Method for stabilising a rotor of a wind turbine

Abstract

The present invention relates to a method for stabilizing a rotor of a wind turbine during a time period following an abnormal grid event, the method comprising the steps of detecting an occurrence of the abnormal grid event, reducing an allowable rotor thrust from a first thrust limit to a second thrust limit, detecting that the abnormal grid event has ended, and maintaining the second thrust limit a selected time period after the abnormal grid event has ended. The present invention further relates to a wind turbine controller and a computer program product for performing this method.

Claims

1. A method for stabilizing a rotor of a wind turbine, the method comprising: detecting, by a controller of a wind turbine, an occurrence of a low voltage event; setting, by the controller, a maximum allowable thrust of a rotor of the wind turbine in response to detecting the occurrence of the low voltage event; in response to detecting that the low voltage event ended, starting, by the controller, a timer; maintaining, by the controller, the set maximum allowable thrust of the rotor until the timer has expired; and increasing the set maximum allowable thrust of the rotor after the timer has expired.

2. The method of claim 1, wherein the timer runs for less than ten seconds before expiring.

3. The method of claim 1, wherein the set maximum allowable thrust when the timer is started is at least 20% lower than the set maximum allowable thrust after the set maximum allowable thrust is increased.

4. The method of claim 1, wherein setting the set maximum allowable thrust comprises reducing the set maximum allowable thrust with a predefined rate.

5. A control system for stabilizing a rotor of a wind turbine, the control system comprising: a voltage detector; and a controller for: detecting, based on the voltage detector, an occurrence of a low voltage event; setting a maximum allowable thrust of a rotor of a wind turbine during at least part of the low voltage event; detecting that the low voltage event ended; starting, in response to detecting that the low voltage event ended, a timer; maintaining the set maximum allowable thrust of the rotor until the timer has expired; and increasing the set maximum allowable thrust of the rotor after the timer has expired.

6. The control system according to claim 5, wherein setting the maximum allowable thrust comprises reducing the set maximum allowable thrust with a predefined rate.

7. A computer program product comprising a non-transitory computer readable medium storing instructions that, when executed by a microprocessor of a control system of a wind turbine, cause the microprocessor to perform an operation comprising: detecting an occurrence of a low voltage event; setting a maximum allowable thrust of a rotor of the wind turbine in response to detecting the occurrence of the low voltage event; in response to detecting that the low voltage event ended, starting a timer; maintaining the set maximum allowable thrust of the rotor until the timer has expired; and increasing the set maximum allowable thrust of the rotor after the timer has expired.

8. The computer program product of claim 7, wherein the set maximum allowable thrust when the timer is started is at least 20% lower than the set maximum allowable thrust after the set maximum allowable thrust is increased.

9. The computer program product of claim 7, wherein setting the set maximum allowable thrust comprises reducing the set maximum allowable thrust with a predefined rate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described in further details with reference to the accompanying figures, in which:

(2) FIG. 1 illustrates a timing diagram showing a sequence of events in connection with an abnormal grid event is detected, and

(3) FIG. 2 illustrates a block diagram showing the control system together with other blocks forming part of the present invention.

(4) While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of examples in the drawings and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS

(5) In general the present invention relates to a method for stabilising a rotor of a wind turbine in a time period following an abnormal grid event. As previously addressed the present invention provides a method and a control system that ensure that wind turbines remain connected to the power grid during abnormal grid events, and that rotor oscillations are prevented, or at least reduced, in connection with such abnormal grid events.

(6) FIG. 1 illustrates examples of an allowable rotor thrust 102 and an associated blade pitch angle 103 as a function of time before, during and after an abnormal grid. As previously addressed an abnormal grid event typically involves a pronounced change in the operating conditions of the grid, such as for example an LVE.

(7) As addressed above an LVE may be defined differently by the TSOs. Thus, an LVE may for example arise when a grid voltage is equal to or below for example 90% of the rated grid voltage. Other threshold values like for example 85%, 80% of the rated grid voltage may also be applicable.

