Abstract:
A variable speed wind turbine utilizes a hydraulic transmission including a hydraulic pump coupled to a rotor and a hydraulic motor coupled to an induction generator to produce variable frequency power that is converted to grid frequency power by electronic controls.

Description:
CROSS-REFERENCE TO RELATED CASES 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/860,773, filed Nov. 21, 2006, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a wind driven electric power production system, and more particularly to a variable speed wind turbine drive and control system which utilizes a hydraulic pump coupled to the rotor to drive a hydraulic motor coupled to an induction generator producing AC power and utilizing electronic controls to produce a grid frequency power generation at variable rotor speeds. 
       BACKGROUND 
       [0003]    The majority of wind turbines utilize a mechanical speed increasing gearbox which is prone to failure resulting in extended down time and high maintenance costs. It is possible to remove the gearbox from the wind turbine system by utilizing a hydrostatic transmission. These prior art wind turbines have generally been limited to fixed speed generators in order to enable the system to be attached to a power grid. This has been accomplished using blade control systems to compensate for changes in wind speed, pressure control systems to control changes in hydraulic flow, as well as variable displacement motors and pumps to compensate for pressure changes in the hydrostatic systems. 
         [0004]    The use of power electronics to convert a variable frequency output of a variable speed AC generator into useable AC power is well known. U.S. Pat. No. 5,083,039 deals with this type of control used on variable speed wind turbines. 
         [0005]    It would therefore be an advantage to provide a variable speed wind turbine that utilizes a hydrostatic transmission that does not require the use of a gearbox. 
       SUMMARY 
       [0006]    At least one embodiment of the invention provides a variable speed wind turbine transmission system comprising: a turbine rotor driven by wind; a fixed displacement hydraulic pump coupled to the rotor; a fixed displacement hydraulic motor coupled to a generator; a hydraulic circuit to transfer fluid from the pump to the motor; wherein the generator rotates at a variable speed directly related to rotation of the turbine rotor and produces an electrical output at a variable frequency; and a power converter to convert the variable frequency electrical output of the generator into grid frequency power. 
         [0007]    At least one embodiment of the invention provides a variable speed wind turbine system comprising: a turbine rotor driven by the wind; a fixed displacement hydraulic pump directly driven by the rotor; a fixed displacement hydraulic motor; a closed loop hydraulic fluid circuit for carrying pressurized oil to the hydraulic motor from the hydraulic pump and to return hydraulic fluid to the hydraulic pump from the hydraulic motor; an induction generator directly driven by the fixed displacement hydraulic motor, the induction generator producing AC power at variable frequency; a power converter including an inverter which converts the AC power to DC power, an active front end which converts the DC power into grid frequency AC power, and an L-C-L filter which reduces power line harmonics. 
         [0008]    At least one embodiment of the invention provides a method for producing a grid frequency power with a variable speed wind turbine, the method comprising the steps of: providing a variable speed wind turbine comprising a turbine rotor driven by wind, a fixed displacement hydraulic pump coupled to the rotor, a fixed displacement hydraulic motor coupled to a generator, and a hydraulic circuit to transfer fluid from the pump to the motor; subjecting the rotor of the variable speed wind turbine to a source of wind such that the generator produces an electrical output at a variable frequency related to a speed of rotation of the rotor; and converting the variable frequency electrical output of the generator into grid frequency power. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of this invention will now be described in further detail with reference to the accompanying drawing, in which: 
           [0010]      FIG. 1  is a conceptual schematic view of an embodiment of a wind energy system in accordance with the present invention; and 
           [0011]      FIG. 2  is a hydraulic schematic view an embodiment of the present invention also showing various possible optional features. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The proposed system  110  is designed as a means of converting kinetic energy from the wind into electrical power suitable to be transferred over the electrical grid. Referring now to  FIG. 1 , a rotor  1  is shown mounted on a nacelle of a wind tower. A hydraulic pump  3  positioned in the nacelle is coupled to the rotor  1 . In one embodiment of the invention, the pump  3  is directly driven by the shaft of the rotor  1  and the pump  3  rotates at the same speed as the rotor  1 . A first hydraulic fluid line  31  connects the hydraulic pump output to a hydraulic motor  11  which may be positioned on the ground or in the nacelle. A second hydraulic fluid line  32  connects the hydraulic motor  11  to the fluid input of the hydraulic pump  3  in the nacelle to complete the fluid circuit. The flow going through the motor  11  turns the motor output shaft, which is directly coupled to an electric generator  12 . The generator  12 , coupled to the rotation of the rotor  1  through the hydraulic transmission, produces variable frequency AC power. A generator drive inverter  27  converts the variable frequency AC power to DC power which is transferred into the Active Front End (AFE)  28 . The AFE  28  is synchronized with the frequency of the grid, so it can convert the DC power into grid frequency AC power. The AFE  28  synthesizes the sinusoidal input voltage by Pulse-Width-Modulation (PWM). A tuned L-C-L filter  38  reduces potential PWM power line harmonics to negligible levels. 
