Abstract:
A small wind-powered electricity generation system, providing regulated AC electric power from wind energy to a power system grid, has a wind turbine that produces less than 10 kW of peak electric power in a permanent magnet generator that produces generator power with a frequency that varies with the wind speed. The wind turbine utilizes a cross-wind type rotor having a power coefficient that varies with the operating tip speed ratio and has an optimal tip speed ratio wherein the power coefficient is maximum. A power converter, for converting generator power to regulated electric power by applying a controlled load to the generator controls the operation of the wind turbine rotor such that the operating tip speed ratio is greater than the optimal tip speed ratio in a low wind speed region, is approximately equal to the optimal tip speed ratio in a medium wind speed region, and is greater than the optimal tip speed ratio in a high wind speed region.

Description:
This invention pertains to a wind turbine system for providing regulated AC power from a low cost small wind turbine to a power system grid, and more particularly to a wind turbine system that increases the annual energy delivery while also having a reliable construction and operation and low cost. 
     BACKGROUND OF THE INVENTION 
     Use of wind turbines for electrical energy generation is currently growing because of their economical power production and environmental benefits, and because of the growing realization that other sources of energy are limited. Large wind turbines, located in off shore or remote wind farms are increasingly being installed worldwide. They can produce megawatts of electric power with lower costs than many other types of power production, and they do not pollute. 
     Another application for wind turbines is in small wind turbines, typically of 10 kilowatts peak power or less. Such small wind turbines have been deployed on farms for providing some electricity production, but their use has generally been limited because of certain deficiencies. An additional emerging market opportunity for small wind turbines is in urban and suburban installations. In these installations, customers expect to be able to produce some of their own electric power and offset their utility bills through net metering. Urban and suburban wind turbines will be located where people live, with installations on rooftops, in yards and along roadsides. They will reduce electricity transmission losses and the need for additional transmission lines. 
     Unfortunately, small wind turbines currently suffer from substantial deficiencies that limit their use and are preventing their widespread adoption. Small wind turbines are currently much too expensive for the energy that they produce. They have less than ideal energy capture, conversion and efficiency. They typically employ complex and costly constructions. Most small wind turbines are also noisy and require the use of towers that are unsightly and difficult to zone and install, both limiting their suitability for urban and suburban installations. Other deficiencies include rotor turbulence sensitivity, overspeed structural failure potential and avian-unfriendliness. Accordingly, a new type of small wind turbine system is needed. 
     SUMMARY OF THE INVENTION 
     The invention provides a small wind turbine system that delivers regulated AC power to a power system grid at reduced costs and increased annual energy production while having very desirable wind turbine operating characteristics. The system uses a low tip speed ratio cross-wind type or drag propelled turbine with peak capacity of less than 10 kilowatts. Unlike horizontal propeller small wind turbines, the drag propelled configuration operates in cross winds and with a low tip speed ratio under 3.0 for silent operation. The tip speed ratio is the ratio of the speed of the rotor tip divided by the speed of the wind. The turbine has robust construction and low construction costs, and does not need pitch speed control. The wind turbine directly drives a permanent magnet generator for high efficiency at low weight and costs. The generator has no field coil and therefore provides for no voltage regulation from field coil control. Instead, a power converter converts the generator power to regulated AC electric power by applying a controlled load to the generator power. The system maximizes the energy production to the grid from the wind turbine and generator and also provides for desirable turbine operation. In accordance with the invention, the power converter utilizes three operating regions including a low wind speed region, a medium wind speed region and a high wind speed region. The power converter provides no power to the power system grid in the low wind speed region. The power converter provides power with an increasing function of the turbine rotational speed in the medium wind speed region. In the high wind speed region, the power converter provides output power with variations of turbine rotational speed by increasing the rotor operating tip speed ratio above the operating tip speed ratio in the medium wind speed region. 
     This operational method is unlike that of conventional turbines. Conventional turbines are controlled to decrease their operating tip speed ratio in high winds. Also, the power converter of conventional small wind turbine systems applies load to the generator that continues increasing the rotor operating tip speed ratio with increasing wind speeds. 
