Abstract:
A wind turbine generator system includes: a wind turbine rotor including a blade having a variable pitch angle; a generator driven by the wind turbine rotor; and a control unit controlling the output power of the generator and the pitch angle of the blade in response to the rotational speed of the wind turbine rotor or the generator. The control unit performs a first control in which the output power is controlled in accordance with a predetermined power-rotational speed curve until the rotational speed is increased to reach a predetermined rated rotational speed, and performs a second control in which the output power is controlled to a predetermined rated power when the rotational speed exceeds the rated rotational speed; the control unit is responsive to the pitch angle for maintaining a state of performing the second control is or for switching to a state of performing the first control, when the rotational speed is reduced below the rated rotational speed after the control unit is once placed into the state of performing the second control.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation of International Application No. PCT/JP2008/068764, filed on Oct. 16, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wind turbine generator system and method for controlling the same, particularly, to control of the output power and the pitch angle of a wind turbine generator system adopting a variable-speed and variable-pitch control method. 
         [0004]    2. Description of the Related Art 
         [0005]    One of the promising control methods for a wind turbine generator system is a variable-speed and variable-pitch control method, in which the rotational speed of the wind turbine rotor (that is, the rotational speed of the generator) is variable and the pitch angle of the blades is variable. Advantages of the variable-speed and variable-pitch control method include increased energy capture from the wind and decreased output fluctuation. 
         [0006]    With respect to the variable-speed and variable-pitch control method, it is of importance to optimize the control of the output power of the generator and the pitch angle of the blades. Japanese of International Publication No. 2001-512804 discloses a control method in which the torque of the generator is controlled with a field orientation control while the pitch angle is controlled independently of the torque of the generator. In the disclosed control method, desired output power of the generator is determined in response to the rotational speed of the generator using a lookup table, and a torque command for the generator is determined from the desired output power. The torque of the generator is controlled by a field orientation control in response to the torque command. On the other hand, the pitch angle of the blades is controlled by PID control, PI control or PD control responsive to the deviation between the rotational speed of the generator and a desired rotational speed. 
         [0007]    One issue for the wind turbine generator system is how to deal with occurrence of a transient wind null, that is, a short-time reduction in the wind speed. Generally, a wind turbine generator system is designed to generate rated power in a case where the rotational speed of the wind turbine rotor is equal to or higher than a rated rotational speed. In such a wind turbine generator system, the output power is reduced below the rated power, when the rotational speed of the wind turbine rotor is reduced below the rated rotational speed due to the occurrence of the transient wind null. This causes output power fluctuation and generation efficiency reduction. 
       SUMMARY OF INVENTION 
       [0008]    Therefore, an object of the present invention is to provide a wind turbine generator system which suppresses output power fluctuation and generation efficiency reduction even when a transient wind null occurs. 
         [0009]    In an aspect of the present invention, a wind turbine generator system includes: a wind turbine rotor including a blade having a variable pitch angle; a generator driven by the wind turbine rotor; and a control unit controlling the output power of the generator and the pitch angle of the blade in response to the rotational speed of the wind turbine rotor or the generator. The control unit performs a first control in which the output power is controlled in accordance with a predetermined power-rotational speed curve until the rotational speed increases and reaches a predetermined rated rotational speed, and performs a second control in which the output power is controlled to a predetermined rated power when the rotational speed exceeds the rated rotational speed; the control unit is responsive to the pitch angle for maintaining a state of performing the second control is or for switching to a state of performing the first control, when the rotational speed is reduced below the rated rotational speed after the control unit is once placed into the state of performing the second control. Here, the pitch angle is an angle formed between a chord of the blade and a rotation plane of the wind turbine rotor. Namely, the wind turbine rotor extracts more energy from wind when the pitch angle is small, and the wind turbine rotor extracts less energy from the wind when the pitch angle is large. 
         [0010]    The wind turbine generator system configured as stated above can suppress the output power fluctuation by using the rotational energy of the wind turbine rotor when the wind speed is reduced only for a short time. This is because the wind turbine generator system according to the present invention keeps the output power at the predetermined rated power in response to the pitch angle of the blade, when said rotational speed is reduced below said rated rotational speed. When it is determined from the pitch angle of the blade that the system is in a state in which the output power can be kept at the predetermined rated power, the output power is kept at the rated power, and this allows effectively extracting the rotational energy of the wind turbine rotor and suppressing the output power fluctuation and the power generation efficiency reduction. 
         [0011]    Preferably, in a case where the rotational speed is reduced below the rated rotational speed after the control unit is once placed into the state of performing the second control, the control unit maintains the state of performing said second control for a case when the pitch angle is larger than a predetermined pitch angle, not switching to the state of performing the first control until the pitch angle reaches the predetermined pitch angle. In this case, said control unit is preferably switched to the state of performing said first control irrespectively of said pitch angle, when said rotational speed is reduced below a predetermined threshold rotation speed which is lower than said rated rotational speed after the control unit is once placed into the state of performing said second control. 
         [0012]    Preferably, the control unit controls said pitch angle in response to the difference between the rotational speed of the wind turbine rotor or the generator and the predetermined rated rotational speed and the difference between the output power and the rated power. 
         [0013]    In this case, the control unit preferably controls the pitch angle to be reduced when the output power is lower than the rated power. 
