Essential Evaluations on Developing Natual Ester Immersed 750kV Power Transformer

Natural ester insulating liquids have gained attention from the global power industry in recent years, and until today, the highest voltage level of in-service natural ester immersed transformer is 420kV. Many concerns are raised to explore higher voltage level and essential evaluations are needed. Such as a 750kV power transformer that was developed in China, with Um 800kV, BIL 900kV/1950kV/1550kV (AC/LI/SI). This single-phase transformer is rated at 120MVA, filled with FR3™ Natural Ester, passed all the type tests at once under witness from an authorized 3$^{rd}$ party laboratory in November 2022. Since there was no experience, either on design or on manufacturing of such high voltage ester filled transformer, evaluations comparing natural ester design with traditional mineral oil solution become essential. This paper covers most of the essential evaluations in the journey of aiming to make the world’s highest voltage level natural ester transformer, including design and manufacturing. Evaluation aspects need attention, such as liquid performance differences on breakdown voltage under different electrode and voltage configurations, the stopping length of streamers, the partial discharge initial voltage, and the impacts on the winding design, main insulation and leads of the transformer. Evaluation of the oil flow electrification performance, viscosity, water saturation, the impacts on cooling design, and evaluation on material compatibility between natural ester and construction materials of 750kV transformer demand attention. This paper can be valuable to the industry for exploring natural ester liquids application in UHVAC transformers.