Study on the Performance Change of Mineral oil Mixed Natural Ester Insulating Liquid for Long-term Service Transformer

With the advantages of environmental friendliness, biodegradability and fire safety, natural ester insulating liquid has gradually become an alternative insulating medium for mineral oil in traditional transformers, and has been rapidly developed and applied. However, after retrofilling, the insulating liquid in the transformer is a mixture of natural ester and residual long-term service mineral oil, and the advantages of natural ester will be affected by residual mineral oil. In this paper, the variation of water content, fire and flash point, acid value, breakdown voltage, dielectric dissipation factor and D.C.resistivity of the mixed insulating liquid of natural ester and long-term service mineral oil was studied. The experimental results show that the water content of the mixed insulating liquid has little effect on the dielectric properties. Retrofilling with natural ester can effectively improve the fire safety of the transformer. It can be quickly judged by the fire and flash point whether the mixed insulating liquid meets the K-level flame retardant after retrofilling. The performance changes of the mixed insulating liquid are affected by the residual amount of mineral oil. The dielectric dissipation factor and kinematic viscosity of the mixed insulating liquid have a good linear relationship with the change of the mass ratio of mineral oil in long-term service, which can be used to predict the residual amount of mineral oil after retrofilling.