Performance and new application of ester liquids

The combination of solid and liquid insulation is the most utilised insulation system in high voltage apparatus, where the insulation consists of impregnated solid material and the demand on heat transfer is not negligible. The requirements on the liquid part of the insulation system are not only the electric and dielectric performance but also the performance regarding environmental requirements and dehydration capability. The use of ester liquid replacing partly or totally mineral oil is useful in order to reduce the endangering of the soil, underground water and the fire risk. The high capacity of ester liquid to pick up water can also be used for the dehydration of the paper component of the insulation system. The contribution presents the performance of ester and mineral oil/ester mixtures concerning the electric behaviour and cover the whole range of mixture between ester and mineral oil. The dielectric parameter like loss factor and permittivity were measured for the whole mixture range in order to judge the performance of the complete insulation system regarding the compatibility of the liquid insulation with different type of papers and the loss factor of the whole apparatus. The efficiency of the hydration was checked using ester liquid as insulation or only as water carrier to fry the paper in a long or short time period.