         [0014]    The control unit preferably increases the output power of the generator in response to said rotational speed when a gust is detected. 
         [0015]    In a case where the wind turbine generator system further includes: a rotation mechanism rotating a direction of the rotational surface of the wind turbine rotor; and a wind direction detector detecting a windward direction and the wind turbine rotor includes a pitch drive mechanism driving the blade, it is preferable that the control unit controls the rotation mechanism so as to move the rotation plane of the wind turbine rotor away from the windward direction when detecting a failure in the pitch drive mechanism. 
         [0016]    Preferably, the control unit controls reactive power outputted from the generator to a power grid connected to the generator in response to a voltage of the power grid, and controls the pitch angle in response to said reactive power. 
         [0017]    In a case where the wind turbine generator system further includes an emergency battery and a battery charger charging the emergency battery with power received from the power grid, wherein the wind turbine rotor includes a pitch drive mechanism driving the blade and wherein the emergency battery supplies power to the pitch drive mechanism and the control unit when the voltage of the power grid connected to the generator is reduced, the control unit preferably controls the output power to be increased while the emergency battery is being charged. 
         [0018]    A method of controlling a wind turbine generator system according to the present invention is a method of controlling a wind turbine generator system provided with: a wind turbine rotor including a blade having a variable pitch angle; and a generator driven by the wind turbine rotor. The control method includes a control step of controlling output power of the generator and a pitch angle of the blade in response to the rotational speed of the wind turbine rotor or the generator. Said control step includes steps of: 
         [0019]    (A) performing a first control in which said output power is controlled in accordance with to a predetermined power-rotational speed curve until said rotational speed increases to reach a predetermined rated rotational speed; 
         [0020]    (B) performing a second control in which said output power is controlled to a predetermined rated power when said rotational speed exceeds said rated rotational speed; and 
         [0021]    (C) in response to said pitch angle, maintaining the state of performing said second control or switching to the state of performing said first control, when said rotational speed is reduced below the rated rotational speed after the state of performing said second control is once established. 
         [0022]    The present invention provides a wind turbine generator system which can suppress output power fluctuation and generation efficiency reduction even when a transient wind null occurs. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  is a side view showing the configuration of a wind turbine generator system in one embodiment of the present invention; 
           [0024]      FIG. 2  is a block diagram showing the configuration of a pitch drive mechanism of the wind turbine generator system of the present embodiment; 
           [0025]      FIG. 3  is a block diagram showing the configuration of the wind turbine generator system of the present embodiment; 
           [0026]      FIG. 4  is a graph showing a power control method performed by the wind turbine generator system of the present embodiment; 
           [0027]      FIG. 5  is a block diagram showing an example of the configuration of a main control unit of the wind turbine generator system of the present embodiment; 
           [0028]      FIG. 6  is a table explaining operations performed by a power controller and a pitch controller of the wind turbine generator system of the present embodiment; 
           [0029]      FIG. 7  is a graph showing an example of an operation performed by the wind turbine generator system of the present embodiment; 
           [0030]      FIG. 8  is a block diagram showing another configuration of the wind turbine generator system of the present embodiment; 
           [0031]      FIG. 9  is a flowchart of a preferred control performed by the wind turbine generator system of the present embodiment; 
           [0032]      FIG. 10  is a flowchart of another preferred control performed by the wind turbine generator system of the present embodiment; 
           [0033]      FIG. 11  is a flowchart of still another preferred control performed by the wind turbine generator system of the present embodiment; and 
           [0034]      FIG. 12  is a flowchart of still another preferred control performed by the wind turbine generator system of the present embodiment. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]      FIG. 1  is a side view showing the configuration of a wind turbine generator system  1  in one embodiment of the present invention. The wind turbine generator system  1  is provided with a tower  2  and a nacelle  3  provided on the top end of the tower  2 . The nacelle  3  is rotatable in the yaw direction and directed to a desired direction by a nacelle rotation mechanism  4 . Mounted in the nacelle  3  are a wound-rotor induction generator  5  and a gear  6 . The rotor of the wound-rotor induction generator  5  is connected to a wind turbine rotor  7  through the gear  6 . 
         [0036]    The wind turbine rotor  7  includes blades  8  and a hub  9  supporting the blades  8 . The blades  8  are provided so that the pitch angle thereof is variable. More specifically, as shown in  FIG. 2 , the hub  9  contains therein hydraulic cylinders  11  driving the blades  8  and servo valves  12  supplying hydraulic pressure to the hydraulic cylinders  11 . The hydraulic pressure supplied to the hydraulic cylinders  11  is controlled by the openings of the servo valves  12 , thereby controlling the blades  8  to a desired pitch angle. 
         [0037]    Referring back to  FIG. 1 , the nacelle  3  additionally includes an anemometer  10 . The anemometer  10  measures the wind speed and the wind direction. As described later, the nacelle  3  is rotated in response to the wind speed and the wind direction measured by the anemometer  10 . 
         [0038]      FIG. 3  is a block diagram showing details of the configuration of the wind turbine generator system  1 . The wind turbine generator system  1  in this embodiment is a sort of doubly-fed variable speed wind turbine system. Namely, the wind turbine generator system  1  of this embodiment is configured to output the power generated by the wound-rotor induction generator  5  to the power grid  13  from both of the stator and rotor windings. Specifically, the wound-rotor induction generator  5  has the stator winding directly connected to the power grid  13  and the rotor winding connected to the power grid  13  through an AC-DC-AC converter  17 . 
