Test Comparison Study on a Natural Ester Retro-filled 220kV Power Transformer

With the growing global interests on natural ester insulating liquids advantages, China is very actively promoting this technology in liquid-filled transformers. The major power grid utilities, like SGCC and CSG, have published their corporate purchase specification standards for 10kV natural ester filled distribution transformers. Small bulk tenders from utilities and industrial end users are requesting natural ester solutions for their projects with voltage level 10kV-35kV. Although the need for applying this same technology in power transformer is obvious, the market is more conservative as the voltage level increases. In order to explore the opportunity of using natural ester liquid in power transformers, CHINT, one of the leading natural ester power transformer manufacturers in China, decided to invest in a test comparison of an actual 220kV 16MVA power transformer which was designed for mineral oil. All the routine and type tests (including switch impulse, full wave and chopped wave lightning impulse tests) were performed for this unit before and after retro-filling with natural ester insulating liquid (FR3™ Natural Ester). The transformer passed all the tests with each liquid and a KEMA certificate was granted. The tests showed satisfactory results according to the respective standards for mineral oil and natural ester liquid. In this study, the differences in the test results on dielectric and oil tests, such as IR, tan delta, temperature rises, DGA, and even noise level were all listed and analyzed. Moreover, to get the utmost benefits from natural ester liquid, such as its higher liquid permittivity and paper-liquid insulation system thermal class, this paper provides suggestions on design optimization based on the combined efforts using FEM electric field and thermal analysis. This actual transformer test comparison will give confidence to end users regarding the safety and reliability of retrofilling a mineral oil designed and filled power transformer with FR3 natural ester insulating liquid.