Abstract: The transformer in service of hydropower station is possible to suffering lightning hit and withstanding the power frequency operating voltage. In order to investigate the overvoltage distribution in the transformer winding in service, using the Bergeron equivalent model with inductance as the basic element, the power frequency voltage of different phases superimposed by the standard lightning wave is analyzed. The results show that: when the standard lightning wave is superimposed on the power frequency voltage in the negative peak phase, the input wave peak value is slightly lower than that of the pure standard lightning value but the peak voltages of the node and every coil are higher than that of the pure standard lightning wave; conversely, the input wave peak value is slightly higher than that of the pure standard lightning value under the positive peak phase. The calculation results are consistent with the simulation results in the SIMULINK environment, which proves the correctness of the method.