PURPOSE: To make it possible to adequately distribute a load current in response to respective rated capacities when operating in parallel three or more transformers having different impedances.

CONSTITUTION: N units (N≥3) to be operated in parallel, for example, (2N-3) units, that is, 3 units of single-phase autotransformers AT1, AT2 and AT3 are connected to the primary side or secondary side of 3 units of single-phase transformers T1, T2 and T3. And the distribution of load current of the single- phase transformers T1 to T3 is controlled to a proper value in response to the reciprocal ratio of the number of turns by adequately adjusting the voltage ratio, that is, the number of turns N1/N2 respectively of each single-phase autotransformers AT1 to AT3. By doing this, the load current can be properly distributed in response to the rated capacity of each transformer when operating three or more transformers in parallel having different impedances.