Arcing resistance of high fire point dielectric liquids

High fire point (HFP) dielectric liquids have been in use since the demise of Askarels (PCB/TCB) in the mid 1970s. Having fire points of at least 300°C, their main application 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 HFP liquids are produced from one of three different chemical bases. These are: (a) polyol ester; (b) high molecular weight hydrocarbon (HMWH); and (c) dimethyl silicone. Samples of these liquids were evaluated in a typical oil switch primarily utilized in capacitor switching applications. Although not an HFP liquid, transformer oil was included in the test matrix to provide a base line to which the other liquids could be compared. Each liquid was exposed to 250 cycles of switching 8000 V at 50 A. The results show that both polyol ester and HMWHs compare favorably to transformer oil 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 dimethyl silicone 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 electrical stress on the fluid and cause dielectric breakdown