Abstract:
An over speed control circuit for a wind turbine generator is disclosed which optimizes the time that the wind turbine generator is operational and thus maximizes the power output over time. The over speed control circuit forms a closed feedback loop which periodically measures the output voltage of the wind turbine generator in order to regulate its speed by electronically controlling the load on the generator. The over speed control circuit in accordance with the present invention is adapted to work in conjunction with known over speed protection lock out relays. More particularly, the over speed control circuit causes a short circuit to be placed the generator terminals when the generator voltage reaches a threshold value, relatively less than the threshold value used to trigger the over speed lockout relay. As such, the over speed control circuit minimizes the operation of the lockout relay, thereby maximizing the power output of the generator over time making such wind turbine generator systems much more practical as a renewable energy source.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an over speed control circuit for optimizing the power output of a wind turbine generator and more particularly to a circuit for optimizing the operational time and thus power output over time of a wind turbine generator which coordinates with known over speed relay lockout protection circuitry and incorporates closed feedback control that periodically measures the output voltage of the generator to regulate its speed by electronically controlling the load on the generator to minimize activation of the over speed relay lockout protection circuitry.  
         [0003]     2. Description of the Prior Art  
         [0004]     Wind turbine generator systems are generally known in the art. Examples of such systems are disclosed in U.S. Pat. Nos. 4,565,929; 5,506,453; 5,907,192; 6,265,785; and 6,541,877. Such wind turbine generator systems are also described in U.S. Patent Application Publication Nos. US2002/0117861; 2005/0034937; 2005/0230979; 2005/0236839; 2006/0006658; and 2006/0012182. Due to the ever-increasing demand and increasing cost for electrical power, renewable energy sources, such as wind turbine generator systems, are becoming more and more popular for generating electrical power. Such wind turbine generator systems are known to be used individually to generate supplemental or excess power for individual, residential or light industrial users to generate electrical power in the range of 1-2 kw. Such wind turbine generator systems are also known to be aggregated together, forming a wind turbine generator farm, to produce aggregate amounts of electrical power. It is also known that unconsumed electrical power generated by wind turbine generators is connected to the utility power grid.  
         [0005]     Such wind turbine generators are known to include a wind turbine, which includes a plurality of turbine blades connected to a rotatable shaft. The rotatable shaft is rigidly connected to a direct current (DC) generator. Wind causes rotation of the wind turbine which acts as the prime mover for a DC generator. The generator, for example, a self-excited generator, generates DC electrical power.  
         [0006]     One problem with such systems is that wind speeds are not constant. As is known in the art, the voltage output of the generator is a cubic function of the speed of rotation of the turbine blades and the direct connected generator. As such, the effect of wind gusts on the wind turbine generator must be controlled to prevent damage to the wind turbine generator.  
         [0007]     Some wind turbine generator systems are known to use some type of mechanical braking to protect the wind turbine generator from an over speed condition. For example, U.S. Pat. No. 5,506,453 utilizes the pitch of the wind turbine blades to protect the wind turbine from over speed. In particular, the blades of the wind turbine are mechanically coupled to a rotatable mechanical hub. The blades are configured so as to be rotatable about their longitudinal axis relative to the hub allowing the pitch of the turbine blades to be varied. The pitch of the blades is turned in such a way as to create braking of the wind turbine.  
         [0008]     Other known systems utilize mechanical brakes, such as disclosed in U.S. Patent Application Publication No. US 2005/0034937. Yet other systems disclose the use of aerodynamic-type brakes as well as mechanical brakes, for example, as disclosed in U.S. Pat. No. 6,265,785, to protect the wind turbine from over speed.  
         [0009]     While mechanical brakes do an adequate job of protecting the wind turbine generator from over speed, mechanical braking systems do little to optimize the operational time and thus power output of the wind turbine generator. Moreover, such mechanical braking systems are mechanically complex and are, thus, relatively expensive.  
         [0010]     As such, electrical braking systems have been developed to control over speed of wind turbine generator systems. For example, Japanese Patent Publication JP2000-179446 discloses an electrical braking system for a wind turbine generator. The system disclosed in the Japanese patent publication includes a relay whose contacts are connected across the output terminals of the generator. When an over speed condition is detected, the relay is energized which, in turn, shorts out the output terminals of the generator, which loads the generator and causes it to slow down and stop.  