         [0039]    The AC-DC-AC converter  17 , which includes an active rectifier  14 , a DC bus  15  and an inverter  16 , converts AC power received from the rotor winding into AC power adapted to the frequency of the power grid  13 . The active rectifier  14  converts the AC power generated by the rotor winding into DC power and outputs the DC power to the DC bus  15 . The inverter  16  converts the DC power received from the DC bus  15  into AC power of a frequency equal to that of the power grid  13  and outputs the AC power to the power grid  13 . The output power which the wound-rotor induction generator  5  outputs to the power grid  13  is controlled by the active rectifier  14  and the inverter  16 . 
         [0040]    The AC-DC-AC converter  17  also has a function of converting AC power received from the power grid  13  into AC power adapted to the frequency of the rotor winding, and the AC-DC-AC converter  17  is used to excite the rotor winding, depending on the operating state of the wind turbine generator system  1 . In this case, the inverter  16  converts the AC power into the DC power and outputs the DC power to the DC bus  15 . The active rectifier  14  converts the DC power received from the DC bus  15  into the AC power adapted to the frequency of the rotor winding and supplies the AC power to the rotor winding of the wound-rotor induction generator  5 . 
         [0041]    A control system of the wind turbine generator system  1  includes a PLG (pulse logic generator)  18 , a main control unit  19 , a voltage/current sensor  20 , a converter drive control unit  21 , a pitch control unit  22 , and a yaw control unit  23 . 
         [0042]    The PLG  18  measures the rotational speed ω of the wound-rotor induction generator  5  (hereinafter, referred to as the generator rotational speed ω). 
         [0043]    The main control unit  19  generates an real power command P*, a reactive power command Q* and a pitch angle command β*, in response to the generator rotational speed ω measured by the PLG  18 , and also generates a yaw command in response to the wind speed and the wind direction measured by the anemometer  10 . As described later in detail, one feature of the wind turbine generator system  1  of this embodiment is a control algorithm for generating the real power command P* and the pitch angle command β*. 
         [0044]    The voltage/current sensor  20 , which is provided on power lines connected between the wound-rotor induction generator  5  and the power grid  13 , measures the voltage V grid  of the power grid  13  (“grid voltage”) and the output current I grid  outputted from the wound-rotor induction generator  5  to the power grid  13 . 
         [0045]    The converter drive control unit  21  controls real power P and reactive power Q outputted to the power grid  13  in response to the real power command P* and the reactive power command Q*, respectively. The converter drive control unit  21  also controls the turn-on-and-off of power transistors within the active rectifier  14  and the inverter  16 . Specifically, the converter drive control unit  21  calculates the real power P and the reactive power Q to be outputted to the power grid  13  from the voltage V grid  and the output current I grid  measured by the voltage/current sensor  20 . Further, the converter drive control unit  21  generates PWM signals for the PWM control in response to the difference between the real power P and the real power command P* and the difference between the reactive power Q and the reactive power command Q*, and supplies the generated PWM signals to the active rectifier  14  and the inverter  16 . The real power P and the reactive power Q outputted to the power grid  13  are thereby controlled. 
         [0046]    The pitch control unit  22  controls the pitch angle β of the blades  8  in response to the pitch angle command β* transmitted from the main control unit  19 . The pitch angle β of the blades  8  is controlled to coincide with the pitch angle command β*. 
         [0047]    The yaw control unit  23  controls the nacelle rotation mechanism  4  in response to the yaw command transmitted from the main control unit  19 . The nacelle  3  is oriented to the direction indicated by the yaw command. 
         [0048]    An AC/DC converter  24  is connected to the power lines connected between the power grid  13  and the wound-rotor induction generator  5 . The AC/DC converter  24  generates DC power from AC power received from the power grid  13 . The AC/DC converter  24  supplies the DC power to the control system of the wind turbine generator system  1 , particularly to the servo valves  12 , the main control unit  19 , and the pitch control unit  22  used to control the pitch angle β of the blades  8 . 
         [0049]    Moreover, the wind turbine generator system  1  is provided with an uninterruptible power supply system  26  which includes a battery charger  27  and an emergency buttery  28  so as to stably supply DC power to the servo valves  12 , the main control unit  19 , and the pitch control unit  22 . From requirements of wind turbine generator system standards, it is necessary for the wound-rotor induction generator  5  to remain connected to the power grid  13  even when the grid voltage V grid  falls. This requires appropriately controlling the pitch angle of the blades  8  to thereby maintain the rotational speed of the wound-rotor induction generator  5  to a desired value even when the voltage of the power grid  13  falls. To satisfy such requirements, when the grid voltage V grid  falls to a predetermined voltage, the uninterruptible power supply system  26  is connected to the servo valves  12 , the main control unit  19 , and the pitch control unit  22  by a switch  25 , to supply power from the emergency battery  28  to the servo valves  12 , the main control unit  19 , and the pitch control unit  22 . The pitch angle of the blades  8  is thereby kept controlled. The emergency battery  28  is connected to the battery charger  27 . The battery charger  27  charges the emergency battery  28  with the DC power supplied from the AC/DC converter  24 . 
