Abstract:
Some general aspects of the invention provide a method for operating a wind energy converter having a variable-ratio gear system ( 906, 172 - 178 ) mechanically coupled between a rotor ( 102 ) and a generator ( 104 ), wherein the variable-ratio gear system ( 906, 172 - 178 ) includes a first transmission unit ( 174 ) coupled to a first shaft ( 168 ) and a second transmission unit ( 172 ) coupled to a second shaft ( 165 ). The method comprises adjusting the variable-ratio gear system ( 906, 172 - 178 ) to a first gear ratio at which the first shaft ( 168 ) substantially does not rotate; determining a wind speed ( 300 ) or a related parameter (n, P, p); detecting whether the wind speed ( 300 ) or the related parameter (n, P, p) has crossed a first threshold value ( 310 ) in a first direction; and adjusting the variable-ratio gear system ( 906, 172 - 178 ), when the wind speed ( 300 ) or the related parameter (n, P, p) has crossed the first threshold value ( 310 ) in the first direction, to a second gear ratio at which the second shaft ( 165 ) substantially does not rotate. Under further aspects, the invention provides a control device ( 190 ) for a wind energy converter having a variable-ratio gear system and a wind energy converter comprising the control device ( 190 ).

Description:
BACKGROUND 
       [0001]    Method for operating a wind energy converter, control device for a wind energy converter, and wind energy converter. 
         [0002]    The present invention relates to a wind energy converter. 
         [0003]    Wind energy can be converted into useful forms, such as electricity, by a wind energy converter that includes a rotor, e.g. a low-speed propeller, coupled to a generator. Since the speed of wind at a given location in general fluctuates with changing weather conditions, conventional wind energy converters are typically operated with variable rotor speed in order to achieve a high aerodynamic efficiency of the rotor at different levels of wind speed. 
         [0004]    In some implementations, to supply electricity into an electric power grid, it is advantageous to use a constant-speed generator such as a synchronous generator directly connected to the grid for power transmission. In addition to economic benefits, the quality of the electric power fed into the grid is usually higher without a power converter, which can be expensive. Furthermore, the constant speed generator can be operated at a medium voltage level such that no additional transformer is needed. 
         [0005]    DE 10 2005 054 539 B3 discloses one example of a wind energy converter including a rotor and an electrical generator for direct connection to an electric power grid. In this example, the rotor and the generator are coupled by a variable-ratio gear system that includes a hydraulic component for controlling the gear ratio of the variable-ratio gear system. As a result, the wind energy converter operates at a variable rotor speed according to the speed of wind while driving the generator at a constant speed. 
         [0006]    In the above example, while aerodynamic losses of the wind energy converter can be reduced by adjusting the rotor speed to changes in the speed of wind, a substantial portion of the power is transported from the rotor to the generator via the hydraulic component of the variable-ratio gear system. As hydraulic components generally suffer power losses by leakage flow, friction losses etc., the overall efficiency of the wind energy converter is reduced due to the power losses in the hydraulic component of the variable-ratio gear system. 
       SUMMARY 
       [0007]    One general aspect of the invention relates to a method for operating a wind energy converter that has a variable-ratio gear system mechanically coupled between a rotor and a generator, wherein the variable-ratio gear system includes a first transmission unit, e.g. a hydraulic unit, coupled to a first shaft, and a second transmission unit, e.g. a hydraulic unit, coupled to a second shaft. The method includes adjusting the variable-ratio gear to a first gear ratio at which the first shaft substantially does not rotate. For example, the first shaft does not rotate at all or rotates at a rotational speed that is negligibly low compared to a range of rotational speeds at which the first shaft rotates when the gear ratio of the variable-ratio gear system is varied over a range of gear ratios, assuming a constant generator speed. 
         [0008]    Embodiments of this method may include one or more of the following features. 
