HVDC POWER TRANSMISSION

This application describes methods and apparatus for HVDC power distribution. A control apparatus (108) is described for controlling a frequency set-point for a first AC network (104) electrically connected to a first HVDC station (101) to regulate active power. The controller has a frequency controller (501) operable in a first mode of operation to determine a frequency set-point (Fref) for the first AC network (104) based on a measured DC voltage (V) at the first HVDC station. A disturbance detector (601) is configured to monitor the measured value of DC voltage at the first HVDC station (V) for a predetermined characteristic indicative that a variation in measured DC voltage does correspond to a known modulation applied to the DC voltage by a second HVDC station (102). The frequency controller is configured to determine the frequency set-point (Fref) for the first AC network (104) based on a measured value of DC voltage (V) if said predetermined characteristic is detected, and to control the frequency set-point to a predetermined default frequency if said predetermined characteristic is not detected.