     Because wind turbines in accordance with this invention use a low tip speed ratio, cross-wind turbine rotor, affording the advantages of low cost and low noise, the rotor does not offer the ability to yaw or furl out of the wind like a conventional propeller type wind turbine. As a result, the speed cannot be controlled through rotor direction and will increase in increasing wind. Moreover, no pitch control of the rotor blades is available to limit speed, although speed control could be maintained by overloading the rotor in high winds. However, it has been surprisingly found that low tip speed ratio rotors can be constructed such that the rotor speed need not be limited for structural safety. In operation, contrary to conventional turbine operation, the wind turbine system of this invention utilizes no active stalling. Even in extreme wind speeds of more than 35 m/sec, the rotor will not have failure from stress induced in operation at high rotational speed. The stress of a given rotor is a squared function of its tip speed. A drag-propelled rotor in accordance with this invention operates with a tip speed ratio of about 1.2 whereas a conventional turbine rotor operates with a tip speed ratio of about 7.0-10.0. Accordingly, the stress in the drag-propelled rotor can be more than 34 times lower and can be economically designed to survive high winds. Speed regulation of the rotor is not required, allowing instead just the use of over voltage prevention of the generator and power converter, as generator voltage increases linearly with rotor speed and wind speed. 
     A further advantage of the construction is that, without the need to control rotor speed through pitch control or stalling, the turbine construction is simplified and noise is also further reduced. Conventional small wind turbines with speed regulation through stalling are very noisy and not compatible for urban and suburban installation where their noise would be unacceptable. The stall control also puts high stresses on the mechanical system and it requires the inverter electronics to be oversized, adding undesirable costs. An additional cost savings is also provided by eliminating the need for an energy diversion load. When a conventional turbine is operated in high wind, the rotor is slowed and driven to a lower operating tip speed ratio by dumping excess energy into a diversion. To allow operation in high wind areas, the diversion load can be large and costly. In contrast, the small wind turbine system in accordance with this invention requires no diversion load. When operated in high wind, the rotor is driven to higher operating tip speed ratios, and no energy needs to be dumped into a diversion load. This reduces the costs of the wind turbine system and eliminates the need to remove unnecessary heat. 
     In a preferred embodiment, the power converter applies a cubically varying load with rotor rotational speed to the generator in the medium wind speed region. This power extraction is more complex than a linear interpolation applied by current permanent magnet generator driven grid tie inverter wind turbine systems. Because the power of the wind varies with the cube of the wind speed, the wind generator with this preferred power converter can extract the maximum potential energy from the wind. The wind turbine rotor has a power coefficient for energy extraction from wind that varies with the tip speed ratio of the rotor. Preferably, in the medium wind speed region, the power converter applies load to the generator in accordance with the rotor and generator designs so as to maintain the rotor operating tip speed approximately equal to the tip speed ratio corresponding to the maximum rotor power coefficient. This can be accomplished by the power converter controlling the load through detection of the generator voltage level or through detection of the generator or rotor frequency. 
     In the high wind region, the power converter can provide a substantially constant output power. It allows the operating tip speed ratio to increase above the tip speed ratio corresponding to the maximum rotor power coefficient by reducing the load to the generator. 
     In an additional embodiment, the power converter includes a fourth region for extreme wind speeds. In the extreme wind speed region, the system disconnects the generator power from the power converter to prevent over voltage damage that could occur. The wind turbine system has an extremely wide voltage range of the generator output to the inverter. If power production begins at 4 m/sec and the highest expected wind speed is 35 m/sec, this results in a more than 8:1 voltage range. A consequence of the large range is the production of very high voltages in high winds that have potential to cause electronics failure. The fourth region maintains the functioning of the wind turbine system despite the occurrence of an unusually high velocity storm. Disconnection of a load to a conventional wind turbine system would be contrary to the teachings in the art, as it would cause a conventional wind turbine to fail. The combination of a properly designed low tip speed rotor with the control prevents failure. Disconnection of the load allows the rotor to rotate even faster and the generator to produce higher voltage. 
