Abstract:
A method for powering a pitch motor drive system for at least one DC pitch motor of a wind turbine includes rectifying a voltage using a bridge circuit to thereby supply a DC link voltage to a bridge comprising active switching devices, and utilizing at least one link capacitor to smooth the DC link voltage and act as an energy sink and source for the DC pitch motor or motors.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to control of DC motors, and more particularly to methods and apparatus that are particularly useful for efficiently controlling DC pitch motors in wind turbines. 
     Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient. 
     Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox or directly coupled to the rotor. The gearbox, when present, steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid. 
     On a pitch controlled wind turbine, an electronic controller is used in conjunction with a blade pitch mechanism to pitch the blades around their respective longitudinal axes to control the power output of the wind turbine. Motors are provided to pitch the blades while the rotor is turning. 
     Some new pitch implementation systems are required to regenerate continuously. For example, the use of pitch control drive systems in wind turbines with one or more DC links as an intermediate link between source and load requires that the DC link(s) absorb regenerative energy under some conditions. For example, DC link(s) supplied by a diode source may be required to absorb regenerative energy when pitch drive motors are decelerating. The requirement to absorb regenerative energy continuously is new and is not believed to have been addressed by previously known wind turbine pitch system configurations. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, some configurations of the present invention therefore provide a method for powering a pitch motor drive system for at least one DC pitch motor of a wind turbine. The method includes rectifying a voltage using a bridge circuit to thereby supply a DC link voltage to a bridge comprising active switching devices, and utilizing at least one link capacitor to smooth the DC link voltage and act as an energy sink and source for the DC pitch motor or motors. 
     In another aspect, some configurations of the present invention provide a system for supplying power to at least one DC pitch motor of a wind turbine. The system includes a bridge circuit coupled to a source of power and configured to produce a rectified DC link voltage, a bridge of active switching devices configured to switch the DC link voltage and supply the switched DC link voltage to the DC pitch motor or motors, and at least one link capacitor in circuit and configured to smooth the DC link voltage and act as an energy sink and source for the DC pitch motor or motors. 
     In yet another aspect, some configurations of the present invention provide a wind turbine having a rotor, which itself has at least one blade operatively coupled to at least one DC pitch motor. Also provided is a power system including a bridge circuit operatively coupled to a source of power and configured to produce a rectified DC link voltage, a bridge of active switching devices configured to switch the DC link voltage and supply the switched DC link voltage to the DC pitch motor or motors, and at least one link capacitor in circuit and configured to smooth the DC link voltage and act as an energy sink and source. 
     It will thus be apparent that various configurations of the present invention realize advantages in system cost, reliability and/or availability, particularly when used in wind turbine pitch control systems. In addition, some configurations of the present invention used in wind turbine pitch control systems can be configured to advantageously provide energy swapping between pitch motor drive systems on a single DC bus and/or provide other ways to dissipate regenerative energy in single pitch motor drive systems and/or advantageously allow energy swapping between pitch control power converters. Energy swapping during operation poses advantages over single converter operation by allowing a reduction or minimization of the number and rating of dynamic brake (DB) resistors and DC link capacitors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of an exemplary configuration of a wind turbine. 
         FIG. 2  is a cut-away perspective view of a nacelle of the exemplary wind turbine configuration shown in  FIG. 1 . 
         FIG. 3  is a block diagram of an exemplary configuration of a control system for the wind turbine configuration shown in  FIG. 1 . 
         FIG. 4  is a block schematic diagram representative of some pitch control power conversion system configurations of the present invention. 
         FIG. 5  is a block schematic diagram representative of some pitch control power conversion system configurations of the present invention in wind turbines having a plurality of pitch motors and pitch motor drive systems. 
         FIG. 6  is a block schematic diagram representative of some pitch control power conversion system configurations of the present invention in wind turbines having a plurality of pitch motors and pitch motor drive systems and having a common dynamic brake resistor. 
         FIG. 7  is a block schematic diagram representative of some pitch control power conversion system configurations of the present invention in wind turbines having a plurality of pitch motors and pitch motor drive systems and in which a set of input power switches replace a non regenerative diode bridge. 
         FIG. 8  is a block schematic diagram representative of some pitch control power conversion system configurations of the present invention similar to those of  FIG. 7 , but with an individual regenerative MOSFET source provided for each pitch drive. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In some configurations and referring to  FIG. 1 , a wind turbine  100  in some configurations comprises a nacelle  102  housing a generator (not shown in  FIG. 1 ). Nacelle  102  is mounted atop a tall tower  104 , only a portion of which is shown in  FIG. 1 . Wind turbine  100  also comprises a rotor  106  that includes a plurality of rotor blades  108  attached to a rotating hub  110 . Although wind turbine  100  illustrated in  FIG. 1  includes three rotor blades  108 , there are no specific limits on the number of rotor blades  108  required by the present invention. 
