Patent Publication Number: US-2013234433-A1

Title: Wind turbine generator system and operation control method thereof

Description:
TECHNICAL FIELD  
     The present invention relates to a wind turbine generator which transmits rotation of a rotor to a generator via a hydraulic transmission including a hydraulic pump and a hydraulic motor, and an operation control method thereof. 
     BACKGROUND ART 
     In recent years, from a perspective of preserving the environment, it is becoming popular to use a wind turbine generator utilizing wind power which is one form of renewable energy. 
     The wind turbine generator converts motion energy of the wind to the rotation energy of the rotor and further converts the rotation energy of the rotor to electric power by the generator. In a common wind turbine generator, the rotation speed of the rotor is about a few rotations per minute to tens of rotations per minute. Meanwhile, a rated speed of the generator is normally 1500 rpm or 1800 rpm and thus a mechanical gearbox is provided between the rotor and the generator. Specifically, the rotation speed of the rotor is increased to the rated speed of the generator by the gearbox and then inputted to the generator. 
     In recent years, the gearbox tends to become heavier and more expensive as the wind turbine generators are getting larger to improve power generation efficiency. Thus, a wind turbine generator equipped with a hydraulic transmission adopting a combination of a hydraulic pump and a hydraulic motor of a variable capacity type is getting much attention. 
     For instance, Patent Document 1 discloses a wind turbine generator using a hydraulic transmission including a hydraulic pump rotated by a rotor and a hydraulic motor connected to a generator. In the hydraulic transmission of this wind turbine generator, the hydraulic pump and the hydraulic motor are connected via a high pressure reservoir and a low pressure reservoir. By this, the rotation energy of the rotor is transmitted to the generator via the hydraulic transmission. Further, the hydraulic pump is constituted of a plurality of sets of pistons and cylinders, and cams which periodically reciprocate the pistons in the cylinders. 
     Further, Patent Document 2 describes a wind turbine generator adopting a hydraulic transmission constituted of a hydraulic pump rotated by a rotor, a hydraulic motor connected to a generator, and an operating oil path arranged between the hydraulic pump and the hydraulic motor. In the hydraulic transmission of this wind turbine generator, the hydraulic pump is constituted of a plurality of sets of pistons and cylinders, cams which periodically reciprocate the pistons in the cylinders, and high pressure valves and low pressure valves which opens and closes with the reciprocation of the pistons. By latching the piston near a top dead center, a working chamber surrounded by the cylinder and the piston is disabled, and then the displacement of the hydraulic pump is changed. 
     Although the hydraulic pump and the hydraulic motor are not variable displacement type, Patent Document 3 discloses a wind turbine generator having a hydraulic pump and a hydraulic motor. The wind turbine generator of Patent Document 3 maintains the rotation speed of the generator constant by adjusting the pressure of operating oil to be supplied from a hydraulic pump to a hydraulic motor. In this wind turbine generator, a discharge side of the hydraulic pump is connected to an intake side of the hydraulic motor via an inner space of the tower functioning as a high pressure tank, and an intake side of the hydraulic pump is connected to a discharge side of the hydraulic motor via low pressure tank arranged below the tower. Further, a proportional valve is provided between the high pressure tank and the hydraulic motor. The pressure of the operating oil to be supplied to the hydraulic motor is adjusted by the proportional valve. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     U.S. 2010/0032959 
     [PTL 2] 
     U.S. 2010/0040470 
     [PTL 3] 
     U.S. Pat. No. 7,436,086 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional wind turbine generator, it is necessary to appropriately perform efficient and stable operation thereof in response to the change of the wind conditions such as the low wind speed or the occurrence of a gust. Further, to synchronized the wind turbine generator with the grid, the wind turbine generator must be equipped with a Ride Through function at Grid low voltage condition according to a grid code set by every country. The Ride Through function at Grid low voltage condition enables an uninterrupted operation without being disconnected from the grid even when the voltage of the grid declined momentarily. 
     However, none of the wind turbine generators disclosed in Patent Documents 1 to 3 takes measures against the situation when the wind speed is low and when there arises a gust. Therefore, the wind turbine generators of Patent Documents 1 to 3 do not necessary achieve an operation efficiency or stability when the wind speed is low and when there arises a gust. 
     Further, none of Patent Documents 1 to 3 discloses a structure to realize the Ride Through function at Grid low voltage condition and there was an issue on how to realize the Ride Through function at Grid low voltage condition in the wind turbine generator using a hydraulic transmission. 
     In view of the problems above, an object of the present invention is to provide a wind turbine generator system and an operation control method thereof, which has a superior operation efficiency and stability at a low wind speed and during the occurrence of a gust and is equipped with Ride Through function at Grid low voltage condition. 
     Solution to Problem 
     A wind turbine generator system in relation to the present invention, comprises: a hub; a main shaft which is coupled to the hub; a generator which is synchronized with a grid; a hydraulic pump of variable displacement type which is driven by the main shaft; a hydraulic motor of variable displacement type which is connected to the generator; a high pressure oil line which is arranged between a discharge side of the hydraulic pump and an intake side of the hydraulic motor; a low pressure oil line which is arranged between an intake side of the hydraulic pump and a discharge side of the hydraulic motor; an accumulator which is connected to the high pressure oil line via an accumulator valve; and a control unit which controls the hydraulic pump, the hydraulic motor and the accumulator valve; wherein the control unit controls opening and closing of the accumulator valve based on at least one of wind speed and a state of the grid. 
     In this wind turbine generator system, the opening and closing of the accumulator valve is controlled based on at least of wind speed and the state of the grid. When the accumulator valve is opened, the accumulator is in communication with the high pressure oil line, and the pressure in the high pressure oil line is absorbed in the accumulator or the pressure in accumulator is released to the high pressure oil line depending on which one of the pressure in the accumulator and the pressure in the high pressure oil line is higher. Meanwhile, when the accumulator valve is closed, the communication between the accumulator and the high pressure oil line is disconnected and thus the pressure in the accumulator is maintained on its own without depending on the pressure in the high pressure oil line. 
     Therefore, the opening and closing of the accumulator valve is controlled base on at least one of the wind speed and the state of the grid, so as to improve the operation efficiency and stability when the wind speed is low and when there arises a gust, and to realize the Ride Through function at Grid low voltage condition. 
     The wind turbine generator system preferably further comprises a grid state determination unit, wherein the state of the grid is determined by the grid state determination unit. 
     The wind turbine generator system may further comprise a bypass oil line which is arranged between the high pressure oil line and the low pressure oil line to bypass the hydraulic motor; and a relief valve which is arranged in the bypass oil line to maintain the hydraulic pressure in the high pressure oil line not more than a set pressure. 
     In this manner, by providing the relief valve in the bypass line, the pressure in the high pressure oil line rises to the prescribed pressure of the relief valve by the high pressure oil supplied from the hydraulic pump raises the pressure, and then the relief valve automatically opens to discharge excessive high pressure oil to the low pressure oil line via the bypass line. 
