Abstract:
The invention provides a wind turbine having a system for positioning the rotor in an azimuthal reference position Az ref  and for maintaining it therein for a predetermined period of time, the wind turbine being arranged in test mode. Said rotor positioning system comprises a first controller ( 31 ) configured to generate a generator speed reference Ω ref  from the difference between the rotor azimuthal reference position Az ref  and the rotor azimuthal measured position Az meas  and a second controller ( 35 ) configured to generate a generator torque reference T ref  from the difference between said generator speed reference Ω ref  and the generator speed measured Ω meas .

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a rotor positioning system of a wind turbine. 
       BACKGROUND 
       [0002]    Over the life of wind turbines there are several tasks that must maintain fixed the rotor in a certain position. 
         [0003]    One of these tasks is the rotor locking to ensure it does not rotate which is needed to perform certain maintenance activities. Rotor locking systems typically comprise one or more pins (often two pins) which are pushed into holes in the stationary part of the wind turbine to prevent rotation of the rotor and therefore require a system to position the rotor so that the pins are perfectly aligned with the holes. The rotor locking process is difficult and time consuming. This task is even more complex when the wind speed increases and become impossible above certain wind speed. 
         [0004]    Another of these tasks is the access of service personnel transported by helicopters to offshore wind turbines that have helihoist platforms where is needed to have the rotor stationary while the helicopter is nearby. 
         [0005]    Another of these tasks is the calibration of the blade load sensors where the rotor shall be maintained fixed in various positions (and also having the blades at different pitch angles in each position) to compare the calibration with the static moments in said positions. 
         [0006]    All these tasks must be performed in narrow ranges of time and within a wide range of speeds. 
         [0007]    Known rotor positioning systems for such tasks have a high manual component and do not therefore allow a remote operation which would be very desirable especially in offshore wind turbines. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention provides a wind turbine having a system for positioning the rotor in an azimuthal reference position Az ref  and for maintaining it therein for a predetermined period of time, the wind turbine being arranged in test mode, so that during that period of time the tasks mentioned in the previous section can be carried out. This system comprises a first controller configured to generate a generator speed reference Ω ref  from the difference between the rotor azimuthal reference position Az ref  and the rotor azimuthal measured position Az meas  and a second controller configured to generate a generator torque reference T ref  from the difference between said generator speed reference Ω ref  and the generator speed measured Ω meas . 
         [0009]    The first and/or the second controller can be PI (Proportional, Integral) controllers or PID (Proportional, Integral, Derivative) controllers being its variable gains dependent of the wind speed V meas  measured at the height of the rotor hub. 
         [0010]    The wind turbine comprises an Uninterruptible Power Supply (UPS) device or a connection to a power grid to provide power to the generator when acting as a motor under the control of the rotor positioning system. 
         [0011]    Other features and advantages of the present invention will be understood from the following detailed description of illustrative and by no means limiting embodiments of its object in relation with the enclosed drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0012]      FIG. 1  is a schematic cross sectional view of a wind turbine. 
           [0013]      FIG. 2  is a schematic block diagram illustrating an embodiment of the rotor positioning system according to the invention with two PI controllers. 
           [0014]      FIGS. 3-6  are schematic block diagrams illustrating how the proportional and integral gains of the two PI controllers are obtained. 
           [0015]      FIG. 7  illustrates the operation of the rotor positioning system to place it in the azimuthal position 90 deg. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    A typical wind turbine  11  comprises a tower  13  supporting a nacelle  21  that houses 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 to the generator  19  and increase the shaft speed in order to achieve a suitable rotational speed of the generator rotor. 
         [0017]    The wind turbine  11  also comprises means allowing the generator  19  acting as a motor receiving power from a suitable source such as an Uninterruptible Power Supply (UPS) device available in the own wind turbine  11  or an electricity grid to which the turbine  11  is connected. Thus, the generator  19  can be used as a rotor driving means. 
         [0018]    The wind turbine power output is 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 controlled. 
         [0019]    For implementing said regulation the control system receives input data such as wind speed V, generator speed Ω, pitch angle θ, power P from well-known measuring devices and send output data θ ref , T ref  to, respectively, the pitch actuator system for changing the pitch of the blades  17  and to a generator command unit for changing the torque reference for the power production. 
         [0020]    According to the invention the wind turbine  11  also comprises a rotor positioning system which allows placing it in a particular position, being the wind turbine  11  in test mode, i.e. when the wind turbine does not produces energy, the rotor and the power train rotate freely by the wind action and the brake system is disabled. 
         [0021]    That particular position is expressed in terms of an azimuthal reference position Az ref . For example, the azimuthal position 0 deg means that the blade  1  of the wind turbine  11  has its tip pointing to the sky, the azimuthal position 90 deg means that, looking at the wind turbine from outside and from an observer in front of it, the blade  1  would be rotated clockwise 90 deg and the azimuthal position 180 deg means that the blade  1  is pointing to the ground. The azimuthal position of the rotor is measured by a sensor located on the low speed side of the drive train which generates a pulse when the blade  1  is in the azimuthal position 0 deg. Depending on the transmission ratio between the low speed shaft and the high speed shaft and this pulse the azimuthal position is calculated by integration. 
         [0022]    In one embodiment of the invention using PI (proportional integral) controllers the rotor positioning system (see  FIG. 2 ) comprises:
       A first PI controller  31  that generates a generator speed reference Ω ref  from the azimuthal error Az err , obtained in a module  29  which is configured to calculate it from the azimuthal reference position Az ref  and the azimuthal measured position Az meas  (by the above-mentioned sensor) and the proportional and integral gains Kp 1  and Ki 1  dependent on the wind speed V (measured with an anemometer located at the height of the rotor hub  15 ).   A second PI controller  35  that generates a generator torque reference T ref  from the generator speed error Ω err , obtained in a module  33  which is configured to calculate it from the generator speed reference Ω ref  and the measured generator speed Ω meas  (upon application of a filter to remove high frequency components) and the proportional and integral gains Kp 2  and Ki 2 .       
 