         [0009]    A wind speed is determined. For example, a wind-speed value that characterises a current wind speed is determined directly from wind measurement or indirectly from operating parameters of the wind energy converter. Further, the method also includes detecting whether the wind speed has crossed a first threshold value in a first direction. For example, the first direction may be predefined as the direction corresponding to a rise in wind speed, such that a detection that the wind speed has crossed the first threshold value occurs when the wind speed has risen beyond, i.e. has become greater than the first threshold value. Alternatively, the first direction may be predefined as the direction corresponding to a decrease in wind speed, such that a detection that the wind speed has crossed the first threshold value occurs when the wind speed has fallen below, i.e. has become less than the first threshold value. 
         [0010]    In some embodiments, when it is determined that the wind speed has crossed the first threshold value in the first direction, the variable-ratio gear system is adjusted to a second gear ratio at which the second shaft substantially does not rotate. In this event the rotational speed of the second shaft is equal to zero, for example, or is negligibly low when compared to a range of rotational speeds at which the second shaft rotates when the gear ratio of the variable-ratio gear system is varied over the range of gear ratios of the variable-ratio gear, assuming a constant generator speed. 
         [0011]    Using the above method, the gear ratio of the variable-ratio gear system is changed from a first gear ratio at which the first shaft coupled to the first hydraulic unit substantially does not rotate to a second gear ratio at which the second shaft coupled to the second hydraulic unit substantially does not rotate. In this way, gear ratios at which both the first and the second shaft are in substantial rotation are avoided, thus reducing energy loss in the hydraulic units, which typically is greater for higher rotational speeds of the shafts coupled to the hydraulic units. Therefore, the method enables to operate a wind energy converter having a variable-ratio gear system that includes a hydraulic component in such a way that the energy loss in the hydraulic units and thereby in the variable-ratio gear system as a whole is significantly reduced while the rotor is still able to operate at different rotor speeds in accordance with varying wind conditions. 
         [0012]    The method can further include driving the generator at a constant generator speed. This allows a synchronous generator to be directly connected to a power grid, without necessarily requiring the use of a power converter that can be both expensive and energy inefficient. In some embodiments, at least one of the first gear ratio and the second gear ratio corresponds to one of a minimum operational speed and a nominal maximum speed of the rotor, when the generator is driven at the constant generator speed. In this way, the lower and/or upper limits of the operational range of physically possible rotor speeds of the wind energy converter can be utilised, thus enabling high efficiency over a particularly wide range of wind speeds by utilising the physical capabilities of the rotor to the respective limit(s). 
         [0013]    In some embodiments, the method further includes uncoupling the first shaft from the first hydraulic unit, when the wind speed has crossed the first threshold value in the first direction. In this situation, as the second shaft, which is coupled to the second hydraulic unit, substantially does not rotate, substantially no power is transmitted hydraulically or mechanically between the first and second hydraulic units. By uncoupling the first shaft from the first hydraulic unit, further power transmission between the first shaft and the first hydraulic unit is disabled, enabling the first hydraulic unit to decelerate to zero speed e.g. by its own friction. In this way, both the first and second hydraulic units are configured to operate at zero speed, thus further reducing hydraulic losses in the variable-ratio gear system and achieving a particularly high efficiency of the wind energy converter. 
         [0014]    In some embodiments, the method further includes braking the first hydraulic unit when the wind speed has crossed the first threshold value in the first direction. This can advantageously increase the operational stability of the wind energy converter in this operational state and can further ease the load on the first hydraulic unit for increasing the life span of the hydraulic unit. 
         [0015]    In some embodiments, when the wind speed has crossed the first threshold value in the first direction, a swivel angle of the first hydraulic unit is adjusted so that a hydraulic flow through the first hydraulic unit is substantially stopped. This effectively stops hydraulic power flow between the first and second hydraulic units, such that the braking of the first hydraulic unit enables to indirectly brake the second hydraulic unit, too, or to dampen the second hydraulic unit, depending on a swivel angle setting of the second hydraulic unit. This can improve the operational stability of the wind energy converter in this operational state. 
