A power transmission network comprising: a variable power source; an AC transmission link for AC power transmission from the variable power source to at least one source side converter; at least one source side converter including: an AC connecting point operably connected to the AC transmission link; and a DC connecting point for connection to a DC transmission link; and a control system (32a,32b) configured to operate the source side converter or at least one of the source side converters in a frequency damping mode to control an AC voltage at its AC connecting point and thereby damp at least one frequency component at its AC connecting point and/or in the AC transmission link; wherein the control system (32a,32b) is further configured to operate the source side converter or at least one of the source side converters in an AC voltage control mode as an AC slack bus to control a magnitude and/or a frequency of an AC voltage of the AC transmission link at a or a respective steady-state value and thereby facilitate variation of a power transfer between its AC and DC connecting points so as to accommodate a variation in power generated by the variable power source, the control system is further configured to actively modify a magnitude reference value for an AC voltage of the AC transmission link or a frequency reference value for an AC voltage of the AC transmission link, and the control system (32a,32b) is further configured to operate the source side converter or at least one of the source side converters in the frequency damping mode in accordance with the modified reference value during its operation in the AC voltage control mode. a variable power source; an AC transmission link for AC power transmission from the variable power source to at least one source side converter; at least one source side converter including: an AC connecting point operably connected to the AC transmission link; and a DC connecting point for connection to a DC transmission link; and a control system (32a,32b) configured to operate the source side converter or at least one of the source side converters in a frequency damping mode to control an AC voltage at its AC connecting point and thereby damp at least one frequency component at its AC connecting point and/or in the AC transmission link; wherein the control system (32a,32b) is further configured to operate the source side converter or at least one of the source side converters in an AC voltage control mode as an AC slack bus to control a magnitude and/or a frequency of an AC voltage of the AC transmission link at a or a respective steady-state value and thereby facilitate variation of a power transfer between its AC and DC connecting points so as to accommodate a variation in power generated by the variable power source, the control system is further configured to actively modify a magnitude reference value for an AC voltage of the AC transmission link or a frequency reference value for an AC voltage of the AC transmission link, and the control system (32a,32b) is further configured to operate the source side converter or at least one of the source side converters in the frequency damping mode in accordance with the modified reference value during its operation in the AC voltage control mode.