     In the wind turbine generator system, when the wind speed is below a cut-in wind speed, the control unit may control a displacement of the hydraulic motor approximately to zero, and keep the accumulator valve open so as to store in the accumulator a pressure of operating oil discharged from the hydraulic pump which pressure can be reached to a set value of a relief valve. 
     Alternatively, in the wind turbine generator system, when the wind speed is below a cut-in wind speed, the control unit controls a displacement of the hydraulic motor approximately to zero, and opens the accumulator valve so as to store in the accumulator a pressure of operating oil discharged from the hydraulic pump and closes the accumulator valve when the pressure in the accumulator detected by the first pressure sensor reaches a threshold value which is decided in advance. 
     In the conventional wind turbine generator system, there is not power generation when the wind speed is below the cut-in wind speed and thus, the wind energy at the low wind speed is wasted. Thus, the accumulator valve is opened when the wind speed is below the cut-in wind speed so as to store pressure in the accumulator. As a result, the wind energy previously wasted at the low wind speed is utilized, resulting in improved operation efficiency. 
     The wind turbine generator system may further comprise a first pressure sensor which detects a pressure in the accumulator and a second pressure sensor which detects a pressure in the high pressure oil line, and the control unit may preferably control a displacement of the hydraulic motor approximately to zero leaving some displacement, opens the accumulator valve so as to store in the accumulator a pressure of operating oil discharged from the hydraulic pump in such a case that the pressure in the accumulator detected by the first pressure sensor is lower than the pressure in the high pressure oil line detected by the second pressure sensor, and closes the accumulator valve when the pressure in the accumulator detected by the first pressure sensor reaches a threshold value. 
     In such a case, the wind turbine generator system may further comprises: a first pressure sensor which detects the pressure in the accumulator; a bypass oil line which is arranged between the high pressure oil line and the low pressure oil line to bypass the hydraulic motor; and a relief valve which is arranged in the bypass oil line to maintain the hydraulic pressure in the high pressure oil line not more than a set pressure, wherein, when the pressure in the accumulator detected by the first pressure sensor reaches the set pressure of the relief valve, the control unit closes the accumulator valve. 
     In this manner, by providing the relief valve in the bypass line, the pressure in the high pressure oil line rises to the prescribed pressure of the relief valve by the high pressure oil supplied from the hydraulic pump raises the pressure, and then the relief valve automatically opens to discharge excessive high pressure oil to the low pressure oil line via the bypass line. Further, when the pressure in the accumulator reaches the prescribed pressure of the relief valve, the wind energy at the low wind speed can be stored maximally as a pressure in the accumulator. 
     Further, when the wind speed is not less than the cut-in wind speed, the control unit keeps the accumulator valve open so as to assist rotation of the hydraulic motor with use of pressure of operating oil in the accumulator. Alternatively, when the wind speed is not less than the cut-in wind speed, the control unit opens the accumulator valve so as to assist rotation of the hydraulic motor with use of pressure of operating oil in the accumulator in such a case that the pressure in the accumulator detected by the first pressure sensor is higher than the pressure in the high pressure oil line detected by the second pressure sensor. 
     By this, with use of the pressure of the fluid in the accumulator stored at the low wind speed, more electric power can be generated, resulting in improving the operation efficiency. 
     The above wind turbine generator system may further comprises: a pitch driving mechanism which adjusts a pitch angle of a blade mounted on the hub, wherein, when the grid state determination unit determines that a voltage of the grid has decreased to a prescribed voltage or lower and the condition based on Gid Code rule continues, the control unit may controls the pitch driving mechanism to change the pitch angle of the blade toward a feathering position, changes the displacements of the hydraulic pump to meet the load of the rotor and reduces the hydraulic motor to an amount required for keeping the generator synchronized with the grid, and keeps the accumulator valve open so as to store in the accumulator a pressure of the operating oil discharged from the hydraulic pump. 
     Alternatively, the wind turbine generator system may further a pitch driving mechanism which adjusts a pitch angle of a blade mounted on the hub, wherein, when the grid state determination unit determines that a voltage of the grid has decreased to a prescribed voltage or lower and the condition based on Gid Code rule continues, the control unit controls the pitch driving mechanism to change the pitch angle of the blade to a feathering position, changes the displacements of the hydraulic pump to meet the load of the rotor and reduces the hydraulic motor to an amount required for keeping the generator synchronized with the grid, and opens the accumulator valve so as to store in the accumulator a pressure of the fluid discharged from the hydraulic pump in such a case that the pressure in the accumulator detected by the first pressure sensor is lower than the pressure in the high pressure oil line detected by the second pressure sensor. 
     As described above, when the voltage of the grid is decreased, the pitch angle is changed to the feathering position, the displacements of the hydraulic pump are changed to meet the load of the rotor and the hydraulic motor is reduced to an amount required for keeping the generator synchronized with the grid, so as to continue the operation of the system without being disconnected from the grid. That is, the Ride Through function at Grid low voltage condition is realized. 
     Herein, after the voltage of the grid has decreased to the prescribed voltage or lower, if the displacement of the hydraulic pump is suddenly reduced, the adjusting of the pitch angle does not catch up with the reduced displacement and thus the force acting from the wind acting on the rotor with respect to the torque required for driving the hydraulic pump becomes too strong and the rotation of the rotor increases dramatically. Meanwhile, after the voltage of the grid has declined, the load of the generator declines dramatically and thus, the rotation of the hydraulic motor gets too fast unless the displacement of the hydraulic motor is reduced instantly. Therefore, after the voltage of the grid has decreased, the displacement of the hydraulic motor needs to be reduced promptly in comparison to the hydraulic pump. Unless the displacement of the hydraulic motor is reduced promptly, the discharge amount from the hydraulic pump momentarily exceeds the discharge amount from the hydraulic motor and thus the pressure in the high pressure oil line rises, which can result in dysfunction of the hydraulic transmission. By providing a relief valve for releasing the excessive pressure from the high pressure oil line to the low pressure oil line, the pressure in the high pressure oil line does not exceed the prescribed pressure. However, the friction heat cause when passing through the relief valve rises the temperature of the fluid, and it is difficult to promptly return to the normal operation once the voltage of the grid is restored. 
     Therefore, when the voltage of the grid has decreased to the prescribed voltage or below, the accumulator valve is opened in such a case that the pressure in the accumulator is lower than the pressure in the high pressure oil line, thereby preventing the pressure in the high pressure oil line from being too high or the temperature of the fluid from rising by the actuation of the relief valve. 
     Furthermore, the wind turbine generator system may further comprise an uninterruptible power supply which supplies electrical power to the pitch driving mechanism-when the voltage of the grid becomes zero. 
     Further, the wind turbine generator system may further comprise an uninterruptible power supply which supplies electrical power to the hydraulic pump and the hydraulic motor when the voltage of the grid becomes zero. 
     By this, even when the voltage of the grid becomes zero, the pitch driving mechanism and/or the hydraulic pump and the hydraulic motor can be operated to put the wind turbine generator system into a Ride Through operation state at Grid low voltage condition or a shutoff state. 