     Several features can be included in the small wind turbine system to reduce total costs. Depending on the wind turbine system, it can be desirable to utilize high voltage (1200 volt) transistors for the power converter to allow for high voltage swing and a wide range of wind speeds for power generation. Alternatively, a wye to delta control can be added to limit the voltage to the power converter to 600 volts. The wye to delta control switches the 3-phase power connection of the generator to limit the voltage when it gets to a predetermined limit. Switching the connection reduces the voltage by a factor of 1.73. The control can be operated by sensing the phase winding voltage or alternatively by sensing the generator frequency. The control can be included inside the generator to limit the high voltage, greater than 600 volts, from needed connection to an external power converter. If a wye to delta converter is utilized, the power converter control can be operated utilizing the generator frequency instead of the rectified generator voltage level. 
     In a preferred configuration the power converter switches between the low wind speed region and medium wind speed region at wind speeds between 3-6 m/sec. The power converter further preferably switches between the medium wind speed region to high wind speed region at wind speeds between 10 and 13 m/sec. This configuration provides good energy capture and low costs for Class 3 and Class 4 wind speeds, and it is compatible with the majority of locations. 
     The generator voltage can be chosen to reduce the costs of the power converter. A typical converter consists of a boost-buck converter that boosts the incoming generator voltage and then forms regulated AC output for grid tied connection. In the simplest form, the power converter consists of a rectifier that rectifies the generator voltage and an inverter that generates grid tie AC output power. The costs of the inverter can be reduced and efficiency increased by eliminating the need for a boosting stage. This simplifies the construction and reduces parts and operating losses. The generator must provide a voltage, at the lowest generating wind speeds that can be directly switched into AC utility power. The generator is designed to provide a rectified voltage that is greater than the peak voltage of the power system grid whenever the wind turbine is operating in the medium wind speed region. 
     The inverter can utilize analog electronics to control the load that is applied to the generator according to the rotor speed and wind speed. Alternatively, the power converter controls the load to the generator using a digital signal processor that varies said load accordingly with the rotational speed of the rotor. One advantage of the use of a DSP for the control is exact adjustment of the operation can be achieved. If a power converter is sold with different sized rotors or with rotors that are installed in different wind regimes, it may be desirable to set the operating parameters on site. In an additional preferred embodiment the power converter includes an adjustment for manually setting the operating parameters of the rotor. 
     The generator for the wind turbine system converts the rotational energy from the rotor into electrical energy. A permanent magnet generator provides the power conversion with high efficiency and compact size, but does not include a field coil that would allow for field coil voltage control. Because a drag propelled rotor can have directions of wind approach that do not provide high torque, it is desirable that the rotor and generator rotate freely so as to be self-starting regardless of the wind speed or direction. Permanent magnet generators utilizing NdFeB magnets can have significant cogging torque that locks the rotor in place when not rotating and prevents easy starting of rotation. Conventional permanent magnet generators also suffer from magnetic induced losses, eddy currents and hysteresis. Further, conventional permanent magnet generators also exert a high rotor to stator magnetic attraction that requires multiple bearings and a heavy rigid structure to resist. As a consequence, the preferred generator for the wind turbine system is a double rotating air core construction. This type of generator construction avoids these deficiencies. Preferably, the permanent magnet generator is directly driven by the rotor at the same rotational speed and comprises two spaced apart co-rotating generator rotors forming therebetween an armature airgap. The permanent magnets are located on the generator rotors and drive magnetic flux across the armature airgap. Located within the armature airgap are multiple generator windings for producing power. The windings are preferably constructed into a substantially non-magnetic structure to eliminate magnetic attraction and magnetically induced losses. The air core construction allows the turbine to begin rotating and energy capture in the lowest of winds. It further eliminates generator noise or vibration. 
     Cross-wind turbines generate power from wind coming from many angles and not just head-on as in a propeller turbine that requires a yaw mechanism for correct wind tracking. Because a cross-wind turbine does not need to track wind direction changes, it can generate more energy in turbulent winds. Low tip speed ratio cross-wind turbine rotors in accordance with the invention can be constructed by several different designs. One design is the traditional Savonius rotor that relies primarily on drag. Another type is a modified Savonius, which has a modified rotor profile to utilize lift for torque generation at some rotational angles and utilizes drag at other angles. New versions or multi bladed turbines are also being constructed and could be utilized with the wind turbine system so long as they operate with a maximum tip speed ratio of below 3.0 and were structurally sound. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic elevation of a wind turbine for use in a small wind turbine system in accordance with the invention. 
         FIG. 1B  is a schematic plan view of the wind turbine shown in  FIG. 1A . 