         [0016]    In some embodiments, the first and second hydraulic units are connected by at least one hydraulic conduit. The method further includes blocking at least one hydraulic conduit when the wind speed has crossed the first threshold value in the first direction. This also stops hydraulic power flow between the first and second hydraulic units, and indirectly brakes or dampens the second hydraulic unit for improving the operational stability of the wind energy converter in this operational state. Furthermore, the hydraulic load on the first hydraulic unit is eased, resulting in an increase in the hydraulic unit&#39;s life span. 
         [0017]    In some embodiments, at least one hydraulic conduit is interconnected by a bypass, and the method further includes regulating the bypass when the wind speed has crossed the first threshold value in the first direction. For example, the bypass may be initially closed and then be opened when the wind speed has crossed the first threshold value in the first direction. This enables to control the damping of the second hydraulic unit while reducing hydraulic load on the first hydraulic unit for increasing its life span. 
         [0018]    In some embodiments, the method further includes detecting whether the wind speed has crossed a second threshold value opposite to the first direction, and if so, adjusting the variable-ratio gear system to the first gear ratio. In this way, gear ratios other than the first and second gear ratio are avoided for both cases of rising and falling wind speed, thereby reducing power losses for achieving a particularly high overall efficiency. 
         [0019]    In some embodiments, the second shaft is uncoupled from the second hydraulic unit when the wind speed has crossed the second threshold value inversely to the first direction for achieving benefits analogous to those achieved by the uncoupling of the first hydraulic unit. Similarly, the second hydraulic unit is braked when the wind speed has crossed the second threshold value inversely to the first direction for achieving benefits analogous to those achieved by the braking of the first hydraulic unit. Another general aspect of the invention provides a system and a method for improving the energy efficiency of a wind energy converter having a first and a second active module. The system includes a sensor system for obtaining a measurement of a wind condition (for example, a local wind speed). A controller is coupled to the sensor system for determining, according to the obtained measurement, whether a pre-determined operational condition is satisfied. Based on a result of the determination, the controller generates a signal for adjusting a configuration of a selected one of the first and the second active module to achieve a predetermined power characteristic of the wind energy converter. 
         [0020]    Embodiments of this aspect may include one or more of the following features. 
         [0021]    Using the obtained measurement, the controller determines whether a first or a second operational condition is satisfied and according to the result, generates a signal for adjust the configuration of a corresponding one of the first and second active modules. 
         [0022]    For example, when a magnitude of the obtained measurement has crossed a first threshold value in a first direction, the controller determines that the first operational condition is satisfied and subsequently generates a signal for adjusting the configuration of the first active module. Similarly, when the magnitude of the obtained measurement has crossed a second threshold value in a second direction opposite to the first direction, the controller determines that the second operational condition is satisfied and subsequently generates a signal for adjusting the configuration of the second active module. 
         [0023]    The systems and methods described herein provide significant advantages over the known prior art. By operating the variable-ratio gear system in such a way that at least one of the transmission units, e.g. hydraulic units, is at or close to zero speed, the transmission of power through the hydraulic units can be significantly reduced. The above-mentioned advantages can also help to increase the efficiency of energy conversion of the wind energy converter, to reduce noise emissions from the hydraulic units, and furthermore to reduce cost including maintenance cost by increasing the lifespan of the hydraulic units. 
         [0024]    Other features and advantages are illustrated in the accompanying drawings and described in detail in the following part of the description. 
     
    
     
       FIGURES 
         [0025]    In the Figures: 
           [0026]      FIG. 1  is a schematic illustration of a wind energy converter according to an embodiment of the invention, the wind energy converter having a variable-ratio gear system including a first hydraulic unit coupled to a first shaft and a second hydraulic unit coupled to a second shaft; 
           [0027]      FIG. 2A  is a schematic diagram showing, for the wind energy converter of  FIG. 1 , a rotational speed of a first shaft, a rotational speed of a second shaft, and an amount of power transferred between the hydraulic units as functions of the rotor speed; 
           [0028]      FIG. 2B  is a schematic diagram showing, for the wind energy converter of  FIG. 2A , a hydraulic power loss in the hydraulic units as a function of the rotor speed; 
           [0029]      FIG. 3  is a schematic diagram showing, for a wind energy converter operated according to a method of an embodiment, a dependency of the rotor speed on the speed of wind; and 
           [0030]      FIG. 4  is a schematic illustration of a wind energy converter according to a further embodiment. 