     It is preferable that, when the grid state determination unit determines that voltage of the grid starts to be restored, the control unit changes the pitch angle of the blade to a fine position, and sets the displacements of the hydraulic pump so that a power coefficient becomes at a maximum thereof, and increases the displacement of the hydraulic motor so that the power generated by the generator into the grid increases. 
     In this manner, when the voltage of the grid starts to be restored, the pitch angle is changed to the fine position and the displacements of the hydraulic pump is set so that a power coefficient becomes at a maximum thereof, and the displacement of the hydraulic motor is increased so that the power generated by the generator into the grid increases. As a result, the operation of the system can promptly returns to the efficient operation. 
     Moreover, in this situation, the accumulator valve may be opened so as to assist rotation of the hydraulic motor with use of pressure of fluid in the accumulator in such a case that the pressure in the accumulator is higher than the pressure in the high pressure oil line. 
     The wind turbine generator system may also comprise: a oil tank which stores fluid and is connected to the low pressure oil line; an accumulator pressure relief line provided between the accumulator valve and the accumulator and connected to one of the low pressure oil line and the oil tank; and a pressure relief valve provided in the accumulator pressure relief line, wherein, when the grid state determination unit determines that voltage of the grid is restored, the control unit opens the pressure relief valve to allow the pressure in the accumulator to escape to the one of the low pressure oil line and the oil tank. 
     As described above, when the voltage of the grid is restored, the pressure relief valve is opened to allow the pressure in the accumulator to escape to the low pressure oil line or the oil tank. As a result, the pressure in the accumulator can be reduced to take measure against the reoccurrence of the voltage decline of the grid. 
     In the above wind turbine generator system, when there arises a gust having a wind speed greater than a prescribed threshold value, the control unit may keep the accumulator valve open so as to store in the accumulator a pressure of operating oil discharged from the hydraulic pump, and when the wind speed of the gust becomes not greater than the prescribed threshold value, the control unit keeps the accumulator valve open so as to assist rotation of the hydraulic motor with use of the pressure of the operating oil in the accumulator. 
     Alternatively, a first pressure sensor which detects a pressure in the accumulator and a second pressure sensor which detects a pressure in the high pressure oil line may also be provided and when there arises a gust having a wind speed greater than a prescribed threshold value, the control unit opens the accumulator valve so as to store in the accumulator a pressure of fluid discharged from the hydraulic pump in such a case that the pressure in the accumulator detected by the first pressure sensor is lower than the pressure in the high pressure oil line detected by the second pressure sensor; and when the wind speed of the gust becomes not greater than the prescribed threshold value, the control unit keeps the accumulator valve open until the pressure in the accumulator detected by the first pressure sensor becomes the same as the pressure in the high pressure oil line detected by the second pressure sensor. 
     When there arises a gust, the rotation of the rotor rises and the discharge amount from the hydraulic pump increases, thereby rising the pressure in the high pressure oil line. Therefore, when there arises a gust, the accumulator valve is opened in such a case that pressure in the accumulator is lower than the pressure in the high pressure oil line so as to absorb the excessive pressure of the high pressure oil line in the accumulator. And once the wind speed of the gust becomes equal to or lower than the threshold value, the accumulator valve is kept open so as to release the pressure in the accumulator. In this manner, the effect of the gust on the wind turbine generator system can be minimized. 
     Further, the prescribed threshold value may be 60m/sec which is obtained as an average wind speed for a period of three seconds or longer. 
     Further, the wind turbine generator system further comprises: an oil tank which stores operating oil and is connected to the low pressure oil line; an accumulator pressure relief line provided between the accumulator valve and the accumulator and connected to one of the low pressure oil line and the oil tank; and a pressure relief valve provided in the accumulator pressure relief line, wherein, when the grid state determination unit determines that voltage of the grid is restored, the control unit opens the pressure relief valve to allow the pressure in the accumulator to escape to the one of the low pressure oil line and the oil tank. 
     As described above, when the wind speed of the gust becomes not greater than the threshold value, the pressure relief valve is opened so as to allow the pressure in the accumulator to escape to the low pressure oil line or the oil tank via the accumulator pressure relief line. As a result, the pressure in the accumulator is reduced and thus, it is possible to take measure against the recurrence of the gust. 
     The wind turbine generator system may further comprise: a oil tank which stores fluid; and a boost pump which replenishes the low pressure oil line with the fluid from the oil tank. 
     By this, even if the fluid leaks in the hydraulic transmission, the boost pump replenishes the low pressure oil line with the fluid from the oil tank so as to keep the amount of the fluid circulating in the hydraulic transmission. 
     In this situation, the wind turbine generator system may also comprise: a return line which returns the fluid in the low pressure oil line to the oil tank; and a low pressure relief valve which is arranged in the return line to maintain the hydraulic pressure in the low pressure oil line approximately at a set pressure. 
     By this, although the boost pump replenishes the low pressure oil line with the fluid, the low pressure relief valve automatically opens once the pressure in the low pressure oil line reaches the prescribed pressure of the low pressure relief valve. Thus, the fluid is returned to the oil tank via the return line and the amount of the fluid circulating in the hydraulic transmission is appropriately maintained. 
     The present invention also proposes an operation control method of a wind turbine generator system which comprises a hub, a main shaft coupled to the hub, a generator which is synchronized with a grid, a hydraulic pump of variable displacement type which is driven by the main shaft, a hydraulic motor of variable displacement type which is connected to the generator, a high pressure oil line which is arranged between a discharge side of the hydraulic pump and an intake side of the hydraulic motor, a low pressure oil line which is arranged between an intake side of the hydraulic pump and a discharge side of the hydraulic motor, an accumulator which is connected to the high pressure oil line via an accumulator valve, and a grid state determination unit which determines a state of the grid, the method comprising the step of controlling opening and closing of the accumulator valve based on at least one of wind speed and the state of the grid determined by the grid state determination unit. 
     According to the operation control method of the wind turbine generator system, the opening and closing of the accumulator valve is controlled based on at least one of wind speed and the state of the grid determined by the grid state determination. As a result, the operation efficiency and stability at the low wind speed or when there arises a gust, can be improved and the Ride Through function at Grid low voltage condition can be achieved as well. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the present invention, the accumulator is connected to the high pressure oil line via the accumulator valve and the opening and closing of the accumulator valve is controlled based on at least one of wind speed and the state of the grid determined by the grid state determination unit and both of the pressure in the accumulator detected by the first pressure sensor and the pressure in the high pressure oil line detected by the second pressure sensor. As a result, the operation efficiency and stability at the low wind speed or when there arises a gust, can be improved and the Ride Through function at Grid low voltage condition can be achieved as well. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       
         FIG. 1 
       