         [0050]    One feature of the wind turbine generator system  1  of this embodiment is optimized control of the output power P of the wound-rotor induction generator  5 .  FIG. 4  is a graph showing the relationship between the real power command P* and the rotational speed ω of the wound-rotor induction generator  5 , depicting a method of controlling the output power P performed by the wind turbine generator system  1  of this embodiment. 
         [0051]    When the generator rotational speed ω is lower than a minimum rotational speed ω min , the real power command P* for the wound-rotor induction generator  5  is controlled to zero. The minimum rotational speed ω min  is a minimum rotational speed at which power can be generated by the wound-rotor induction generator  5 , and the minimum rotational speed ω min  is determined in accordance with characteristics of the wind turbine generator system  1 . 
         [0052]    When the generator rotational speed ω is higher than the minimum rotational speed ω min , the real power command P* is controlled in one control mode selected from two control modes: an optimum curve control mode and a rated value control mode. 
         [0053]    In the optimum curve control mode, the real power command P* is controlled to coincide with an optimized power P opt  defined by the following Equation ( 1 ): 
         [0000]      P opt =Kω 3 ,   (1)
 
         [0000]    where K is a predetermined constant. It is known that it is optimum for the wind turbine generator system  1  to control the output power to be proportional to the cube of the generator rotational speed, and accordingly, the output power P is controlled to be proportional to the cube of the generator rotational speed ω in the first control mode. 
         [0054]    The optimum curve control mode is used mainly in a range in which the generator rotational speed ω is higher than the minimum rotational speed ω min  and lower than a rated rotational speed ω max . Note that the rated rotational speed ω max  is the rotational speed at which the wound-rotor induction generator  5  operates in the steady operation. The generator rotational speed ω is controlled to the rated rotational speed ω max  (if possible) by controlling the pitch angle of the blades  8 . 
         [0055]    In the rated value control mode, on the other hand, the output power P is controlled to the rated power P rated . The rated value control mode is used mainly in a range in which the generator rotational speed ω is equal to or higher than the rated rotational speed ω max . In a steady condition in which wind blows with a rated wind speed, the generator rotational speed ω is controlled to the rated rotational speed ω max  and the output power P is controlled to the rated power P rated . 
         [0056]    An important feature of the wind turbine generator system  1  of this embodiment is in a fact that switching from the rated value control mode to the optimum curve control mode is made in response to the pitch angle β of the blades  8 . When the generator rotational speed ω is increased to reach the rated rotational speed ω max , the power control is switched from the optimum curve control mode to the rated value control mode. When the generator rotational speed ω is decreased below the rated rotational speed ω max , on the other hand, the pitch angle β is first reduced. The power control is not switched from the rated value control mode to the optimum curve control mode until the pitch angle β reaches a minimum value β min . Namely, the real power command P* is switched from the rated power P rated  to the optimized power value P opt . In other words, the real power command P* is kept at the rated power P rated  unless the pitch angle β reaches the minimum value β min  (that is, the pitch angle command β* reaches the minimum value β min ). It should be noted that the fact that the pitch angle β is set to the minimum angle β min  implies that the output coefficient of the wind turbine rotor  7  is maximum with the pitch angle β set to the fine-side limit value, since the pitch angle β is the angle formed between the chords of the blades  8  and the rotation plane of the wind turbine rotor. 
         [0057]    The control in which the output power P is kept at the rated power P rated  until the pitch angle β reaches the minimum value β min  is effective for suppressing output power fluctuation and avoiding generation efficiency reduction when a transient wind null occurs. Under the above-described control, the real power command P* is kept at the rated power P rated  when the generator rotational speed ω is reduced below the rated rotational speed ω max  if such state continues only for a short time, and thereby the fluctuation in the output power P is suppressed. 
         [0058]    Furthermore, the wind turbine generator system  1  of this embodiment allows making effective use of rotational energy of the wind turbine rotor  7 , effectively improving the generation efficiency, since the output power P is not reduced from the rated power P rated  until the increase in the output coefficient of the wind turbine rotor  7  through the reduction in the pitch angle β becomes impossible, when the generator rotational speed ω is reduced below the rated rotational speed ω max . 
         [0059]    It should be noted that the power control is switched from the rated value control mode to the optimum curve control mode irrespectively of the pitch angle β (or the pitch angle command β*), when the generator rotational speed ω is reduced below a predetermined threshold rotational speed ω′ M  which is lower than the rated rotational speed ω max . It is unpreferable for securing the control stability to maintain the output power P at the rated power P rated  when the generator rotational speed ω is excessively low. It is preferable that the threshold rotational speed ω′ M  is determined by the following equation: 
         [0000]      ω′ M =(ω M +ω max )/2,
 
         [0000]    where ω M  is an intermediate rotational speed defined as: 
         [0000]      ω M =(ω max +ω min )/2.
 
         [0060]      FIG. 5  is a block diagram showing an example of a configuration of the main control unit  19  for realizing a control shown in  FIG. 4 . It should be noted that  FIG. 5  shows only one example of the configuration of the main control unit  19 ; the main control unit  19  may be implemented as hardware, software, or a combination of hardware and software. The main control unit  19  includes a power control module  31  generating the real power command P* and the reactive power command Q* and a pitch control module  32  generating the pitch angle command β*. 
