A method of controlling one of a first voltage source converter and a second voltage source converter, said first voltage source converter connected to a first AC system and said second voltage source converter connected to a second AC system, said first voltage source converter and said second voltage source converter further connected by a DC link in an HVDC power transmission network, said DC link comprising DC lines at opposite polarities DC+ and DC- or configured for unipolar transmission, wherein to energise the DC link, the method comprises: controlling the one voltage source converter to generate a DC voltage on the DC link based on a voltage order; wherein the voltage order is based on a time varying voltage reference signal (Rv) and the rate of change of the voltage reference signal varies over time; wherein the time varying voltage reference signal (Rv) and the rate of change of the voltage reference signal (Rv) are selected to prevent overvoltage on the DC link during the energisation process, characterized in that the rate of change of the time varying voltage reference signal (Rv) decreases over time; the time varying voltage reference signal (Rv) comprises a ramp signal having a slope that varies over time; the ramp signal ramps from an initial value corresponding to the voltage of the DC link when the voltage source converter is initially de-blocked to a final value corresponding to the nominal voltage of the DC link; the ramp signal has a first slope for a first period and has a second slope for a second period following the first period, wherein the second slope is decreased compared to the first slope; and wherein there is at least one additional period subsequent to the second period, wherein the slope of the ramp signal in each period is decreased from the slope in the previous period. controlling the one voltage source converter to generate a DC voltage on the DC link based on a voltage order; wherein the voltage order is based on a time varying voltage reference signal (Rv) and the rate of change of the voltage reference signal varies over time; wherein the time varying voltage reference signal (Rv) and the rate of change of the voltage reference signal (Rv) are selected to prevent overvoltage on the DC link during the energisation process, characterized in that the rate of change of the time varying voltage reference signal (Rv) decreases over time; the time varying voltage reference signal (Rv) comprises a ramp signal having a slope that varies over time; the ramp signal ramps from an initial value corresponding to the voltage of the DC link when the voltage source converter is initially de-blocked to a final value corresponding to the nominal voltage of the DC link; the ramp signal has a first slope for a first period and has a second slope for a second period following the first period, wherein the second slope is decreased compared to the first slope; and wherein there is at least one additional period subsequent to the second period, the DC link based on a voltage order; wherein the voltage order is based on a time varying voltage reference signal (Rv) and the rate of change of the voltage reference signal varies over time; wherein the time varying voltage reference signal (Rv) and the rate of change of the voltage reference signal (Rv) are selected to prevent overvoltage on the DC link during the energisation process, characterized in that the rate of change of the time varying voltage reference signal (Rv) decreases over time; the time varying voltage reference signal (Rv) comprises a ramp signal having a slope that varies over time; the ramp signal ramps from an initial value corresponding to the voltage of the DC link when the voltage source converter is initially de-blocked to a final value corresponding to the nominal voltage of the DC link; the ramp signal has a first slope for a first period and has a second slope for a second period following the first period, wherein the second slope is decreased compared to the first slope; and wherein there is at least one additional period subsequent to the second period, wherein the slope of the ramp signal in each period is decreased from the slope in the previous period.