         FIG. 1  is a general structure of a wind turbine generator system. 
       
         FIG. 2 
       
         FIG. 2  is a diagram showing a structure of a pitch driving mechanism. 
       
         FIG. 3 
       
         FIG. 3  is a diagram showing a structure of a hydraulic transmission. 
       
         FIG. 4 
       
         FIG. 4  is a diagram showing a system structure of the wind turbine generator system. 
       
         FIG. 5 
       
         FIG. 5  is a flow chart showing an operation control of an accumulator at a low wind speed. 
       
         FIG. 6 
       
         FIG. 6  is a flow chart showing an operation control of releasing pressure in the accumulator. 
       
         FIG. 7 
       
         FIG. 7  is a flow chart showing an operation control when a voltage of a grid has decreased. 
       
         FIG. 8 
       
         FIG. 8  is a flow chart showing an operation control when there arises a gust. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present. 
     A general structure of the wind turbine generator system in relation to a preferred embodiment is described.  FIG. 1  illustrates a general structure of a wind turbine generator system. 
     As an example of the wind turbine generator, a three-bladed wind turbine is used. However, the preferred embodiment is not limited to this example and can be applied to various types of wind turbine generator systems. 
     As illustrated in  FIG. 1 , a wind turbine generator system  1  comprises a rotor  2  rotated by the wind, a hydraulic transmission  10  for increasing rotation speed of the rotor  2 , a generator  20  for generating electric power, a nacelle  22 , a tower  24  system for supporting the nacelle  22  and a control unit  30  for controlling each controller of the wind turbine generator system  1 . 
     The rotor  2  is constructed such that a main shaft  8  is connected to a hub  6  having blades  4 . Specifically, three blades  4  extend radially from the hub  6  and each of the blades  4  is mounted on the hub  6  connected to the main shaft  8 . By this, the power of the wind acting on the blades  4  rotates the entire rotor  2 , the rotation of the rotor  2  is inputted to the hydraulic transmission  10  via the main shaft  8 . 
     The hydraulic transmission  10  includes a hydraulic pump  12  of a variable displacement type which is rotated by the main shaft  8 , a hydraulic motor  14  of a variable displacement type which is connected to the generator  20 , and a high pressure oil line  16  and a low pressure oil line  18  which are arranged between the hydraulic pump  12  and the hydraulic motor  14 . The detailed structure of the hydraulic transmission  10  is described later. 
     The generator  20  is connected to the hydraulic motor  14  of the hydraulic transmission  10 . An existing asynchronous generator or synchronous generator can be used as the generator  20 . The torque having the rotation speed that is almost constant is inputted from the hydraulic motor  14  to the generator  20  and then the generator  20  generates alternating current having a frequency that is almost constant. Further, the generator  20  is synchronized with a grid  50  which will be described later, and the current generated by the generator  20  is transmitted to the grid  50 . 
     The nacelle  22  supports the hub of the rotor  2  rotatably and houses a variety of devices such as the hydraulic transmission  10  and the generator  20 . The nacelle  22  is further supported on the tower  24  rotatably and may be turned in accordance with the wind direction by a yaw motor which is not shown. 
     The tower  24  is formed into a column shape extending upward from a base  26 . For instance, the tower  22  can be constituted of one column member or a plurality of units that are connected in a vertical direction to form a column shape. If the tower  24  is constituted of the plurality of units, the nacelle  22  is mounted on the top-most unit. 
     The control unit  30  comprises a pump controller  32  for adjusting the displacement of the hydraulic pump  12 , a motor controller for adjusting the displacement of the hydraulic motor  14 , a pitch controller for adjusting the pitch angle of the blade  4 , and an ACC valve controller  38  for controlling the opening and closing of an accumulator valve described later. 
     Further, a pitch driving mechanism  40  for changing the pitch angle of the blade  4  is housed in the hub  6 .  FIG. 2  is a diagram showing a structure of the pitch driving mechanism  40 . The figure illustrates the pitch driving mechanism  40  comprising a hydraulic cylinder  42 , a servo valve  44 , a hydraulic pressure source  46  and an accumulator  48 . The servo valve  44  adjusts the supply of the high pressure oil generated by the hydraulic pressure source  46  and the high pressure oil stored in the accumulator  48  to the hydraulic cylinder  42  so as to achieve a desired pitch angle of the blade  4  under the control of the pitch controller  36 . 
     The pitch controller  36  normally receives the electric power from the grid  50 . However, when the voltage of the grid becomes zero, the electric power is supplied from an uninterruptible power supply  52 . Further, a state of the grid  50  is monitored by a grid state determination unit  54 . The grid state determination unit  42  preferably includes a voltage sensor for measuring a voltage of the grid  50 . However, the voltage sensor can be replaced with a power-factor meter or a reactive volt-ampere meter to indirectly determine the state of the grid  50 . 
     Next, a detailed structure of the hydraulic transmission  10  is explained.  FIG. 3  is a diagram showing a structure of the hydraulic transmission  10 . 
     As described above, the hydraulic transmission comprises the hydraulic pump  12 , the hydraulic motor  14 , the high pressure oil line  16  and the low pressure oil line  18 . The discharge side of the hydraulic pump  12  is connected to the intake side of the hydraulic motor  14  and the intake side of the hydraulic pump  12  is connected to the discharge side of the hydraulic motor  14 . 
       FIG. 3  illustrates the hydraulic transmission  10  including only one hydraulic motor  14 . However, the hydraulic transmission  10  may comprise a plurality of hydraulic motors  14  and the hydraulic motors  14  may be connected to the hydraulic pump  12  via the high pressure oil lines  16  and the low pressure oil lines  18  respectively. In this case, the high pressure oil line  16  having one end thereof connected to the discharge side of the hydraulic pump  12 , splits midway and each of the split lines of the high pressure oil line  16  is connected to the intake side of each of the hydraulic motors  14  while a plurality of lines of the low pressure oil line  18  having one end thereof connected to the discharge side of each of the hydraulic motors  14  flow into one and then connected to the discharge side of the hydraulic pump  12 . 
     Accumulators  60  ( 60 A,  60 B) are connected to the high pressure oil line  16  via accumulator valves  62  ( 62 A,  62 B). The accumulator  60  may be, for instance, a bladder-type in which air and the operating oil are separated by a deformable bag, e.g. a bladder or a piston. In the accumulator  60 , the operating oil of high pressure is introduced during an accumulating process so as to deform the bladder or the piston and compress the air. In contrast, during a pressure-releasing process, the compressed air expands or the high pressure air from outside pushes the bladder or piston and then the operating oil is pushed out of the accumulator  60 . 
     Further, at least one set of the accumulator  60  and the accumulator valve  62  is needed. However, if a plurality of sets of the accumulators  60  ( 60 A,  60 B) and the accumulator valves  62  ( 62 A,  62 B) are arranged as shown in  FIG. 3 , the plurality of accumulators  60  can be adequately used depending on intended purposes. 
     Furthermore, at least one of the accumulators  60  may have an air part whose displacement is greater than that of an operating oil part at the time of initially sealing the air, and the pressure of the initially-sealed air of each accumulator  60  may be different from one another. Moreover, it is possible that the operating oil is introduced into the accumulator  60  via the accumulator valve  62  instantaneously and the pressure in the accumulator  60  changes to follow the pressure change in the high pressure oil line  16  without delay in terms of time, and the pressure in the accumulator  60  may be equal to the pressure in the high pressure oil line  16  in such state that the accumulator valve  62  is open. 
     Between the accumulator valve  62  ( 62 A,  62 B) and the accumulator  60  ( 60 A,  60 B), provided is a first pressure sensor P 1 . In the high pressure oil line  16 , provided is a second pressure sensor P 2 . The first pressure sensor P 1  measures the pressure of the operating oil in the accumulator  60  ( 60 A,  60 B). Meanwhile, the second pressure sensor P 2  measure the pressure of the operating oil in the high pressure oil line  16 . 
     Measurement results of the first pressure sensor P 1  and the second pressure sensor P 2  are sent to the ACC valve controller  38  to be used for controlling the opening and closing of the accumulator valves  62  ( 62 A,  62 B). The ACC valve controller  38  controls the opening and closing of the accumulator valves  62  based on the wind speed and determination results of the grid state determination unit  54  in addition to the results of the first pressure sensor P 1  and the second pressure sensor P 2 . Specifically, the ACC valve controller  38  controls the opening and closing of the accumulator valves  62  based on at least one of the wind speed and the state of the grid  50  determined by the grid state determination unit  54  and the pressure in the high pressure oil line  16  measured by the second pressure sensor P 2 . 
     Further, separately of the accumulator  60 , an anti-pulsation accumulator  64  is provided in the high pressure oil line  16  and the low pressure oil line  18 . These suppress the pressure change (pulsation) in the high pressure oil line  16  and the low pressure oil line  18 . In the low pressure oil line  18 , provided are an oil filter  66  for removing impurities from the operating oil and an oil cooler for cooling the operating oil. 