       
    
    
       [0031]    Throughout the figures the same reference numbers indicate the same or functionally equivalent means. 
       DETAILED DESCRIPTION 
       [0032]      FIG. 1  is a schematic illustration of a wind energy converter  800 , which includes a control device  190  configured for improving the energy efficiency of wind energy converter  800 . Prior to describing the control device  190  in detail, the following section first discusses the structure and the operation of the wind energy converter  800 . 
         [0033]    As shown in  FIG. 1 , the wind energy converter  800  includes a rotor  102  (e.g. a low speed propeller) that drives a generator  104  (preferably a constant speed synchronous generator) through a gear train  130 ,  906 ,  802 . The gear train  130 ,  906 ,  802  includes a constant-ratio gear box  130  and a variable-ratio gear system  906 ,  802  the gear ratio of which is controllable by the control device  190 . In this description, the gear ratio of the variable-ratio gear system  906 ,  802  corresponds to, or is defined in association with, the ratio between the rotational speeds at the input terminal (shaft  126 ) and the output terminal (shaft  158 ) of the variable-ratio gear system  906 ,  802 . 
         [0034]    The rotor  102  is connected to the constant-ratio gearbox  130  through a rotor shaft  106 . The constant-ratio gearbox  130  is configured to transfer power from the low speed, high torque rotor shaft  106  to a high speed, low torque interconnecting shaft  126 . Purely as an example, the constant ratio gearbox  130  is shown to include a first epicyclic gearing  110  and a second epicyclic gearing  120 . Power from the rotor shaft  106  is transmitted, e.g., first to a planet carrier  112  of the first epicyclic gearing  110 , and then via a shaft  116  to the planet carrier  122  of the second epicyclic gearing  120 . Both epicyclic gearings  110  and  120  are configured to give constant step-up ratios so that the speed of rotation gradually increases from the rotor shaft  106  at the input of the constant ratio gearbox  130  to the interconnecting shaft  126  at the output of the constant ratio gearbox  130 . In other embodiments, the constant ratio gearbox  130  may include additional or fewer gears configured in a different way to transmit power from the rotor shaft  106  to the interconnecting shaft  126 . 
         [0035]    The interconnecting shaft  126  connects the torque output of the constant ratio gearbox  130  to a torque input of the variable-ratio gear system  906 ,  802 . The variable-ratio gear system  906 ,  802  functions as a superposition gear having a mechanical component  906  and a hydraulic component  802 . The mechanical component  906  includes a spur gear  140  driven by the interconnecting shaft  126 . The spur gear  140  meshes with a pinion  142  and transmits power via a shaft  144  to a third epicyclic gearing  150 . The shaft  144  is rotatably held by a bearing  942  resting on a rigid support  944 , which is e.g. attached to a housing (not shown) of the wind energy converter  800 , such as a nacelle rotatably disposed on a tower. 
         [0036]    The third epicyclic gearing  150  driven by the shaft  144  has three basic components: a planet carrier  940  carrying a set of planet gears  932 , an annulus  928 , and a sun gear  934 . The planet carrier  940  receives input from the shaft  144  and thus rotates at a speed proportional to the speed of the rotor  102 . The annulus  928  of the third epicyclic gearing  150  is coupled to the generator  104  via an output shaft  158  of the variable ratio gear system  906 ,  802 , and rotates at the same speed as the generator  104 . 