         [0061]    The power control module  31  includes a selector  33 , a subtracter  34 , a PI controller  35 , a power limiter  36 , and a power setting calculator  37 . On the other hand, the pitch control module  32  includes a subtracter  38 , a PI controller  39 , a subtracter  40 , a PI controller  41 , and an adder  42 . The selector  33 , the subtracter  34 , the PI controller  35 , the power limiter  36 , the power setting calculator  37 , the subtracter  38 , the PI controller  39 , the subtracter  40 , the PI controller  41 , and the adder  42  perform respective calculation steps synchronously with a clock used in the main control unit  19 , and the real power command P*, the reactive power command Q*, and the pitch angle command β* are thereby generated. 
         [0062]    In detail, the selector  33  selects one of the minimum rotational speed ω min  and the rated rotational speed ω max  as a power control rotational speed command ω P * in response to the generator rotational speed ω. More specifically, the selector  33  sets the power control rotational speed command ω P * to the minimum rotational speed ω min , when the generator rotational speed ω is equal to or lower than the intermediate rotational speed ω M , and sets the power control rotational speed command ω P * to the rated rotational speed ω max , when the generator rotational speed ω is higher than the intermediate rotational speed ω M . 
         [0063]    The subtracter  34  calculates the deviation Δω P  by subtracting the power control rotational speed command ω P * from the generator rotational speed ω. 
         [0064]    The PI controller  35  performs PI control in response to the deviation Δω P  to generate the real power command P*. Note that the range of the generated real power command P* limited by an power command lower limit P min  and an power command upper limit P max  supplied from the power limiter  36 . Namely, the real power command P* is limited to be equal to or higher than the power command lower limit P min  and limited to equal to or lower than power command upper limit P max . 
         [0065]    The power limiter  36  determines the power command lower limit P min  and the power command upper limit P max  to be supplied to the PI controller  35  in response to the generator rotational speed ω and the pitch angle command β*. Further, the power limiter  36  supplies the rated power P rated  to the subtracter  40  of the pitch control module  32 . As described later, the power control shown in  FIG. 4  is implemented by appropriately determining the power command lower limit P min  and the power command upper limit P max  generated by the power limiter  36  as well as the power control rotational speed command ω P * determined by the selector  33 . 
         [0066]    The power setting calculator  37  generates the reactive power command Q* from the real power command P* generated by the PI controller  35  and a power factor command indicating the power factor of the AC power outputted from the wind turbine generator system  1 , and outputs the real power command P* and the reactive power command Q*. As described above, the real power command P* and the reactive power command Q* are used to control the real power P and the reactive power Q outputted from the wind turbine generator system  1 , respectively. 
         [0067]    On the other hand, the subtracter  38  of the pitch control module  32  calculate the deviation Δω β  by subtracting a pitch control rotational speed command ω β * from the generator rotational speed ω. The pitch control rotational speed command ω β * is coincident with the rated rotational speed ω max , and therefore the deviation Δω β  represents the difference between the generator rotational speed ω and the rated rotational speed ω max . 
         [0068]    The PI controller  39  performs PI control in response to the deviation Δω β  to generate a pitch angle command baseline value β in *. The pitch angle command baseline value β in * mainly controls the finally generated pitch angle command β*, but the pitch angle command baseline value β in * does not always coincide with the pitch angle command β*. The pitch angle command baseline value β in * is determined so that the generator rotational speed ω is controlled to the rated rotational speed ω max . 
         [0069]    The subtracter  40  generates a deviation ΔP by subtracting the rated power P rated  from the real power command P*. The PI controller  41  performs the PI control in response to the deviation ΔP to generate a correction value Δβ*. The adder  42  adds up the pitch angle command baseline value β in * and the correction value Δβ* to generate the pitch angle command β*. 
         [0070]    The subtracter  40  and the PI controller  41  of the pitch control module  32  have a role to prevent the pitch control module  32  from undesirably interfering with the power control when the generator rotational speed ω increases up to the rated rotational speed ω max  and the power control is switched from the optimum curve control mode to the rated value control mode. The PI controller  39  of the pitch control module  32  is designed to adjust the generator rotational speed ω to the rated rotational speed ω max . This may result in that the aerodynamic energy to be extracted as the power is undesirably abandoned. Therefore, in this embodiment, the PI controller  41  generates the correction value Δβ* in response to the difference between the rated power P rated  and the real power command P*, and the pitch angle command β* is corrected with the correction value Δβ*. The correction value Δβ* is determined so that the pitch angle command β* is smaller than the pitch angle command baseline value β in *, i.e., the pitch angle β is set closer to the fine-side limit value, when the real power command P* is lower than the rated power command P rated , i.e., the deviation ΔP (=P*−P rated ) is negative. Such control allows avoiding the pitch angle β from being closer to the feather-side limit value just before the generator rotational speed ω reaches the rated rotational speed ω max . After the real power command P* reaches the rated power P rated , the deviation ΔP becomes zero and the correction value Δβ* becomes zero. 
         [0071]      FIG. 6  is a table showing operations performed by the power control module  31  and the pitch control module  32  of the main control unit  19 . The operations performed by the power control module  31  and the pitch control module  32  will be described for the following five cases:
   Case (1): The generator rotational speed ω is equal to or higher than the minimum rotational speed ω min  and equal to or lower than the intermediate rotational speed ω M (=(ω min +ω max )/2).   
