Investigations on cavities inside liquid-immersed paper and pressboard insulation materials and their influence on the electrical behavior

Although the process of impregnating cellulosic materials with liquid insulants is well known and has been in use for many decades sometimes problems with gas-filled cavities inside the impregnated material still arise. Therefore extensive experimental investigations on the effects of gas-filled cavities inside an impregnated pressboard, like they appear after an imperfect impregnation, have been performed. The filling of hidden voids with surrounding insulation liquid during and after impregnation process was examined and the results are presented in this contribution. Using defined failures inside a transformerboard also the influence of these defects on the electrical behavior of the insulation in a semi-uniform field was simulated.