         [0013]    The hydraulic circuit for this system  110  is shown in  FIG. 2  and the system is discussed in additional detail. The rotor  1  is placed in an air stream to turn a shaft. The shaft is shown directly coupled to the drive shaft of a hydraulic pump  3 . A high pressure hydraulic line  31  connects the outlet of the pump to the inlet of the motor  11 , and a low pressure return line  33  connects the outlet of the motor  11  to the inlet of the pump  3 . Flow is generated through this hydraulic loop as the pump shaft is driven by the rotor  1 . The flow going through the motor  11  turns the motor output shaft, which is directly coupled to an electric generator  12 . The pump  3  has a much larger displacement than the motor  11 , so the hydraulic transmission acts as a speed increaser. This allows the use of a relatively small generator  12  even though the rotor input speed is slow. 
         [0014]    As the generator  12  turns, it produces electric power which is fed into the power electronics for frequency correction before it is sent to the power grid. In addition to frequency correction, the power electronics control the loading of the system proportional to rotor speed. This allows the rotor  1  to operate at its most efficient speed relative to wind velocity and therefore optimize the amount of power captured from the wind. 
         [0015]    The induction generator rotor speed is precisely measured by means of a quadrature encoder or resolver feedback. This feedback is provided to the generator inverter control, which regulates the amount of negative slip (rpm difference from true synchronous speed) of the induction generator by means of Flux Vector (FV) control. The Flux Vector algorithm optimally controls frequency and excitation to the generator at all speed and load conditions. This is accomplished by optimizing the Magnetization Current Vector (motor “field’) component, relative to the Torque Current Vector component. 
         [0016]    Alternatively, the generator inverter control can be operated without the need for a rotor speed feedback device. This mode of operation is known as Sensorless Vector (SV) control. In the SV mode, the control algorithm approximates rotor rpm, based on applied frequency, current, and voltage, and the known slip and torque characteristics of the induction generator. Upon initial commissioning, a self-tuning algorithm locks itself into the unique generator characteristics by interpolating the results by stimulating electrical impulses to the generator. 
         [0017]    The generator inverter control provides additional ability to operate the wind turbine generator above its designated base speed and frequency ratings, at a constant torque level, thereby widening the power capture bandwidth. This additional power capture bandwidth is effective up to the mechanically safe operating limits of any component of the turbine, with the capability to adjust the speed safe-limit point. 
         [0018]    The power electronics used on this system are capable of controlling both speed and torque of the generator shaft. Increasing the torque required to drive the generator will increase the pressure in the hydraulic system, which in turn will increase the amount of torque required to drive the pump. A plc can be used to command the generator torque to match the maximum power output available for a given rotor speed, so that the turbine is operating at its most efficient point over a wide range of wind speeds. 
         [0019]    The power electronics consist of two major components. The active front end  28  is connected to the grid, and the generator drive inverter  27  is connected to the output leads of the generator. During normal operation, the generator drive inverter receives variable frequency AC power from the generator and converts it to DC power which is transferred into the Active Front End (AFE). The AFE is synchronized with the frequency of the grid, so it can convert the DC power into grid frequency AC power. The AFE synthesizes the sinusoidal input voltage by Pulse-Width-Modulation (PWM). Power generated into the grid is performed at near-unity Power Factor (PF). A tuned L-C-L filter reduces potential PWM power line harmonics to negligible levels as discussed above with regard to  FIG. 1 . 
