Current feedback control for a power converter with multiple feedback loops

1. A method performed by a control system of a power electronics converter that comprises a valve arrangement and is connected to a power grid, the method comprising:

executing a first feedback control algorithm with an input based on a difference between a predetermined current reference and a first feedback current measurement of a current in the converter, wherein the first feedback control algorithm has a first control cycle time and includes at least proportional control using a proportional gain;

executing a third feedback control algorithm with an input based on the first feedback current measurement, wherein the third feedback control algorithm has the first control cycle time and acts on an output from the first feedback control algorithm after Safe Operating Area limits have been applied to the first feedback control algorithm output, the third feedback control algorithm including proportional control using the proportional gain but with a different polarity compared to the first feedback control algorithm; and

executing a second feedback control algorithm with an input based on a second feedback current measurement of the current, wherein the second feedback control algorithm has a second control cycle time and acts on an output from the third feedback control algorithm with the same polarity as the first feedback control algorithm and includes proportional control using the proportional gain, the second control cycle time being less than the first control cycle time;

wherein the first, second and third feedback control algorithms are executed by a grid-side current controller to control power output from the converter to the power grid.

2. The method of claim 1 , wherein the second feedback control algorithm acts on the output after voltage co-ordinations/transformations thereof.

3. The method of claim 1 , wherein the first feedback control algorithm further includes integral or resonance control, using an integral gain or a resonance gain.

4. The method of claim 1 , wherein the first control cycle time is at least two times longer than the second control cycle time.

5. The method of claim 4 , wherein the first control cycle time is at least ten times longer than the second control cycle time.

6. The method of claim 1 , wherein the first control cycle time is within a range from 50 μs to 250 μs.

7. The method of claim 1 , wherein the second control cycle time is within a range from 5 μs to 50 μs.

8. The method of claim 1 , wherein the first control cycle time is within a range from 50 μs to 250 μs and the second control cycle time is within a range from 5 μs to 50 μs.

9. The method of claim 8 , wherein the first control cycle time is at least five times longer than the second control cycle time.

10. The method of claim 8 , wherein the first control cycle time is within a range from 100 μs to 200 μs and the second control cycle time is within a range from 5 μs to 10 μs.

11. The method of claim 1 , wherein the first and third feedback control algorithms operate in direct quadrature frame.

12. The method of claim 1 , wherein the second feedback control algorithm operates in ABC frame.

13. The method of claim 1 , wherein the first and second feedback current measurements are measurements of current between the valve arrangement and the power grid.

14. The method of claim 13 , wherein the first and second feedback current measurements are measurements of current on either a valve or a grid side of a transformer or reactor of the converter.

15. The method of claim 13 , wherein the first and second feedback current measurements are measurements of current within the valve arrangement.

16. A computer program product comprising computer-executable components for causing the control system to perform the method of claim 1 when the computer-executable components are run on processing circuitry comprised in the grid-side current controller.

17. A control system for a power electronics converter comprising a valve arrangement and connected to a power grid, the control system comprising:

processing circuitry; and

data storage storing instructions executable by the processing circuitry to cause the processing circuitry to:

run a first feedback control algorithm with an input based on a difference between a predetermined current reference and a first feedback current measurement of a current in the converter, wherein the first feedback control algorithm has a first control cycle time and includes at least proportional control using a proportional gain;

run a third feedback control algorithm with an input based on the first feedback current measurement, wherein the third feedback control algorithm has the first control cycle time and acts on an output from the first feedback control algorithm after Safe Operating Area limits have been applied to the first feedback control algorithm output, the third feedback control algorithm including proportional control using the proportional gain but with a different polarity compared to the first feedback control algorithm; and

run a second feedback control algorithm with an input based on a second feedback current measurement of the current, wherein the second feedback control algorithm has a second control cycle time and acts on an output from the third feedback control algorithm with the same polarity as the first feedback control algorithm and includes proportional control using the proportional gain, wherein the second control cycle time is less than the first control cycle time.

18. A power electronics converter comprising the control system of claim 17 and the valve arrangement.

19. The power electronics converter of claim 18 , wherein the power electronics converter is a neutral point clamped three level converter or a modular multilevel converter.

20. The power electronics converter of claim 18 , wherein the power electronics converter is configured for the power grid being a single phase or three-phase grid.

21. The power electronics converter of claim 18 , wherein the power electronics converter is configured to operate as a three-to-one phase rail intertie, as a Static Compensator or as a three-to-three phase converter.

22. A method performed by a control system of a power electronics converter that comprises a valve arrangement and is connected to a power grid, the method comprising:

providing a grid-side current controller for controlling power output from the converter to the power grid;

as a first part of the controller, running a first feedback control algorithm with an input based on a difference between a predetermined current reference and a first feedback current measurement of a current in the converter, wherein the first feedback control algorithm has a first control cycle time and includes at least proportional control using a proportional gain;

as a third part of the controller, running a third feedback control algorithm with an input based on the first feedback current measurement, wherein the third feedback control algorithm has the first control cycle time and acts on an output from the first feedback control algorithm after Safe Operating Area limits have been applied to the first feedback control algorithm output, the third feedback control algorithm including proportional control using the proportional gain but with a different polarity compared to the first feedback control algorithm; and

as a second part of the controller, running a second feedback control algorithm with an input based on a second feedback current measurement of the current, wherein the second feedback control algorithm has a second control cycle time and acts on an output from the third feedback control algorithm with the same polarity as the first feedback control algorithm and includes proportional control using the proportional gain, wherein the second control cycle time is less than the first control cycle time.