         [0037]    The sun gear  934  of the third epicyclic gearing  150  is connected to a sun gear  912  of a fourth epicyclic gearing  910  via a spur gear  162  and a pinion  166 . In addition to the sun gear  912 , the fourth epicyclic gearing  910  includes a planet carrier  916  carrying planet gears  920  and an annulus  918  as basic components. The annulus  918  of the fourth epicyclic gearing  910  is connected to the shaft  144  and thus also to the planet carrier  940  of the third epicyclic gearing  150  via a further pinion  924  mashing with a further spur gear  926 . Thus, the annulus  918  rotates at a speed proportional to the speed of the rotor  102 . 
         [0038]    The sun gear  912  of the fourth epicyclic gearing  910  is furthermore connected to a first shaft  168  coupled to a first hydraulic unit  174  of the hydraulic component  802  of the variable ratio gear system  906 ,  802 . The hydraulic component  802  furthermore includes a second hydraulic unit  172  and hydraulic conduits  176 ,  178  interconnecting the first  174  and second  172  hydraulic units, thereby forming a hydraulic circuit  174 ,  176 ,  172 ,  178  in which power is transferable between the hydraulic units  174 ,  172  by a circulating hydraulic fluid. The second hydraulic unit  172  is coupled to a second shaft  165 , which is connected via a pinion  922  and a spur gear  914  to the planet carrier  916  of the fourth epicyclic gearing  910 . Both hydraulic units  174 ,  172  have a respective swash plate  174   a ,  172   a  with a respective controllable swivel angle. For example, the hydraulic units  174 ,  172  are hydraulic motors or pumps such as axial piston pumps. Each hydraulic unit performs a conversion between the rotational motion of a respective one of the first  168  and second  165  shafts and the hydraulic flow through the hydraulic circuit  174 ,  176 ,  172 ,  178 . 
         [0039]    The setting of the swivel angle of the swash plate  174   a  of the first hydraulic unit  174  sets the relationship between an amount Q of the hydraulic fluid flowing through a cross section of the hydraulic circuit  174 ,  176 ,  172 ,  178  (e.g., a cross section of one of the hydraulic conduits  176 ,  178 ) during a given time span and an angular distance  9  through which the first shaft  168  rotates during the same time span. Similarly, the swivel angle of the swash plate  172   a  of the second hydraulic unit  172  sets the relationship between the amount Q of hydraulic fluid flowing through a cross section of the hydraulic circuit  174 ,  176 ,  172 ,  178  and an angular distance θ 2  through which the second shaft  165  rotates during a corresponding time span. Therefore, the hydraulic component  802  of the variable-ratio gear system  906 ,  802  functions as an infinitely variable gear system in which torque (and thus power) is transmitted between the first  168  and second  165  shafts. The direction and amount of the power transmitted between the shafts can be controlled by the setting of the swivel angles of the swash plates  174   a ,  172   a.    
         [0040]    The variable-ratio gear system  906 ,  802  as shown in  FIG. 1  functions as a superposition gear system in which two additional torque paths are superimposed upon a main torque path that connects the output of the constant ratio gearbox  130  to the input of the generator  104 . The main torque path includes the interconnecting shaft  126 , the spur gear  140 , the pinion  142 , and the planet gears  932  and annulus  928  of the third epicyclic gearing  150 . 
         [0041]    The first superimposed torque path branches off the main torque path at the shaft  144  and leads through the spur gear  926 , the pinion  924 , the fourth epicyclic gearing  910 , the pinion  166 , the spur gear  162  and the sun gear  934  of the third epicyclic gearing  150  before rejoining the main torque path at the planet gears  932  of the third epicyclic gearing  150 . 
         [0042]    The second superimposed torque path branches off the first superimposed torque path at the planet gears  920  of the fourth epicyclic gearing  910  and leads through the spur gear  914 , the pinion  922 , and the second shaft  165  coupled to the second hydraulic unit  172  where the torque on the second shaft  165  is converted into a hydraulic flow through the hydraulic conduits  176 ,  178 . After the hydraulic flow is re-converted by the first hydraulic unit  174  into a torque on the first shaft  168 , the second superimposed torque path continues through the first shaft  168  to the pinion  166  where it rejoins the first superimposed torque path. 
