If the capacity of the transformer is too large, it will not only increase the initial investment, but also make the transformer run at no load or light load for a long time, so that the proportion of no-load loss increases, the power factor decreases, and the network loss increases. This operation is neither economical nor reasonable. If the transformer capacity is too small, it will overload the transformer for a long time and damage the equipment. Therefore, the rated capacity of the transformer should be selected according to the needs of the electrical load, and should not be too large or too small.