     Between the high pressure oil line  16  and the low pressure oil line  18 , provided is a bypass line  70  for bypassing the hydraulic motor  14 . And, a relief valve  72  is provided in the bypass line  70  to maintain the pressure of the operating oil in the high pressure oil line  16  the same as a prescribed pressure or below. In this manner, when the pressure in the high pressure oil line  16  rises to the prescribed pressure of the relief valve  72 , the relief valve  72  automatically opens so as to release the high pressure oil to the low pressure oil line  18  via the bypass line  70 . 
     Furthermore, in the hydraulic transmission  10 , provided are an oil tank  80 , a supplementary line  82 , a boost pump  84 , an oil filter  86 , a return line  88  and a low pressure relief valve  89 . 
     The oil tank stores supplementary operating oil. The supplementary line connects the oil tank  80  to the low pressure oil line  18 . The boost pump  84  is arranged in the supplementary line  82  so as to replenish the low pressure oil line  18  with the supplementary operating oil from the oil tank  80 . In such a case, the oil filter  86  arranged in the supplementary line  82  removes impurities from the operating oil to be supplied to the low pressure oil line  18 . 
     Even when the operating oil leaks in the hydraulic transmission  10 , the boost pump  84  replenishes the low pressure oil line with the operating oil from the oil tank  80  and thus, the amount of the operating oil circulating in the hydraulic transmission can be maintained. 
     The return line  88  is installed between the oil tank  80  and the low pressure oil line  18 . The low pressure relief valve  89  is arranged in the return line  88  and the pressure in the low pressure oil line  18  is maintained near the prescribed pressure. 
     By this, even though the boost pump  84  supplies the operating oil to the low pressure oil line  18 , once the pressure in the low pressure oil line  18  reaches the prescribed pressure of the low pressure relief valve  89 , the low pressure relief valve  89  automatically opens so as to release the operating oil to the oil tank  80  via the return line  88 . Thus, the amount of the operating oil circulating in the hydraulic transmission  10  can be adequately maintained. 
     Further, an accumulator pressure relief line  61  is arranged between the accumulator valve  62  ( 62 A,  62 B) and the accumulator  60  ( 60 A,  60 B). The accumulator pressure relief line  61  is equipped with a pressure relief valve  63  and connected to the oil tank  80 . When the pressure relief valve  63  is opened under the control of the control unit  30 , the accumulator  60  becomes in communication with the oil tank  80  and the pressure in the accumulator  60  is released to a side of the oil tank  80 . 
       FIG. 3  illustrates the case in which the accumulator pressure relief line  61  is connected to the oil tank  80 . However, the accumulator pressure relief line  60  may be connected to the low pressure oil line  18 . 
     Furthermore, the hydraulic pump  12  and the hydraulic motor  14  are variable displacement type, the displacement of which can be adjusted. Thus, actuating the same requires electric power. Therefore, in the wind turbine generator system  1 , the electric power is normally supplied from the grid  50  to the hydraulic pump controller  32  and the hydraulic motor controller  34 . In such a case that a solenoid valve is used to control the displacements of the hydraulic pump  12  and the hydraulic motor  14 , the electric power is supplied to the solenoid valve as well. When the voltage of the grid  50  becomes zero, the uninterruptible power supply  52  supplies electric power to the hydraulic pump controller  32  and the hydraulic motor controller  34  and may further be supplied to the solenoid valve for adjusting the displacements of the hydraulic pump  12  and the hydraulic motor  14 . 
     In the hydraulic transmission  10 , when the hydraulic pump  12  is driven by the rotation of the main shaft  8 , a pressure difference occurs between the high pressure oil line  16  and the low pressure oil line  18 . This pressure difference drives the hydraulic motor  14 . In such a case, the displacement of the hydraulic pump  12  is adjusted by the pump controller  32  so that a power coefficient becomes at a maximum thereof. Meanwhile, the displacement of the hydraulic motor  14  is adjusted by the motor controller  34  depending on the rotation speed of the main shaft  8  and the displacement of the hydraulic pump  12  so that a rotation speed of the hydraulic motor  14  becomes constant. 
     The wind turbine generator system  1  has the above structure and each controller thereof is controlled by the control unit  30 .  FIG. 4  is a diagram showing a system structure of the wind turbine generator system  1 . As shown in the drawing, the following items are sent to the control unit  30 , which are the wind speed V obtained in a wind speed obtaining unit  56 , a grid state S of the grid  50  determined by the grid state determination unit  54 , a pressure P ACC  in the accumulator  60  measured by the first pressure sensor P 1  and a pressure P H  in the high pressure oil line  16  measured by the second pressure sensor P 2 . And each controller ( 32 ,  34 ,  36 ,  38 ) of the control unit  30  controls the hydraulic pump  12 , the hydraulic motor  14 , the pitch driving mechanism  40  (i.e. servo valve  44 ) and the accumulator valve  62  based on the wind speed V, the grid state S, the pressure P ACC , the pressure P H  and so on. 
     Moreover, the wind speed obtaining unit  56  is not limited as long as being capable of measuring or estimating the wind speed V. For instance, the wind speed obtaining unit  56  may be a wind speed meter mounted on the nacelle, or a combination of a rotation meter for measuring the rotation speed of the main shaft  8  and a calculator for estimating the wind speed V from measuring results of the rotation meter. 
     Next, an operation control of the wind turbine generator system  1  by the control unit  30  is explained. Hereinafter, three types of the operation control are explained in the order of a low wind speed control wherein the wind speed V is below a cut-in wind speed, a low voltage control wherein the voltage of the grid  50  decreases and a gust control wherein there arises a gust. Then, the operation method for efficiently operating the three types of the operation control is explained. 
     (Low Wind Speed Control) 
       FIG. 5  is a flow chart showing an operation control of the accumulator  60  to accumulate pressure therein at a low wind speed.  FIG. 6  is a flow chart showing an operation control of releasing the pressure of the accumulator  60 . 
       FIG. 5  illustrates the following process. First, the wind speed obtaining unit  56  obtains the wind speed V in a step S 2 , and the wind speed V and the cut-in wind speed Vc are compared in a step S 4 . Then, in the step S 4 , if it is determined that the wind speed V is smaller than the cut-in speed Vc, the process advances to a step S 6 . In the step S 6 , the motor controller  34  controls the displacement of the hydraulic motor  14  approximately to zero and the power generation by the generator  20  is stopped. In contrast, if it is determined that the wind speed V is not less than the cut-in speed Vc, the process returns to the step S 2  to obtain the wind speed V again. 
     The cut-in wind speed Vc is defined as a wind speed to start the power generation by the generator  20 . 
     After the step S 6  in which the displacement of the hydraulic motor  14  is controlled approximately to zero, the pressure P ACC  in the accumulator  60  is measured by the first pressure sensor P 1  and the pressure P H  in the high pressure oil line  16  is measured by the second pressure sensor P 2  in a step S 8 . Then, the process advances to a step S 10  in which the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  are compared. 
     In the step S 10 , if it is determined that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 , the process advances to a step S 12  in which the ACC valve controller  38  closes the accumulator valve  62 . In contrast, if it is determined in the step S 10  that the pressure P ACC  in the accumulator  60  is not lower than the pressure P H  in the high pressure oil line  16 , the process returns to the step S 2  so that the wind speed obtaining unit  56  obtains the wind speed V again. 
     Once the accumulator  62  is opened in the step S 12 , the pressure P ACC  in the accumulator  60  is measured by the first pressure sensor P 1  in a step S 13 . Then, it is determined in a step S 14  if the pressure P ACC  has reached a threshold value. If it is determined in the step S 14  that the pressure P ACC  in the accumulator  60  has reached or exceeds the threshold value, the process advances to a step S 16  in which the ACC valve controller  38  closes the accumulator valve  62 . In contrast, if it is determined in the step S 14  that the pressure P ACC  in the accumulator  60  is less than the threshold value, the process returns to the step S 12  to keep the accumulator valve  62  open. 
     As the threshold value, with which the pressure P ACC  is compared with in the step S 14 , a small amount, e.g. 5 bar, less than the prescribed pressure of the relief valve  72  arranged in the bypass line  70  may be used. 
     In this manner, the accumulator valve  62  is opened at the low wind speed when the wind speed V is below the cut-in wind speed Vc to charge the accumulator, and then when the wind speed becomes the same as the cut-in wind speed Vc or above again, the pressure in the accumulator  60  is released so as to assist the rotation of the hydraulic motor  14 . 
     As  FIG. 6  illustrates, first the wind speed obtaining unit  56  obtains the wind speed V in a step S 17 , and in a step S 18  it is determined if the wind speed V is again the same as or above the cut-in wind speed Vc. If it is determined that the wind speed V is again the same as or above the cut-in wind speed Vc, the displacement of the hydraulic motor  14  is increased by the motor controller  34  and the power generation by the generator  20  starts again. In contrast, if it is determined that the wind speed V remains less than the cut-in wind speed Vc, the process returns to the step S 17  to obtain the wind speed V again. 