     In some configurations and referring to  FIG. 2 , various components are housed in nacelle  102  atop tower  104  of wind turbine  100 . The height of tower  104  is selected based upon factors and conditions known in the art. In some configurations, one or more microcontrollers within control panel  112  comprise a control system are used for overall system monitoring and control including pitch and speed regulation, high-speed shaft and yaw brake application, yaw and pump motor application and fault monitoring. Alternative distributed or centralized control architectures are used in some configurations. 
     In some configurations, the control system provides control signals to a variable blade pitch drive  114  (which includes a DC pitch drive motor, not shown in  FIG. 2 ) to control the pitch of blades  108  (also not shown in  FIG. 2 ) that drive hub  110  as a result of wind. In some configurations, hub  110  receives three blades  108 , but other configurations can utilize any number of blades. In some configurations, the pitches of blades  108  are individually controller by blade pitch drive  114 . Hub  110  and blades  108  together comprise wind turbine rotor  106 . 
     The drive train of the wind turbine includes a main rotor shaft  116  (also referred to as a “low speed shaft”) connected to hub  110  and a gear box  118  that, in some configurations, utilizes a dual path geometry to drive a high speed shaft enclosed within gear box  118 . The high speed shaft (not shown in  FIG. 2 ) is used to drive a first generator  120  that is supported by main frame  132 . In some configurations, rotor torque is transmitted via coupling  122 . First generator  120  may be of any suitable type, for example and without limitation, a wound rotor induction generator. Another suitable type by way of non-limiting example is a multi-pole generator that can run at the speed of the low speed shaft in a direct drive configuration, without requiring a gearbox. 
     Yaw drive  124  and yaw deck  126  provide a yaw orientation system for wind turbine  100 . In some configurations, the yaw orientation system is electrically operated and controlled by the control system in accordance with information received from sensors used to measure shaft flange displacement, as described below. Either alternately or in addition to the flange displacement measuring sensors, some configurations utilize a wind vane  128  to provide information for the yaw orientation system. The yaw system is mounted on a flange provided atop tower  104 . 
     In some configurations and referring to  FIG. 3 , a control system  300  for wind turbine  100  includes a bus  302  or other communications device to communicate information. Processor(s)  304  are coupled to bus  302  to process information, including information from sensors configured to measure displacements or moments. Control system  300  further includes random access memory (RAM)  306  and/or other storage device(s)  308 . RAM  306  and storage device(s)  308  are coupled to bus  302  to store and transfer information and instructions to be executed by processor(s)  304 . RAM  306  (and also storage device(s)  308 , if required) can also be used to store temporary variables or other intermediate information during execution of instructions by processor(s)  304 . Control system  300  can also include read only memory (ROM) and or other static storage device  310 , which is coupled to bus  302  to store and provide static (i.e., non-changing) information and instructions to processor(s)  304 . Input/output device(s)  312  can include any device known in the art to provide input data to control system  300  and to provide yaw control and pitch control outputs. Instructions are provided to memory from a storage device, such as magnetic disk, a read-only memory (ROM) integrated circuit, CD-ROM, DVD, via a remote connection that is either wired or wireless providing access to one or more electronically-accessible media, etc. In some embodiments, hard-wired circuitry can be used in place of or in combination with software instructions. Thus, execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. Sensor interface  314  is an interface that allows control system  300  to communicate with one or more sensors. Sensor interface  314  can be or can comprise, for example, one or more analog-to-digital converters that convert analog signals into digital signals that can be used by processor(s)  304 . 
     In some configurations of the present invention and referring to  FIG. 4 , a single pitch motor drive system  400  is powered from a power source (not shown) using a transformer (also not shown) operatively coupled to a non regenerative diode bridge  402  that rectifies a voltage at the secondary of the transformer and that supplies a DC link voltage to an H-bridge  404  comprising four active switching devices  406 , for example, paralleled MOSFETs or individual or paralleled IGBTs. At least one DC link capacitor  408  smooths DC link voltage V DL  and act as an energy sink and source for a series DC motor  410 , which operates variable blade pitch drive  114  (which itself is shown in  FIG. 2 ). An emergency pitch system  412  comprising at least one battery  414  and contactors  416  is also provided in some configurations to pitch blades of the wind turbine (not shown in  FIG. 1 ) to a feathered position when DC power is not otherwise available. 
     System  400  of  FIG. 4  has some capability to absorb regenerative energy from series DC motor  410 . Thus, single pitch motor drive system  400  is sufficient for applications that require blades  108  to be pitched to a desired angle and that then perform very small adjustments to the pitch angle while wind turbine  100  is operating. 
