POWER TRANSMISSION NETWORK

A power transmission network comprises, an AC electrical system AC transmission link from the AC electrical system to at least one AC-DC converter which includes: an AC connecting point connected to the AC transmission link and a DC connecting point for connection to a DC transmission link and a control system configured to operate each AC-DC converter 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 facilitating a power transfer between its connecting points to accommodate power generated or supplied to the AC electrical system. The control system is further configured to operate each first AC-DC converter in a protection mode to protect each AC-DC converter from an overvoltage and/or an overcurrent.