Abstract:The no - load test of main transformer often occurs in power plants and substations,due to differences in factory parameters of equipment,there is three - phase voltage unbalance in low voltage side of main transformer under no - load condition. Through vector analysis and equivalent circuit method,the reasons for three - phase voltage unbalance are analyzed,and the parameters of different units are measured. The data computation provides a reliable basis for three - phase voltage unbalance in low voltage side of main transformer under no - load condition.