         [0011]     In many countries, for example, in Europe, such relay protection is dictated by industrial standards, for example, the Energy Networks Association, an engineering association in the UK, promulgated Engineering Recommendation G83/1, September 2003, Recommendations For the Connection of Small-Scale Embedded Generators (Up to 16 A Per Phase) In Parallel With Low Voltage Distribution Networks”, specifies an over speed lockout relay connected across the generator terminals. Upon detection of an over speed condition, the lock out relay shorts out the generator terminals, which causes the generator to slow down and stop. The standard specifies a three-minute wait period before the relay can be de-energized so that the wind turbine generator can be restarted.  
         [0012]     Although the electrical brake is effective in preventing damage to the wind turbine generator due to over speed, such outages frustrate the practicality of using such wind turbine generator and connecting them to a utility power grid. Thus, there is a need for a control circuit for a wind turbine generator that not only protects the wind turbine generator from over speed, but also optimizes the time that the wind turbine generator is operational and thus maximizes the output power from the generator.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention relates to an over speed control circuit for a wind turbine generator which optimizes the time that the wind turbine generator is operational and thus maximizes the power output over time. The over speed control circuit forms a closed feedback loop which periodically measures the output voltage of the wind turbine generator in order to regulate its speed by electronically controlling the load on the generator. The over speed control circuit in accordance with the present invention is adapted to work in conjunction with known over speed protection lock out relays. More particularly, the over speed control circuit causes a short circuit to be placed the generator terminals when the generator voltage reaches a threshold value, relatively less than the threshold value used to trigger the over speed lockout relay. As such, the over speed control circuit minimizes the operation of the lockout relay, thereby maximizing the power output of the generator over time making such wind turbine generator systems much more practical as a renewable energy source. 
     
    
     DESCRIPTION OF THE DRAWING  
       [0014]     These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein:  
         [0015]      FIG. 1  is a block diagram of a known control circuit for a wind turbine generator. Illustrating a lockout relay across the generator terminals.  
         [0016]      FIG. 2  is a block diagram as illustrated in  FIG. 1 , further illustrating the over speed control circuit in accordance with the present invention.  
         [0017]      FIG. 3  is a schematic diagram of an analog embodiment of the over speed control circuit in accordance with the present invention.  
         [0018]      FIG. 4  is a schematic diagram of an alternative digital embodiment of the over speed control circuit in accordance with the present invention.  
         [0019]      FIG. 5  is a flow chart for the embodiment illustrated in  FIG. 4   
         [0020]      FIG. 6A  is a graphical illustration of wind speed and power output of a wind turbine generator as a function of time for a wind turbine generator system that utilizes an over speed lock out relay as illustrated in  FIG. 1 .  
         [0021]      FIG. 6B  is similar to  FIG. 6A  but illustrates the power output of the generator over time for a wind turbine generator control system which includes the over speed control circuit in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0022]     The present invention relates to an over speed control circuit for a wind turbine generator that is configured to co-ordinate with a conventional over speed lock-out relay to optimize the operating time and thus the power exported by the generator over time in an environment of varying wind conditions. More particularly, available electrical power for export from a wind turbine generator is approximately equal to the cube of the generator speed. Since the generator is rigidly coupled to the wind turbine, the generator rotational speed (i.e. revolutions per minute or RPM) is directly proportional to the wind speed. With a wind speed of, for example, 10 meters/second a conventional generator can support a 1.0 kWatt output, for example. Because the generator voltage output curve is a cube function, a relatively small change in wind speed can create a large change in the generator output voltage. Such changes in the wind speed can cause damage to the turbine as well as the generator attached to the turbine and the circuitry connected to the generator terminals. In known wind turbine generator systems, a electromechanical braking system is applied during an over speed condition which stops the turbine, reducing the DC output of the generator to zero. Unfortunately, some known systems utilize a lockout relay which, as discussed above, locks out the wind turbine generator system for a nominal period, such as 3 minutes, any time the generator voltage exceeds a threshold indicative of an over speed condition. Thus, during conditions when high wind speeds exist and the opportunity to export maximum power, the generator must be shut down. The over speed control circuit in accordance with the present invention solves this problem by applying electronic braking to the generator when the output voltage at the generator terminals exceeds a first predetermined threshold indicative of an over speed condition. In accordance with an important aspect of the invention, the first threshold is relatively lower than a second predetermined threshold, used to trigger the over speed lock out relay. As such, the over speed control circuit in accordance with the present invention minimizes the operation of the over speed lockout relay, thereby maximizing the power output of the generator over time making such wind turbine generator systems much more practical as a renewable energy source.  
