An Electrode Model to Ascertain the Effects of Voltage and Tip Radius with Gap Distance on Electric Field of Transformer Oil

Dielectric liquids are a major part of the insulation system in high voltage equipment like the transformer. The reliability of the liquid is very much critical for proper operation of the power apparatus. With the increase in electrical stress, the oil begins to ionize and a streamer develops. In due course, the growing streamer causes an electric breakdown. These streamer discharges are not desirable as they ruin the life of the insulation system. In this paper, a 2D COMSOL Multiphysics electrode model is proposed for streamer formation with needle-plane geometry with a different tip radius of the needle. A needle tip is considered so as to replicate any impurity present in the oil. The method of Finite element analysis (FEA) is applied for discretization of the COMSOL model. This analysis helps to understand the effect of variation of the electric field along the axial length of the geometry considering changing tip radius of the needle. For the simulation study, various voltage levels are used and the distance between the needle and the plane is also changed in steps of 5mm, from 15mm to 30mm. The simulation results show that higher electric field magnitude is obtained with smaller needle tip radius. The higher the field magnitude, lower is the voltage required to initiate streamer. As the applied voltage increases, the magnitude of electric field also increases. It is inferred that for the development of discharges in the transformer oil, the needle with lower radius is the suitable as the electric field produced from such surfaces is very high, which produces streamers at low voltages. A comparative study of the simulation results is done for varying voltage levels and tip radius at different gap distances to understand the mechanism behind the streamer formation.