Abstract:
A method for controlling a pitch control system of a wind turbine includes providing a charged backup battery configured to supply no energy to a DC link when full AC input power is available, wherein the DC link includes a DC link capacitor. The method further includes using energy stored in the DC link capacitor to operate a pitch control system during a loss or dip of AC input power, and maintaining charge on the DC link capacitor using the charged backup battery as voltage across the DC link capacitor drops during the operation of the pitch control system.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to wind turbine energy systems and more particularly to pitch control systems for wind turbines. 
     In one known wind turbine, a pitch control system having a fully regenerative silicon controlled rectifier (SCR) bridge drives a 4.2 KW series DC motor. This type of system has been used in servo motor drives for many years and is commercially available. The SCR drive has the advantage of simplicity, but may not be able to deliver the level of pitch system performance that may be needed in newer and/or larger wind turbines. 
     In the event of a loss of AC input power, at least one known wind turbine system pitches the blades of the wind turbine using emergency pitch batteries. The blades are pitched to a position that would prevent blade overspeed. The AC voltage drop is sensed by the pitch control system and the emergency pitch system is activated. The wind turbine control system modulates the emergency pitch system and attempts to keep the hub rotational speed below overspeed limits. In many cases, the turbine control issues a fault and stops the turbine. However, known wind turbine systems use DC link capacitors and an H bridge power converter circuit, and do not have the ability to pitch the blades using this circuit once the small amount of energy stored in the DC link capacitors is depleted. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One aspect of the present invention therefore provides a method for controlling a pitch control system of a wind turbine. The method includes providing a charged backup battery configured to supply no energy to a DC link when full AC input power is available, wherein the DC link includes a DC link capacitor. The method further includes using energy stored in the DC link capacitor to operate a pitch control system during a loss or dip of AC input power, and maintaining charge on the DC link capacitor using the charged backup battery as voltage across the DC link capacitor drops during the operation of the pitch control system. 
     In another aspect, the present invention provides an apparatus for controlling pitch of a blade of a wind turbine. The apparatus includes a pitch control system and a DC link having a DC link capacitor. The DC link is configured to provide power to the pitch control system. Also included is a source of AC input power to provide power to the DC link, and a backup battery configured to supply no energy to the DC link when full AC input power is available. The apparatus is configured to use energy stored in the DC link capacitor to operate the pitch control system during a loss or dip of AC input power, and maintain charge on the DC link capacitor using the backup battery as voltage across the DC link capacitor drops during the operation of the pitch control system. 
     In yet another aspect, the present invention provides a wind turbine that includes at least one blade and a generator coupled to the blade and configured to generate AC power. The wind turbine further includes a pitch control system configured to control a pitch of the blade about an axis, a DC link having a DC link capacitor and configured to provide power to the pitch control system, and a source of AC input power to provide power to the DC link. The source of AC power is not necessarily the generator. The wind turbine also includes a backup battery configured to supply no energy to the DC link when full AC input power is available. The wind turbine is configured to use energy stored in the DC link capacitor to operate the pitch control system during a loss or dip of AC input power, and maintain charge on the DC link capacitor using the backup battery as voltage across the DC link capacitor drops during the operation of the pitch control system. 
     Configurations of the present invention are thus able to provide battery supported operation of pitch control motor drives, which increases the availability of the wind turbine by allowing operation through grid disturbances. 
    
    
     
       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 a configuration of a control system for the wind turbine configuration shown in  FIG. 1 . 
         FIG. 4  is a block schematic diagram representative of some configurations of the present invention for controlling a pitch control system of a wind turbine. 
         FIGS. 5A and 5B  are a block schematic diagram representative of some configurations of the present invention for controlling a plurality of pitch control systems of a wind turbine using a non-regenerative source. 