         [0023]      FIG. 1  illustrates a conventional wind turbine generator system, generally identified with the reference numeral  20 . The wind generator system  20  includes a generator  22 , such as, a self-excited DC generator. A wind turbine (not shown) functions as a prime-mover for the generator  22 . The generator  22  generates a DC voltage across its output terminals  24 ,  26  as a cubic function of the rotational speed of the generator  22 . In as much as the generator  22  is directly coupled to the wind turbine, the rotational speed of the turbine and generator is directly proportional to the wind speed. As such, the output voltage at the generator terminals  24  and  26  is a cubic function of the wind speed.  
         [0024]     The output terminals  24 ,  26  of the generator  22  are coupled to an inverter, shown within the block  28 . The inverter  28  converts the DC output voltage, available at the output terminals  24 ,  26  of the generator  22 , to an AC voltage suitable for connection to a utility AC power grid, generally identified with the reference numeral  30 . The AC power grid  30  may be a phase to phase 230/240 Volts AC, suitable for residential, commercial and industrial application. In the exemplary embodiment shown, shown, the inverter  28  generates a phase to phase voltage across two output phases L 1  and L 2 , for example, 230/240 Volts AC.  
         [0025]     Depending on the configuration of the utility AC power grid  30 , the inverter  28  may also include a ground conductor for use with utility AC power grids which are 230/240 Volts AC with a center tap ground, for providing 230/240 Volts AC phase to phase and 115/120 Volts AC phase to ground. In such a system, the inverter ground conductor (not shown) would be electrically coupled to the utility center tap ground. The principles of the present invention are applicable to wind turbine generator systems  20  configured to be connected to various configurations of the utility AC power grid  30 .  
         [0026]     The phase to phase output L 1  and L 2  of the inverter  28  is connected to the utility AC power grid  30  by way of a grid relay  32 . The grid relay  32  ensures that the output of the inverter  28  is in phase with the utility AC power grid before enabling any connection between the two. The grid relay  32  is under the control of an AC Relay Control Circuit  34 . The AC Relay Control Circuit  34  monitors the phase of the output of the inverter  28  and the phase of the utility AC power grid  30 . When the phase of the inverter output is synchronized with the phase of the utility AC power grid  30 , the AC Relay Control Circuit  34  causes the grid relay  32  to connect the two together.  
         [0027]     In order to protect the wind turbine generator system  20  from damage from over speed resulting from wind gusts, some wind turbine generator systems  20  include a brake relay  36 , as mentioned above. The brake relay  36  is connected across the output terminals  24 ,  26  of the generator  22 . The brake relay  36  may be an electromechanical relay, for example, as specified by G83/1, that shorts the terminals  24 ,  26  of the generator  22  together when the relay is activated. Shorting the terminals  24 ,  26  of the generator  22  together creates a load on the generator  22  and slows down and eventually stops the generator  22 , thus acting as an electronic brake. Due to the variability of the wind speed, many known wind turbine generator systems  20 , such as those systems designed to the Engineering Recommendation G83/1, discussed above, continuously monitor the output voltage of the generator  22  at a DC Measurement Point. When the output voltage of the generator  22  exceeds the lockout threshold voltage, for example, 310 Volts DC, indicative of an over speed condition, a Brake Relay Control Circuit  38  activates the brake relay  36 , which shorts the terminals  24 ,  26  of the generator  22  and maintains the short circuit condition, thus locking out the generator  22 , for a time period of 3 minutes, for example. This lock out condition causes the wind turbine generator system  20  to be off-line during a wind condition in which the system could be delivering maximum power to the utility AC power grid  30 . The lock out condition also makes wind turbine energy systems  20  less desirable as a renewable energy source.  
