Abstract:
A wind turbine is provided that minimizes the size of the drive train and nacelle while maintaining the power electronics and transformer at the top of the tower. The turbine includes a direct drive generator having an integrated disk brake positioned radially inside the stator while minimizing the potential for contamination. The turbine further includes a means for mounting a transformer below the nacelle within the tower.

Description:
FEDERAL RESEARCH STATEMENT 
     This invention was made with Government support under contract DE-FC36-03GO13131 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     FIELD OF INVENTION 
     This disclosure relates generally to wind turbine and especially to wind turbines with a direct connection between the turbine and the electrical generator. 
     BACKGROUND OF INVENTION 
     The wind has historically been one of the most widely used natural resources to provide the energy necessary to power our needs. As the demand for energy has increased and the supplies of fossil dwindled, the result has been a renewed look by electrical utility companies at alternative methods for producing electrical power. One method of electrical production involves the harnessing of wind by a turbine to drive an electrical generator. 
     Wind turbines typically involve using a series of blades fixed to the top of a tower to rotate about a horizontal axis. The blades have an aerodynamic shape such that when a wind blows across the surface of a blade, a lift force is generated causing the series of blades to rotate a shaft about an axis. The shaft is connected, typically via a gearing arrangement, to an electrical generator located in a structure called a nacelle which is positioned behind the blades. The gear box converts the rotation of the blades into a speed usable by the generator to produce electricity at a frequency that is proper for the electrical grid it is providing power. 
     The nacelle houses a number of components which are needed for operation of a modern high capacity wind turbine. In addition to the aforementioned gear box and generator, other components include a yaw drive which rotates the wind turbine, various controllers, and a brake that is used to slow the generator. Since it is desirable to keep the nacelle as small as possible, and given the number of relatively large pieces of equipment which must be located in the nacelle, space becomes very valuable. This often results in difficulties in both manufacturing the wind turbine and in conducting maintenance operations in the nacelle once the wind turbine is installed. 
     Accordingly, it is considered desirable to provide a wind turbine which minimizes the size of the nacelle while providing adequate accessibility to components during maintenance operations. 
     SUMMARY OF INVENTION 
     A wind turbine is provided that includes a nacelle with a rotor hub adjacent thereto. The turbine has a main shaft coupled to the hub and the nacelle. A generator is coupled to the shaft between the nacelle and the hub, wherein the generator includes a rotor adjacent to the shaft. Also a stator is positioned adjacent to and radially outward from the rotor and, a brake is coupled to the generator and the shaft, such that the brake is positioned radially inward from the stator. 
     A wind turbine is also provided including a tower having a yaw bearing attached at one end. A nacelle having a bedplate is connected to the yaw bearing and a transformer is positioned within the tower opposite the nacelle. In a first alternate embodiment, the transformer is suspended by a chain. In a second alternate embodiment, the transformer is suspended in a viscous fluid in a container connected to the tower. 
     A wind turbine is further provided having a nacelle and a blade rotor hub adjacent to the nacelle. A main shaft is coupled to the blade rotor hub and the nacelle. Also a generator is coupled to the shaft between the nacelle and the hub, the generator having a housing containing a generator rotor adjacent to the shaft and a stator positioned adjacent to and radially outward from said rotor. A cylindrical roller bearing is coupled between the shaft and the housing adjacent to the nacelle. A second bearing is coupled between the shaft and the housing adjacent to the hub. 
     Also, a method for transferring electrical power from a wind turbine is provided including the steps of rotating blades using wind. Rotating a generator and generating electricity with the generator. Supporting the generator with a tower and suspending a transformer adjacent to the generator. Damping the movement of the tower by contacting the transformer and transmitting the electricity through the transformer. 
     The above discussed and other features will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike: 
         FIG. 1  is a plan view illustrating a direct drive wind turbine of the present invention; 
         FIG. 2  is a side plan view of the wind turbine of  FIG. 1 ; 
         FIG. 3  is a side plan view, partially in section of the wind turbine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Electrical power may be generated by many different methods. The most common methods involve the boiling of water using fossil or nuclear based fuels. The steam produced by the boiling is used to rotate a turbine that drives an electrical generator to create the electrical power. While these common methods are very efficient, they also have undesirable side effects, such as the production of toxic pollutants, or the rely on a dwindling natural resource. One alternate method of creating electrical power is to harness a renewable natural resource such as the wind to be a driving force to rotate the electrical generator to produce the electricity. 
