Abstract: Two rectifiers and two inverters are operated under the state under which they are connected in parallel by common transmission lines. These rectifiers and inverters are controlled an independent control apparatus respectively which can selectively apply either the constant current control or the constant voltage control.The control apparatus of each converter has bestowed thereon the reference value of a current to flow through the particular converter and the reference value of a terminal voltage of the particular converter. A signal for lowering the current reference value is added to the control apparatus of one or all of the converters which are to be operated as the inverters. On the other hand, a signal for essentially rendering the terminal voltage of the converter smaller than the voltage reference value is added to the control apparatus of the converter which is to be operated as the converter for determining the voltage of a direct current system.

Abstract: Control units are provided associative with forward and inverse converters in a multiterminal DC power transmission system, respectively. The control units set different constant voltages in the converters. Further, the minimum value of the constant voltage set values in the forward converters is set at a value larger than the maximum value of the constant voltage set values in the inverse converters.