Method for operating a wind power plant in a weak grid environment and a wind power plant

1. A method for operating a wind power plant in a weak grid environment, the wind power plant coupled to an electrical grid and comprising a power plant controller for controlling a plurality of wind turbine generators of the wind power plant, the method comprising:

providing, using the power plant controller, a first active power reference to the plurality of wind turbine generators;

determining whether the electrical grid fulfills a weak grid criterion;

when the weak grid criterion is fulfilled, setting the power plant controller to operate in a weak grid mode, wherein operating the power plant controller to operate in the weak grid mode comprises:

when a grid voltage deviates from a normal operating voltage beyond a threshold voltage and before the grid voltage begins to return to the normal operating voltage, providing a second active power reference to the plurality of wind turbine generators, wherein the second active power reference is less than the first active power reference; and

when the electrical grid recovers from the voltage deviation, controlling each wind turbine generator of the plurality of wind turbine generators to generate a reduced amount of active power based on the second active power reference.

2. The method of claim 1 , wherein fulfilling the weak grid criterion comprises:

determining a short-circuit ratio of the electrical grid; and

determining that the short-circuit ratio is less than a predetermined value.

3. The method of claim 1 , further comprising:

maintaining the generation of the reduced amount of active power for a predetermined time period after the electrical grid recovers from the voltage deviation.

4. The method of claim 3 , further comprising ramping up the active power generated by the plurality of wind turbine generators after the predetermined time period, the ramping up of the active power being carried out at a predetermined ramp up rate.

5. The method of claim 4 , further comprising ramping up the active power to a maximum capability of each of the plurality of wind turbine generators.

6. The method of claim 4 ,

wherein ramping up the active power generated is responsive to determining that the grid voltage is within a range defined by the threshold voltage and the normal operating voltage for a predetermined time period prior.

7. The method of claim 3 , further comprising:

providing a varying amount of reactive power injection for the predetermined time period, the varying amount being based on a difference between a predetermined grid voltage reference level and a measured grid voltage amount.

8. The method of claim 4 , further comprising:

determining, after ramping up the active power, whether the electrical grid fulfills the weak grid criterion; and

when the weak grid criterion is not fulfilled, setting the power plant controller to operate in a normal grid strength operating mode.

9. The method of claim 1 , further comprising:

when the grid voltage deviates beyond a second threshold voltage from the normal operating voltage, reducing the active power generated by each of the plurality of wind turbine generators to generate the reduced amount of active power.

10. The method of claim 1 , wherein, when the grid voltage deviates beyond the threshold voltage, controlling the power plant controller to provide the second active power reference to the plurality of wind turbine generators prior to the plurality of wind turbine generators entering a fault ride-through mode.

11. The method of claim 10 , further comprising:

controlling the power plant controller to provide an indication to the plurality of wind turbine generators to exit the fault ride-through mode.

12. The method of claim 10 , wherein a first wind turbine generator of the plurality of wind turbine generators is designated to represent a wind power plant fault ride-through state, and wherein the first wind turbine generator provides an indication to the power plant controller to exit the fault ride-through mode.

13. The method of claim 10 , wherein during the fault ride-through mode the power plant controller freezes a plurality of controls and allows independent operation of each of the plurality of wind turbine generators.

14. The method of claim 10 , wherein when a predetermined number of the plurality of wind turbine generators have exited the fault ride-through mode and the power plant controller remains in the fault ride-through mode, the power plant controller receives a communication from the plurality of wind turbine generators and the power plant controller exits the fault ride-through mode.

15. The method of claim 4 , wherein any one of the threshold voltage, the second active power reference, the reduced amount of active power, and the ramp up rate is determined by an electrical site study.

16. The method of claim 1 , wherein the deviation of the grid voltage from the normal operating voltage comprises a drop in voltage; and

wherein the return to the normal operating voltage comprises a rise in voltage.

17. The method of claim 1 , wherein the deviation of the grid voltage from the normal operating voltage comprises a rise in voltage; and

wherein the return to the normal operating voltage comprises a drop in voltage.

18. A wind power plant coupled to an electrical grid, the wind power plant comprising:

a plurality of wind turbine generators; and

a power plant controller configured to control the plurality of wind turbine generators according to an operation, comprising:

a first active power reference provided by the power plant controller to the plurality of wind turbine generators;

determining whether the electrical grid fulfills a weak grid criterion;

when the weak grid criterion is fulfilled, setting the power plant controller to operate in a weak grid mode, wherein operating the power plant controller to operate in the weak grid mode comprises:

when a grid voltage deviates from a normal operating voltage beyond a threshold voltage, and before the grid voltage begins to return to the normal operating voltage, providing a second active power reference to the plurality of wind turbine generators, wherein the second active power reference is less than the first active power reference; and

when the electrical grid recovers from the voltage deviation, controlling each wind turbine generator of the plurality of wind turbine generators to generate a reduced amount of active power based on the second active power reference.

19. A computer program product directly loadable into an internal memory of at least one digital computer of a wind power plant, the wind power plant coupled to an electrical grid and comprising a plurality of wind turbine generators, the computer program product comprising software code portions for performing an operation when the computer program product is run on the at least one digital computer, the operation comprising:

providing a first active power reference to the plurality of wind turbine generators;

determining whether the electrical grid fulfills a weak grid criterion;

when the weak grid criterion is fulfilled and a grid voltage deviates from a normal operating voltage beyond a threshold voltage, and before the grid voltage begins to return to the normal operating voltage, providing an a second active power reference to the plurality of wind turbine generators, wherein the second active power reference is less than the first active power reference; and

when the electrical grid recovers from the voltage deviation, controlling each wind turbine generator of the plurality of wind turbine generators to generate a reduced amount of active power based on the second active power reference.