Abstract:
A method of controlling the operation of a variable speed wind turbine ( 11 ), tracking a power curve ( 25, 27 ) including a nominal operational region ( 3  ) and sub-nominal operational regions ( 2, 1, 0 ), comprising steps of: a) implementing a control strategy to follow said power curve in said nominal operational region ( 3 ) based on the use of demanded torque T d  for controlling power P and on the use of demanded pitch θ d  for controlling demanded torque T d ; b) implementing a control strategy to follow said power curve in sub-nominal operational regions ( 2, 1, 0 ) based on the use of demanded torque T d  for controlling power P and on the setting of a constant optimum value for demanded pitch θ d  in each sub-nominal operational region ( 2, 1, 0 ). The invention also refers to a control system comprising one or more Adaptive Predictive Controllers ( 51, 53, 55, 59 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to variable speed wind turbine control methods and systems and, in particular, to variable speed wind turbine control methods and systems using an adaptive predictive approach. 
     BACKGROUND 
     Wind turbines are devices that convert mechanical energy to electrical energy. A typical wind turbine includes a nacelle mounted on a tower housing a drive train for transmitting the rotation of a rotor to an electric generator and other components such as a yaw drive which rotates the wind turbine, several controllers and a brake. The rotor supports a number of blades extending radially therefrom for capturing the kinetic energy of the wind and causing the driving train rotational motion. The rotor blades have an aerodynamic shape such that when a wind blows across the surface of the blade, a lift force is generated causing the rotation of a shaft which is connected—directly or through a gearing arrangement—to the electrical generator located inside the nacelle. The amount of energy produced by wind turbines is dependent on the rotor blade sweeping surface that receives the action from the wind and consequently increasing the length of the blades leads normally to an increase of the power output of the wind turbine. 
     Under known control methods and systems the power produced by a wind turbine increases with wind speed until a rated nominal power output is reached and then it is maintained constant. This is done regulating the pitching action of the blades so that the rotor blade&#39;s pitch angle is changed to a smaller angle of attack in order to reduce power capture and to a greater angle of attack to increase the power capture. Therefore the generator speed, and consequently, the power output may be maintained relatively constant with increasing wind velocities. 
     However in case of gusts and turbulences wind speed may change drastically in a relatively small interval of time requiring relatively rapid changes of the pitch angle of the blades to maintain constant the power output that are difficult to implement taking into account the dynamics of the pitch control actuator and the inertia of the mechanical components. As a result, generator speed may exceed the over speed limit and the wind turbine is shut down to avoid damages. 
     The power and rotor speed regulation implemented in most of the known commercial wind turbine control systems is based on a Proportional-Integral-Derivative (PID) approach which reacts to already produced errors between measured variables and its set points with its associated limitations. 
     In order to solve this problem there are known several proposals of control systems improving its performance particularly under wind speed varying conditions such as the proposal disclosed in WO 2008/046942 A1. 
     On the other hand there are known many general-purpose control systems. One of them is the adaptive predictive control system disclosed in Spanish Patents 460649 and 2206315 but the applicant does not know any proposal of an adaptive predictive control system for wind turbines. 
     Adaptive predictive controllers drive the controlled variable to desired values (set points) reacting to non-already produced errors. These controllers are based on an internal plant model in order to predict its future states. A second functionality is introduced when adapting the internal plant dynamic model parameters in order to take into account the plant evolutions. This kind of controllers require information in execution time which differs from the one used by the PID controllers. Consequently the use of these controllers in particular areas can not be carried out without deep strategy studies. 
     Therefore the known proposals involve the use of more information (particularly statistical data) than in commercial control systems and/or improved tools for the analysis of the relevant information but none of them provide a clear control strategy, easy to implement, that can cope with situations of rapid changes of the wind speed. 
     This invention is intended to solve this drawback. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide wind turbine control methods and systems that can cope with situations of rapid changes of the wind speed. 
     In is another object of the present invention to provide wind turbine control methods and systems allowing an adaptation to the eventual wind turbine dynamic evolutions. 
     In one aspect, these and other objects are met by providing a method of controlling the operation of a variable speed wind turbine, tracking a power curve including a nominal operational region and sub-nominal operational regions, comprising steps of: 
     a) Implementing a control strategy to follow said power curve in said nominal operational region based on the use of demanded torque T d  for controlling power P and on the use of demanded pitch θ d  for controlling demanded torque T d . 
     b) Implementing a control strategy to follow said power curve in sub-nominal operational regions based on the use of demanded torque T d  for controlling power P and on the setting of a constant optimum value for demanded pitch θ d  in each sub-nominal operational region. 