     After the step S 20  in which the displacement of the hydraulic motor  14  is increased, the pressure sensor P 1  and the pressure sensor P 2  measure the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  respectively in a step S 22 , and then the measured pressure P ACC  and the measured pressure P H  are compared in a step S 24 . And if it is determined that the pressure P ACC  in the accumulator  60  is greater than the pressure P H  in the high pressure oil line  16  in the step S 24 , the process advances to a step S 26  in which the ACC valve controller  38  opens the accumulator valve  62 . In contrast, if it is determined in the step S 24  that the pressure P ACC  in the accumulator  60  is not greater than the pressure P H  in the high pressure oil line  16 , the process returns to the step S 17  to obtain the wind speed V again. 
     In this manner, the accumulator valve  62  is opened when the wind speed V becomes the same as or above the cut-in wind speed Vc. As a result, it is possible to utilize the pressure stored in the accumulator when the wind speed is small and thus, more electric power can be generated and the operation efficiency is improved. 
     Explained so far is the case wherein, the pressure of the fluid is stored at the low wind speed operation when the wind speed V is below the cut-in wind speed Vc and once the wind speed V becomes again the same as or above the cut-in wind speed Vc, the rotation of the hydraulic motor  14  is assisted with use of pressure of fluid in the accumulator. However, the pressure accumulated in the accumulator at the low wind speed may be used in different situations. For instance, the rotation of the hydraulic motor  14  may be assisted with the use of the accumulator  60  having the pressure reserved in advance at the low wind speed to resume the operation of the wind turbine generator system  1  after the maintenance. 
     Further, the case was also explained in reference to  FIG. 5  and  FIG. 6 , in which the accumulator  60  reserves the pressure when the wind speed V is less than the cut-in wind speed Vc and once the wind speed V becomes again not less than the cut-in wind speed Vc, the rotation of the hydraulic motor  14  is assisted with the use of the accumulator  60 . However, there is a case sometimes wherein the pressure P ACC  is already high enough when the wind speed V becomes less than the cut-in wind speed Vc. In such a case, instead of reserving the pressure in the accumulator  60  as shown in  FIG. 5 , it is also possible to wait for the wind speed V to return to the same as or above the cut-in wind speed Vc and then release the pressure of the accumulator  60  in the process shown in  FIG. 6  so as to assist the rotation of the hydraulic motor  14 . 
     Moreover,  FIG. 5  illustrates the case wherein, when the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16  (YES in the step S 10 ), the accumulator valve  62  opens so as to save the fluid in the accumulator  60 . However, it is also possible to control the opening and closing of the accumulator  62  based on the result of comparing the pressure P ACC  in the accumulator  60  and a pressure threshold value Pth in the accumulator. The pressure threshold value Pth may be the same value as the prescribed pressure of the operating oil in the high pressure oil line  16 , or lower than the prescribed pressure of the operating oil in the high pressure oil line  16 . 
     It is also possible to skip the steps S 8  through S 10  of  FIG. 5  so that, after the displacement of the hydraulic motor  14  is reduced in the step S 6  (reduced approximately to zero but leaving some displacement), the accumulator valve  62  is opened to save the fluid in the accumulator  60 . If the accumulator valve  62  is already open, the step S 12  may be skipped as well the steps S 8  through S 10 . 
     Further,  FIG. 5  illustrates the case in which the accumulator valve  62  opens when the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16  (YES in the step S 10 ), but the accumulator valve  62  may be kept open consistently. More specifically, when the wind speed V is below the cut-in wind speed Vc, the control unit may control the displacement of the hydraulic motor  14  approximately to zero, and keeps the accumulator valve  62  open so as to store in the accumulator  60  a pressure of operating oil discharged from the hydraulic pump  12 . 
     Further,  FIG. 6  illustrates the case in which the accumulator valve  62  opens when the pressure P ACC  in the accumulator  60  is higher than the pressure P H  in the high pressure oil line  16  (YES in the step S 24 ), but the accumulator valve  62  may be kept open consistently. More specifically, when the wind speed V is not less than the cut-in wind speed Vc, the control unit may keep the accumulator valve  62  open so as to assist rotation of the hydraulic motor  14  with use of the pressure of the operating oil in the accumulator  60 . 
       FIG. 7  is a flow chart showing an operation control when a voltage of a grid has decreased. As described in the drawing, it is determined by the grid state determination unit  54  in a step S 30  if the voltage of the grid  50  has decreased to the prescribed voltage or lower. If it is determined in the step S 30  that the voltage of the grid  50  has decreased to the prescribed voltage or lower, the process advances to a step S 32  in which the pitch controller  36  controls the pitch driving mechanism  40 , i.e. a servo valve  44  so as to control the supply of the high pressure oil to the hydraulic cylinder  42  and change the pitch angle of the blade  4  to a feathering position. In contrast, if it is determined in the step S 30  that the voltage of the grid  50  is greater than the prescribed voltage, which means that the grid  50  is in a normal state, the process returns to the steps S 30  to continue monitoring the state of the grid  50  by the grid state determination unit  54 . 
     The prescribed voltage herein refers to a low voltage set defined in the grid code and may include the case wherein the voltage is practically zero temporarily. And the changing of the pitch angle to the feathering position means that the pitch angle of the blade  4  is changed to such a position that the wind acting on the blades  4  provides no net torque to the rotor shaft  8 . 
     After the pitch angle of the blade  4  is changed to the feathering position in the step S 32 , the process advances to a step S 34 . In the step  34 , the displacements of the hydraulic pump  12  and the hydraulic motor  14  are reduced to an amount required for keeping the generator  20  synchronized with the grid  50 . 
     The displacement herein refers to a geometric volume that a capacitive pump or a capacitive motor pushes per one rotation according to JIS B0142, Japanese Industrial Standard. Specifically, the displacement of the hydraulic pump  12  means the volume of the operating oil being discharged from the hydraulic pump  12  to the high pressure oil line  16  while the main shaft  8  makes one rotation, and the displacement of the hydraulic motor  14  means the volume of the operating oil discharged from the hydraulic motor  14  to the low pressure oil line  18  while the output shaft of the hydraulic motor  14  makes one rotation. 
     Next in a step S 36 , the first pressure sensor P 1  measures the pressure P ACC  in the accumulator  60  and the second pressure sensor P 2  measures the pressure P H  in the high pressure oil line  16 . Then the process advances to a step S 38  in which the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  are compared. If it is determined in the step S 38  that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 , the process advances to a step S 40  to open the accumulator valve  62  by the ACC valve controller  38 . In contrast, it is determined in the step S 38  that the pressure P ACC  in the accumulator  60  is not lower than the pressure P H  in the high pressure oil line  16 , the process skips the step  40  and advances to a step S 42  which is described later. 
     In this manner, after it is determined in the step S 30  that the voltage of the grid  50  has decreased, the accumulator valve is opened in a step S 40  in such a case that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 . Thus, the pressure in the high pressure oil line  16  is absorbed in the accumulator  60 , thereby preventing the pressure in the high pressure oil line  16  from being too high or the temperature of the operating oil from rising due to the actuation of the relief valve  72 . 
     After the accumulator valve  62  is opened in the step S 40 , it is determined in a step S 42  by the grid state determination unit  54  if the voltage of the grid  50  starts restoring to be greater than the prescribed voltage again. Specifically, in the step  42 , it is determined whether the voltage of the grid  50  has started to be restored. 
     In the similar manner, if it is determined in the step S 38  that the pressure P ACC  in the accumulator  60  is not lower than the pressure P H  in the high pressure oil line  16 , it is judged if the voltage of the grid  50  has started to be restored in the step S 42 . 
     If it is determined in the step S 42  that the grid  50  has started to be restored, a restoration operation to the normal operation mode is performed as described below in the steps S 44  to S 52 . 
     First, in a step S 44 , the pitch controller  36  controls the pitch driving mechanism  40  (specifically, the servo valve  44 ) to adjust the supply of the high pressure oil to the hydraulic cylinder  42  and changes the pitch angle of the blade  4  to the fine position. The changing of the pitch angle to the fine position means that the pitch angle is changed so that the blade  4  receives the wind and applies torque to the rotor  8 . 
     Next in a step S 46 , the pump controller  32  and the motor controller  34  set the displacements of the hydraulic pump  12  so that the power coefficient becomes at a maximum thereof and increases the displacements of the hydraulic motor  14  so that the power generated by the generator  20  into the grid increases. 
     In a step S 48 , the pressure sensor P 1  measures the pressure P ACC  in the accumulator and the second pressure sensor P 2  measures the pressure P H  in the high pressure oil line  16 . The process advances to a step S 50  in which the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  are compared. If it is determined in the step S 50  that the pressure P ACC  in the accumulator  60  is greater than the pressure P H  in the high pressure oil line  16 , the process advances to a step S 52  to open the accumulator valve  62  by the ACC valve controller  38 . In contrast, if it is determined in the step S 50  that the pressure P ACC  in the accumulator  60  is not greater than the pressure P H  in the high pressure oil line  16 , a step S 52  is not performed. 
