PURPOSE: To quickly detect the short circuit or the grounding of a line and prevent the damage on a converter by monitoring the current level and the transmitting power command value, which are sought by the neutral line current detector and the earth point current detector provided in a dipole DC transmission system, and inputting them into an AND circuit.

CONSTITUTION: By the command of a common controller 50, ac to dc converters 1-4 are driven through converter controllers 10-40 so as to perform dipole DC transmission. When the current value of a neutral line 6 detected by a detector 60 is zero or thereabout, a monitor means 61 outputs a signal 1. When the current value at the earth point detected by a detector 70 is zero or thereabout, a monitor means 71 outputs a signal 1. It monitors 51 the command value of dipole transmitting power and it outputs a signal 1 if the command values are not equal. If a short accident or a grounding accident occurs in the power transmission line, the AND condition materializes immediately, and it outputs a signal 1, and controls the transmitting power command value to stop it softly. Hereby, the damage on the converter or the adverse function to the AC system can be prevented.