Method and controller for controlling a power transmission network

1 . 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.

2 . The method of claim 1 , wherein the power converter further comprises a DC side that is connected to a power transmission medium; the method further comprising:

issuing, by the controller, a DC voltage command to change or adjust a DC side voltage of the power converter to a DC reference value.

3 . The method of claim 1 , wherein detecting the fault in the second branch comprises detecting a fault between the third switch and the fourth switch.

4 . The method of claim 1 , wherein the fault comprises any one of:

a break in an insulation in the second transformer, and/or

an insulation failure in a component in the second branch; and/or

an abnormal current flow through the converter station; and/or

an abnormal current flow through the second branch; and/or

an abnormal voltage in the converter station; and/or

an abnormal voltage in the second branch; and/or

a current path in the second branch that allows a current to flow from the power transmission network to a ground or Earth.

5 . The method of claim 1 , wherein

the first command includes instructions to reduce an AC side voltage of the power converter to a first reference value;

wherein the first reference value is zero, such that issuing the first command to reduce the AC side voltage of the power converter causes the AC side voltage of the power converter to converge to zero; or

wherein the first reference value is a current opening capability of the fourth switch.

6 . The method of claim 1 , wherein the converter station is an asymmetrical monopole, or a bipole, or a symmetrical monopole HVDC transmission scheme, and initially controlling the power converter comprises initially controlling, by the controller, the power converter for the asymmetrical monopole, or the bipole, or the symmetrical monopole HVDC transmission scheme.

7 . The method of claim 1 , wherein the method is for controlling a power transmission network; the method further comprising:

in response to the first switch closing:

issuing, by the controller, a fifth command to the power transmission network, and/or to a windfarm controller, to transfer or connect wind-turbine feeders to the first AC network.

8 . A controller 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 controller configured to:

initially control the power converter to transfer power between the AC network and the transmission network via the first branch and the second branch;

detect a fault in the second branch;

in response to detecting the fault in the second branch:

issue a first command to reduce an AC side voltage of the power converter;

issue a second command to open the first switch, the third switch, and the fourth switch;

in response to the fourth switch opening:

issue a third command to close the first switch; and

issue a fourth command to restore an AC side voltage of the power converter to an AC reference value.

9 . A converter station for a power transmission network, the converter station comprising:

a first branch comprising: a first switch in series with a first transformer in series with a second switch;

a second branch comprising: a third switch in series with a second transformer in series with a fourth switch;

a power converter comprising an AC side and a DC side; and

the controller of claim 8 , configured to control the converter station;

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 the AC side of the power converter.

10 . The converter station of claim 9 , wherein the power converter is a Voltage Source Converter for providing power to a power transmission medium.

11 . The converter station of claim 9 , wherein the converter station is located onshore.

12 . The converter station of claim 9 , wherein the converter station is located offshore on a fixed or floating platform.

13 . The converter station of claim 9 , wherein:

the first and third switches have higher current breaking capability compared to the second and fourth switches; and/or

the first and third switches are circuit breakers and the second and fourth switches are isolators or disconnectors.

14 . An asymmetrical monopole or a symmetrical monopole HVDC transmission scheme comprising the converter station of claim 9 .

15 . A power transmission network, comprising:

an AC network;

a HVDC power transmission medium; and

the converter station of claim 9 ;

wherein the converter station is connected between the AC network and the HVDC power transmission medium.