In the process of commissioning a 2＊600MW Coal - fired power plant, The secondary circuit voltage test of # 1 main transformer voltage transformer was carried out. During the test it was found that the vacuum switch MCB 9 of the third winding was not closed,but A voltage is generated at the non-charged side of the switch. After a number of experimental analysis, the problem was found and solved. The secondary circuit voltage test of # 1 main trmasformer voltage transformer is carried out again. The test results are correct, ensuring the commissioning process.