Controlling a wind turbine during over-voltage ride through

1. A method for operating a wind turbine comprising a power generator, a generator side converter, a grid side converter, and a DC link electrically connected to an output of the generator side converter and an input of the grid side converter, the method comprising:

monitoring a wind turbine signal to detect an operational condition requiring an increase of an output voltage of the grid side converter; and

responsive to detecting the operational condition:

initiating an over-modulation mode, wherein during a first sub-period of the over-modulation mode, the grid side converter is operated with a modulation index being increased to a value in an over-modulation range, wherein the first sub-period follows a second sub-period of the over-modulation mode during which the grid side converter is operated with the modulation index in the over-modulation range; and

initiating a DC voltage adjustment mode, wherein during a third sub-period of the DC voltage adjustment mode, a DC voltage of the DC link is decreased from a second voltage level towards a first voltage level, wherein the third sub-period follows a fourth sub-period of the DC voltage adjustment mode during which the DC voltage is increased from the first voltage level towards the second voltage level.

2. The method according to claim 1 , wherein a rate of decreasing the DC voltage and a rate of increasing the modulation index are matched in order to achieve a desired value of the output voltage at least during the first sub-period or the third sub-period.

3. The method according to claim 1 , wherein the first sub-period and the third sub-period are at least partly overlapping.

4. The method according to claim 1 , wherein the first sub-period depends on a duration of the detected operational condition.

5. The method according to claim 1 , further comprising:

obtaining accumulated DC voltage data based on a level of the increased DC voltage and time data, where the accumulated DC voltage data relates to an accumulated time where the DC voltage is above a predetermined voltage level.

6. The method according to claim 5 , further comprising:

determining, based on the accumulated DC voltage data, one or more of: the second voltage level, a duration of the third sub-period, and a rate of decreasing the DC voltage.

7. The method according to claim 1 , wherein the detected operational condition is an over-voltage situation where a grid voltage is above a nominal grid voltage level.

8. The method according to claim 1 ,

wherein in the second sub-period the grid side converter is operated with the modulation index in the over-modulation range for a predetermined or variable period of time.

9. The method according to claim 8 , wherein the second sub-period and the fourth sub-period are concurrent.

10. The method according to claim 8 , further comprising:

reducing the modulation index during the second sub-period from a value in the over-modulation range to a value in a linear modulation range depending on a level of the DC voltage.

11. The method according to claim 10 , wherein the modulation index is reduced to the value in the linear modulation range in response to the DC voltage reaching the second voltage level.

12. The method according to claim 10 , wherein a rate of decreasing the modulation index during the second sub-period and a rate of increasing the DC voltage during the fourth sub-period are matched in order to achieve a desired value of the output voltage.

13. The method according to claim 1 , further comprising:

changing the modulation index dependent on a level of the DC voltage between the first voltage level and the second voltage level.

14. A control system for operating a wind turbine, wherein the wind turbine comprises a power generator, a generator side converter, a grid side converter, and a DC link electrically connected to an output of the generator side converter and an input of the grid side converter; wherein the control system comprises:

one or more computer processors configured to:

monitor a wind turbine signal to detect an operational condition requiring an increase of an output voltage of the grid side converter; and

responsive to detecting the operational condition:

initiate an over-modulation mode, wherein during a first sub-period of the over-modulation mode, the grid side converter is operated with a modulation index being increased to a value in an over-modulation range, wherein the first sub-period follows a second sub-period of the over-modulation mode during which the grid side converter is operated with the modulation index in the over-modulation range; and

initiate a DC voltage adjustment mode, wherein during a third sub-period of the DC voltage adjustment mode, a DC voltage of the DC link is decreased from a second voltage level towards a first voltage level, wherein the third sub-period follows a fourth sub-period of the DC voltage adjustment mode during which the DC voltage is increased from the first voltage level towards the second voltage level.

15. A wind turbine, comprising:

a power generator;

a generator side converter;

a grid side converter;

a DC link electrically connected to an output of the generator side converter and an input of the grid side converter; and

a control system configured to:

monitor a wind turbine signal to detect an operational condition requiring an increase of an output voltage of the grid side converter;

responsive to detecting the operational condition:

initiate an over-modulation mode, wherein during at least a first sub-period of the over-modulation mode, the grid side converter is operated with a modulation index being increased to a value in an over-modulation range, wherein the first sub-period follows a second sub-period of the over-modulation mode during which the grid side converter is operated with the modulation index in the over-modulation range; and

initiate a DC voltage adjustment mode, wherein during at least a third sub-period of the DC voltage adjustment mode, a DC voltage of the DC link is decreased from a second voltage level towards a first voltage level, wherein the third sub-period follows a fourth sub-period of the DC voltage adjustment mode during which the DC voltage is increased from the first voltage level towards the second voltage level.

16. The control system according to claim 14 , wherein the detected operational condition is an over-voltage situation where a grid voltage is above a nominal grid voltage level.

17. The control system according to claim 14 , wherein the second sub-period and the fourth sub-period are concurrent.

18. The control system according to claim 14 , wherein the one or more computer processors are further configured to:

reduce the modulation index during the second sub-period from a value in the over-modulation range to a value in a linear modulation range depending on a level of the DC voltage.

19. The wind turbine according to claim 15 , wherein the second sub-period and the fourth sub-period are concurrent.

20. The wind turbine according to claim 15 , wherein the control system is further configured to:

reduce the modulation index during the second sub-period from a value in the over-modulation range to a value in a linear modulation range depending on a level of the DC voltage.