Controlling a modular converter in two stages

1. A method for controlling a modular converter, the modular converter including a plurality of converter modules configured for converting an input voltage into an output voltage to be supplied to a load, the method comprising:

receiving a control input reference vector, a control input vector and a control input parameter vector;

determining a control output reference vector from the control input reference vector, the control input vector and the control input parameter vector in a first control stage; and

controlling the converter modules by generating switching signals based on the control output reference vector in a further control stage;

wherein the control output reference vector is determined by:

predicting at least one future state of the modular converter with a prediction model of the modular converter, wherein the prediction model is adapted for calculating the future state of the modular converter based on an actual state of the modular converter, a state of the modular converter having current values and/or voltage values of the modular converter;

enhancing the at least one future state with respect to an objective function by minimizing the objective function with respect to a dynamical evolution in time of the prediction model, wherein the objective function is based on a cost value associated with switching costs of the converter modules, the objective function penalizes differences between the control input reference vector at a time step and a predicted evolution of the control input vector at the time step, and the objective function minimizes a change in evolution of the control output reference vector; and

determining the control output reference vector from the future state.

2. The method of claim 1 , wherein the modular converter controlled by the method is a modular multi-level converter wherein each converter module of the modular multi-level converter includes two power connectors, at least two power semiconductors and a capacitor, wherein the power connectors are short-circuited in a first switching state of the power semiconductors and are connected to the capacitor in a second switching state of the power semiconductors.

3. The method of claim 2 , comprising:

determining a voltage vector from the control output reference vector in a second control stage with a modulator; and

controlling the converter modules by generating switching signals from the voltage vector in a third control stage.

4. The method of claim 3 ,

wherein the control input reference vector is a current reference vector, the control input vector is an actual current vector and the control input parameter vector is an actual voltage vector; and/or

wherein the control output reference vector is a voltage reference vector.

5. The method of claim 4 ,

wherein a sequence of future states is predicted for a plurality of time steps in the future; and

wherein the control output reference vector is determined from a next future state associated with a next time step.

6. The method of claim 5 ,

wherein the prediction model is based on linear equations relating voltages and/or currents at a time step with voltages and/or currents at a next time step.

7. The method of claim 1 , comprising:

determining a voltage vector from the control output reference vector in a second control stage with a modulator; and

controlling the converter modules by generating switching signals from the voltage vector in a third control stage.

8. The method of claim 1 ,

wherein the control input reference vector is a current reference vector, the control input vector is an actual current vector and the control input parameter vector is an actual voltage vector; and/or

wherein the control output reference vector is a voltage reference vector.

9. The method of claim 1 ,

wherein a sequence of future states is predicted for a plurality of time steps in the future; and

wherein the control output reference vector is determined from a next future state associated with a next time step.

10. The method of claim 1 ,

wherein the prediction model is based on linear equations relating voltages and/or currents at a time step with voltages and/or currents at a next time step.

11. The method of claim 1 ,

wherein the prediction model includes a model of the converter modules and/or a model of the load.

12. The method of claim 1 , comprising:

compensating a time delay caused by the determining of the control output reference vector by predicting currents at a next time step using actual voltages and/or currents.

13. The method of claim 1 ,

wherein the objective function is based on a vector norm.

14. The method of claim 1 ,

wherein the objective function is based on a quadratic and/or linear norm.

15. The method of claim 1 , comprising:

controlling the converter modules by generating switching signals from a rounded control output reference vector.

16. The method of claim 1 , comprising:

detecting a converter module with a fault;

short-circuiting the detected converter module; and

removing the short-circuited converter module from the prediction model.

17. A controller for controlling a modular converter, wherein the controller is configured for performing the steps of:

predicting at least one future state of the modular converter with a prediction model of the modular converter, wherein the prediction model is adapted for calculating the future state of the modular converter based on an actual state of the converter, a state of the modular converter having current values and/or voltage values of the modular converter;

enhancing the at least one future state with respect to an objective function by minimizing the objective function with respect to a dynamical evolution in time of the prediction model, wherein the objective is based on a cost value associated with switching costs of converter modules of the modular converter, the objective function penalizes differences between the control input reference vector at a time step and a predicted evolution of the control input vector at the time step, and the objective function minimizes a change in evolution of the control output reference vector; and

determining a control output reference vector from the future state.

18. A modular converter for supplying a load with electrical voltages, the modular converter comprising:

a plurality of converter modules having semiconductor switches and a capacitor;

a first controller according to claim 17 for generating a control output reference vector; and

a further controller for generating switching signals for the converter modules based on the control output reference vector.

19. The modular converter of claim 18 ,

wherein the modular converter is a modular multi-level converter.