     In a preferred embodiment, in step a) said demanded pitch θ d  is determined by means of an adaptive predictive algorithm having as inputs torque set point T SP , demanded torque T d , measured generator speed Ω and measured pitch θ. Hereby it is achieved a control method that improves the wind turbine power production in the nominal region because of its better adaptation to the wind turbine dynamic evolution and the consequent reduction of the control variables standard deviation with respect to its set points. 
     In another preferred embodiment, in step a) wind speed V and nacelle fore-aft acceleration a x , are used as perturbations in said adaptive predictive algorithm. Hereby it is achieved a control method that allows a reduction of the wind turbine loads taking into account relevant specific load factors. 
     In another preferred embodiment, in step a) it is also used as an additional control variable the demanded pitch rate θr d  corresponding to the demanded pitch θ d  which is determined by means of adaptive predictive algorithms having as inputs the demanded pitch θ d  and the measured pitch θ. Hereby it is achieved a control method that allows an improved control of the pitch regulation. 
     In another preferred embodiment, in step b) the demanded torque T d  is determined by means of an adaptive predictive algorithm having as inputs the generator speed set point Ω SP , the measured generator speed Ω and the measured torque T. Hereby it is achieved a control method that improves the wind turbine power production in the sub-nominal regions because of its better adaptation to the wind turbine dynamic evolution and the consequent reduction of the control variables standard deviation with respect to its set points. 
     In another preferred embodiment, the control method also comprises an step for implementing a control strategy in region pre- 0  based on the use of demanded pitch θ d , determined by means of an adaptive predictive algorithm for controlling generator speed Ω. Hereby it is achieved a control method that improves the wind turbine start-up procedure. 
     In another aspect, the above mentioned objects are met by a control system for a variable speed wind turbine comprising measuring devices for measuring at least wind speed V, generator speed Ω, pitch angle θ, power P and the nacelle fore-aft acceleration a x , and also a control unit connected to said measuring devices and to the wind turbine control pitch and generator torque actuators comprising one or more of the following controllers implementing adaptive predictive algorithms taking into account the dynamics of the wind turbine physical components involved:
         A Torque with Pitch Adaptive Predictive Controller having as inputs torque set point T SP , demanded torque T d , measured generator speed Ω measured pitch θ, and as output the demanded pitch θ d  in the nominal operation region.   A Speed with Pitch Adaptive Predictive Controller having as inputs measured generator speed Ω, and as output the optimum pitch θ in the sub-nominal operation regions.   A Speed with Torque Adaptive Predictive Controller having as inputs the generator speed set point Ω SP , the measured generator speed Ω and the measured torque T and as output the demanded pitch T d , in the sub-nominal operation regions.   A Pitch Rate Adaptive Predictive Controller having as inputs the demanded pitch θ d  and the measured pitch θ and as output the demanded pitch rate θr d .       

     Hereby it is achieved a wind turbine control system easy to implement because it does not need a previous knowledge of the dynamic of each individual component as it happen in known control systems. It also allows a reduction of wind turbine component costs because involves lesser requirements regarding dimensional tolerances than known control systems. 
     Other characteristics and advantages of this invention will be seen from the detailed description which follows from an illustrative but not limitative embodiment of its aim, based on the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  shows schematically the main components of a wind turbine. 
         FIG. 2  shows the ideal power curve of a variable speed wind turbine. 
         FIG. 3  shows the ideal torque vs. rotor speed curve normally used in the control system of a variable speed wind turbine. 
         FIG. 4  is flow chart of functional blocks illustrating a wind turbine control method according to this invention. 
         FIG. 5  is a block diagram illustrating a wind turbine control system according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A typical wind turbine  11  comprises a tower  13  supporting a nacelle  21  housing a generator  19  for converting the rotational energy of the wind turbine rotor into electrical energy. The wind turbine rotor comprises a rotor hub  15  and, typically, three blades  17 . The rotor hub  15  is connected either directly or through a gearbox to the generator  19  of the wind turbine for transferring the torque generated by the rotor  15  to the generator  19  and increase the shaft speed in order to achieve a suitable rotational speed of the generator rotor. 
     The power output from a modern wind turbine is typically controlled by means of a control system for regulating the pitch angle of the rotor blades and the generator torque. The rotor rotational speed and power output of the wind turbine can hereby be initially controlled e.g. before a transfer to a utility grid through a converter. 
     The control strategy in nominal region aims to produce power as closed as possible to nominal value. 
