Method and controller for controlling a power transmission network

A method for controlling a converter station in a power transmission network, the converter station comprising a first branch, a second branch, and a power converter;

• wherein the first branch comprises:
• a first switch in series with a first transformer in series with a second switch;
• wherein the second branch comprises:
• a third switch in series with a second transformer in series with a fourth switch;
• wherein the first branch and the second branch are in parallel, the first and third switches are connected to an AC network, and the second and fourth switches are connected to an AC side of the power converter;
• the method comprising:
• initially controlling, by a controller, the power converter to transfer power between the AC network and the transmission network via the first branch and the second branch;
• detecting, by the controller, a fault in the second branch;
• in response to detecting the fault in the second branch:
• issuing, by the controller, a first command to reduce an AC side voltage of the power converter;
• issuing, by the controller, a second command to open the first switch, the third switch, and the fourth switch;
• in response to the fourth switch opening:
• issuing, by the controller, a third command to close the first switch;
• and
• issuing, by the controller, a fourth command to restore an AC side voltage of the power converter to an AC reference value.