PURPOSE: To inhibit damage due to trouble minimally, and to improve reliability remarkably by installing integral type multipole disconnectors to the input-output sections of a converter, easily detaching the connecting sections of input-output buses and connecting them again without stopping plant operation.

CONSTITUTION: An integral type five-pole disconnector 12 is mounted to the input-output sections of a converter. When the converter gets trouble or is brought to an abnormal state, a group of a cubicle-side output bus P pole 13, a cubicle-side output bus N pole 14, a cubicle-side input bus R phase 15, a cubicle-side input bus S phase 16 and a cubicle-side input bus T phase 17 and a group of a converter-side output bus P pole 18, a converter-side input bus T phase 19, a converter-side input bus S phase 20, a converter-side input bus R phase 21 and a converter-side output bus N pole 22 are separated by one- operation by the disconnector.