         [0043]    In the example of  FIG. 1 , the variable-ratio gear system  906 ,  802  functions as a superposition gear system having at least two superimposed torque paths in addition to a main torque path, wherein one of the superimposed torque paths includes a hydraulic circuit having at least a first and a second hydraulic unit hydraulically interconnected. 
         [0044]      FIGS. 2A and 2B  illustrate the operation of this variable-ratio gear system  906 ,  802  in detail, assuming that the generator  104  is driven at a substantially constant speed by the output of the variable-ratio gear system  906 ,  802 . 
         [0045]    In  FIG. 2A , a horizontal axis represents the rotational speed  200  of the rotor  102  of the wind energy converter. Based on the assumption of constant generator speed, the rotor speed  200  is directly—proportionally or inversely, according to definition—related to the gear ratio of the variable-ratio gear system  906 ,  802 , which is controllable via the settings of the hydraulic units  172  and  174 , e.g. the combined settings of the swivel angles of the hydraulic units. 
         [0046]    In this figure, three variables are shown as functions of the rotor speed  200 , each in arbitrary units along a common vertical axis  202 . The three variables include a rotational speed  214  of the first hydraulic unit  174 , a rotational speed  212  of the second hydraulic unit  172 , and an amount of power  216  transferred hydraulically between the two hydraulic units. For each hydraulic unit, the respective rotational speed  212 ,  214  shows a near linear relationship to the rotor speed  200 . 
         [0047]    The rotational speed  214  of the first hydraulic unit crosses the horizontal axis  200 , i.e. changes its rotational direction, at an upper zero point  208  of the variable-ratio gear system. Likewise, the rotational speed  212  of the second hydraulic unit crosses the horizontal axis  200 , i.e. changes its rotational direction, at a lower zero point  206  of the superposition gear system. Here, the respective correspondence of the first and second hydraulic unit to the upper  208  and lower  206  zero points has been chosen arbitrarily purely for illustrative reasons. The power  216  transferred between the hydraulic units reverses direction at both the lower  206  and upper  208  zero points, and it exhibits a local minimum  218  in terms of its absolute amount approximately midway between the lower  206  and upper  208  zero points. 
         [0048]      FIG. 2B  shows the amount of hydraulic power loss  205  as a function of rotor speed for the variable-ratio gear system  906 ,  802  of  FIG. 1  operating in the same range of rotor speed  200  as in  FIG. 2A . The hydraulic power loss  205  is seen to exhibit a respective local minimum at each of the lower  206  and upper  208  zero points. Due to leakage flow etc. in the hydraulic units, the power loss is still greater than zero at the zero points  206 ,  208 . 
         [0049]      FIG. 3  illustrates a method of operating a wind energy converter of the aforementioned kind under conditions of changing wind speed. Above a horizontal axis representing the wind speed  300 , which is assumed to fluctuate arbitrarily, solid curves  302 ,  304  represent the way by which the rotational speed of the rotor  200  is adjusted according to the wind speed  300 . Herein, the first solid curve  304  applies to a situation where the wind speed  300  rises, whereas the second solid curve  302  applies to a situation where the wind speed  300  falls. 
         [0050]    In this example, when the wind speed  300  is below a first threshold value  310 , the rotor speed  200  (as shown by both curves  302 ,  304 ) is fixed at a lower rotor speed limit  322  that corresponds to the lower zero point  206  as shown in  FIGS. 2A-B . Preferably, the lower rotor speed limit  322  also corresponds to a minimum operational speed at which the wind energy converter is capable of producing electricity. When the wind speed  300  is above a second threshold value  311 , the rotor speed  200  (as shown by both curves  302 ,  304 ) is fixed at an upper rotor speed limit  320  that corresponds to the upper zero point  208  as shown in  FIGS. 2A-B . Preferably, the upper rotor speed limit  320  also corresponds to a maximum operational speed at which the wind energy converter is capable of producing electricity, such as a nominal operation speed at which the rotor can safely operate. 
