Cells control in a multilevel converter

1. A method of controlling cells in a phase leg of a multilevel converter operating to convert between alternating current and direct current,

the method being performed by a cell control unit of the converter and comprising

regulating, using tap-changer control, a modulation index of the converter to above 0.80,

providing a group of carrier waves for the phase leg, where the carrier waves of a phase leg are identical but displaced in time from each other with a time delay,

providing a voltage reference for the phase leg being separate from voltage references provided for other phase legs of the converter,

stopping switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform,

controlling switching of the cells based on comparisons of the respective carrier waves with the voltage reference when the waveform values of the power transmission converter waveform are detected to be outside said selected interval, and

obtaining measurements of a phase arm current, filtering out harmonic components of the phase arm current and adjusting the voltage reference based on the filtered harmonic components.

2. The method according to claim 1 , wherein an interval is in the range of ±15-±30 degrees around the corresponding peak value.

3. The method according to claim 2 , wherein the modulation index is in the range 0.80-0.86.

4. The method according to claim 2 , wherein the power transmission converter waveform is the voltage reference.

5. The method according to claim 2 , wherein the power transmission converter waveform is a phase arm current.

6. The method according to claim 2 , wherein the adjusting comprises PI regulating voltage reference through PI regulation of the filtered harmonic components.

7. The method according to claim 1 , wherein the modulation index is in the range 0.80-0.86.

8. The method according to claim 7 , wherein the power transmission converter waveform is the voltage reference.

9. The method according to claim 7 , wherein the power transmission converter waveform is a phase arm current.

10. The method according to claim 1 , wherein the power transmission converter waveform is the voltage reference.

11. The method according to claim 1 , wherein the power transmission converter waveform is a phase arm current.

12. The method according to claim 1 , wherein the adjusting comprises PI regulating voltage reference through PI regulation of the filtered harmonic components.

13. The method according to claim 12 , wherein the filtered harmonic components comprise the fifth/and or seventh harmonic components.

14. A control device for a multilevel converter operating to convert between alternating current and direct current, the converter being connected to a transformer controllable via a tap changer and comprising three phase legs with cells, the control device comprising

a central control element configured to

regulate, using tap-changer control, a modulation index of the converter to above 0.80, and

provide a voltage reference for the phase leg being separate from voltage references provided for other phase legs,

a carrier providing block providing a group of carrier waves for a phase leg, where the carrier waves of the phase leg are identical but displaced in time from each other with a time delay,

a group of cell control blocks, where each cell control block is configured to compare a corresponding carrier wave with the voltage reference and control a corresponding cell based on the comparison,

a switching control block configured to

stop the cell control blocks from switching the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform, and

allow the cell control blocks to control switching of the cells when the waveform values of the power transmission converter waveform are detected to be outside said selected interval, and

a reference voltage adjusting block configured to

obtain measurements of a phase arm current,

filter out harmonic components of the phase arm current and

adjust the voltage reference based on the filtered harmonic components.

15. The control device according to claim 14 , wherein an interval is in the range of ±15-±30 degrees around the corresponding peak value.

16. The control device according to claim 14 , wherein the power transmission converter waveform is the voltage reference.

17. The control device according to claim 14 , wherein the power transmission converter waveform is a phase arm current.

18. A multilevel converter operating to convert between alternating current and direct current, the converter being connected to a transformer controllable via a tap changer and comprising:

three phase legs with cells, and

a control unit comprising a central control element, a group of cell control blocks a carrier providing block, a switching control block and a reference voltage adjusting block according to claim 14 .

19. The multilevel converter according to claim 18 , further comprising a phase arm current detector in the first phase leg configured to measure a phase arm current running through a phase arm of the phase leg.

20. A computer program product for controlling cells in a phase leg of a multilevel converter operating to convert between alternating current and direct current,

said computer program product being provided as a data carrier comprising computer program code configured to cause a control device to, when said computer program code is loaded into the control device

regulate, using tap-changer control, a modulation index of the converter to above 0.80,

provide a group of carrier waves for the phase leg, where the carrier waves of a phase leg are identical but displaced in time from each other with a time delay,

provide a voltage reference for the phase leg being separate from voltage references provided for other phase legs of the converter,

stop switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform,

control switching of the cells based on comparison of the respective carrier waves with the voltage reference when the waveform values of the power transmission converter waveform are detected to be outside said selected interval, and

obtain measurements of a phase arm current, filtering out harmonic components of the phase arm current and adjusting the voltage reference based on the filtered harmonic components.