For exploring the reason of insulation fault of oil-immersed power transformer of wind farms in severe cold regions in winter

the transient operating temperature of the transformer with different load rates under low temperature is calculated by IEEE standard. The simulation model of dynamic migration and diffusion of moisture in oil-paper insulation under dynamic hot spot temperature is set up on the basis of Fick's second law

and the transient distribution characteristics of moisture in insulation board and oil is compared and analyzed. The study shows that in case of temperature rise the moisture migrates from insulation board to oil and vice verse in case of temperature decrease. However

the internal temperature of transformer is generally low due to the cold environment. In case of temperature rise

the moisture migrated out of the insulation board has no time to enter into the insulation board in the temperature reduction stage

leading to gradual increase of moisture concentration in the oil with the time. However

with the sudden decrease of load in the wind farm

the internal operation temperature of the transformer and the moisture dis-solution ability are reduced accordingly. Under the same moisture content

the increase of relative humidity in the oil leads to the the polymerization and even precipitation of water molecules

which seriously endangers the safe and stable operation of oil-immersed transformer.