         FIG. 2  is a plot of power versus wind speed along with a power extraction interpolation of prior art. 
         FIG. 3  is a plot of power coefficient versus tip speed ratio for a small wind turbine system in accordance with the invention. 
         FIG. 4  is a graph of annual percentage of time for given wind speeds in a Class 3 wind regime. 
         FIG. 5  is a graph of annual energy available for given wind speeds in a Class 3 wind regime. 
         FIG. 6  is plot of power extraction for a small wind turbine system in accordance with the invention. 
         FIG. 7  is graph comparing annual energy generation using prior art wind turbines with what it could be using wind turbines in accordance with the invention. 
         FIG. 8  is a plot showing the operating power coefficients for a wind turbine in a small wind turbine system in accordance with the invention. 
         FIG. 9  is a plot of voltage versus wind speed for a small wind turbine system in accordance with the invention. 
         FIG. 10  is a schematic drawing of a power system for use in a small wind turbine system in accordance with the invention. 
         FIG. 11  is a schematic diagram of a wye to delta generator control for use in a small wind turbine system in accordance with the invention. 
         FIG. 12  is a schematic diagram of an inverter for use in a small wind turbine system in accordance with the invention. 
         FIG. 13  is a schematic elevation of the top end of a wind turbine in a small wind turbine system in accordance with the invention. 
         FIG. 14  is a schematic elevation of the bottom end of a wind turbine in a small wind turbine system in accordance with the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to the drawings, wherein like reference characters designate identical or corresponding parts,  FIGS. 1A and 1B  shows a wind turbine  30  for use in a small wind turbine system in accordance with the invention. The wind turbine  30  includes a drag-propelled rotor  31  that is driven by the drag force of the wind. The rotor  31  is constructed from two curved blades  32 ,  33  that catch the wind to self-start. Radial rib members  34  reinforce the blades  32 ,  33  to hold the desired curved profile and to increase the bending stiffness and centrifugal structural strength of the rotor  31 . The rotor  31  is journalled for rotation about a vertical axis pole  35  and captures wind energy from any direction, without the need for a tower. The rotor  31  drives a permanent magnet generator  36  that converts the rotation energy into unregulated generator power. One embodiment of a suitable wind turbine is shown in  FIGS. 13 and 14  and is described in more detail below. 
     A plot of power versus wind speed along with a power extraction interpolation of prior art is shown in  FIG. 2 . As shown in curve  40 , wind power is a cubic function of the speed of the wind. Efficient capture of the energy of the wind at the higher wind speeds in a given wind speed regime is significantly more important than capture of low speed wind to the costs per annual energy generation of the small wind turbine system. The wind energy extraction from a conventional small wind turbine inverter for use with a permanent magnet generator wind turbine is a linear interpolation, indicated by the line  41  . The system is very simple to implement because the inverter duty cycle for switching load power to the generator is a direct linear function of the generator voltage. Unfortunately, when no stall or pitch control mechanism is utilized in the small wind turbine, such as a Savonius type, significant wind energy in the operating speed range is lost. Energy at the lower range end  43  and energy at the upper range end  42 , as illustrated in the divergence between line  41  and curve  40 , is not captured, which substantially affects the costs per annual energy generation of the small wind turbine system. 
     A plot of power coefficient versus tip speed ratio for a small wind turbine system in accordance with the invention is shown in  FIG. 3 . Power coefficient is the percent of wind energy acting on the rotor that is actually captured by the rotor. A drag propelled wind turbine provides the benefits of capturing energy from any direction of wind utilizing a vertical axis, reduced sensitivity from turbulence induced from nearby structures without the use of a tower, and operates at a low tip speed ratio for low noise operation. The baseline drag propelled turbine rotor is the Savonius wind turbine. It utilizes two semicircular cup blades to catch the wind and has a peak power coefficient that occurs around 0.8 tip speed ratio. New modifications of the blade profile from semicircular to airfoil-contoured shape have increased the power coefficient and tip speed ratio. These increases have come with the addition of wind-induced lift contribution to the drag contributions that drive the rotor. As shown, the power coefficient of the rotor has a peak in the middle of its operating tip speed ratio range. With no inverter load applied to the generator, the rotor speed increases and operates at a no load tip speed ratio of about 1.8. The peak power coefficient, or peak percent of wind energy captured by the rotor per available wind energy acting on the rotor, occurs at tip speeds of between about 0.8 to about 1.3. The higher the tip operating tip speed ratio and power coefficient the lower the costs for the generator and turbine rotor to provide a given level of energy generation. 
