Abstract: The abnormal noise data from a three-phase five-pole transformer of a 220 kV substation is analyzed, which is the first case occurred in initial operation of such substation in southern region of Hebei Province, the noise component tests and spectrum analysis of DC current through the transformer neutrals are carried out, and after the noise levels are revised, the test conclusion that the transformer noise is caused by small DC current of about 1 A flowing through is obtained, which has been verified by simulation, meanwhile, the pertinent operation and maintenance strategy is formulated and the corresponding control measures are studied.