When the busbar has short trouble at any side of the transformer, the overcurrent protection device will act, because the three sides of the transformer all are provided with overcurrent protection, to enable them to eliminate the trouble selectively, and the transformer it not required to stop. The overcurrent protection at each side can be used as the backup protection of the busbar and circuit of this side, the overcurrent protection of main power supply side can be used as the backup protection of other two sides and the transformer.