     The control strategy in sub-nominal regions (when there is not enough wind available for nominal production) aims to achieve an operation at the ideal aerodynamic output. The control strategy generally used in sub-nominal region for variable speed wind turbines is based on electrically adjusting the generator&#39;s torque to achieve the maximum output and this is carried out using a controller which receives signals indicating the speed of the generator and the power produced by the generator and which provides a torque reference signal to the converter to obtain the required power. 
     Accordingly, the wind turbine controller uses a curve which defines the desired functional relationship between power and speed to achieve ideal output. A curve of this type is curve  25  in  FIG. 2 . In order to track the power curve of  FIG. 2 , the control strategy of wind turbines is usually divided into the operational regions  3 ,  2 ,  1 ,  0 , shown also in the Torque-Rotor speed diagram  27  of  FIG. 3 , corresponding to pre-defined wind speed/rotor speed intervals, plus a pre-region  0  corresponding to the start up. 
     As was already said, known wind turbine controllers are not able to regulate the power output as close as possible to the power output prescribed by said power curve in all wind situations and particularly in cases of gusts and turbulences. 
     The wind turbine control methods and systems according to this invention are based on a new control strategy which is implemented using adaptive predictive control techniques. 
     In a preferred embodiment the control strategy in said regions implemented in the wind turbine control system is the following: 
     Nominal Operation: Region  3   
     The control objective for this region is to maintain nominal rotor speed and nominal power conditions, avoiding the capture of wind exceeding energy through pitch operation. 
     Control strategy: Generated power P is controlled with “measured” torque T 3 , and this “measured” torque T 3  is controlled with pitch through an adaptive predictive controller which identifies in real time the dynamics between the pitch action and the “measured” torque T 3 . Generated power set point P sp , is equal to the machine nominal power. Torque set point T sp  is equal to the machine nominal torque. It is to be noted that in the prior art the control strategy is based on two independent control mechanisms: power P is controlled with torque T, and generator speed Ω is controlled with pitch θ. 
     Following  FIG. 4 , it can be seen that in region  3 :
         The input to the Pitch&amp;Torque Select&amp;Control Unit  49  is the operational mode determined in Mode Switch Unit  41  according to the measured values of wind speed V, pitch θ, generator speed Ω and generated power P. The outputs in region  3  are the demanded pitch θ d  and the demanded torque T d  corresponding to the “measured” torque T 3  and the demanded pitch θ 3  provided by Power Controller  43  and Torque with Pitch Adaptive Predictive Controller  51  (see below).   The inputs to Pitch Rate Adaptive Predictive Controller  59  are said demanded pitch θ d  and the measured pitch θ. The output is the demanded pitch rate θr d  so that the measured pitch θ can converge efficiently towards demanded pitch θ d . The Pitch Rate Adaptive Predictive Controller  59  is used to identify the pitch rate actuator dynamics in order to follow the demanded pitch θ d  through predetermined trajectories configured in the Pitch Rate Adaptive Predictive Controller  59 .   The inputs to Power Controller  43  are power set point P sp , (the nominal power) and measured generator speed Ω. The output is “measured” torque T 3  calculated according to the equation T=P/Ω.   The inputs to Torque with Pitch Adaptive Predictive Controller  51  are torque set point T sp  (the torque corresponding to nominal power), “measured” torque T 3 , measured generator speed Ω and measured pitch θ. Measured wind speed V and measured nacelle fore-aft acceleration a x , are introduced as perturbations. The output is demanded pitch θ 3 . The Torque with Pitch Adaptive Predictive Controller  51  identifies in real time operation the cause-effect relation between “measured” torque T 3  and measured pitch θ as well as the dynamics of wind speed V and of nacelle fore-aft acceleration a x , on the “measured” torque T 3  in order to update its internal dynamic model.   Wind turbine  11  is therefore controlled in region  3  through two control variables: the demanded torque T d  and the demanded pitch θ d .       

     Sub-Nominal Operation: Regions  2 ,  1 ,  0   
     Region  2   
     The control objective in this region is to keep the generator nominal speed generating the maximum possible power, while capturing the maximum available energy in the wind. 
     Control strategy: Generator speed Ω is controlled with torque T 2  through the Speed with Torque Adaptive Predictive Controller  55 . Therefore the dynamics between both variables is online identified. Generator speed set point Ω SP  is constant and equal to the generator nominal speed. Pitch θ 2  is positioned at its optimum value. Measured wind speed V and measured pitch angle θ are used as perturbations, because controller downwards logic can introduced pitch movements. 
     Region  1   
     The control objective in this region is to maximise wind power capture and therefore to maintain the lambda relation constant and equal to its analytical optimum value. 
