PURPOSE: To continue an operation in a dual system by a controller even when a malfunction occurs in one system of a communication channel during the operation of a converter, and to connect it in parallel with the system without disturbing the system when the malfunction is recovered.

CONSTITUTION: A first control system is composed of a communication channel 191, transmission control circuits 26AA, 26BA, and controllers 200AA, 200BA, and a second control system is composed of a communication channel 192, and transmission control circuits 26AA, 26BB. Power converters 1A, 1B connected to both ends of a DC electric line 3 are controlled by either one of the first and second control systems at the normal time. Now, it is assumed that a malfunction occurs in the channel 191 during controlling by the first system. In this case, it can be controlled by the control system formed of the 200AA, 26AB, 192, 26BB and 200BB. Or, it can be controlled by the control system formed of the 200AB, 26AB, 192, 26BB and 200BA.