 
         [0073]    In this case, the power control rotational speed command ω p * is set to the minimum rotational speed ω min  by the selector  33 , and the power command lower limit P min  and the power command upper limit P max  are set to zero and P opt  (=Kω 3 ), respectively. Besides, the real power command P* is always set to the power command upper limit P max , since the deviation Δω P (=ω−ω min ) is positive and the generator rotational speed ω is controlled to the rated rotational speed ω max . The real power command P* is eventually set to the optimized power value P opt , since the power command upper limit P max  is P opt . In other words, the power control is set to the optimum curve control mode. 
         [0074]    In this case, the pitch angle command β* is eventually set to the fine-side limit value, i.e., the minimum pitch angle β min , since the pitch control module  32  controls the generator rotational speed ω to the rated rotational speed ω max .
   Case (2): The generator rotational speed ω exceeds the intermediate rotational speed ω M , whereby the generator rotational speed ω is in a range where the generator rotational speed ω is higher than the intermediate rotational speed ω M  and lower than the threshold rotational speed ω′ M .   
 
         [0076]    In this case, the power control rotational speed command ω P * is set to the rated rotational speed ω max  by the selector  33 , and the power command lower limit P min  and the power command upper limit P max  are set to P opt  and P rated , respectively. In this case, the real power command P* is always set to the power command lower limit P min , since the deviation Δω P (=ω−ω max ) is negative and the generator rotational speed ω is controlled to the rated rotational speed ω max  by the pitch control module  32 . The real power command P* is eventually set to the optimized power value P opt , since the power command lower limit P min  is P opt . In other words, the power control is set into the optimum curve control mode. 
         [0077]    The above-described correction of the pitch angle command β* with the correction value Δβ* validly works in the case (2). In the case (2), since the real power command P* is lower than the rated power P rated , the deviation ΔP is negative and the correction value Δβ* is therefore negative. Accordingly, the pitch angle command β* is reduced below the pitch angle command baseline value β in *, that is, the pitch angle β is set closer to the fine-side lower limit. This allows converting the aerodynamic energy into the power more effectively.
   Case (3): The generator rotational speed ω is equal to or higher than the threshold rotational speed ω′ M , and the pitch angle β does not reach the minimum pitch angle β min .   
 
         [0079]    In this case, the power control rotational speed command ω P * is set to the rated rotational speed ω max  by the selector  33 , and the power command lower limit P min  and the power command upper limit P max  are both set to P rated . 
         [0080]    When the generator rotational speed ω is equal to or higher than the threshold rotational speed ω′ M  and lower than the rated rotational speed ω max , the deviation Δω P (=ω−ω max ) is negative and the real power command P* is always set to the power command lower limit P min . The power command lower limit P min  is P opt  and as a result, the real power command P* is set to P opt . 
         [0081]    When the rotational speed ω exceeds the rated rotational speed ω max , the deviation Δω P (=ω−ω max ) is positive and the real power command P* is always set to the power command upper limit P max . Therefore, the real power command P* is set to the rated power P rated . In other words, the power control is set into the rated value control mode. 
         [0082]    On the other hand, when the generator rotational speed ω is in a range in which the generator rotational speed ω is equal to or higher than the threshold rotational speed ω′ M  and lower than the rated rotational speed ω max , the generator rotational speed ω is controlled to the rated rotational speed ω max  by the PI control and therefore the pitch angle command β* is set to the fine-side limit value, that is, the minimum pitch angle β min . 
         [0083]    The correction of the pitch angle command β* with the above-described correction value Δβ* effectively works when the generator rotational speed ω is higher than the rated rotational speed ω max  and the real power command P* does not reach the rated power P rated . Since the real power command P* is smaller than the rated power P rated , the deviation ΔP is negative and therefore the correction value Δβ* is also negative. As a result, the pitch angle command β* becomes smaller than the pitch angle command baseline value β in *, that is, the pitch angle β becomes closer to the fine-side. This allows converting the aerodynamic energy into electric power more efficiently. When the real power command P* reaches the rated power P rated , the generator rotational speed ω is controlled to the rated rotational speed ω max  by the PI control.
   Case (4): The generator rotational speed ω is higher than the threshold rotational speed ω′ M  and the pitch angle β does not reach the minimum pitch angle β min .   
 
         [0085]    In this case, the power control rotational speed command ω P * is set to the rated rotational speed ω max . Furthermore, the power command lower limit P min  is set to smaller one of the one-operation-step previous real power command P* and the power command upper limit P max  at the current operation step, and the power command upper limit P max  is set to the rated power P rated . As a result, the real power command P* is set to the rated power P rated . In other words, the power control is kept in the rated value control mode even when the generator rotational speed ω is reduced below the rated rotational speed ω max . It is determined whether or not the pitch angle β reaches the minimum pitch angle β min  on the basis of whether the pitch angle command β* coincides with the minimum pitch angle β min . 
         [0086]    On the other hand, when the generator rotational speed ω is in a range in which the generator rotational speed ω is equal to or higher than the threshold rotational speed ω′ M , and lower than the rated rotational speed ω max , the pitch angle command β* is controlled to the rated rotational speed ω max  by the PI control, and therefore the pitch angle command β* is set to the fine-side limit value, that is, the minimum pitch angle β min . 