     Referring to  FIG. 1  and  FIG. 2 , a wind turbine  10  capable of generating electrical power in the 100 kW to 2000 kW range is shown. The wind turbine  10  is includes a tower  12  which is anchored to the ground by means of a bolted connection to a steel and concrete foundation. On the opposing end of the tower  12 , the nacelle  14  is mounted to rotate about the tower  12  to allow the nose cone  16  and the plurality of blades  18  to face into the wind. As will be described in more detail herein, a generator  20  is positioned between the nose cone  16  and the nacelle which allows the size of the nacelle to be minimized while still allowing all the necessary power electronics and controls to located either in the nacelle  14  itself, or adjacent the top of the tower  12 . 
     Typically for this size turbine, the tower  12  is between 20 and 100 meters in height and constructed of tapered tubular steel of up to 4 meter diameter at the ground and 1–2 meters diameter at the top. The tapered tubular steel tower is constructed in sections to facilitate the transportation and assembly of the wind turbine  10  at its point of use. Alternatively, the tower may be made from a lattice structure or from concrete sections. In the preferred embodiment, there are three turbine blades  18  of 10–45 meters in length that equally spaced around the nose cone  16 . While the blades may be made of any suitable material, typically a glass fiber reinforced plastic or epoxy is typically used to reduce weight while still providing the necessary mechanical strength required to withstand the wind loads. To reduce the complexity of the wind turbine  10  the blades  18  are preferably of a fixed pitch type, though variable pitch blades could also be used as well. 
     Turning to  FIG. 3 , the nacelle  14  and generator  20  will be described in more detail. The nacelle  14  has a bedplate  22  which forms the floor of the nacelle  14  and a cover  15  which encloses the bedplate  22 . The bedplate  22  is mounted to a yaw bearing  24  that is mounted a top the tower  12 . The yaw bearing  24  allows the nacelle  14  to rotate relative to the tower  12  to allow the blades  18  to orient correctly relate to the wind ensuring maximum energy production. A yaw drive  26  mounted inside the nacelle  14  drives a pinion  28  which interacts with gear teeth  35  on the outer race of yaw bearing  24  to provide the necessary force to rotate the structure. The controller  62  receives information on the wind direction from a wind sensor  66 . In response to a chaing in the wind direction, the controller  62  activates the yaw drive  26 . The safety system of the wind turbine uses an anemometer  27 . Whenever the wind speed exceeds a pre-determined safe value, the wind turbine shuts down. A typical wind speed for shut down is 25 meters/second. It is desirable to transfer the electrical power from the nacelle  14  to the grid at a high voltage to reduce the required cable size. In the preferred embodiment, a transformer  30  is suspended below the bedplate  22  inside the tower  12  by a chain  29 . It should be appreciated that the transformer  30  may be mounted to the bedplate  22  by any suitable means, preferably a means that allows some flexure to compensate for vibratory/oscillatory movement of the wind turbine  10 . 
     By arranging the transformer  30  beneath the nacelle  14  inside the tower  12 , the transformer  30  is allowed to rotate with the nacelle  14  while reducing the required size of the nacelle. Preferably, the transformer  30  will also have an opening  31  in the center to allow access to the nacelle  14  by maintenance personnel from within the tower  12 . In an alternative embodiment, the transformer is sized to allow periodic contact between the transformer  30  and the tower  12  which will act to mechanically damp any oscillations of the tower which may occur. The transformer  30  may be of any electrical type suitable for a wind turbine, including both the dry-type and oil-filled, 3-phase Wye or 3-phase delta, high voltage or low voltage. In another alternate embodiment, the transformer is of a rectangular shape, and placed to one side in the tower  12  to allow access to the nacelle  14  by maintenance personnel. In another alternate embodiment, the transformer is suspended in a bath of viscous fluid that is attached to the tower  12  to provide viscous damping of any oscillations of the tower  12 . 
     The transformer  30  connects via cable  33  to the power electronics  32  mounted inside the nacelle  14 , typically on the cover  15 . As will be described in more detail below, the power electronics  32  receives electricity from the generator  20  and converts the variable frequency electricity to match the frequency required by the electrical grid that wind turbine  10  is connected. For a typical application, the generator  20  produces at a frequency between 10–30 Hz and the power electronics  32  uses conventional means to produce the frequency of the electrical grid, typically 50 Hz or 60 Hz. The power electronics  32  may utilize an intermediate conversion of alternating current (“AC”) power from the generator to direct current (“DC”) power before converting to AC power at the grid frequency. Power throughput and terminal power factor are adjustable via controller commands (not shown). 
