Investigating the Effects of Moisture Distribution on the Electric Field and Leakage Current of Power Transformer Bushings

Bushings are one of the most important and essential equipment of power transformers. Statistics show that one of the main causes of transformer failures is due to bushing problems. One of the main reasons for bushing failures is the penetration of moisture into its internal insulation system. The moisture affects the dielectric properties and leads to an increase in its leakage current. These effects lead to changes in the electric field of the inner insulating layers of the bushings, so it is necessary to investigate these effects. In this paper, the changes of the electric field in the insulating layers of a sample bushing have been investigated with considering the different moisture distribution between the layers. This evaluation has been done during the moisture diffusion time, from the start of penetration to the time of uniformity of moisture in all layers. Also, the moisture effects on the bushing leakage current under above conditions have been studied. For this purpose, an OIP bushing is modeled in COMSOL Multiphysics software and the simulations are carried out based on Finite Element Method (FEM) and finally the obtained results are analyzed. The results show that the moisture distribution in the bushing can have an important effect on the internal electric fields. Also, according to the results, the moisture distribution inside the bushing does not have a great effect on the leakage current and this current is only affected by the volume of the moisture penetrated inside the bushing.