The influence of concentrated heat flux on dielectric properties of synthetic and natural esters

The short time of existing the ester fluids in the transformer market and also the short time of their exploitation in power transformers cause, that the knowledge about their dielectric properties is not still full. Each new produced and operated unit brings a new information giving the impulse for additional studies. One of the observed phenomena is connected with the changing of ester properties as a result of heat treatment. A parameter such a dielectric dissipation factor (DDF) which is an important index speaking about the dielectric properties of insulating liquids, changes as a result of this treatment, and in the case of synthetic ester, DDF may rise even to the level (0.03) disqualifying it, according to the IEC 61099 standard, from the use as an insulating liquid in power transformers. For the natural ester, DDF also rises, but there is no possibility to assess its usefulness because requirements of IEEE C57.147-2008 Guide relate to the different temperature of DDF measurement than European Standards (IEC 61099, IEC 60247). In this paper, the above mentioned phenomena connected with the behavior of synthetic and natural esters in the condition of concentrated heat flux, are presented.