     The generator  20  includes a housing  34  which is mounted to the bedplate  22 . The housing  34  connects to a main drive shaft  36  through front bearing  38  and rear bearing  40 . In the preferred embodiment, the front bear  38  is a double-tapered roller bearing sized to carry a majority bending moment and axial thrust generated by the blades  18 . Alternatively, the front bearing  38  may be a crossed roller bearing or a three row type roller bearing. If the bearing  38  was required to support large bending moments by itself, the distance between the rollers would be large requiring a larger drive shaft  36  which would dramatically increase the cost of the wind turbine  10 . To make this arrangement more cost effective, a second rear bearing  40  is used to assist the front bearing  38  in carrying the bending moment. Preferably, the rear bearing  40  is a cylindrical type bearing. 
     By properly spacing the bearings  38 ,  40  the forces generated by the blades  18  can be carried while minimizing the size of the drive shaft  36 . In the preferred embodiment, the front bearing  38  and the rear bearing  40  are spaced apart a distance equal to the diameter of the drive shaft  36 . Between the bearings  38 ,  40 , the generator rotor  52  is mounted via a hub  54 . The rotor  52  rotates inside the housing  34  adjacent to the stator  56 . The rotor has electrical coils which are energized with direct current, creating a magnetic field. As the shaft  36  is driven by the blades  18 , the rotor  52  rotates a magnetic field which induces electrical current in the stator  56 . The electrical current flows from the stator  56  through cable  58  to power electronics  32  in the nacelle  14 . 
     In order to provide electric current to the generator rotor  56 , a slip ring assembly  42  is provided at the end of the drive shaft. The slip ring assembly  42  is mounted to the bedplate  22  by strut  43 , which prevents rotation of the housing of the slip ring assembly  42  relative to the shaft  44 . Mounted on the slip ring assembly is a speed sensor  60 , which measures the rotational speed of the shaft  44 . Further along the shaft, a disk  46  is mounted to the shaft  36  adjacent to the housing  34 . For reasons that will be made clearer herein, the disk  46  interacts with a brake  48  which is used to slow the turbine blades  18  and generator  20 . The brake  48  may be of any conventional type such as caliper actuated by hydraulic, pneumatic or electrical pressure. In the preferred embodiment, the disk  46  and brake  48  are positioned in a recess  50  in the housing  34  The shaft  36  terminates in a flange  44  to which the nose cone  16  mounts. 
     In operation, the turbine controller  62  receives information from wind direction sensor  66  indicating the direction of the wind. If the blades  18  are not oriented correctly with the respect to the wind, the wind turbine controller  62  activates and powers a yaw drive  26  which energizes a motor to drive pinion  28  to rotate the nacelle  14  and blades  18  to the correct position. If there is sufficient wind to drive the blades  18 , typically 4–25 meters per second, the rotation of the blades  18  will turn the shaft  36  and the rotor  52  to generate the electrical current as described herein above. The wind turbine controller  62  periodically checks the wind direction, typically multiple times per second. 
     Since over speeding of the wind turbine  10  due to excessively high wind speeds could damage the generator, it is desirable to have a means for slowing down the blades  18  and the shaft  36 . Unlike in a variable pitch turbine which has blades that can be rotated to reduce the amount of lift generated on the blades, the blades  18  of the preferred embodiment are of a fixed pitch. The aerodynamic design of the fixed-pitch blades causes stall at higher wind speeds to reduce lift, provided the rotational speed of the blade rotor is limited. The speed is controlled under normal conditions by adjusting the generator torque using the power converter or the rotor current. In the event that an over speed condition occurs, two independent braking systems are normally applied, both with the capability to stop the rotor. The first system is an electrical dynamic brake, which uses a resistor to dump energy and create a high torque on the generator  20 . The second system uses a mechanical brake  48  to slow the blades  18 . In the event that an over speed condition is detected by speed sensor  60 , or alternatively by a rotary encoder (not shown), located adjacent the slip rings downwind of the main shaft, the caliper  49  on the brake  48  is actuated causing the caliper  49  to contact the disk  46 . The resulting friction between the brake  48  and the disk  46  causes the shaft to decrease in speed. By locating the brake in the recess  50  of the generator  20 , space is saved in the nacelle  14  without risking contamination of the generator  20  components. Potential contamination is further reduced by placing this recess on the down-wind side of the generator  20 . 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, may modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention.