A power transmission network, for interconnecting at least one variable power source (28) and at least one AC electrical network (30), comprises: at least one DC transmission link (20) for DC power transmission between at least one network side converter (24) and at least one source side converter (24); at least one AC transmission link (26) for AC power transmission from the or the respective variable power source (28) to at least one source side converter (24); at least one source side converter (24) including: a DC connecting point operably connected to the or the respective DC transmission link (20); and an AC connecting point operably connected to the or the respective AC transmission link (26); at least one network side converter (22) including: an AC connecting point for connection to the or the respective AC electrical network (30); and a DC connecting point operably connected to the or the respective DC transmission link (20); and a control system (32a,32b,34), wherein at least one network side converter (22) is designated as a first converter, and the control system (32a,32b,34) is configured to operate the or each first converter in a DC voltage control mode as a DC slack bus to vary a DC voltage at its DC connecting point with respect to the power generated by the or the respective variable power source (28).