         [0087]    The correction of the pitch angle command β* with the above-described correction value Δβ* effectively works when the generator rotational speed ω is higher than the rated rotational speed ω max  and the real power command P* does not reach the rated power P rated . Since the real power command P* is smaller than the rated power P rated , the deviation ΔP is negative and therefore the correction value Δβ* is also negative. As a result, the pitch angle command β* becomes smaller than the pitch angle command baseline value β in *, that is, the pitch angle β becomes closer to the fine-side. This allows converting the aerodynamic energy into electric power more efficiently. When the real power command P* reaches the rated power P rated , the generator rotational speed ω is controlled to the rated rotational speed ω max  by the PI control.
   Case (5): The generator rotational speed ω is reduced below the threshold rotational speed ω′ M , whereby the generator rotational speed ω is in a range higher than the intermediate rotational speed ω M .   
 
         [0089]    In this case, the power control rotational speed command ω p * is set to the rated rotational speed ω max  by the selector  33 , and the power command lower limit P min  and the power command upper limit P max  are set to P opt  and P rated , respectively. In this case, the real power command P* is always set to the power command lower limit P min , since the deviation Δω P (=ω−ω max ) is negative and the generator rotational speed ω is controlled to the rated rotational speed ω max  by the pitch control module  32 . The real power command P* is eventually set to the optimized power value P opt , since the power command lower limit P max  is P opt . In other words, the power control is switched from the rated value control mode to the optimum curve control mode. 
         [0090]      FIG. 7  is a graph showing an example of the operation performed by the wind turbine generator system  1  of this embodiment. The real power command P* is set to the optimized power value P opt  until the generator rotational speed ω reaches the rated rotational speed ω max  after the wind turbine generator system  1  starts operating (the above-described Case (2)). Accordingly, the outputted real power P is increased as the generator rotational speed ω increases. The pitch angle command β* is set to the minimum pitch angle β min , so as to allow the generator rotational speed ω to reach the rated rotational speed ω max . 
         [0091]    When the generator rotational speed ω exceeds the rated rotational speed ω max , the real power command P* is set to the rated power P rated  (the above-described Case (3)). Accordingly, the outputted real power P is kept at the rated power P rated . Since the generator rotational speed ω exceeds the rated rotational speed ω max , the pitch angle command β* increases and the pitch angle β is varied toward the feather-side limit value. 
         [0092]    When a transitional wind null occurs, the generator rotational speed ω sharply decreases. The pitch control module  32  reduces the pitch angle command β* so as to maintain the generator rotational speed ω at the rated rotational speed ω max  to thereby reduce the pitch angle β, that is, to vary the pitch angle β toward the fine side. Even when the generator rotational speed ω is reduced below the rated rotational speed ω max , the real power command P* is kept at the rated power P rated  as long as the pitch angle β does not reach the minimum pitch angle β in . Therefore, the outputted real power P is also kept at the rated power P rated . 
         [0093]    In the operation shown in  FIG. 7 , the generator rotational speed ω returns to the rated rotational speed ω max  again before the pitch angle β reaches the minimum pitch angle β min , so that the real power P is kept at the rated power P rated . In this way, the wind turbine generator system  1  of this embodiment suppresses the output power fluctuation when a transient wind null occurs. Furthermore, the wind turbine generator system  1  of this embodiment makes effective use of the rotational energy of the wind turbine rotor  7  and improves the generation efficiency, since the output power P is not reduced below the rated power P rated  until the increase in the output coefficient of the wind turbine rotor  7  through the reduction in the pitch angle β becomes impossible, when the generator rotational speed ω is reduced below the rated rotational speed ω max . 
         [0094]    It is preferable that the wind turbine generator system  1  of this embodiment is configured to perform various control methods in accordance with various operating situations.  FIG. 8  shows a preferred configuration of the wind turbine generator system  1  configured to perform controls accordingly to various operating situations. 
         [0095]    First, in the wind turbine generator system  1  shown in  FIG. 8 , the main control unit  19  detects an occurrence of a gust (rush of wind) by the wind speed an the wind direction measured by the anemometer  10 . The main control unit  19  may detect the occurrence of the gust on the basis of the generator rotational speed ω in place of the wind speed and the wind direction. When the main control unit  19  detects the occurrence of the gust, the real power command P* is controlled so as not to excessively increase the rotational speed of the wind turbine rotor  7 . Specifically, as shown in  FIG. 9 , when the occurrence of the gust is detected based on the wind speed and the wind direction (Step S 01 ), the acceleration of the wind turbine rotor  7  (rotor acceleration) or the rotational speed of the wind turbine rotor  7  (rotor rotational speed) is monitored. When the rotor acceleration or the rotor rotational speed exceeds a predetermined limit value (Step S 02 ), the real power command P* is increased (Step S 03 ). When the real power command P* is controlled to the rated power P rated  until just before the step S 03 , the real power command P* is controlled to be increased above the rated power P rated . The rotational energy of the wind turbine rotor  7  is thereby converted into electric energy and consumed by the power grid  13 . This decelerates the wind turbine rotor  7 . 
         [0096]    Moreover, the wind turbine generator system  1  shown in  FIG. 8  is configured so that the nacelle rotation mechanism  4  moves the rotation plane of the wind turbine rotor  7  away from the windward direction to thereby stop the wind turbine rotor  7 , when the pitch control unit  92  detects a failure in the pitch drive mechanism that drives the blades  8 . To achieve this goal, the wind turbine generator system  1  shown in  FIG. 8  is configured so that the pitch control unit  22  is adapted to detect a failure in the hydraulic cylinders  11  and/or the servo valves  12  shown in  FIG. 2 . The main control unit  19  generates a yaw command in response to the detection of the failure, when a failure is detected in the hydraulic cylinders  11  and/or the servo valve  12 . 