     Control strategy: In order to keep the lambda relation in its optimum value an analytically deduced torque T 1  is applied to the wind turbine. Generator speed Ω is kept between connexion and nominal generator speed. Pitch θ 1  is positioned at its optimum value. 
     Region  0   
     The control objective in this region is to keep generator speed Ω at connexion speed in order to proceed with the start-up procedure. 
     Control strategy: Generator speed Ω is controlled with torque T 0  through the Speed with Torque Adaptive Predictive Controller  55 . Generator speed set point Ω sp  is constant and equal to the generator connection speed. Pitch θ 0  is positioned at its optimum value. 
     Following  FIG. 4 , it can be seen that in Regions  2 ,  1 ,  0 :
         The input to the Pitch&amp;Torque Select&amp;Control Unit  49  is the operational mode determined in Mode Switch Unit  41  according to the measured values of wind speed V, pitch θ, generator speed Ω and generated power P. The outputs are the demanded pitch θ d  and the demanded torque T d  corresponding to the demanded torques T 2 , T 1 , T 0  and the demanded pitch θ 2 , θ 1 , θ 0 , provided by the Speed with Torque Adaptive Predictive Controller  55  and the Speed with Pitch Adaptive Predictive Controller  53  in each Region.   The demanded pitch θ d  is kept constant in its optimum value in Regions  2 ,  1  and  0 , therefore the Pitch Rate Adaptive Controller  59  should not be used but actually it is because downward controller logic can ask for pitch changes.   The inputs to Speed with Pitch Adaptive Predictive Controller  53  are measured generator speed Ω, measured wind speed V, and measured pitch θ, these two last ones are used as perturbations. The output is demanded pitch θ 2 , θ 1  θ 0  which are set in its optimum value in Regions  2 ,  1  and  0 .   The inputs to Speed with Torque Adaptive Predictive Controller  55  are the value of K opt  provided by K opt  Controller  47 , the measured torque T and the measured generator speed Ω. The measured wind speed V and the measured pitch θ are used as perturbations of the generator speed Ω. The measured pitch θ is used as a perturbation because although it is not supposed to change within Regions  2 ,  1  and  0 , some changes may be required in special circumstances. The Speed with Torque Adaptive Predictive Controller  55  identifies the dynamics between the demanded torque T 2 , T 1 , T 0  and the measured generator speed Ω.   Wind turbine  11  is therefore controlled in regions  2 ,  1  and  0  through the demanded torque T d .       

     Region pre- 0   
     The control objective in this region is to speed up the generator speed Ωfrom stop to the connexion speed and keep it around this value. 
     Control strategy: Generator speed Ω is controlled with pitch action through the Speed with Pitch Adaptive Predictive Controller  53 . Generator speed set point Ω sp  is the connexion speed. The Speed with Pitch Adaptive Predictive Controller  53  identifies the dynamics between the applied pitch θ pre-0  and the measured generator speed Ω. The measured wind speed V and the measured torque T when connecting are used as perturbations of the generator speed Ω, which is the control variable in this Region. 
     The above-mentioned Torque with Pitch Adaptive Predictive Controller  51 , Speed with Pitch Adaptive Predictive Controller  53 , Speed with Torque Adaptive Predictive Controller  55  and Pitch Rate Adaptive Predictive Controller  59  are controllers based on adaptive predictive control algorithms according to the teachings of ES 460649. These controllers together with Mode Switch Unit  41 , Power Controller  43 , K opt  Controller  47  (implementing all of them analytical models between output and input variables) and Pitch&amp;Torque Select&amp;Control Unit  49  are the basic components of the control system according to this invention. 
     The use of such controller technology and associated strategy in a wind turbine control system improves wind power capture and power quality with respect to PID controllers due to a better adaptation to the dynamic evolutions of the wind turbine. 
     A wind turbine control system according to the present invention combines control means available in known variable speed wind turbines with the above mentioned controllers as schematically shown in  FIG. 5 . 
     Pitch control means involve blades  17 , actuators  61 , adjusting transmissions  63  and the main control unit  65 . Similarly torque control means involve the generator  19 , a generator command unit  67  and the main control unit  65 . 
     The main control unit  65 , that include all above mentioned controllers, receives input data such as wind speed V, generator speed Ω, pitch angle θ, power P, nacelle fore-aft acceleration a x , from measuring devices  71 ,  73 ,  75 ,  77  and send output data θ d , T d  to, respectively, the actuator  61  for changing the angular position of the blades  17  and the generator command unit  67  for changing the reference for the power production. 
     Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering this as limited by these embodiments, but by the contents of the following claims.