Insulating liquid in power equipment becomes increasly contaminated as the equipment ages in service. Insulation liquids’ ageing is a four dimensional problem due to dielectric, chemical, thermal and electro mechanic stresses, which are highly depending on operational conditions. The aging rate depends thereby on several factors such as temperature, light, oxygen, catalysts, loadings as well as gas and water content of the insulating liquid. Among those parameters, temperature is recognized to play a key role in the ageing of liquid insulation used in power systems. The aim of this paper is regarded as a main task to investigate the effect of temperature gradient on the dielectric properties of insulating fluids. The consequences for power equipments are discussed. For these investigations, mineral oil and a synthetic ester liquid were considered.
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