Control of DC transmission lines

This application relates to methods and apparatus for control apparatus of a voltage converter (401) for direct current transmission of electrical power over a transmission line (402). The control apparatus comprises regulation circuitry (403) for generating a regulation signal for the converter to regulate an electrical parameter of the transmission line, such as voltage or power. The regulation signal is based on the difference between a measured value of said parameter and a reference value of said parameter, as may be supplied from a reference control block (404) and may be derived from a demand signal indicating the voltage or power demand. The apparatus also includes damping circuitry (405) configured to generate a damping signal for modulating said regulation signal to reduce the effect of resonance within the bandwidth of control of the converter. The damping signal is generated as a function of a measured voltage or current of the transmission line and an impedance value for the transmission line. The damping signal may be high-pass filtered (407) so as to have no substantial impact on steady state operation.