Study of electrode for measuring dielectric properties of oil immersed material

Dielectric properties including relative permittivity and dc resistivity has been used widely as a design parameter of insulation system. In measuring dielectric properties of oil immersed material (OIM), we have to make some adaptations from IEC 60093 and IEC 60250 i.e. the detail of live electrode curvature corner shape and the effect of electrode's weight. The effect of the electrode curvature shape on the electric field stress distribution should be reduced as small as possible when measuring the dielectric properties of the pressboard. Meanwhile the weight of electrode plays an important role to reduce the effect of non-flatness surface of the pressboard. This paper focused on electric field analysis distribution for three electrode configurations and pressure simulation of different electrode's weight. Then the simulation results were deepened with the laboratory experiment, permittivity, 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 bath in a test vessel. For PDIV measurement, 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. Meanwhile for the permittivity measurements, a guard electrode was used to prevent edge capacitance. 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 PD experiment was set up according to IEC 60270, the PDIV test procedure was performed in accordance with IEC 61294 and the permittivity and tan δ measurements performed in accordance with IEC 60250. 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. Meanwhile, the D/R (ratio of the sample thickness to corner radius of electrode) parameter does not seem to have any real important meaning. It was also found that electrode with pressure higher than 1.9 N/cm2 give enough pressure to prevent a thin oil layer that present on surface. These results showed the effect of electrode corner shape on electric field enhancement and pressure of electrode at the contact point of dielectric interface. Furthermore, the results 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. 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 sharpcornered electrode. Meanwhile, the D/R (ratio of the sample thickness to corner radius of electrode) parameter does not seem to have any real important meaning. It was also found that electrode with pressure higher than 1.9 N/cm2 give enough pressure to prevent a thin oil layer that present on surface. These results showed the effect of electrode corner shape on electric field enhancement and pressure of electrode at the contact point of dielectric interface. Furthermore, the results 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.