     In this manner, after it is determined in the step S 42  that the voltage of the grid  50  is restored, the accumulator valve  62  is opened in a step S 52  in such a case that the pressure P ACC  in the accumulator  60  is greater than the pressure P H  in the high pressure oil line  16 , so as to assist the rotation of the hydraulic motor  14  with use of the pressure of the operating oil in the accumulator  60 . 
     Although not shown in  FIG. 7 , after opening the accumulator valve  62  in the step S 52 , it is possible to take measure against the recurrence of the voltage decline of the grid  50  by reducing the pressure in the accumulator  60 . 
     For example, the pressure relief valve  63  is opened while the accumulator valve  62  is closed so that the pressure in the accumulator  60  is allowed to escape to the oil tank via the accumulator pressure relief line  61  (if the accumulator pressure relief line  61  is connected to the low pressure oil line  18 , the pressure is released to the low pressure oil line  18 ) to reduce the pressure in the accumulator  60 . Alternatively, it is possible to reduce the pressure of the air in the bladder-type or the piston-type accumulator  60  while the accumulator valve  62  is closed so as to reduce the pressure of the operating oil in the accumulator  60 . 
     In contrast, if it is determined in the step S 42  that the grid  50  is not restored and the voltage of the grid  50  remains the same as or below the prescribed voltage, the process advances to a step S 54 . In the step S 54 , it is determined if a condition in which the voltage of the grid  50  is decreased continues for a set period of time. And if it determined that the condition continues for a set period of time, the process moves to a shutoff mode (steps S 56  to S 62 ) as described below. In contrast, if it is determined in the step S 54  that the condition does not continue for a set period of time, the process returns to the step S 42  in which it is determined again by the grid state determination unit  54  if the gird  50  is restored. 
     The set period of time here refers to the time defined in the grid code. 
     The transition to the shutoff mode is performed in the steps S 56  to S 62 . First, in the step S 56  the pump controller  32  and the motor controller  34  reduce the displacements of the hydraulic pump  12  and the hydraulic motor  14 . (approximately to zero). 
     Next, in the step S 58  the first pressure sensor P 1  measures the pressure P ACC  in the accumulator  60  and the second pressure sensor P 2  measures the pressure P H  in the high pressure oil line  16 . After the step S 58 , the process advances to the step S 60  in which the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  are compared. If it is determined in the step S 60  that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 , the process advances to the step S 62  in which the ACC valve controller  38  opens the accumulator valve  62 . In contrast, if it is determined in the step S 60  that the pressure P ACC  in the accumulator  60  is not lower than the pressure P H  in the high pressure oil line  16 , the process returns to the step S 42  and it is determined again if the voltage of the grid  50  return to the prescribed voltage or higher. 
     In this manner, after it is determined in the step S 54  that the condition in which the voltage of the grid  50  is decreased continues for a set period of time, the accumulator valve  62  is opened in the step S 62  in such a case that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 . As a result, the pressure in the high pressure oil line  16  is absorbed in the accumulator  60 , thereby preventing the pressure in the high pressure oil line  16  from being too high or the temperature of the operating oil from rising due to the actuation of the relief valve  72 . 
     And after opening the accumulator valve  62  in the step S 62 , the process returns to the step S 42  so that the grid state determination unit  54  determines again if the grid  50  is restored. Subsequently, if the grid  50  is restored, the restoration operation to the normal operation mode is performed in the step S 44  to S 51 . If the grid  50  is not restored, the wind turbine generator system  1  stands by in a shutoff state in the steps S 56  to S 62 . 
     In this manner, when the voltage of the grid is decreased, the pitch angle of the blade  4  is changed to the feathering position and the displacements of the hydraulic pump  12  and the hydraulic motor  14  are reduced to the amount required for keeping the grid  50  synchronized with the generator  20 . As a result, the operation of the wind turbine generator system can continue without being disconnected from the grid  50 . In other words, it is possible to achieve the Ride Through function at Grid low voltage condition. 
     Further, when the voltage of the grid  50  is decreased, the accumulator valve  62  is opened in such a case that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 , thereby preventing the pressure in the high pressure oil line  16  from being too high or the temperature of the operating oil from rising due to the actuation of the relief valve  72 . 
     Furthermore, when the condition in which the voltage of the grid  50  is decreased continues for a set period of tie, the displacements of the hydraulic pump  12  and the hydraulic motor  14  are controlled approximately to zero and the operation of the wind turbine generator system  1  is shutoff. However, during this process, if the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16 , the accumulator valve  62  is opened, thereby preventing the pressure in the high pressure oil line  16  from being too high or the temperature of the operating oil from rising due to the actuation of the relief valve  72 . 
     Moreover, when the grid  50  is restored, the pitch angle of the blade  4  is changed to the fine position and the displacements of the hydraulic pump  12  and the hydraulic motor  14  are set so that the power coefficient becomes at a maximum thereof and a rotation speed of the generator  20  becomes constant. As a result, it is possible to promptly restore the efficient operation thereof. In this process, the accumulator valve  62  is opened in such a case that the pressure P ACC  in the accumulator  60  is greater than the pressure P H  in the high pressure oil line  16  so as to assist the rotation of the hydraulic motor  14  with use of the pressure of the operating oil in the accumulator  60 . 
     Further,  FIG. 7  illustrates the case in which the accumulator valve  62  opens in step S 40  when the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16  (YES in the step S 38 ), but the accumulator valve  62  may be kept open regardless of which one of the pressure P ACC  or the pressure P H  is lower. More specifically, when it is determined that the voltage of the grid  50  has decreased to a prescribed voltage or lower and the condition based on Gid Code rule continues, the control unit may controls the pitch driving mechanism  40  to change the pitch angle of the blade  4  toward the feathering position, changes the displacements of the hydraulic pump  12  to meet the load of the rotor  2  and reduces displacements of the hydraulic motor  14  to an amount required for keeping the generator  20  synchronized with the grid  50 , and keeps the accumulator valve  62  open so as to store in the accumulator  60  a pressure of the operating oil discharged from the hydraulic pump  12 . 
     (Gust Control) 
       FIG. 8  is a flow chart showing an operation control when there arises a gust. As shown in the drawing, first, the wind speed obtaining unit  56  obtains the wind speed V in a step S 70 . And in a step S 72 , it is determined if the wind speed V is greater than the threshold value Vth, i.e. if there is a gust. If it is determined in the step S 72  that the wind speed V is greater than the threshold value Vth, the process advances to a step S 74  in which the first pressure sensor P 1  measures the pressure P ACC  in the accumulator  60  and the second pressure sensor P 2  measures the pressure P H  in the high pressure oil line  16 . In contrast, if it is determined in the step S 72  that the wind speed V is not greater than the threshold value Vth, the process returns to the step S 70  to obtain the wind speed V again. 
     Herein, the threshold value Vth may be, for example, 60 m/sec which is obtained as an average wind speed for a period of three seconds or longer. 
     After measuring the pressure P ACC  in the accumulator  60  and the pressure P H  in the pressure oil line  16  in the step S 74 , the process advances to a step S 76  in which the pressure P ACC  and the pressure P H  are compared. If it is determined in the step S 76  that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure line  16 , the process advances to a step S 78  to open the accumulator valve  62  by the ACC valve controller  38 . 
     In this manner, when there arises a gust (the wind speed V&gt;the threshold value Vth), the accumulator valve  62  is opened in such a case that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure line  16  so as to absorb the pressure from the high pressure line in the accumulator  60 / 
     Meanwhile, if it is determined in the step S 76  that the pressure P ACC  in the accumulator  60  is not lower than the pressure P H  in the high pressure line  16 , the process returns to the step S 70  to obtain the wind speed V again by the wind speed obtaining unit  56 . 
     After opening the accumulator valve  62  in the step S 78 , the wind speed obtaining unit  56  obtains the wind speed V in a step S 80 . In a step S 82 , it is determined if the wind speed V has decreased to the threshold value Vth or below. If it is determined in the step S 82  that the wind speed V has decreased to the threshold value Vth or below, the process advances to a step S 84  to keep the accumulator valve  62  open. In contrast, if the wind speed V still remains greater than the threshold value Vth, the process returns to the step S 80  to obtain the wind speed V again. 
     After keeping the accumulator valve  62  open in the step S 84 , the first pressure sensor P 1  measures the pressure P ACC  in the accumulator  60  and the second pressure sensor P 2  measures the pressure P H  in the high pressure oil line  16  in a step S 86 . Then, in a step S 88 , it is determined if the pressure P ACC  in the accumulator  60  becomes the same as the pressure P H  in the high pressure oil line  16 . 
