As the industry of renewable energies growing rapidly across the globe, mega wind farms with large sized wind turbines are being installed. In China, power generation developers have been seeking ways to improve the operational reliability and economic properties of wind turbine transformers. The increase of turbine size brings the opportunity to position transformers inside the wind tower or upward to the nacelle, and while the voltage class of turbine continuing to climb up (to 66kV or even 110kV), the inherent constraints of dry-type transformers make it impractical to be applied, due to the confined space in tower or nacelle. To cope with those issues, combined the purpose on not sacrificing fire safety property and environmental friendliness, and meanwhile to enhance load capability, a new type of liquid filled transformer for wind turbines has been designed and the full-scale model was manufactured. The prototype is 37kV 3300kVA, filled with FR3™ natural ester insulating oil, KFAF cooling type, transformer tank is designed to be installed inside wind tower and cooler outside the tower, tank and cooler are connected by oil pipes. The prototype passed all FATs with satisfied results. To evaluate the transformer performance under cold climate, specifically, under the scenario that transformers need to be energized after a long period of storage in cold weather, a cold start test was performed during January of 2022, the transformer was tested inside a -40℃ climate chamber, temperatures of core, coils, oil pipes and coolers were continuously monitored. It took nearly 10 days to really cool down the transformer with around 1.5 tons of insulating liquid. The cold start procedure was designed referring to IEEE C57.93, firstly energize the transformer with no load for 2 hours, then, gradually increase the loading rate by sequence, reaching 40%, 59%, 81%, and holding for 30 minutes at each step, finally 105% loading rate was applied. The hottest spot of the transformer arrived at 127.4℃ after 5 hours under 105% loading, then the test was ended. It was later discovered that, unfortunately, due to inappropriate selection of the cooler’s control cable, which was damaged during the cooling stage, and causing that the cooler was actually not activated during the whole process of cold start. However, no electrical faults, no issues on tank integrity or leakage were detected during and after the cold start process， with considerations of the long days of cooling phase and the practical cold start procedure of wind turbines, the test itself was recognized as a convincing prove for applying natural ester filled transformers in cold regions.

cold start,natural ester,wind turbine,transformer