Properties of ester liquid midel 7131 as an alternative liquid to mineral oil for transformers

The combination of a solid and a liquid insulation is the most frequently used insulating system in high voltage apparatus, where components have to he insulated and loss heat has to be dissipated. The requirements on the liquid part of the insulating system are not only the electric and dielectric performance but also the performance regarding environmental requirements and dehydration capability as well as low in flammability. The use of ester liquid midel 7131, partly or totally replacing mineral oil, reduces the risk of environmental pollution, increases the lifetime of the component and reduces the fire risk. In distribution transformers midel 7131 has been used for several years with very good experience and therefore the manufacturer are encouraged to use midel 7131 also in power transformers even if the viscosity of the new liquid is higher than the viscosity of mineral oil. Due to larger dimensions of power transformers in comparison to distribution transformers special consideration is necessary. Some results concerning the electric and dielectric behaviour of midel 7131 is presented and pure midel 7131 as well as mixtures with mineral oil fulfil the requirements on the electrical performance of liquid insulating materials.