The pressure wave propagation (PWP) method has been used to measure the space charge and the electric field distributions in transformer oil impregnated pressboard under DC electric field at a moisture concentration in the order of 0.1%. The development of space charge versus the amplitude and the polarity of applied voltage is investigated. An electric field below a critical value injects heterocharge near the positive electrode. Near the negative electrode homocharge is injected. On the other hand, when the electric field exceeds the critical field, homocharge is injected near the negative electrode; however, both hetero and homo charges are injected near the positive electrode. In order to analyse the results obtained with the PWP method, a computational tool which takes into account attenuation, dispersion and reflections of a pressure wave propagating in a sample is used to calculate the electric field from the raw data. The results have implication whenever an interface between two materials exists. The field at the interface between the electrode (Al) and the sample can be up to 2 times the average field in the bulk