Method and apparatus for determining a weakened grid condition and controlling a power plant in a manner appropriate to the grid condition

1. A method comprising:

over a time horizon during which a varying output occurs in at least one selected wind turbine generator in a wind power plant comprising a plurality of wind turbine generators, measuring output parameters of the at least one selected wind turbine generator comprising reactive power and voltage;

determining a relationship between the measured output parameters of the at least one selected wind turbine generator, the relationship between the measured output parameters of the at least one selected wind turbine generator being indicative of a condition of a power grid to which the wind power plant supplies power;

controlling the wind power plant in a manner responsive to the relationship between the measured output parameters of the at least one selected wind turbine generator;

wherein the relationship between the measured output parameters of the at least one selected wind turbine generator is effective to estimate a short circuit ratio in the power grid; and

wherein the controlling of the wind power plant comprises selecting a predefined normal control mode in a controller for the wind power plant when the estimate of the short circuit ratio is above a predefined first limit, selecting a predefined weak grid control mode in the controller for the wind power plant when the estimate of the short circuit ratio is below the predefined first limit and above a predefined second limit, and reducing output of the wind power plant when the estimate of the short circuit ratio is below the predefined second limit.

2. The method of claim 1 , wherein the varying output occurs in response to a natural variation in the output of the at least one selected wind turbine generator.

3. The method of claim 1 , wherein the varying output occurs in response to a commanded variation in the output of the at least one selected wind turbine generator.

4. The method of claim 1 , wherein the relationship between the measured output parameters of the at least one selected wind turbine generator is based on a response over the time horizon of the reactive power to changes in the voltage, or is based on a response over the time horizon of the voltage to changes in the reactive power.

5. The method of claim 1 , wherein the relationship between the measured output parameters of the at least one selected wind turbine generator is characterized by a slope defined by said parameters.

6. The method of claim 1 , wherein the controlling of the wind power plant comprises reducing the output of the wind power plant when the estimate of the short circuit ratio is below the predefined second limit, but not when it is above the predefined second limit.

7. The method of claim 1 , wherein a response of the measured output parameters of the at least one selected wind turbine generator during the varying output in the selected wind turbine generator is compensated for by control of other wind turbine generators in the wind plant.

8. The method of claim 1 , wherein the wind turbine plant comprises a plurality of collector systems being fed by respective different sets of wind turbine generators, wherein the steps of measuring and determining are performed in at least one of the wind turbine generators in the respective different sets of the wind turbine generators feeding the plurality of collector systems.

9. The method of claim 1 , wherein the predefined normal control mode differs from the predefined weak grid control mode in at least one of a gain and a time delay of a plant control algorithm.

10. A method comprising:

over a time horizon during which a varying output occurs in at least one selected generator of a power plant, measuring output parameters of the at least one selected generator comprising reactive power and voltage;

determining a relationship between the measured output parameters of the at least one selected generator, the relationship between the measured output parameters of the at least one selected generator being indicative of a condition of a power grid to which the power plant supplies power; and

selecting a predefined normal control mode in a controller for the wind power plant when the relationship is above a predefined first limit, selecting a predefined weak grid control mode in the controller for the wind power plant when the relationship is below the predefined first limit and above a predefined second limit, and reducing output of the wind power plant when the relationship is below the predefined second limit.

11. The method of claim 10 , wherein the at least one selected generator comprises a wind turbine generator and the varying output occurs in response to a natural variation in the wind turbine generator output, or in response to a commanded variation in the wind turbine generator output.

12. The method of claim 10 , wherein the at least one selected generator comprises a solar photovoltaic generator and the varying output occurs in response to a natural variation in the solar photovoltaic generator output, or in response to a commanded variation in the solar photovoltaic generator output.

13. The method of claim 10 , wherein the relationship between the measured generator output parameters is based on a response over the time horizon of the reactive power to changes in the generator voltage over the time horizon, or is based on a response over the time horizon of the generator voltage to changes in the reactive power.

14. An apparatus comprising:

a measuring device coupled to at least one selected generator in a power plant, wherein over a time horizon when a varying output occurs in the at least one selected generator, the measuring device is arranged to measure output parameters of the at least one selected generator comprising reactive power and voltage;

a processing module configured to determine a relationship between the measured output parameters of the at least one selected generator, the relationship between the measured output parameters of the at least one selected generator being indicative of a condition of a power grid to which the power plant supplies power; and

a controller configured to control the power plant in a manner responsive to the relationship between the measured output parameters of the at least one selected generator, wherein the relationship between the measured output parameters of the at least one selected generator is effective to estimate a short circuit ratio in the power grid, wherein the controller is configured to control the power plant in a predefined normal control mode when the short circuit ratio is above a predefined first limit, in a predefined weak grid control mode when the short circuit ratio is below the predefined first limit and above a predefined second limit, and to reduce output of the wind power plant when the short circuit ratio is below the predefined second limit;

wherein the predefined normal control mode differs from the predefined weak grid control mode in at least one of a gain and a time delay of a plant control algorithm.

15. The apparatus of claim 14 , further comprising a logic unit responsive to the relationship determined between the measured output parameters of the at least one selected generator to select a control mode appropriate to the grid condition.

16. A wind turbine power plant comprising the apparatus of claim 14 .

17. A solar photovoltaic power plant comprising the apparatus of claim 14 .