Overload Investigation on Retrofilling Mineral Oil Distribution Transformer with Soybean-Based Natural Ester

The retrofilling of mineral-oil-immersed transformers with natural ester can improve the fire safety and environmental characteristics of power grids and extend the life of cellulose insulations. In China, although several retrofillings were performed by different users, there is still insufficient understanding of the retrofilling operation procedure for minimizing residual mineral oil and on the effect of the temperature rise of transformers. In this paper, following specific procedures, an out-off-service S11-M-315/10 and a new S13-M-400/10 distribution transformers were selected to retrofill their mineral oil with FR3™, a soybean-based natural ester. Before and after the retrofilling process, oil tests, temperature rise tests and overload tests were performed to detect the differences. The fire point of the mixed oil after retrofilling was 352 °C, which meets K-class less-flammable requirements and indicates the proper control of residual mineral oil. For the 315 kVA and 400 kVA unit, the two retrofilled transformers temperature rises of top oil grow by 4.1 K and 5 K, and temperature rises of average oil by 3.3 K and 4.0 K, respectively, compared with mineral oil transformers. The temperature rises of high-voltage windings grow by 11.5 K and 4.3 K separately, and low-voltage windings by 7.8 K and 8.8 K. According to the IEC 60076-14 standard, the thermal class of the oil-paper insulation system can be promoted by changing mineral oil to natural ester, thus resulting in the temperature rise limits of the top oil to reach 75 K and the average windings to reach 80 K, respectively. The results of the overload test of S13-M-400/10 transformer showed that the highest temperature rise values of top oil of the mineral oil transformer was 109.1 K and the retrofilled transformer was 121.9 K. After retrofillng, the time of reaching the limits of the 1.5 times rated load was 30% higher than that of the mineral oil transfomer. The research results showed that conventionally designed mineral oil transformers cannot fulfill the severe overloading test patterns of State Grid Corporation of China (SGCC) but the overload capacity of the 1.5 times rated load can be improved to some extent by retrofilling. Also, after a series of tests, the retrofilled S11-M-315/10 transformer was put in service, and it has been in a good condition since November 2019.