Method for energizing a chain-link converter, controller, computer programs and computer program products

1. A method for energizing a chain-link converter comprising one or more phase legs, each phase leg comprising a number of series-connected converter cells, each converter cell comprising four valves arranged in an H-bridge connection with a DC capacitor, and each valve in turn comprising a semiconductor switch in parallel with a diode, the method comprising the following steps in listed order:

charging the DC capacitor of each converter cell to a voltage level below their nominal voltage so that the semiconductor switches are controllable,

diagnosing the converter cells so as to detect failed components thereof,

discharging the DC capacitors of faulty cells,

bypassing faulty components in a controlled manner, and

charging the DC capacitors to their nominal voltage.

2. The method as claimed in claim 1 , wherein the step of charging comprises closing an AC breaker arranged between a power network and the chain-link converter and charging the DC capacitor through a starting resistor arranged between the AC breaker and the chain-link converter.

3. The method as claimed in claim 1 , wherein the step of diagnosing comprises receiving, in the controller, data from converter cell control and protection units connected to gate units of the converter cells, and determining based on the data whether a converter cell comprises a faulty component.

4. The method as claimed in claim 3 , wherein faulty components are identified by monitoring voltage of healthy components.

5. The method as claimed in claim 1 , wherein the step of bypassing failed components comprises:

bypassing converter cells comprising faulty component by providing an alternative current path through healthy components thereof.

6. The method as claimed in claim 1 , wherein the step of bypassing failed components comprises:

bypassing converter cells comprising faulty component by closing a switch arranged in parallel with the converter cell comprising the faulty component.

7. The method as claimed in claim 1 , comprising the further step of closing a starting resistor switch arranged in parallel with a starting resistor, the starting resistor arranged between a power network and the chain-link converter, whereby the DC capacitors are charged to their nominal voltage.

8. The method as claimed in claim 1 , wherein the voltage level at which the semiconductor switches are controllable comprises the minimum at which the semiconductor switches become controllable.

9. The method as claimed in claim 1 , wherein the phase legs are connected in a delta configuration or in a wye configuration.

10. A controller for energizing a chain-link converter comprising one or more phase legs, each phase leg comprising a number of series-connected converter cells, each converter cell comprising four valves arranged in an H-bridge connection with a DC capacitor, and each valve in turn comprising a semiconductor switch in parallel with a diode, the controller arranged to enable the following steps in listed order:

enable the charging of the DC capacitor of each converter cell to a voltage level at which the semiconductor switches are controllable but below their nominal voltage,

diagnose the converter cells so as to detect failed components thereof,

enable discharging of the DC capacitors of faulty cells,

enable bypassing of faulty components in a controlled manner, and

enable charging of the DC capacitors to their nominal voltage.

11. A non-transitory computer readable storage medium storing a program executed by a computer for energizing a chain-link converter comprising one or more phase legs, each phase leg comprising a number of series-connected converter cells, each converter cell comprising four valves arranged in an H-bridge connection with a DC capacitor, and each valve in turn comprising a semiconductor switch in parallel with a diode, the computer program comprising computer program code, which, when run on a controller, causes the controller to perform the following steps in listed order:

enable the charging of the DC capacitor of each converter cell to a voltage level at which the semiconductor switches are controllable but below their nominal voltage,

diagnose the converter cells so as to detect failed components thereof,

enable discharging of the DC capacitors of faulty cells;

enable bypassing of faulty components in a controlled manner, and

enable charging of the DC capacitors to their nominal voltage.