Method and device for damping voltage harmonics in a multilevel power converter

1. A method for reducing lower order harmonics of a power converter, the method comprising, for each phase leg of the converter, the steps of:

obtaining a voltage reference for the phase leg, wherein the phase leg has a plurality of cells;

for a present sample, obtaining a flux error of the output voltage of the phase leg, wherein the obtaining of the flux error comprises:

calculating a voltage error of the present sample by subtracting an obtained voltage reference for the phase leg for a preceding sample from an estimated voltage output of the phase leg for said preceding sample; and

integrating the calculated voltage error over a sampling time period to obtain the flux error of the present sample;

dividing the present sample flux error by a cycle time period to obtain an average voltage error over said time period;

subtracting an obtained processed average voltage error, based on the average voltage error, from the voltage reference to obtain a modified voltage reference for the phase leg; and

providing the modified voltage reference to a modulation unit of the converter for controlling the phase leg in a present cycle time period,

generating, by the modulation unit, switching state signals to switch on or off of all of the cells in the phase leg using the modified voltage reference, thereby compensating for the average voltage error that is not compensated in the preceding sample to control the phase leg in the present cycle time period, and reducing the lower order harmonics of the phase leg.

2. The method of claim 1 , wherein obtaining the processed average voltage error comprises multiplying the average voltage error by a gain.

3. The method of claim 2 , wherein obtaining the processed average voltage error comprises applying a filter to the average voltage error.

4. The method of claim 2 , wherein the cycle time period is the half switching cycle duration.

5. The method of claim 2 , wherein sampling is performed every 10 μs or less.

6. The method of claim 1 , wherein obtaining the processed average voltage error comprises applying a filter to the average voltage error.

7. The method of claim 6 , wherein the cycle time period is the half switching cycle duration.

8. The method of claim 6 , wherein sampling is performed every 10 μs or less.

9. The method of claim 1 , wherein the cycle time period is the half switching cycle duration.

10. The method of claim 9 , wherein sampling is performed every 10 μis or less.

11. The method of claim 1 , wherein sampling is performed every 10 μs or less.

12. The method of claim 1 , wherein the method is performed by a respective control unit of each phase leg.

13. The method of claim 1 , wherein the modulation unit is for pulse-width modulation, PWM.

14. The method of claim 13 , wherein the modulation unit is for phase shifted carrier based PWM.

15. A non-transitory computer program product comprising computer-executable components for causing a control unit for a phase leg of a power converter to perform the method of claim 1 when the computer-executable components are run on processor circuitry comprised in the control unit.

16. A control unit for a phase leg of a power converter, the control unit comprising:

processor circuitry; and

a storage unit storing instructions executable by said processor circuitry whereby said control unit is operative to:

obtain a voltage reference for the phase leg, wherein the phase leg has a plurality of cells;

for a present sample, obtain a flux error of the output voltage of the phase leg by:

calculating a voltage error of the present sample by subtracting an obtained voltage reference for the phase leg for a preceding sample from an estimated voltage output of the phase leg for said preceding sample; and

integrating the calculated voltage error over a sampling time period to obtain the flux error of the present sample;

divide the present sample flux error by a time period to obtain an average voltage error over said time period;

subtract an obtained processed average voltage error, based on the average voltage error, from the voltage reference to obtain a modified voltage reference for the phase leg; and

provide the modified voltage reference to a modulation unit of the converter for controlling the phase leg in a present cycle time period, such that the modulation unit generates switching state signals to switch on or off of all of the cells in the phase leg using the modified voltage reference, thereby compensating for the average voltage error that is not compensated in the preceding sample to control the phase leg in the present cycle time period, and reducing the lower order harmonics of the phase leg.

17. A power converter comprising a plurality of phase legs, each of which comprising a control unit of claim 16 .

18. The converter of claim 17 , wherein the converter is a three-phase converter in delta configuration or Y configuration.

19. A non-transitory computer program product for reducing lower order harmonics of a power converter, the computer program product comprising computer program code which is able to, when run on processor circuitry of a control unit for a phase leg of the power converter, cause the control unit to:

obtain a voltage reference for the phase leg, wherein the phase leg has a plurality of cells;

for a present sample, obtain a flux error of the output voltage of the phase leg by:

calculating a voltage error of the present sample by subtracting an obtained voltage reference for the phase leg for a preceding sample from an estimated voltage output of the phase leg for said preceding sample; and

integrating the calculated voltage error over a sampling time period to obtain the flux error of the present sample;

divide the present sample flux error by a time period to obtain an average voltage error over said time period;

subtract an obtained processed average voltage error, based on the average voltage error, from the voltage reference to obtain a modified voltage reference for the phase leg; and

provide the modified voltage reference to a modulation unit of the converter for controlling the phase leg in a present cycle time period, such that the modulation unit generates switching state signals to switch on or off of all of the cells in the phase leg using the modified voltage reference, thereby compensating for the average voltage error that is not compensated in the preceding sample to control the phase leg in the present cycle time period, and reducing the lower order harmonics of the phase leg.

20. The non-transitory computer program product according to claim 19 , wherein the non-transitory computer program product is embodied on a non-transitory computer readable medium.