This paper presented the idea and plan to replace the forced-oil and air cooling systems of two SFPSZ7-180000/220 main transformers into oil-immersed forced-air cooling systems in Zidong substation of Guangdong power grid according to the computation of oil temperature rise and the analysis of the cooling system and oil ducts.The monitoring results of temperature rise and load for the improved transformers indicate that at full load the maximum oil temperature rise keeps under allowable value
and the loss of the improved cooling system and noise reduce.