Influence of water content on dielectric response of pressboard impregnated with different insulating fluid

Synthetic ester is amongst insulating fluid which have been thoroughly investigated to be used as an alternative for mineral oil in power transformer. Due to its low polarity, interaction between synthetic ester and water is different from interaction between non polar mineral oil and water. This interaction results in high water saturation limit, which is another beneficial property that can be used in maintaining the water content of insulation of power transformer. To get the behavior of this interaction, we measured dielectric properties of dry and wet pressboard impregnated with synthetic ester. As a comparison, dielectric properties of dry and wet pressboard impregnated with non-polar mineral oil is also measured. The dielectric properties consist of d.c. conductivity, relative permittivity and tan δ. To get the behavior of their polarization process, relative permittivity and tan δ were measured in wide frequency range, from 1 mHz up to 1 kHz. To get the influence of temperature on the dielectric response, the volume d.c. resistivity, relative permittivity and tan δ were measured at two different temperature, 21.5°C and 58.5°C.