         [0051]    If the wind speed  300  starts at a level above the first threshold value  310  and then falls below it, the gear ratio of the variable-ratio gear system  906 ,  802  is adjusted such that the rotor speed  200  falls to the lower rotor speed limit  322 , as shown by the second solid curve  302 . If the wind speed  300  starts at a level below the second threshold value  311  and then rises beyond it, the gear ratio of the variable-ratio gear system is adjusted such that the rotor speed  200  rises to the upper rotor speed limit  320 , as shown by the first solid curve  304 . 
         [0052]    In this example, the gear ratio is adjusted only when the wind speed  300  crosses the threshold values  310 ,  311 , causing the rotor speed  200  to change, for example, from the lower rotor speed limit  322  to the upper rotor speed limit  320 , or in the reverse direction. Note that the upper rotor speed limit  320  and lower rotor speed limit  322  respectively correspond to the zero points  206 ,  208  of the variable-ratio gear system as shown in  FIGS. 2A-B , and the variable-ratio gear system passes gear ratios corresponding to the region of the local maximum  210  of the hydraulic losses only transitionally. Thus, the hydraulic losses averaged over a given time interval of operation, such as one day, are lower than e.g. in an alternative method of operation illustrated by a dashed line  306 , where within a predefined wind speed interval  314 ,  316  the rotor speed  200  is continuously regulated according to the wind speed  300  to minimize aerodynamic losses of the rotor. Using typical rotors and hydraulic units, the reduction in hydraulic power losses according to the method of the solid curves  302 ,  304  generally outweighs the increase in aerodynamic power losses compared e.g. to the method of operation illustrated by the dashed line  306 . 
         [0053]    Together, the threshold values  310 ,  311  enclose a hysteresis region  308  that ensures that the frequency at which the rotor speed (and therefore the gear ratio) is adjusted between the upper  320  and lower  322  rotor speed limits is kept low. In other embodiments, the first  310  and second  311  threshold values are configured to be equal. 
         [0054]    Referring now back to  FIG. 1 , in order to control the speed of the rotor  102 , for example, according to the method described above with reference to  FIG. 3 , the wind energy converter  800  further includes a control device  190  for controlling the swivel angles of the swash plates  174   a ,  172   a  of the first  174  and second  172  hydraulic units. The control device  190  includes a sensor system  902  that obtains measurements of a local wind condition. In some examples, the sensor system  902  may include a wind speed determiner, which continuously determines the current wind speed by means of a measuring signal received from an anemometer  900 . Furthermore, the control device  190  includes a threshold detector  904 , which monitors the wind speed obtained from the wind speed determiner  902  and detects when the wind speed rises above and/or falls below a predefined threshold value. Different threshold values may be predefined for falling and rising wind speed in alternative embodiments. In some other examples, the sensor system  902  may additionally or alternatively measure parameters characterizing the operational state of the wind energy converter  800 , such as rotor speed n, power P, and hydraulic pressure p. These parameters may also be used by the control device  190  for adjusting the gear ratio of the variable-ratio gear system  906 ,  802 . 
         [0055]    In this example, the control device  190  includes a first control unit  192  connected to the threshold detector  904 . When the wind speed has risen beyond the threshold value, the first control unit  192  adjusts the variable-ratio gear system  906 ,  802  to a first gear ratio at which the first shaft  168  substantially does not rotate. The control device  190  further includes a second control unit  194  likewise connected to the threshold detector  904 . When the wind speed has fallen beyond the threshold value, the second control unit  194  adjusts the variable-ratio gear system  906 ,  802  to a second gear ratio at which the second shaft  165  substantially does not rotate. 
