Oil Tap On Load Tap Changer Filled with Natural Ester Liquid

On-load tap changers (OLTC or LTC) are indispensable in regulating power transformers used in electrical energy networks and industrial applications. Historically mineral oil has been used as the dielectric liquid. Aspects such as its high flammability, limited biodegradability, and deterioration of contacts due to coking are main triggers for the use of alternative liquids. The global trend towards sustainable and safer solutions, the better tolerance to moisture and temperature and the extremely low tendency of coking, associated with the over 25 years of commercial application, led to natural ester insulating liquids as the fluid of choice. A comprehensive test program and large experience of field applications confirms natural ester liquids are an effective alternative to mineral oil for the main transformer tank and for the tap changer including retro-fill. Studies and tests are performed jointly by Cargill and Easun-MR in Central Power Research India (CPRI) are presented in this paper. The analysis also included additional tests performed in Cargill USA laboratory and Easun MR Laboratory in India. The switching test was performed with an Easun-MR make OLTC Model 220/D filled with FR3 fluid from Cargill, following the procedure and criteria of IEC 60214-1:2014. After completing 50,000 operation under mineral oil, the OLTC was retrofilled with natural ester liquid and other 10,000 operations were completed on same OLTC. Samples were collected and tested every 2000 operations, allowing to generate a detailed comparative study of the properties of the mineral oil side-by-side with the natural ester liquid. Study was also requested by few utilities for their comprehensive retro-fill program. The test results confirmed the excellent performance of natural ester dielectric liquids on the OLTC, validating the concept of upgrading a mineral oil filled regulated power transformer with oil-switching type OLTC to a modern, less-flammable and environmentally friendly natural ester filled transformer. The two initial motivations for using natural ester liquids (improved fire and environmental safety) are increasingly considered as a “bonus”, overshadowed by the better lubricity and lower coking tendency, which may reduce the wearing of the contacts and improve the long-term performance of the OLTC. Also after transformer is retro-filled OLTC use to be with mineral oil which was concern for most of users to make complete substation with minimal fire risk.