Abstract: A method, and apparatus implementing the method, for controlling the current and voltage of a high voltage, direct current transmission link connected at one end to an alternating voltage system via a rectifier and at the other end to another alternating voltage system via an inverter, wherein a first regulating circuit is connected to a rectifier to regulate one of the direct current and direct voltage output of the rectifier, a second regulating circuit is connected to the inverter to regulate one of the extinction angle of and current through the inverter, and a power regulator provides a signal representing a desired current magnitude to the first and second circuit means.

Abstract: A method of operating a high voltage DC transmission system with any number of transformer stations. Transformer stations which operate as rectifier stations operate with voltage regulation having a current dependent control input. In the range from 1 per unit to 1-.delta. per unit of rated voltage, a hard current limit is in effect, and in the range from 1-.delta. per unit to 1-.delta.-.epsilon. per unit rated voltage, the current limiting value is reduced to a minimum current, where .delta. and .epsilon. are preselected fractions. Transformer stations which operate as inverter stations principally operate with current regulation and in the voltage range from 1 per unit to 1-.delta. per unit of rated voltage, the maximum desired current value is reduced from rated current value down to zero.

Abstract: In a control apparatus for connecting three-phase current systems having respectively different frequencies to a high-voltage D.C.

Abstract: A direct current, high-speed circuit breaker which is connected in series at the direct current side of one of a plurality of inverters connected in parallel in a high voltage transmission system, and which is caused to open when the magnitude of the direct current to the one inverter exceeds a threshold value, is further controlled to open in response to the occurrence of a commutation failure only when such failure occurs simultaneously with reduction of the commutation voltage of the one inverter below a rated value by more than a first predetermined magnitude such that proper operation of the one inverter is no longer assured, and the threshold value of the direct current is increased from a normal value to a selected higher value in response to a reduction of the commutation voltage below the rated value by more than a second predetermined magnitude which is less than the first predetermined magnitude.