         [0028]     These problems are solved by the over speed control circuit in accordance with the present invention. With reference to  FIG. 2 , the over speed control circuit in accordance with the present invention includes a pulse width modulated (PWM) Brake switch  40  that is under the control of a PWM brake control circuit  42 . The PWM Brake switch  40  is connected across the output terminals  24 ,  26  of the generator  22  and is thus in parallel with the brake relay  24 . The PWM Brake control circuit  42  continuously monitors the generator output voltage at the DC Measurement Point and compares the generator output voltage with an over speed threshold voltage, for example 300 Volts DC, relatively less than the lock out threshold voltage used to trigger the brake relay  36 . As will be described in more detail below, the over speed control circuit in accordance with the present invention minimizes operation of the brake relay  36 , thus optimizing the operation of the wind turbine generator system  20  and maximizing the power exported to the utility AC power grid  30 .  
         [0029]     The DC output voltage of the generator  22  may be measured by a DC Measurement Circuit  58  or a sensor. In particular, the DC Measurement Circuit  58  may include a diode  44  and a capacitor  46 . With such a configuration, the DC Measurement Point (i.e. cathode of the diode  44 ) is separated from the generator  22  by way of the diode  44 . The measurement side of the diode  44  may be coupled to relatively large metal film hold up capacitor  46 , for example, 1000 microfarads, which holds the generator output voltage relatively constant during measurement once the capacitor  46  is fully charged defining the DC Measurement Point. When the generator output voltage at the DC Measurement Point reaches the maximum rated design voltage (i.e. over speed threshold), the PWM Relay Control Circuit  42  generates a drive signal to actuate the PWM Brake  40 . As will be discussed in more detail below, the PWM Brake  40  may be configured as an n-channel MOSFET, coupled across the output terminals  24 ,  26  of the generator  22 . In such a configuration, the drive signal from the PWM Brake Control circuit  42  is applied to the gate terminal of the n-channel MOSFET. When the drive signal is pulled high, the MOSFET is turned on. This condition looks like a short to the generator  22 . The short across the generator  22  slows the turbine down with a corresponding decrease in the generator output voltage. At this point, the voltage from the generator  22  falls below the voltage of the DC Measurement Point (i.e. the voltage on the capacitor  46 ). This condition back biases the series diode  44 , effectively isolating the generator  22  from the DC Measurement Point. The hold up capacitor  46 , coupled to the DC measurement point, is used to supply current to a flyback section of the inverter  28  during a flyback mode. While the capacitor  46  supplies current to the inverter  28 , the voltage at the DC measurement point (i.e. voltage on the capacitor  46 ) will decrease to a point below the over speed threshold voltage. When the voltage on the capacitor  46  drops below the over speed threshold value, the PWM Brake Control circuit  42  generates a low signal that is applied to the gate of the MOSFET causing the MOSFET to turn off. Once the MOSFET is turned off, the turbine can now spin freely and the DC input voltage from the generator will change according to the available wind speed.  
         [0030]     The ramp-up voltage of the generator  22  is moderated by the load presented to the generator  22  through recharge of the holdup capacitor  46 . The recharge time of the capacitor  46  allows ample time for the MOSFET to turn off. The effect is to set up a PWM regulator whose duty cycle is inversely proportional to the DC voltage. The controlled voltage allows for the generator  22  to operate under a much wider band of wind speed than would normally be possible with the electromechanical method.  
         [0031]      FIG. 3  illustrates an exemplary analog embodiment of the over speed control circuit in accordance with the present invention, generally identified with the reference numeral  50 . The over speed control circuit  50  includes the PWM Brake  40 , for example, a MOSFET, coupled across the output terminals  24 ,  26  ( FIG. 2 ) of the generator  22  and the PWM Relay Control Circuit  42 A ( FIG. 3 ), shown within the dashed box. The PWM Brake Control Circuit  42 A is an analog circuit and includes a comparator  52  and a driver circuit, generally identified with the reference numeral  54 . The over speed threshold signal or reference  56  is applied to an inverting input of the comparator  52 . The generator output (i.e cathode of the diode  44 ), identified in  FIG. 3  as the DC Measurement Point, is applied to a non-inverting input of the comparator  52 .  
         [0032]     The generator output voltage may alternatively be sensed by a sensor or virtually any means for providing a signal representative of the generator output voltage. For example, the sensors may include a step down transformer.  
         [0033]     When the output voltage of the generator  22  at the DC Measurement Point exceeds the Over Speed Threshold Reference  56 , the output of the comparator  52  goes high, thus actuating the PWM Brake  40  to effectively short the output terminals  24 ,  26  of the generator  22 . As mentioned above, the output of the comparator  52  will remain high until the voltage on the capacitor  46  ( FIG. 2 ) drops below the Over Speed Threshold Reference  56 . At that point, the output of the comparator  52  will go low, thus providing PWM control of the PWM Brake  40 .  