         [0025]    The proportional gain Kp 1 , expressed in rpm/deg, is obtained (see  FIG. 3 ) in a module  43  which is configured to calculate it multiplying a variable gain value Δ v1  dependent of the measured wind speed V mean  at the height of the rotor hub  15 , averaged at 600 s, by a parameter P 1 , expressed in rpm/deg, which defines the proportional gain of the first PI controller  31 . The value of the variable gain Δ v1  is obtained in a module  41  which is configured to calculate it from V mean  using an interpolation table. 
         [0026]    The integral gain Ki 1 , expressed in s*rpm/deg, is obtained (see  FIG. 4 ) in a module  45  which is configured to calculate it from the proportional gain Kp 1  and a parameter P 2 , expressed in s, which defines the integral time on the first proportional integral controller  31 . 
         [0027]    The proportional gain Kp 2 , expressed in Nm/rpm, is obtained (see  FIG. 5 ) in a module  53  which is configured to calculate it multiplying a variable gain value Δ v2  dependent of the measured wind speed V mean  at the height of the rotor hub  15 , averaged at 600 s, by a parameter P 3 , expressed in Nm/rpm, which defines the proportional gain of the second PI controller  31 . The variable gain value Δ v2  is obtained in a module  51  which is configured to calculate it from V mean  using an interpolation table. 
         [0028]    The integral gain Ki 2 , expressed in s*rpm/deg, is obtained (see  FIG. 6 ) in a module  55  which is configured to calculate it from the proportional gain Kp 2  and a parameter P 4 , expressed in s, which defines the integral time on the second proportional integral controller  35 . 
         [0029]    The following curves (see  FIG. 7 ) illustrate the operation of the rotor positioning system to place it on the azimuthal position 90 deg:
       The curve  61  represents the azimuthal reference position Az ref (90 deg).   The curve  63  represents the evolution in time of the azimuthal measured position Az meas .   The curve  65  represents the evolution in time of the generator speed reference Ω ref .   The curve  67  represents the evolution in time of the measured generator speed Ω meas .   The curve  69  represents the evolution in time of the azimuthal error Az err .   The curve  71  represents the evolution in time of the generator torque reference T ref .       
 
         [0036]    As shown, the rotor positioning system begins to demand an initial generator speed reference Ω ref  of 20 rpm (curve  65 ) and, since time t 1 , an azimuthal reference position Az ref  of 90 deg (curve  61 ). 
         [0037]    Once the azimuthal measured position Az meas  (curve  63 ) matches the azimuthal reference position Az ref  (curve  61 ) at time t 2 , the controllers  31 ,  35  are activated to achieve the generator torque reference T ref  (curve  71 ) needed to keep Az ref  at 90 deg. The azimuthal error Az err  reaches 0 at 200 s and the generator torque reference T ref  varies with time taking positive and negative values. 
         [0038]    The main advantage of the invention is that it allows automation of the wind turbine operation to maintain fixed the rotor in a given azimuthal position for some time to perform operations such as rotor blocking, personnel access to the wind turbine from helicopters and blade load sensors calibration. 
         [0039]    Although the present invention has been described in connection with various embodiments, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention as defined by the appended claims.