         [0097]      FIG. 10  shows a procedure of moving the rotation plane away from the windward direction. When the pitch control unit  22  detects a failure in the hydraulic cylinders  11  and/or the servo valves  12  (Step S 06 ), a pitch failure signal is activated. In response to the activation of the pitch failure signal, the main control unit  19  controls the yaw angle of the nacelle  3 , thereby moving the rotation plane of the wind turbine rotor  7  away from the windward direction (step S 07 ). The windward direction can be determined from the wind direction measured by the anemometer  10 . By moving the rotation plane of the wind turbine rotor  7  away from the windward direction, the wind speed of wind flowing in the wind turbine rotor  7  is reduced and the rotational torque is reduced (Step S 08 ). As a result, the wind turbine rotor  7  is decelerated and stopped. 
         [0098]    In addition, the wind turbine generator system  1  shown in  FIG. 8  is configured so as to control the reactive power Q supplied to the power grid  13  when the grid voltage V grid  is excessively increased or decreased, and to perform a pitch control in response to the reactive power Q.  FIG. 11  is a flowchart showing procedures of such control. 
         [0099]    When the grid voltage V grid  is increased above X % of a predetermined rated voltage V rated  (where X is a predetermined value larger than 100) or is reduced below Y % of the predetermined rated voltage V rated  (where Y is a predetermined value smaller than 100) (Step S 11 ), the power factor command fed to the power control module  31  is modified (Step S 12 ). The modified power factor command may be fed from a control system of the power grid  13 ; instead, the main control unit  19  may in itself modify the power factor command in accordance with the grid voltage V grid . This results in that the reactive power command Q* is reduced when the grid voltage V grid  exceeds X % of the predetermined rated voltage V rated , and that the reactive power command Q* is increased when the grid voltage V grid  is reduce below Y % of the predetermined rated voltage V rated . Since the apparent power S supplied from the wind turbine generator system  1  to the power grid  13  is constant, the real power command P* is increased when the reactive power command Q* is reduced, while the real power command P* is reduced when the reactive power command Q* is increased. The AC-DC-AC converter  17  is controlled in response to the real power command P* and the reactive power command Q*, thereby controlling the reactive power Q supplied to the power grid  13  (step S 13 ). 
         [0100]    When the reactive power command Q* is largely increased, the real power command P* is reduced, and this causes reduction in the output of the wind turbine generator system  1 . To avoid such a problem, the pitch angle command β* is reduced (that is, the pitch angle command β* is varied toward the fine side) when the increase in the reactive power command Q* is larger than a predetermined increase amount, thereby increasing the real power P (step S 15 ). 
         [0101]    When the reactive power command Q* is largely reduced, the real power command P* is increased, and this unnecessarily increases the output of the wind turbine generator system  1 . To avoid such a problem, the pitch angle command β* is increased (that is, the pitch angle command β* is varied toward the feature side) when the decrease in the reactive power command Q* is larger than a predetermined decrease amount, thereby reducing the real power P. 
         [0102]    Furthermore, the wind turbine generator system  1  shown in  FIG. 8  is configured so as to increase the outputted real power P while the emergency battery  28  is charged. This addresses compensation for power used to charge the emergency battery  28 . Specifically, as shown in  FIG. 12 , when the battery charger  27  starts charging the emergency battery  28  (step S 21 ), the battery charger  27  activates a charge start signal. The main control unit  19  increases the real power command P* in response to the activation of the charge start signal (step S 22 ). The increase amount of the real power command P* is set to be equal to the amount of the power used to charge the emergency battery  28 . When the emergency battery  28  is not charged, the real power command P* generated by the PI controller  35  is used to control the AC-DC-AC converter  17 . 
         [0103]    It should be noted that the present invention is not to be interpreted to be limited to the above-stated embodiments. For example, although the wind turbine generator system  1  of this embodiment is a doubly-fed variable speed wind turbine system, the present invention is also applicable to other kinds of wind turbine generator system capable of varying both the rotational speed of the wind turbine rotor and the pitch angle. For example, the present invention is applicable to a wind turbine generator system configured so that an AC-DC-AC converter converts all the AC power generated by the generator into AC power adapted to the frequency of the power grid. 
         [0104]    Further, the emergency battery  28  may be charged not with the power received from the power grid but with the power outputted from the generator. 
         [0105]    Moreover, it is apparent for the person skilled in the art that the rotational speed of the wind turbine rotor  7  may be used in place of the generator rotational speed ω, since the rotational speed of the wind turbine rotor  7  depends on the generator rotational speed ω. For example, as is the case of this embodiment, the rotational speed of the wind turbine rotor  7  has one-to-one correspondence to the generator rotational speed ω when the wind turbine rotor  7  is connected to the wound-rotor induction generator  5  through the gear  6 . The rotational speed of the wind turbine rotor  7  can be used in place of the generator rotational speed ω, even when a continuously variable transmission such as a toroidal transmission is used in place of the gear  6 ; the generator rotational speed ω increases in accordance with an increase in the rotational speed of the wind turbine rotor  7 .