A power transmission network, for interconnecting a variable power source and a AC electrical network including: a DC transmission link for power transmission between a network side converter and a source side converter; a AC transmission link for power transmission from the respective variable power source to a source side converter; a source side converter including: a DC connecting point operably connected to the respective transmission link; and an AC connecting point operably connected to the respective transmission link; a network side converter including: an AC connecting point for connection to the respective electrical network; and a DC connecting point operably connected to the respective transmission link; and a control system, where a network side converter is designated as a first converter, and the control system is configured to operate each first converter as a DC slack bus to vary a DC voltage at its DC connecting point.