         [0034]     The output of the comparator  52  may be applied to a driver circuit  54 . The driver circuit  54  illustrated in  FIG. 3  is merely exemplary and includes a pair of serially coupled resistors  60  and  62 . The output of the comparator  52  is applied to a node defined between the serially coupled resistors  60 ,  62 . One resistor is coupled to a voltage source V 1 . The resistors  60  and  62  act as a voltage divider to pull up the output of the comparator  52  to a predetermined value. The driver circuit  54  also includes a pair of complementary bipolar junction transistors  64  and  66  connected in a push-pull configuration. More particularly, the transistor  64  is a NPN transistor while the transistor  66  is a PNP. The bases and emitters of the transistors  64  and  66  are coupled together. The collector of the transistor  64  is pulled high by way of a pull up resistor  68 . The collector of the transistor  66  is pulled low and is connected to ground. The emitters of the transistors  64  and  66  are coupled to the PWM Brake  40 .  
         [0035]     In operation, when the output of the comparator  52  is low, the PNP transistor  66  is turned on, connecting the PWM Brake  40  to ground, in which case n-channel MOSFETS used as the PWM Brake  40 , remain off. When the output of the comparator  52  goes high, the PNP transistor  66  turns off and the NPN transistor  64  turns on. This causes the PWM Brake to be pulled high, thus causing the n-channel MOSFET, used for the PWM Brake  40  to be turned on, effectively shorting the generator  22 .  
         [0036]     An exemplary alternate digital embodiment of the over speed control circuit in accordance with the present invention is illustrated in  FIG. 4  and generally identified with the reference numeral  70 . The over speed control circuit  70  includes the PWM Brake  40  and the PWM Brake Control Circuit  42 D. The PWM Brake Control Circuit  42 D includes a microprocessor  72  and a driver circuit  74 . A flow diagram for the microprocessor is illustrated in  FIG. 5 . The voltage at the DC Measurement Point (i.e. voltage at the cathode of the diode  44 , as illustrated in  FIG. 2 ) is monitored by the microprocessor  72 .  
         [0037]     Referring to  FIG. 5 , monitoring of the voltage at the DC Measurement Point may be interrupt driven, as indicated by step  76 . Upon an interrupt, the analog DC voltage from the DC Measurement Circuit  58  is converted to a digital value by an on-board analog to digital converter (not shown), as indicated in step  78 . The system then checks in step  80  if the value of the voltage at the DC Measurement Point is greater than a PWM upper limit (i.e. over speed threshold plus a constant). If so, the PWM Brake  40  is actuated in step  82  and the n-channel MOSFET is turned on to short the generator  22 . The system then continues its processing in step  84  after servicing the interrupt.  
         [0038]     If the system determines in step  80  that the voltage at the DC Measurement Point is not greater than the PWM upper limit (i.e. over speed threshold plus a constant), the system checks in step  86  whether the voltage at the DC Measurement Point is less than or equal to a PWM lower limit (i.e. over speed threshold minus a constant) in step  86 . If not, the system returns to step  84  and continues its processing. If it is determined in step  86  that the voltage at the DC Measurement Point is less than the PWM lower limit, for example, due to a voltage on the capacitor  46 , the PWM Brake  40  is turned off in step  88 . The upper and lower PWM limits are used to set the duty cycle of the PWM.  
         [0039]     The driver circuit  74  ( FIG. 4 ) includes a current limiting resistor  76 , a pair of BJTs  78 ,  80 , configured as a voltage enhancement circuit, a pair of load resistors  82 ,  84  coupled to the collector terminals of the transistors  78  and  80  and a pair of complementary BJTs,  86 ,  88 , connected in a push-pull configuration. The base and emitter terminals of the transistors  86  and  88  are coupled together. The base terminals of the transistors  86  and  88  are coupled to the collector of the NPN transistor  80 . The emitter terminals of the transistors  86  and  88  are tied to the PWM Brake  40 . The emitter terminals of the NPN transistors  78  and  80  are connected to ground.  
