The power transformer is a static electrical device, which is used to transform a numerical value of alternating voltage (current) into other voltage (current) with same frequency or several voltage (current) in different value. The rated voltage of the secondary winding of a power transformer refers to the no-load voltage of the secondary winding when the primary winding is under the rated voltage.

When the transformer is at full load, the impedance of the primary and the secondary winding of the transformer will cause the voltage drop of the transformer itself, which is equivalent to 5% of the rated voltage of the power grid, so that the terminal voltage of the secondary winding is smaller than the no-load voltage. In order to compensate for the voltage loss in the line, the rated voltage of the secondary winding of the transformer should be higher than the rated voltage of the power grid by 5%. Therefore, the rated voltage of the secondary winding of the transformer is 10% higher than the rated voltage of the same-level power grid.

When the transformer is put into practice and the power supply radius is smaller, the voltage loss of the line can be ignored since the line is relatively short. At that time, the rated voltage of the transformer's secondary winding should be 5% higher than the rated voltage of the grid to compensate for the voltage loss of the transformer itself.