Wind turbine control for a weak grid by reducing active power output

1. A method for controlling a wind turbine connected to a node connected to a utility grid, wherein to the node a plurality of other wind turbines is connected, the method comprising:

measuring a quantity indicative of a slope of a voltage at the node in dependence of active power delivered to the node;

determining that the slope is smaller than a negative slope limit;

performing a measure, in order to increase the slope above the slope limit.

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

obtaining another quantity indicative of the voltage at the node;

determining that the voltage is smaller than a voltage limit;

wherein the measure performed, in order to increase the slope above the slope limit, is adapted to increase the voltage.

3. The method according to claim 2 , wherein the voltage limit is 0.95 of a rated voltage.

4. The method according to claim 2 , wherein the voltage limit is 0.9 of a rated voltage.

5. The method according to claim 1 , wherein the measuring the quantity and/or the obtaining the other quantity comprises measuring the respective quantity at the node, at a wind turbine output terminal, at a high voltage side of a park transformer, at a high voltage side of a wind turbine transformer and/or at a low voltage side of a wind turbine transformer.

6. The method according to claim 1 , wherein performing the measure comprises performing a first measure comprising:

changing a transformation ratio of a wind park transformer connected between a wind turbine output terminal and the node such that a voltage at the low voltage side of the wind park transformer increases, the high voltage side being connected to the node.

7. The method according to claim 6 , wherein the first measure further comprises, in order to increase the slope above the slope limit:

increasing reactive power output by the wind turbine using a capacitor or a wind turbine converter connected between a wind turbine generator and the wind turbine output terminal; and/or

increasing reactive power absorbed by the wind turbine using a capacitor or a wind turbine converter connected between a wind turbine generator and the wind turbine output terminal.

8. The method according to claim 6 , wherein performing the measure comprises, if the first measure is not successful, performing a second measure ( 215 ), comprising:

reducing active power output by the wind turbine,

in order to increase the slope above the slope limit.

9. The method according to claim 8 , wherein an amount of reducing the active power is based on a simulation of the wind turbine, a transmission line between the wind turbine output terminal and the node taking into account an impedance of the utility grid.

10. The method according to claim 8 , wherein performing the second measure comprises keeping the wind turbine connected to the utility grid.

11. The method according to claim 8 , wherein performing the second measure comprises increasing the voltage.

12. The method according to claim 9 , wherein the reducing the active power output by the wind turbine is performed such that the voltage is in a predetermined range by taking into account a result of the simulation.

13. The method according to claim 1 , wherein the measuring the quantity and/or the obtaining the other quantity and/or the determining that the slope is smaller than a negative slope limit and/or the determining that the voltage is smaller than a voltage limit is performed repeatedly.

14. The method according to claim 13 , wherein said repeatedly performing of the determining that the voltage is smaller than a voltage limit is performed continuously or periodically with a repeating period between 1 ms and 10 s.

15. The method according to claim 14 , wherein the repeating period is between 10 ms and 1 s.

16. The method according to claim 1 , wherein an increase of the impedance of the utility grid leads to a decrease of the slope.

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

determining that the slope is equal or larger than the negative slope limit;

increasing the active power output by the wind turbine.

18. An arrangement for controlling a wind turbine connectable to a node connectable to the utility grid, wherein to the node a plurality of other wind turbines is connectable, the arrangement comprising:

an input terminal for receiving a measured quantity indicative of a slope of voltage at the node in dependence of active power delivered to the node;

a determining section adapted to determine that the slope is smaller than a negative slope limit;

an output terminal for outputting a control signal for performing a measure, in order to increase the slope above the slope limit.