     In some configurations of wind turbine  100 , a pitch controller could require blade position(s) to change significantly during every revolution of hub  110 , and therefore require pitch motor drive system  400  to dissipate regenerative energy continuously. Thus, some (but not necessarily all) configurations of pitch motor drive system  400  are further provided with at least one dynamic brake (DB) resistor(s)  418  that are used to dissipate regenerative energy from motor  410 . Dynamic brake resistor(s)  418  are electrically coupled to the DC link  419  through a power switch  420  (MOSFET or IGBT) when the DC link voltage V DL  increases to a predefined limit. This approach has been used in some LV and MV induction motor drives by General Electric in the past. 
     Some configurations of pitch control systems for wind turbines have unique environmental requirements that make dissipation of regenerative energy in dynamic braking resistors a disadvantage. Thus, some (but not necessarily all) configuration of pitch motor drive system  400  are also provide with additional capacitors  408  added to DC link  419  to advantageously allow the link voltage V DL  to stay within predefined limits without excessive power dissipation and attendant heating of the environment near pitch motor drive system  400 . 
     In some configurations of wind turbines  100 , plural pitch motor drive systems  400  are used to pitch different blades  108 . Each pitch motor drive system  400  is decoupled from the others by input transformers (not shown in the Figures), which isolate each diode-based source bridge  422  from three phase grid AC power  424 . 
     In some configurations and referring to  FIG. 5 , extra heat in wind turbine hub  110  produced by dynamic brake resistors  418  and increased parts count resulting from plural DC link capacitors  408  and/or dynamic brake resistors are avoided. More particularly, a common DC link  419  and DC link voltage V DL  are shared between a plurality of pitch motors  410  and drive systems  400 . For example, three such systems  400  share a common DC link  419  in the configuration represented in  FIG. 5 . Common DC link  419  allows energy swapping between systems  400 . 
     In some configurations of the present invention represented by  FIG. 5 , a plurality of pitch drive systems  400  are interconnected with a common DC link  419 , and individual DB resistors  418  are provided for each of the plurality of pitch motor drive systems  400 . The interconnection provided by DC link  419  permits DB resistors  418  to have a lower power dissipation rating than would otherwise be necessary. Some configurations provide plural DC link capacitors  408 . In such configurations, energy swapping between the plural pitch motor drive systems  400  allows for a reduction in the total capacitance on common DC link  419  and reduces the rating required for DB resistors  418 . 
     In some configurations of the present invention and referring to  FIG. 6 , a common DB resistor  418  is provided. Some configurations also provide a common power switch  420  used to control the magnitude of current through DB resistor  418 . 
     In some configurations of the present invention and referring to  FIG. 7 , the need to add DB resistors  418  or a plurality of DC link capacitors  408  can be avoided by adding a regenerative source for the DC link voltage V DL . For example, the example configuration represented in  FIG. 7  is configured to absorb all regenerative energy not circulated between the pitch motor drive systems  400  using a set  700  of input power switches  420  that replaces non regenerative diode bridge  402 . Thus, the DC link voltage V DL  that is applied to all pitch motor drives systems  400  is regulated. 
     In some configurations and referring to  FIG. 8 , a circuit  800  a separate regenerative MOSFET source bridge  700  using MOSFETs  420  is provided for each pitch drive. IGBT devices can be used in place of MOSFETs  420  if more rating is desired. The circuit illustrated in  FIG. 7  differs from that of  FIG. 8  in that the circuit of  FIG. 7  is generally more cost-effective and uses a single regenerative source configured to absorb all or most of the regenerative energy not circulated between the pitch motor drive systems. In circuit  800  of  FIG. 8 , separate branches  802 ,  804 ,  806  are provided for axis  1  pitch control, axis  2  pitch control, and axis  3  pitch control, respectively. 
     Thus, in some configurations of the present invention, motor drives for one or more (for example, three) pitch motors can comprise MOSFETs or IGBTs. Non-regenerative sources are provided in some configurations for one or more pitch motors, whereas in other configurations regenerative sources comprising MOSFETs or IGBTs are provided. In some configurations, a single source (for example, a regenerative source) is provided for a plurality of pitch motors (e.g., three pitch motors), whereas in some configurations, an individual source (for example, a regenerative source) is provided for each individual pitch motor. 
     The use of pitch control drive systems with a DC link as an intermediate link between source and load requires, in some configurations, that the DC link absorb regenerative energy under some conditions, such as when the motors are decelerating when the DC link(s) are supplied by a diode source. As can now be appreciated from the example configurations discussed herein, configurations of the present invention advantageously absorb this energy using capacitors and/or switched resistors on the DC link, and/or by using a fully regenerative active source of DC voltage. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.