PURPOSE: To provide an operation system of transformers, wherein the parallel operation of a plurality of transformers can be performed and when a fault is generated on a distribution line in the course of the parallel operation, only that distribution line can be separated from others surely.

CONSTITUTION: A plurality of buses 2, 2 of transformers 1 are linked to each other by semiconductor devices 3. On the respective buses 2, 2, fault current sensing parts 6 are provided respectively, and when a fault current flowing through the bus 2 exceeds a predetermined value in the state wherein the semiconductor device 3 is switched on, the semiconductor device 3 is switched off by a supervision-controlling part 7 before circuit-breakers 5 for the power distributions of respective distribution lines 4 are switched off.