         [0040]     In operation, whenever the microprocessor  74  outputs a high signal on its I/O port, the NPN transistor  78  is turned on, the NPN transistor  80  is turned off, connecting the base terminal of the PNP transistor  88  and the base terminal of the PNP transistor  86  to the high DC rail by way of the resistor  84 , thus turning off the PNP transistor  88  and turning on the NPN transistor  86 . As mentioned above, the PWM Brake  40  may be configured as an n-channel MOSFET. As such when the PNP transistor  86  is turned on, the MOSFET will be turned on. Thus allowing it to turn on and connect the positive voltage DC voltage rail to the DC Brake  40 . This causes the n-channel MOSFET, used as the PWM Brake  40 , to turn on. Alternatively, when the I/O port of the microprocessor  72  is forced low, the NPN transistor  78  is turned off, the NPN transistor  80  is turned on. During this condition, the base of the transistor  86  goes to ground, the transistor  88  is turned on and the MOSFET will be turned off.  
         [0041]     Referring to  FIG. 2 , a wind turbine generator system  20  in accordance with the present invention includes a Brake Relay  36 , a Brake Relay Control Circuit  38 , a PWM Brake  40 , a PWM Brake Control Circuit  42 , a DC Measurement circuit  58 , for example, the diode  44  and the capacitor  46 , an inverter  28 , a grid relay  32  and a AC Relay Control Circuit  34 . Inverters are extremely well known in the art and are used to convert DC electrical power to AC electrical power. Various inverters  28  may be used with the present invention. Exemplary inverters which may be used with the present invention are disclosed in U.S. Pat. Nos. 5,552,712; 5,907,192 and 6,256,212 and US Patent Application Publication No. US 2005/0012339 A1, all hereby incorporated by reference.  
         [0042]      FIG. 6A  illustrates the power exported by a conventional wind turbine generator system as illustrated in  FIG. 1 .  FIG. 6B  illustrates the power exported by a wind turbine generator system in accordance with the present invention. Referring first to  FIG. 6A , the curve  90  is an exemplary curve of the wind speed as a function time. The line  92  represents the lockout threshold value, for example, 10 meters per second. As shown, as the wind speed increases above the lockout threshold, the Brake Relay  36  locks out the generator  22  resulting in no power being exported to the utility AC power grid  30  for the lockout period of 3 minutes. After the lockout period expires, as the wind speed drops below the lockout threshold  92 , the wind turbine generator system exports power, as indicated by the curve  94 , until the wind speed goes above the lockout threshold  92 . As shown in  FIG. 6A , this occurs at about 12 minutes. The wind turbine generator system is again locked out for 3 minutes. After the second lockout period, as the wind speed drops below the lockout threshold, the wind turbine generator system again begins exporting power at about 21 minutes, as indicated by the curve  96 . Thus for the 24 minute time period illustrated in  FIG. 6A , the total power exported to the utility AC power grid  30  is the sum of the areas under the curves  94  and  96 . For the exemplary data indicated in  FIG. 6A , the total power exported is 91 watts-hours.  
         [0043]      FIG. 6B  illustrates the power exported by a wind turbine generator system in accordance with the present invention. For the same wind speed curve  90  illustrated in  FIG. 6A . In this case, the dotted line  96  represents the over speed threshold, for example 10 meters per sec. The over speed threshold is selected to be lower than the lockout threshold. As shown, any time the wind speed exceeds the over speed threshold  96 , the PWM Brake  40  electronically brakes the generator  22  to allow maximum power, for example, 1000 watts, to be exported by the generator from about 0.5 minutes to about 6 minutes, as indicated by the segment  98  of the curve  100 . With the conventional system, as illustrated in  FIG. 6A , the wind turbine generator system was locked during this same time period and exported no power. As the wind speed drops off during the time period from about 6 minutes to 12 minutes, the power exported drops below the maximum as a function of the wind speed. From 14 minutes to 18 minutes, the system exports maximum power, as indicated by the line segment  102 . During this same time period, the conventional wind turbine generator system was locked out because the wind speeds exceeded the lockout threshold and thus exported no power during this period.  
         [0044]     From 18 minutes to 24 minutes, the wind turbine generator system exported power to the utility AC power grid  30  as a function of the wind speed, which remained below the lockout threshold and the over speed threshold. The total power exported by the wind turbine generator in accordance with the present invention is 350 Watt-hours, significantly higher than the conventional system illustrated in  FIGS. 1 and 6 A.  
         [0045]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.