Arcing resistance of high fire point dielectric liquids

High Fire Point ) dielectric liquids have been in use since the demise of Askarels (PPGBITCB) in the mid - 1970ʹs. Having fire points of at least 300°C, their main applicat~on has been in the role of transformer impregnants particularly in units located in, on, or near buildings. This study was aimed at evaluating the suitability of such liquids for application in arcing environments. At present, most wE"p liquids are produced from one of three different chemical bases. These are : a) Poly01 Ester, b) High and c) Dimethyl Silicone. Samples of these liquids were evaluated in a typical oil switch primarily utilized in capacitor switching applications . though not an HIT liquid, Transformer Oil was included in the test matrix to provide a base line to which the other liquids could be compare^ Each liquid was exposed to 250 cycles of switching 8,ooO Volts at 50 Amps. The results show that both Polyol-Ester and High ~ o ~ e c u ~Wa eri g ~ t ~ydrocarbons compare favorably to T r a n s f o ~ e rO il in this application. However, the dielectric breakdown value of Dimethyl Silicone was so rapidly degraded that its use under such conditions requires considerable caution. Further analysis of the particulate arc by-products formed in the ~ i ~ e t hSiylic~on e revealed that this material had an extremely high dielectric constant. Through computer modeling, it was shown that the great disparity in dielectric constants between the fluid and the particulate would greatly intensify the ~l e c t r i c ast~re ss on the fluid and cause dielectric breakdown.