Wind energy installation and method of controlling the output power from a wind energy installation

1. A method of controlling the output power from a wind energy installation to a utility grid having a specified nominal frequency, comprising:

determining an actual grid frequency;

reducing the output power to the utility grid by a first linear function when the actual grid frequency is higher than a first value; and

reducing the output power to the utility grid by a second linear function, different from said first linear function, when the actual grid frequency is higher than a second value of the grid,

wherein the second value being greater than the first value, and

wherein a rate of reducing the output power to the utility grid by the first linear function is a non zero value and a rate of reducing the output power to the utility grid by the second linear function is a non zero value.

2. The method as claimed in claim 1 , wherein the first value is the nominal frequency of the grid so that the output power to the utility grid is reduced as soon as any increase of the grid frequency above the nominal frequency is detected.

3. The method as claimed in claim 1 , wherein the rate of reducing the output power to the utility grid by the second linear function is greater than the rate of reducing the output power to the utility grid by the first linear function.

4. A wind energy installation connected to a utility grid for delivering output power to the utility grid comprising:

a wind driven generator;

a generator electronics;

a frequency sensor constructed and arranged to measure the grid frequency present in the utility grid and to output a frequency signal representing the grid frequency; and

a control unit connected to the generator electronics for delivering a control signal and to the frequency sensor for receiving the frequency signal, and which calculates the control signal depending on the grid frequency such that:

the output power to the utility grid is reduced by a first linear function as soon as any increase of the grid frequency above a first value is detected, and

the output power to the utility grid is reduced by a second linear function, different from said first linear function, as soon as any increase of the grid frequency above a second value is detected,

wherein the first value being at most two per mill higher than the nominal frequency of the grid,

wherein the second value being greater than the first value, and

wherein a rate of reducing the output power to the utility grid by the first linear function is a non zero value and a rate of reducing the output power to the utility grid by the second linear function is a non zero value.

5. The wind energy installation as claimed in claim 4 , wherein first value is nominal frequency of the grid and the control unit is adapted to calculate the control signal depending on the grid frequency such that the output power is reduced as soon as any increase of the grid frequency above the nominal grid frequency is detected.

6. The method as claimed in claim 1 , wherein the first value is 2 per mill higher than the nominal frequency of the grid.

7. The method as claimed in claim 6 , wherein the second value is 6 per mill higher than the nominal frequency of the grid.

8. The wind energy installation as claimed in claim 4 , wherein the rate of reducing the output power to the utility grid by the second linear function is greater than the rate of reducing the output power to the utility grid by the first linear function.

9. The wind energy installation as claimed in claim 4 , wherein the first value is 2 per mill higher than the nominal frequency of the grid.

10. The wind energy installation as claimed in claim 9 , wherein the second value is 6 per mill higher than the nominal frequency of the grid.

11. A method of controlling the output power from a wind energy installation to a utility grid having a specified nominal frequency, comprising:

determining an actual grid frequency;

reducing the output power to the utility grid by a non linear function when the actual grid frequency is higher than a predetermined value.

12. The method as claimed in claim 11 , wherein the predetermined value is the nominal frequency of the grid and the output power to the utility grid is reduced as soon as any increase of the grid frequency above the nominal frequency is detected.

13. The method as claimed in claim 11 , wherein the rate of reducing the output power to the utility grid varies with the magnitude of a deviation of the grid frequency from the nominal frequency such that as the deviation increases the rate increases.