Study on Fire Safety, Dielectric Properties and Temperature Rise Characteristics of Mineral Oil Transformers Retrofilled with Ester

The excellent fire safety and environmentally friendly properties of esters have attracted much attention in the transformer industry. Recently, retrofilling the mineral oil transformers with natural ester and synthetic ester has become a trend in the power industry. However, retrofilling faces practical challenges, such as whether residual mineral oil in the transformer after retrofilling will affect the physicochemical or dielectric properties of ester insulating liquid, and whether high viscosity will reduce the heat dissipation efficiency of the transformer. In the article, the impacts of various mineral oil contents on the fire safety and dielectric properties of ester were investigated. When the mineral oil content is around 3% and 7%, the fire point of synthetic ester and natural ester will no longer meet the requirement of K-class liquids. With the increase of mineral oil content, the breakdown voltage and dielectric loss factor of esters fluctuate up and down. Additionally, two 10 kV distribution transformers were retrofilled with natural ester and synthetic ester in accordance with the retrofilling process, and transformers after the experiment passed the routine test. Comparing the temperature rise results of four retrofilled transformers, it was found that although the temperature rises of retrofilled transformers increased, the results still met the temperature rise limit for semi-hybrid insulation system specified in GB/T 1094.14.