(8) In FIG. 1 the abnormal grid event starts at around t=20 s (t.sub.1) and ends at around t=23 s (t.sub.2). The allowable rotor thrust before, under and after the abnormal grid event is illustrated by curve 102. The allowable rotor thrust 102 may be a value setting a minimum or maximum limit on the allowable rotor thrust. Alternatively, the allowable rotor thrust 102 may be a set-point that the wind turbine is selected to work around.

(9) When an abnormal grid event is detected at t.sub.1, a signal flag is raised, cf. curve 101 in FIG. 1. This signal flag may be a binary 1 or a logic true. At the same time, i.e. at t.sub.1, the allowable rotor thrust 102 starts to decrease from a first thrust limit to a second thrust limit, resulting in an increase in the pitch angle 103 as the wind turbine, due to the reduced allowable rotor thrust, needs to pitch out of the wind. The first and second thrust limits are mutually related as previously discussed. Lowering the allowable rotor thrust 102 gives the rotor of the wind turbine some time to stabilise after the abnormal grid event has been detected.

(10) When the abnormal grid event has ended at t.sub.2, the controller of the wind turbine sends a signal flag indicating the end of the abnormal grid event. Such a signal flag may be a binary 0 or a logic false.

(11) The allowable rotor thrust 102 is maintained low, i.e. at the second thrust limit, for a selected time period after the abnormal grid event has ended at t.sub.2. This is done using for example a timer or counter defining the selected time period before the allowable rotor thrust 102 may be increased to a higher allowable thrust limit, such as to the first thrust limit. It should be noted that the allowable rotor thrust 102 may alternatively be increased to intermediate allowable thrust limits, such as an intermediate allowable thrust limit between the first thrust limit and the second thrust limit.

(12) The selected time period may be given in seconds or any other units that may indicate a time period, such as a digit for use in a counter. The selected time period may be smaller than 20 s, such as smaller than 15 s, such as smaller than 10 s, such as smaller than 8 s, such as smaller than 5 s, such as around 3 s as shown in FIG. 1, cf. curve 102.

(13) As mentioned above, the allowable rotor thrust 102 is increased to a higher thrust limit after the selected time period has ended at t.sub.3. This higher thrust limit may undergo a transient time period, but eventually it reaches a steady-state operating condition at t.sub.4. At t.sub.3 the pitch angle 103 also starts decreasing, which is a result of the higher allowable rotor thrust limit.

(14) In FIG. 1 the allowable rotor thrust 102 reaches a steady-state operating condition after a time period of around 7 s. However, this steady-state period is not fixed and may thus depend on the severity of the abnormal grid event and the ability of the wind turbine controller to act properly when subjected to such an event.

(15) FIG. 2 illustrates a block diagram of a part of the overall control system of a wind turbine. In general, the overall control system of the wind turbine is responsible for operating the wind turbine in response to various parameters.

(16) The controller 202 of the control system 201 shown in FIG. 2 is configured to operate the wind turbine in accordance with a safe mode of operation. This safe mode implies that 1) a rotor thrust limit is calculated or selected in response to detecting an occurrence or an ending of an abnormal grid event, and 2) the calculated or selected rotor thrust limit is used to compute a pitch reference angle 207 for pitching the rotor blades of the wind turbine in order to stabilise the rotor of the wind turbine.

(17) Thus, operating the controller 202 in a safe mode of operation prevents, or at least reduces, rotor oscillations of the wind turbine in connection with abnormal grid events. The controller 202 typically comprises a number of inputs 203. These inputs 203 may receive measurements and/or signals from measurement devices and/or other control systems. These measurements and/or signals may involve grid current, grid voltage, grid frequency and power.

(18) The safe mode of the controller 202 provides an output thrust limit 204 that is processed in the data processor 205. The data processor 205 may optionally include other input signals 206 during its data processing. Again, these other input signals 206 may involve grid current, grid voltage, grid frequency and power.

(19) As a result the output of the data processor 205 is a pitch angle reference 207 which is applied as a pitch setting of the rotor blades in order to comply with the rotor thrust limit computed by the by the controller 202.