     If it is determined in the step S 88  that the pressure P ACC  in the accumulator  60  becomes the same as the pressure P H  in the high pressure oil line  16 , the process advances to a step S 90  to close the accumulator valve  62  by the ACC valve controller  38 . In contrast, if it is determined in the step S 88  that the pressure P ACC  in the accumulator  60  is not the same as the pressure P H  in the high pressure oil line  16 , the process returns to the step S 84  to keep the accumulator valve  62  open. 
     In this manner, when there arises a gust (the wind speed V&gt;the threshold value Vth), the accumulator valve  62  is opened in such a case that the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure line  16  so as to absorb the pressure of the high pressure line  16  in the accumulator  60 . When the wind speed V of the gust is decreased to the threshold value Vth or below, the accumulator valve  62  is kept open until the pressure P ACC  in the accumulator  60  becomes the same as the pressure P H  in the high pressure oil line  16  so as to release the pressure in the accumulator  60 . In this manner, it is possible to minimize the impact of the gust on the wind turbine generator system  1 . 
     As described above,  FIG. 8  illustrates the case wherein once the wind speed V becomes as slow as equal to or lower than the threshold value Vth, the accumulator valve  62  is kept open until the pressure P ACC  in the accumulator  60  becomes the same as the pressure P H  in the high pressure oil line  16  so as to release the pressure P ACC  in the accumulator (S 84  through S 88 ). If the operating oil flows instantaneously into the accumulator  60  via the accumulator valve  62  and the pressure P ACC  in the accumulator  60  changes following the change of the pressure P H  in the high pressure oil line  16  without delay, the steps S 84  through S 88  may be skipped as well. 
     Further,  FIG. 8  illustrates the case in which the accumulator valve  62  opens when the pressure P ACC  in the accumulator  60  is lower than the pressure P H  in the high pressure oil line  16  (YES in the step S 76 ), but the accumulator valve  62  may be kept open regardless of which one of the pressure P ACC  or the pressure P H  is lower. More specifically, when there arises a gust having the wind speed V greater than the prescribed threshold value Vth, the control unit keeps the accumulator valve  62  open so as to store in the accumulator  60  the pressure of the operating oil discharged from the hydraulic pump  12 , and when the wind speed V of the gust becomes not greater than the prescribed threshold value Vth, the control unit keeps the accumulator valve  62  open so as to assist rotation of the hydraulic motor  14  with use of the pressure of the operating oil in the accumulator  60 . 
     (Operation Control Method to efficiently Perform Three Types of the Operation Control) 
     Next, the operation control method for performing the above three types of the operation control efficiently. According to the operation control method, two accumulators  60 A and  60 B (ref.  FIG. 3 ) are used for different control purposes and the pressures of the accumulators  60 A and  60 B are adjusted in advance as a preparation for performing each control efficiently. Explained herein is the example in which the accumulator  60 A is used for the control when the wind speed is low, and the accumulator  60 B is used for the control when the grid voltage is decreased and when there arises a gust. 
     The accumulator  60 A used for the control when the wind speed is small is referred to as a first accumulator  60 A and the accumulator  60 B used for the control when the grid voltage is decreased and the control when there is a gust is referred to as a second accumulator  60 B. Further, the accumulator valve  62   a  provided for the accumulator  60 A is referred to as a first accumulator valve  62 A and the accumulator valve provided for the accumulator  60 B is referred to as a second accumulator valve  62 B. 
     First, the pressure sensors P 1  and P 2  measure the pressure P ACC   1  in the first accumulator  60 A and the pressure P ACC   2  in the second accumulator  60 B, and the pressure P H  in the high pressure oil line  16 . 
     And, in such a case that the pressure P ACC   1  in the first accumulator  60 A is lower than the pressure P ACC   2  in the second accumulator  60 B, the first accumulator valve  62 A is opened so as to store the pressure in the first accumulator  60 A. The storing of the pressure in the first accumulator  60 A is preferably performed at a low wind speed not greater than the cut-in wind speed in which there is no power generation by the generator  20 , or in the rated operation to control the output constant by adjusting the pitch angle so as to prevent the impact on the power generation efficiency of the wind turbine generator system  1 . 
     In this manner, the pressure P ACC   1  in the first accumulator  60 A is always kept comparatively high. 
     In contrast, in such a case that the pressure P ACC   1  in the first accumulator  60 A is higher than the pressure P ACC   2  in the second accumulator  60 B, the second accumulator valve  62 B is opened to release the pressure of the second accumulator  60 B. 
     In this manner, the pressure P ACC   2  in the second accumulator  60 B is always kept comparatively low. 
     In this manner, by keeping the pressure P ACC   1  of the first accumulator  60 A comparatively high to be used for the control when the wind speed is low, the rotation of the hydraulic motor  14  can be efficiently assisted with the use of the first accumulator  60 A when the wind speed V becomes not less than the cut-in wind speed Vc again after being below the cut-in wind speed Vc. 
     Further, by always keeping the pressure P ACC   2  comparatively low to be used for the control when the grid voltage is decreased and when there arises a gust, the pressure in the high pressure oil line  16  can be efficiently absorbed with the use of the second accumulator  60 B when the voltage of the grid  50  is decreased or when the wind speed V exceeds the threshold value Vth. 
     Above explained is the case wherein the use of the first accumulator  60 A and the use of the second accumulator  60 B are decided in advance (the first accumulator  60 A for the control at the low wind speed, and the second accumulator  60 B for the control at the low grid voltage or at the occurrence of the gust). However, the uses of the first accumulator  60 A and the second accumulator  60 B may be adequately changed depending on a situation. 
     For instance, the pressure P ACC1  in the first accumulator  60  and the pressure P ACC   2  in the second accumulator  60 B are regularly measured and the accumulator with higher measured pressure is used for the control when the wind speed is low and the accumulator with lower measured pressure is used for the control when the voltage of the gird is decreased or when there arises a gust. 
     As described above, in the preferred embodiment, the ACC valve controller  38  of the control unit  30  controls the opening and closing of the accumulator valve  62  based on at least one of the wind speed and the state of the grid  50  determined by the grid state determination unit  54 , and the pressure in the accumulator  60  measured by the first pressure sensor P 1  and the pressure in the high pressure oil line  16  measured by the second pressure sensor P 2 . 
     Therefore, not only the operation efficiency and the safety at the low wind speed and at the occurrence of a gust can be improved but also the Ride Through function at Grid low voltage condition can be achieved. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments and that modifications and variations are possible within the scope of the appended claims. 
     For instance, in the above preferred embodiment, the example of controlling the opening and closing of the accumulator valve  62  by the ACC valve controller  38  was explained. However, the opening of the accumulator valve  62  may be adjusted in accordance with the pressure difference between the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16 . For example, when the pressure between the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  is large, the accumulator valve  62  is gradually opened (the opening amount of the accumulator valve  62  is gradually made large) so as to suppress the pressure fluctuation of the high8jun pressure oil line caused by the opening of the accumulator valve  62 . Meanwhile, when the pressure between the pressure P ACC  in the accumulator  60  and the pressure P H  in the high pressure oil line  16  is small, the accumulator valve  62  is promptly opened (the opening amount of the accumulator valve  62  is promptly made large) so as to respond promptly to the change of the situation. 
     REFERENCE SIGNS LIST 
     
         
           1  wind turbine generator 
           2  rotor 
           4  blade 
           6  hub 
           8  main shaft 
           10  hydraulic transmission 
           12  hydraulic pump 
           14  hydraulic motor 
           16  high pressure oil line 
           18  low pressure oil line 
           20  generator 
           22  nacelle 
           24  tower 
           26  base 
           30  control unit 
           32  pump controller 
           34  motor controller 
           36  pitch controller 
           38  ACC valve controller 
           40  pitch driving mechanism 
           42  hydraulic cylinder 
           44  servo valve 
           46  oil pressure source 
           48  accumulator 
           50  grid system 
           52  uninterruptible power supply 
           54  grid status judging unit 
           60 A first accumulator 
           60 B second accumulator 
           61  Accumulator pressure relief line 
           62 A first accumulator valve 
           62 B second accumulator valve 
           63  Accumulator pressure relief line 
           64  anti-pulsation accumulator 
           66  oil filter 
           68  oil cooler 
           70  bypass line 
           72  relief valve 
           80  oil tank 
           82  supplementary line 
           84  boost pump 
           86  oil filter 
           88  return line 
           89  low pressure relief valve