Study On Thermal Performance Of Natural Oil Circulation Power Transformer Winding

Oil immersed transformer is widely used in power transmission because of its good coolant, low wastage and price, large capacity. And about 80% of these transformers use natural oil circulation. Directed oil circulation is widely used in natural oil transformer winding. Whereas the flowing and heat transfer characteristics in it had not been clear, especial for the hottest spot temperature and its location. Experimental and numerical calculation results of the temperature rise of an oil- filled natural convection transformer winding with directed-blockwashers are presented in this paper. A comprehensive test was performed using a non-inductive winding with imbedded thermocouples. When parallel duct height of winding is minished, the oil flux reduces. As a result, winding coolant conditions go bad. And then winding average temperature rise, copper-oil temperature rise and average winding oil temperature rise are higher than those of larger parallel duct. The oil flow stagnant phenomenon in natural convection transformer winding, which setting blockwashers, is greatly improved. And disc surface oil velocity is quickening, so the heat transfer is better. Then winding average temperature rise, copper-oil temperature rise and average winding oil temperature rise are lower than without blockwashers . Furthermore, the winding average temperature rising is proportioned with the number of blockwashers in winding. The location of hot spot is not at top disc. And that there are intricate relation between the hot spot temperature , parallel duct size and amount of .blockwashers. With smaller parallel duct and more blockwashers in winding, hot spot moves up. The natural convection in parallel duct is important and intricate. The oil flow direction in top oriented zone effect winding temperature rise. When oil stream from outer vertical duct to inner in this zone, the top disc temperature rise III is lower about lOO/o~2O% than that of reversed oil flow direction. The hot spot temperature also reduces, and its location moves up. When the number of disc in top oriented zone reduce, which with same number of blockwashers, the temperature of several top discs in this zone go down. However, the winding average temperature and copper oil temperature difference go up. The thermal driving force is greater with larger height difference between the center of radiator and that of winding. Consequently, the average and hot spot temperature rises descend. On all accounts, the adoption of directed oil circulation is helped to reduce average temperature rise. While the hot-spot temperature rise is complicatedly effected by the amount of blockwashers in the winding, the parallel duct size, the number of discs in one pass and the direction of oil flow, the cooling condition besides of the transformer, etc. Moreover, these factors would influence the flux, temperature, and viscosity of the oil. It should be synthetically considered so that to set directed-blockwashers reasonably in transformers with natural oil circulation cooling structure designing.