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A method of operating at partial load a wind turbine (1) supplying electrical power to an electrical grid (20) includes monitoring stability of the grid; and upon detection of a reduced grid stability, changing operation of the wind turbine by limiting wind-caused fluctuations of the active-power su

A method of operating at partial load a wind turbine (1) supplying electrical power to an electrical grid (20) includes monitoring stability of the grid; and upon detection of a reduced grid stability, changing operation of the wind turbine by limiting wind-caused fluctuations of the active-power supply to the electrical grid, or by reducing an already existing limit to fluctuations of the active-power supply.

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1. A method of operating at partial load a wind turbine supplying an output power to an electrical grid, the electrical grid being equipped with regulation capacity against grid-frequency fluctuations, the method comprising: monitoring a grid stability of the electrical grid, wherein monitoring the

1. A method of operating at partial load a wind turbine supplying an output power to an electrical grid, the electrical grid being equipped with regulation capacity against grid-frequency fluctuations, the method comprising: monitoring a grid stability of the electrical grid, wherein monitoring the grid stability comprises determining a correlation parameter relating the supplied output power and grid frequency; andupon determining a reduction of the grid stability indicated by a predefined change to the correlation parameter, changing operation of the wind turbine by imposing a limit for wind-caused fluctuations in the supplied output power relative to a reference output power level. 2. The method of claim 1, wherein monitoring the grid stability comprises determining at least one of a grid-frequency fluctuation range and a variance of grid-frequency fluctuations. 3. The method of claim 1, wherein a larger value of the correlation parameter corresponds to a smaller value of the grid stability, and wherein determining a reduction of the grid stability is indicated by an increase to the correlation parameter. 4. The method of claim 1, wherein determining the correlation parameter comprises performing a test variation of the supplied output power and measuring a grid-frequency response to the test variation. 5. The method of claim 1, wherein the correlation parameter is determined according to a correlation function Rxy=1TF⁢∫-TF/2TF/2⁢x⁡(t)·y⁡(t)⁢⁢ⅆt, where x(t) represents the supplied output power as a function of time t, y(t) represents the grid frequency as a function of time t, and TF represents a time period for which the correlation parameter is determined. 6. The method of claim 2, wherein the limit is selected such that: grid-frequency fluctuations caused by the supplied output power are maintained inside a range (Fmax/Fmin) from a minimum to a maximum allowable frequency, or a variance of grid-frequency fluctuations caused by the supplied output power is maintained below a variance limit. 7. The method of claim 2, wherein the limit is continuously adjusted to maintain the grid frequency within a range (Fmax/Fmin) from a minimum to a maximum allowable frequency, or within a variance of grid-frequency fluctuations below a variance limit. 8. The method of claim 1, wherein changing operation of the wind turbine comprises: switching from a normal-operation mode to a reduced-fluctuation mode in which the limit is imposed or reduced, in response to determining a reduction of the grid stability below a mode-switch threshold value. 9. The method of claim 2, wherein a larger value of a correlation parameter relating supplied output power and the grid frequency corresponds to imposing a stricter limit for wind-caused fluctuations in the supplied output power. 10. The method of claim 1, wherein positive fluctuations in the supplied output power are limited by cutting high-output peaks. 11. The method of claim 1, wherein the wind turbine has an optimal working point, and wherein the operating of the wind turbine with an imposed limit comprises: operating the wind turbine at a non-optimal working point,wherein negative fluctuations in the power supplied by the wind turbine are counteracted by shifting the non-optimal working point towards the optimal working point. 12. The method according to claim 1, wherein the imposed limit is based on a wind forecast and is reduced responsive to the wind forecast forecasting increased wind-power fluctuation. 13. The method according to claim 1, wherein the imposed limit is based on an estimate of power consumption in the electrical grid and is reduced responsive to the power consumption estimate including a predicted increase of grid-frequency fluctuation. 14. The method of claim 1, wherein imposing the limit comprises reducing a previously-imposed limit for wind-caused fluctuations in the supplied output power. 15. The method of claim 8, wherein the mode-switch threshold value is increased responsive to a wind forecast forecasting increased wind-power fluctuation. 16. The method of claim 8, wherein the mode-switch threshold value is increased responsive to an estimate of power consumption in the electrical grid that includes a predicted increase of grid-frequency fluctuation. 17. The method of claim 4, wherein the measured grid-frequency response is used to determine whether to resume a previous operation mode of the wind turbine by cancelling the imposed limit. 18. The method of claim 1, wherein imposing the limit includes one or more of blade-pitch adjustments and electrical controls of a power converter of the wind turbine. 19. The method of claim 1, wherein the reference output power level is time-varying and comprises an average of the supplied output power. 20. A control system for controlling at least one wind turbine supplying an output power to an electrical grid, the control system configured to: monitor a grid stability of the electrical grid, the electrical grid being equipped with regulation capacity against grid-frequency fluctuations, wherein monitoring the grid stability comprises determining a correlation parameter relating the supplied output power and grid frequency; andupon determining a reduction of the grid stability indicated by a predefined change to the correlation parameter, change operation of the wind turbine by imposing a limit for wind-caused fluctuations in the supplied output power relative to a reference output power level. 21. A wind park, comprising: a wind turbine configured for supplying an output power to an electrical grid equipped with regulation capacity against grid-frequency fluctuations; anda control system for operating the wind turbine at partial load, the control system configured to: monitor a grid stability of the electrical grid, wherein monitoring the grid stability comprises determining a correlation parameter relating the supplied output power and grid frequency; andupon determining a reduction of the grid stability indicated by a predefined change to the correlation parameter, change operation of the wind turbine by imposing a limit for wind-caused fluctuations in the supplied output power relative to a reference output power level.