Considerations for Dielectric Properties Measurements of Oil Immersed Pressboard

Abstract

Dielectric properties including relative permittivity, dielectric dissipation factor (DDF) 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.This paper is focused on dielectric properties measurement of oil immersed pressboard (OIP) i.e. related to the effect of live electrode corner shape, the effect of electrode's weight, and the applied the electric field stress and temperature.In this experiment, the solid and liquid dielectric wereoil immersed transformer board type B 3.1A and operated under mineral oil bath, Nynas Nytro 4000x. We concluded that, round-cornered electrode yields steep electric field enhancement at the oil side. This phenomena makes it easier to generate oil discharge in the wedgewhen compared to sharp-cornered electrode. For measuring dielectric properties at high voltage level, this phenomenon should take into account. It was also found that electrode with pressure higher than 1.9 N/cm2 gives enough pressure to prevent a thin oil layer that present on surface.We also notice that for small g h ⁄ ratio (the gap of guard and guarded electrode is small compared to the sample) or thick sample, the fringing effect is high. Therefore calculation of effective area according to IEC 60093 is considerably applicable for thick sample. These results could be used as a consideration when measuring dielectric properties of oil immersed pressboardin compliment with IEC 60093 and IEC 60250.