An investigation on the parameters affecting the conductivity of pressboard in oil by measuring leakage current under DC

Many researches have been done all over the world since last decades in order to increase its efficiency and reliability of the HVDC transformer used for long distance HVDC transmission. Regarding DC characteristics of the oil impregnated pressboard, the complicate structure of the transformer causes several undesirable effects: inhomogeneous distribution of a high temperature caused by a heavy load and change of the insulation conductivity by the non-uniform electric field distribution. In addition, the accumulated space charges could cause any type of service failure under the transient voltage. Therefore, these parameters have been mainly investigated for the related insulation design of the HVDC transformer. In this work, the mobility of space charges has been focused since the pressboard has the composite insulation system where space charges are easily accumulated due to many unknown reasons. For this purpose, the conductivity of the oil impregnated pressboard was obtained by measuring leakage current and then the influence of the temperature and the electric field intensity on the conductivity have been investigated. The result shows that the conductivity seems to be proportional to the electric field intensity and increased exponentially with the temperature. Moreover, based on the measurement of polarization current of the pressboard, the mobility of the space charge in the pressboard has been assumed to be decreased with increasing the electric field intensity.