     A graph of annual percentage of time for given wind speeds in a Class 3 wind regime is shown in  FIG. 4 . The average wind speed is 5.35 m/sec in this regime. However, the wind speed that occurs most often by time is about only 4 m/sec, at about 14% of the time. This would suggest that capturing the energy at low wind speeds is critical. However, the power available from the wind is a cubic function of the speed of the wind. 
     The available wind energy is made apparent by plotting the wind energy distribution. A graph of annual energy available for given wind speeds in a Class 3 wind regime is shown in  FIG. 5 . Looking at the annual energy generation distribution, which will directly affect the energy generation costs of the small wind turbine system, the more important wind speeds for power generation is shifted to the higher wind speeds. The greatest energy generation potential occurs at the wind speeds between 8-9 m/sec. This is despite the fact that they occur only 5-6% of the time. 
     A plot of power extraction for a small wind turbine system in accordance with the invention is shown in  FIG. 6 . A power converter of a power system for use in a small wind turbine system, shown in  FIG. 10  and described in detail below, converts the varying generator power to regulated AC electric power by applying a controlled load to the generator. The power converter utilizes three operating regions including a low wind speed region, a medium wind speed region and a high wind speed region. The power converter provides no power to the power system grid in the low wind speed region. These wind speeds are below the level that provides significant wind energy, and to extract power from this low wind would require undesirably increased generator costs. Because of the simple turbine design, pitch control is not available to increase the generator speed in the low wind speed region. 
     The power converter provides power with a nonlinear increasing function of the turbine rotational speed in the medium wind speed region. As the wind energy increases, the turbine speed and generator voltage increase linearly with the wind speed. The converter utilizes a nonlinear interpolation, preferably a cubic function of the wind speed, to capture most of the available wind energy. The use of the nonlinear function can maintain the rotor operating tip speed ratio such that the rotor power coefficient is near the maximum value in the medium wind speed range. 
     In the high wind speed region, the power converter provides constant output power with variations of turbine rotational speed by increasing the rotor operating tip speed ratio above the operating tip speed ratio in the medium wind speed region. Although more wind energy is available than is being collected in the high wind speed region, collection of additional energy would adversely affect the small wind turbine costs per annual energy generation. To collect more energy in the high wind speed region would require undesirably increased inverter costs. 
     The wind turbine system may also include a fourth or extreme wind speed region. Because the rotor is drag propelled, it does not effectively stall to limit speed. Little danger is present from rotor structural failure because a drag propelled rotor operates at a tip speed ratio of about 1/7 th  a conventional wind turbine, and stress are proportional to the square of the tip speed for a given design structure. However, the wind turbine system can be damaged by over voltage to the power converter. In the extreme wind speed region, generator power can be disconnected from the power converter in the rare occurrence of extreme wind storms. 
     A comparison of potential annual energy generation between the invention and prior art is shown in  FIG. 7 . The small wind turbine system can provide as much as 30% increased annual energy generation over a current conventional horizontal propeller wind turbine with permanent magnet generator and conventional voltage controlled inverter. The small wind turbine system also has lower initial costs resulting from the simpler construction and elimination of a requirement for a tower, thus substantially reducing the costs per annual energy generation of the system. The small wind turbine system also is quiet and avian friendly in operation. 
     Unlike conventional small wind turbine systems employing horizontal axis, high tip speed ratio propellers, a drag propelled wind turbine provides regulation for different reasons. Over voltage protection is the reason for regulation instead of structural integrity, which requires the conventional turbine system to actually limit the rotational speed of the rotor. This is conventionally done through pitch control or by stall of the blades. Stalling of the blades inherently increases the turbine noise over the already loud noise from the high tip speed ratio. In contrast, the small wind turbine system operates with a low tip speed ratio and in high winds, the rotor does not stall and is allowed to accelerate, eliminating any stall induced noise. 
