Investigation of the formation of wax-like substances in silicone and ester fluids

The formation of wax-like substances in mineral oil based insulation liquids has been known for decades. It is most common in applications with high electric field strength, such as oil impregnated cables and capacitors. Silicone fluids are known to form solid siloxanes under heavy partial discharge (PD) stress, but for ester fluids few reports of solidification exist. However, in recent years, there has been a number of outages where a wax-like substance was found in the high voltage insulation of ester and silicone insulated transformers. A scaled model of the high voltage insulation of a compact transformer was designed and built to investigate the influence of temperature and partial discharges on the generation of wax-like substances in the laboratory. An ester, a silicone insulation fluid and a mineral oil were stressed with partial discharges in this setup for 200h. The results of the wax formation and changes in the electric and dielectric properties as well as results of dissolved gas analysis (DGA) of the stressed insulation liquid are discussed in this paper.