         [0056]      FIG. 4  is a schematic illustration of a wind energy converter  800  according to a further embodiment. Compared with the wind energy converter of  FIG. 1 , in this example, a clutch  400  is disposed between the first hydraulic unit  174  and the first shaft  168 , which connects—in an operational condition where the clutch  400  is closed—the first hydraulic unit  174  to the mechanical component  906  of the variable-ratio gear system  906 ,  802 . The clutch  400  is connected to the first control unit  192 , which is configured to uncouple the first shaft  168  from the first hydraulic unit  174  when the threshold detector  904  has detected that the wind speed has fallen below the threshold value. 
         [0057]    The wind energy converter  800  further includes a brake  404  for braking the first hydraulic unit  174 . The brake  404  is connected to the first control unit  192 , which is configured to brake the first hydraulic unit  174  together with uncoupling the first shaft  168  from the first hydraulic unit  174 . In alternative embodiments, the clutch  400  and the brake  404  may be combined in the form of a double clutch switchable between a first clutch position in which the first hydraulic unit  174  is coupled to the first shaft  168 , and a second clutch position in which the first hydraulic unit is coupled to a rigid support such as a housing of the variable-ratio gear system  906 ,  802 , thereby effectively braking the first hydraulic unit  174 . 
         [0058]    A security brake  402  is provided for braking the first shaft  168  in the event of any of the hydraulic units  174 ,  172  becoming inoperative, e.g. when hydraulic fluid is lost in the event of a break of one of the hydraulic conduits  176 ,  178 . In alternative embodiments, the brake  404  is configured to be activated by a security brake activator (not shown) in the event of any hydraulic unit  174 ,  174  becoming inoperative, such that a separate security brake  402  is not needed. In some implementations, the activation of the security brake  402  is always performed in combination with the opening of a hydraulic bypass  406  between the hydraulic conduits  176  and  178 . 
         [0059]    The wind energy converter  800  further includes a faceplate  408  or valve for at least partially blocking the one of the hydraulic conduits  176 ,  178 , e.g. when the hydraulic conduit  176  conducting the hydraulic fluid is under high pressure. The brake  404  is connected to the first control unit  192 , which is configured to shut the faceplate  408  together with brake  404  braking the first hydraulic unit  174  and the first shaft  168  being uncoupled from the first hydraulic unit  174 . Furthermore, the wind energy converter  800  includes a bypass  406  that interconnects at least one hydraulic conduit on the side of the second hydraulic unit relative to the faceplate  408 . The bypass  406  is connected to the first control unit  192 , which is configured to open the bypass  406  at least partially together with shutting the faceplate  408 . 
         [0060]    The first control unit  192  may further be configured to regulate the bypass  406  for dampening torque variations such that the gear ratio of the variable-ratio gear system  906 ,  802  is allowed to dynamically vary within a limited interval around the respective zero point, such as the interval  220  shown in  FIGS. 2A-B , which includes the upper zero point  208  and has a width that is small compared to the distance between the lower  206  and the upper  208  zero point. For example, the rotor  102  may in this way be operated at a nearly fixed speed close the lower zero point  206  of the variable-ratio gear system  906 ,  802 , while allowing the gear ratio to be regulated in a confined gear-ratio interval  220  close to the upper zero point  208 , in order to protect the rotor by dampening torque variations at the rotor close to its nominal maximum speed. 
         [0061]    Although the present invention has been described with reference to embodiments, it is not limited thereto, but can be modified in various manners which are obvious for a person skilled in the art. 
         [0062]    In particular, the respective association of the first and second hydraulic units with the lower and upper zero points, and with features such as gear-ratio intervals, clutches, brakes, bypasses etc. in the above description may be reversed, or such features be provided in association with both hydraulic units. 
         [0063]    The general techniques described above can be applied to a variety of variable rotor speed/constant generator speed drive trains with a variable ratio gear system controlled by an infinitely variable gear, in particular by a hydrostatic circuit consisting of two hydraulic units connected by two pressure lines. The general techniques can also be applied to an infinitely variable gear system controlled by a hydrodynamic circuit (e.g., a VOITH system). Other embodiments are within the scope of the following claims.