     A plot showing the operating power coefficients for a wind turbine in a small wind turbine system in accordance with the invention is shown in  FIG. 8 . In the low wind speed region, the rotor operates at its no load maximum tip speed ratio. No load is applied to the generator by the power converter. In the medium wind speed region, the power converter applies a nonlinear increasing load to the generator in accordance to the generator speed, based on the generator and rotor design parameters. The applied load maintains the rotor operating tip speed ratio at the tip speed ratio corresponding to near the maximum rotor power coefficient. In the high wind speed region, the power converter applies a decreasing load to the generator that increases the rotor operating tip speed ratio above the operating tip speed ratio in the medium wind speed region. The effect is that the rotor speed is yet further increased as the speed of the wind increases. This is in contrast with and is the opposite of convention small wind turbines wherein the rotor speed is reduced by reducing the operating tip speed ratio in high winds. 
     The voltage of the generator per wind speed can be utilized to further reduce the costs of the small wind turbine system. At any given power level, high voltage operation reduces the current and hence the wiring costs for connecting the wind turbine to the power converter, if mounted separately. In addition, the power converter design can be significantly simplified, efficiency increased and costs reduced through elimination of a boosting stage. The generator preferably is designed to produce an output voltage high enough for the rotor rotational speed at the transition to the medium wind speed region such that it does not require boosting to produce the AC power to the grid. A plot of generator output voltage versus wind speed for a small wind turbine system in accordance with the invention is shown in  FIG. 9 . At 4 m/sec, the rectified generator voltage is 155 volts, which is above the 110 volts of the AC power system grid. As the wind speed increases, the generator voltage increases linearly. At the 12 m/sec transition to the high wind speed region, the generator voltage is 465 volts. As the wind speed continues to increase in the high wind speed region, the slope of the voltage with wind speed increases nonlinearly because the operating tip speed ratio is not held constant and is instead increased by the power converter. At 21 m/sec, the voltage increases to 940 volts. Alternatively, a wye to delta converter can switch the generator windings connection and lower the output voltage to 543 volts at 21 m/sec. Depending on the wind speed regime of the installation location, the maximum wind speed set in the high wind speed region of a digital signal processor  69  can altered or reduced to limit the output voltage. 
     A schematic drawing of a power system for use in a small wind turbine system in accordance with the invention is shown in  FIG. 10 . The wind turbine system  60  comprises a Savonius wind turbine  61 , such as the wind turbine  30  shown in  FIGS. 1A and 1B , with a permanent magnet generator that provides generator power via lines  62  to an over voltage protection relay  63 . The relay  63  disconnects the generator power in extreme wind speeds that would cause damage to the rest of the small wind turbine system. The transmitted generator power is supplied via lines  64  to a power converter  65  that converts generator power to regulated AC power in lines  68  for grid tie connection. The power converter  65  can utilize a matrix or cycloconverter topology. In a more simple alternative, the power converter  65  is constructed of a rectifier  66  that converts the generator power to DC and an inverter  67  that produces the regulated AC output  68 . The inverter  67  is controlled by a digital signal processor  69  utilizing information about the output of the generator in the turbine  61  via a line  71 . The digital signal processor  69  controls the firing of the inverter switches to apply the load to the generator power  64  according to wind speed. A suitable programmable digital signal processor (DSP) for this application would be the Texas Instruments TMS320C2000 Digital Signal Controller. 
     To limit the voltage from the generator, it is sometimes advantageous to utilize a wye to delta control for the generator output. A schematic diagram of a wye to delta generator control for use in a small wind turbine system in accordance with the invention is shown in  FIG. 11 . The control  80  comprises the three phase generator windings  81 ,  82 ,  83  that provide the generator power supplied by the rotor rotation. The windings  81 ,  82 ,  83  are switched by a relay  84  between wye and delta three phase connection. A controller  86 , based on a phase voltage or frequency measurement sensed in the lines  87  from the generator, operates the coil  85  of the relay  84 . When the generator phase leg winding voltage achieves a threshold, the connection is switched from a wye to a delta, resulting in a 1.73 times voltage drop. As the rotor speed continues to increase from increasing wind speed, the generator output  88  remains reduced by 1.73 times. This limits the voltage requirements for the interconnection cabling with the power converter. 