         [0020]    While the turbine is generating power, the electrical grid is used to load the generator  12 . In the event of a power grid failure, this load is no longer available, but it would be advantageous to perform a controlled shutdown of the turbine to limit mechanical wear on the system. Once the power grid is down, the AFE  28  is disabled so that the system is no longer feeding power onto the grid, eliminating the possibility of islanding or asynchronous faults, should the grid be re-initiated. In this instance the rotor  1  would still be turning to drive the pump  3 , and generator  12  would still be feeding electrical power into the generator inverter  27 . The generator inverter  27  and controls are kept operational via a Switched-Mode-Power-Supply (SMPS) fed from the DC bus. This system will use this power to keep the plc and other control components operational while a controlled shutdown is automatically performed. Excess power delivered from the turbine will be sent from the DC bus across a load resistor and turned into heat, and can also be used to power yaw motor controls to position the turbine wind-neutral until such time as the grid is reinstated. 
         [0021]    Referring again to  FIG. 2 , an optional brake  2  may be installed on the rotor shaft to prevent rotation. The brake  2  can be either spring set and hydraulically released, or it can be spring released and hydraulically set. Hydraulic pressure for the brake  2  can be supplied by a brake release power unit  6  located near the brake  2 . 
         [0022]    A hot oil shuttle valve  7  may be installed on the system  110  to remove some of the hydraulic fluid from the main loop during operation so that it can be replaced with cooler fluid from the reservoir  23 . A charge pump  21  transfers fluid through a replenishing filter  25  and replenishing check valves  16 ,  17  into the low pressure side of the loop to replace any fluid that is removed by the hot oil shuttle valve  7  and maintain positive pressure at the inlet to the pump  3 . The charge pressure feeding into the replenishing check valves  16 ,  17  is limited by a relief valve  15 . The charge pump flow may also be used to drive a hydraulic motor  24  connected to a cooling fan which blows air across a heat exchanger  26  to remove excess heat from the system. The cooling fan motor speed can be limited using a relief valve  20 , or the motor  24  can be completely bypassed using a directional valve  19  when no cooling is required. 
         [0023]    The pump  3  is located in the nacelle, which is free to rotate to face into the wind. The motor  11  and generator  12  can be located either in the nacelle or at some other location down the tower. In installations where the motor  11  and generator  12  are located apart from the pump  3 , a rotary union  8  can be installed to allow the pump  3  to rotate relative to the motor  11  without twisting the hydraulic lines  31 ,  32 . 
         [0024]    The mechanical speed increasing gearboxes commonly used on existing turbines are a frequent point of failure in these machines. Replacing this failure prone component with a hydrostatic transmission should provide longer service life and reduced maintenance cost. The hydrostatic transmission can be built from components that are smaller, lighter, and easier to handle than the mechanical gearboxes. Since the hydrostatic transmission uses fluid as the medium to transfer power, it also has some built in shock absorbing capabilities that should reduce the wear on the rest of the turbine as it responds to sudden wind gusts and turbulent wind conditions. 
         [0025]    The controls package used on this system allows for grid frequency power generation at a variety of rotor speeds. This enables the rotor to capture more energy from the wind than many of the fixed speed systems currently in use. Along with the speed increasing hydrostatic transmission, the controls system also allows the use of a standard, low-cost 4 or 6 pole induction generator. This is more cost-effective than some of the low speed generators that are currently used, and makes it possible to stock a replacement or obtain one quickly in the event of a failure. 
         [0026]    The generator inverter control provides additional ability to operate the wind turbine generator above its designated base speed and frequency ratings, at a constant torque level, thereby widening the power capture bandwidth. For example, a 6-pole 60 Hz induction generator is typically limited to 1200 rpm (+slip rpm) when only grid-connected. By allowing the generator to continue to 70 Hz, or 1400 rpm (+slip), more power can be captured. This additional power capture bandwidth is effective up to the mechanically safe operating limits of any component of the turbine, with the capability to adjust the speed safe-limit point. 
         [0027]    Although the principles, embodiments and operation of the present invention have been described in detail herein, this is not to be construed as being limited to the particular illustrative forms disclosed. They will thus become apparent to those skilled in the art that various modifications of the embodiments herein can be made without departing from the spirit or scope of the invention. Accordingly, the scope and content of the present invention are to be defined only by the terms of the appended claims.