A power transmission network comprises: an AC electrical system (28); an AC transmission link (26) for AC power transmission from the AC electrical system (28) to at least one first AC-DC converter (24); at least one first AC-DC converter (24) including: an AC connecting point operably connected to the AC transmission link (26); and a DC connecting point for connection to a DC transmission link (20); and a control system (32a) configured to operate the or each first AC-DC converter (24) 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 (26) at a or a respective steady- state value and thereby facilitate a power transfer between its AC and DC connecting points so as to accommodate a power generated by or supplied to the AC electrical system (28), wherein the control system (32a) is further configured to operate the or each first AC-DC converter (24) in a converter protection mode to regulate the or the respective steady-state value of the magnitude and/or frequency of the AC voltage of the AC transmission link (26) to protect the or each first AC-DC converter (24) from an overvoltage and/or an overcurrent.