Comparison of the electric and dielectric behaviour of different insulating fluids

The authors consider ester fluid Midel 7131 and silicon fluid for use in distribution transformers. The electrical and dielectric properties of the two fluids were determined in new condition and after an aging period of 1000 h under severe conditions. The dielectric dissipation factor tan δ was almost the same for the silicon and ester fluid in the temperature range between 0 and 120°C. A temperature change from 20 to 120°C increased these values from about 7×10^-2 to about 3×10^-1. The dielectric strength of the fluids is highly dependent on humidity. A 1000-h aging process at 150°C under the influence of 3 g/1 Cu, 3 g/1 Fe, 3 g/l Al, and atmospheric oxygen caused an increase in tan δ by two orders of magnitude, whereas there was no significant change to be observed with the silicon fluid. The electric strength was reduced for both fluids by a 1000-h aging process. The effect of this aging process was more pronounced with the silicon fluid than with the ester fluid