     Inverters have been designed with many different topologies depending on the desired operational conditions, performance and costs. A preferred inverter construction for the wind turbine system is one that does not require voltage boosting. This can reduce size and costs and can eliminate the need for a step up transformer. A schematic diagram of a power converter for use in a small wind turbine system in accordance with the invention is shown in  FIG. 12 . The power converter  100  receives generator input power from the generator  90  via lines  101 . Generator power is rectified by a rectifier  102  to provide unregulated DC power via lines  103 . The unregulated DC power is inverted to output regulated AC grid tie power  108  by an H-bridge inverter block  111 . The H bridge  111  includes four transistors (typically IGBT&#39;s)  104 ,  105 ,  106 ,  107  that switch the DC power from lines  103 . A control or digital signal processor  109  (corresponding to the DSP  69  in  FIG. 10 ) utilizes input information via a line  95  about the generator output power, e.g output voltage or frequency, and provides a control signal  110  to the transistors  104 - 106  to both regulate the load applied back to the generator power via lines  101  and to provide synchronized output AC power  108 . 
     The drag propelled wind turbine for use in the small wind turbine system can utilize different constructions so long as it is self starting, rotates about a vertical axis and operates without pitch control for limiting the operating speed. A preferable construction is a modified Savonius construction utilizing higher efficiency rotor blade shapes to utilize some lift force. A tall rotor increases the energy capture and allows for reduced diameter to increase the rotational rate and reduce the generator costs. A schematic elevation of the top end of a wind turbine in a small wind turbine system in accordance with the invention is shown in  FIG. 13 . The wind turbine  120  is comprised of a stationary center pole shaft  121 . The rotor  129  rotates about the center shaft  121  and is journalled by an upper end bearing  123  that is fixed upon a small shaft end  124 . The bearing  123  is held in an upper rotor end plate  122 . Vertical rigid blade support tubes  125 ,  126 ,  127 ,  128  are affixed to the upper rotor end plate  122 . Blades (not shown for simplicity) are attached between each of the vertical blade support tubes  125 ,  126 ,  127 ,  128 . 
     The wind turbine utilizes a permanent magnet generator to convert the turbine rotor rotational energy into electrical power. It is desirable to place the generator at the lower end of the wind turbine so that its weight is closer to the ground. A schematic drawing of the bottom end of a wind turbine  140  in a small wind turbine system in accordance with the invention is shown in  FIG. 14 . This wind turbine  140  could use as its upper end the upper end shown in  FIG. 13 , or could use a different upper end. The wind turbine  140  has comprised a stationary center pole shaft  141  and a rotor  155  that rotates about the shaft  141 . The rotor  155  is journalled by a lower bearing  146 , which is attached to a center mounting tube  144 . The mounting tube  144  is clamped onto the center pole  141  by a clamping nut  145 . The bearing  146  is affixed to the generator  143  through an upper back iron  147 . The generator  143  is constructed of upper and lower back irons  147 ,  148  with vertically aligned circumferential arrays of magnets  150 ,  151  of alternating polarity. The magnets  150 ,  151  drive magnetic flux across an armature airgap  152 . An air core armature  153  is located in the armature airgap  152  and attached to the center tube  144  for generation of electrical power. The power is removed by windings connections  154  to the air core armature  153 . An outer tube  149  maintains the distance of the armature airgap  152 . 
     The rotor  155  is constructed of a lower rotor end plate  142  that is bolted to the upper back iron  147 . Vertical blade support tubes  155 ,  156 ,  157 ,  158  are attached to the lower rotor end plate  142  for mounting of the blades, not shown for simplicity. The blades are curved panels that are affixed between the vertical tubes  155 ,  156 ,  157 ,  158  to capture the energy from the wind and to drive the rotor  155  and generator  143 . 
     A wind turbine that would be suitable for use with this invention is disclosed in more detail in a PCT Application entitle “Low Cost Wind Turbine” by Christopher W. Gabrys filed in the U.S. Receiving Office of the PCT concurrently herewith, the disclosure of which is incorporated by reference herein. 
     Obviously, numerous modifications and variations of the described preferred embodiment are possible and will occur to those skilled in the art in light of this disclosure of the invention. Accordingly, I intend that these modifications and variations, and the equivalents thereof, be included within the spirit and scope of the invention as defined in the following claims, wherein