A power transmission network comprising: an AC electrical system; an AC transmission link for AC power transmission from the AC electrical system to at least one first AC-DC converter; said at least one first AC-DC 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 or each first AC-DC converter in an AC voltage control mode as an AC slack bus to control a magnitude and a frequency of an AC voltage of the AC transmission link 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, wherein the control system (32a, 32b) is further configured to operate the or each first AC-DC converter in a converter protection mode to regulate the or the respective steady-state value of the magnitude and frequency of the AC voltage of the AC transmission link to protect the or each first AC-DC converter from an overvoltage and/or an overcurrent, and wherein the control system (32a, 32b) is further configured to operate the or each first AC-DC converter in the converter protection mode to regulate the or the respective steady-state value of the magnitude and frequency of the AC voltage of the AC transmission link as a function of one or more power transmission parameters, the one or more power transmission parameters comprising: • an alternating current at the AC connecting point of the or each first AC-DC converter; • an AC voltage at the AC connecting point of the or each first AC-DC converter; • a direct current at the DC connecting point of the or each first AC-DC converter; and/or • a DC voltage at the DC connecting point of the or each first AC-DC converter. an AC electrical system; an AC transmission link for AC power transmission from the AC electrical system to at least one first AC-DC converter; said at least one first AC-DC 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 or each first AC-DC converter in an AC voltage control mode as an AC slack bus to control a magnitude and a frequency of an AC voltage of the AC transmission link 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, wherein the control system (32a, 32b) is further configured to operate the or each first AC-DC converter in a converter protection mode to regulate the or the respective steady-state value of the magnitude and frequency of the AC voltage of the AC transmission link to protect the or each first AC-DC converter from an overvoltage and/or an overcurrent, and wherein the control system (32a, 32b) is further configured to operate the or each first AC-DC converter in the converter protection mode to regulate the or the respective steady-state value of the magnitude and frequency of the AC voltage of the AC transmission link as a function of one or more power transmission parameters, the one or more power transmission parameters comprising: • an alternating current at the AC connecting point of the or each first AC-DC converter; • an AC voltage at the AC connecting point of the or each first AC-DC converter; • a direct current at the DC connecting point of the or each first AC-DC converter; and/or • a DC voltage at the DC connecting point of the or each first AC-DC converter. • an alternating current at the AC connecting point of the or each first AC-DC converter; • an AC voltage at the AC connecting point of the or each first AC-DC converter; • a direct current at the DC connecting point of the or each first AC-DC converter; and/or • a DC voltage at the DC connecting point of the or each first AC-DC converter.