The study of electrode corner shape for measuring dielectric properties of oil immersed material

Abstract

Dielectric properties including dissipation factor and dc resistivity has been used widely as a diagnostic parameter of insulation system. In measuring dielectric properties according to IEC 60093 and IEC 60250, the detail of cylindrical live electrode curvature corner shape is not fully explained. The effect from the electrode curvature shape on the electric field distribution should be reduced as small as possible when measuring the dissipation factor and dc resistivity of the oil immersed material. This paper focused on comparative study of the electric field distribution for three electrode configurations. The calculation of the electric field strength was done using finite element method. Then the simulation results were deepened with the laboratory experiment, partial discharges inception voltage (PDIV) and partial discharge (PD) pattern respectively, In this experiment, the dielectric interface configurations consists of oil immersed pressboard operated under oil in a test vessel. The pressboard samples were placed on a grounded plane under cylindrical live electrode with three types of corner shape. The live electrodes used were sharp-cornered electrode and rounded-cornered with radius 3 mm and 5 mm. The solid dielectric were oil immersed transformer board type B 3.1A with 2 mm and 4 mm thickness operated under mineral oil, Nynas Nytro 4000x. The test experiment was set up according to IEC 60270 and the PDIV test procedure performed in accordance with IEC 61294. We concluded that, eventhough round-cornered electrode yields more uniform electric field at the pressboard side, it also yields steep electric field enhancement at the oil side. This nature of rounded-corner electrode makes it easier to generate oil discharge in the wedge (small oil gap) when compared to sharp-cornered electrode. For measuring dielectric properties at high voltage level, this phenomenon should take into account. These results showed the effect of electrode corner shape on electric field enhancement at the contact point of dielectric interface. Furthermore, it could be used as a consideration on which electrode is suitable for measuring dielectric properties of pressboard immersed in oil in compliment with IEC 60093 and IEC 60250.