         FIGS. 6A and 6B  are a block diagram representative of some configurations of the present invention for controlling a plurality of pitch control systems of a wind turbine using a regenerative source. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In some configurations of the present invention, a single wind turbine pitch control design has cost and performance advantages over known systems. A single wind turbine motor drive is provided with a non-regenerative bridge supplying a DC voltage to an H bridge that comprises four active switching devices (e.g., paralleled MOSFETs in some configurations). A DC link capacitor smooths the DC link voltage and acts as an energy sink and source for the series DC motor. This design also includes an emergency pitch system using batteries and contactors to pitch the blades to a featured position. 
     In some configurations and referring to  FIG. 1 , a wind turbine  100  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 one or more 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. 
     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  to control the pitch of blades  108  (not shown in  FIG. 2 ) that drive hub  110  as a result of wind. In the illustrated configuration, 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 controlled 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 supported by a main bearing  130  and, at an opposite end of shaft  116 , to a gear box  118 . Gear box  118 , in some configurations, utilizes a dual path geometry to drive an enclosed high speed shaft. The high speed shaft (not shown in  FIG. 2 ) is used to drive generator  120 , which is mounted on main frame  132 . In some configurations, rotor torque is transmitted via coupling  122 . Generator  120  may be of any suitable type, for example, a wound rotor induction generator. 
     Yaw drive  124  and yaw deck  126  provide a yaw orientation system for wind turbine  100 . Wind vane  128  provides information for the yaw orientation system, including measured instantaneous wind direction and wind speed at the wind turbine. In some configurations, 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 , in the event of a loss of AC input power from a source  400  and to prevent turbine overspeed, blades are pitched using a MOSFET  402  based power converter  404 . Power converter  404  comprises part of a pitch control system  406 . (For notational convenience, as used herein, AC input source  400  refers to a rectifier bridge or an IGBT or MOSFET bridge. It is understood that this bridge is intended to be electrified by a generator, a power line, a power grid, or some other source of AC power, which may or may not comprise generator  120 .) A backup battery  408  is provided to allow pitching of blades  108  (shown in  FIG. 1 ) in the event of AC input power loss or a power dip. Battery  408  (which can comprise one or more electrical cells or a plurality of multicell batteries, or any combination thereof) is connected to DC link  410  of through a diode  412  and a fuse  414 . Under normal conditions, diode  412  is reverse biased and no current flow occurs from battery  408  to DC link  410 . In this condition, battery  408  is charged and its condition monitored, but it supplies no energy to DC link  410 . 
     When the DC link  410  voltage dips to below the voltage of battery  408 , current flows out of battery  408  through diode  412  and fuse  414  to maintain charge on DC link capacitor  416 . Diode  412  prevents uncontrolled charging of battery  408  when the DC link voltage is higher than the battery voltage. Fuse  414  prevents damage to battery  408  in the event of a short circuit on DC link  410 . Backup battery  408  of DC link  410  allows pitch control system  406  to maintain active control of blade  108  position throughout an AC power loss or dip event. 
     In some configurations of the present invention and referring to  FIGS. 5A and 5B , a multi-drive wind turbine pitch control system is provided. One non-regenerative source  400  of AC power can supply multiple pitch control systems  406  using a common DC link  510 . Common DC link  510  is supported by the use of a battery  408 . In some configurations, pitch control systems  406  on common DC link  510  swap real power during operation and reduce power demand on battery  408  in the event of an AC power outage or dip. Also, in some configurations and referring to  FIGS. 6A and 6B , pitch control systems  406  with a common DC link  510  supplied by a fully regenerative IGBT or MOSFET  602  controlled source  600 . Battery  408  is used in this configuration to support DC link  510  in the event of power outages. 
     It will thus be appreciated that configurations of the present invention are able to provide battery supported operation of pitch control motor drives, which increases the availability of the wind turbine by allowing operation through grid disturbances. 
     Configurations of the present invention are not limited to wind turbines having any specific number of blades. For example, turbines with one, two, or three blades (or more) can use configurations of the present invention to control blade angle in the event of an AC power loss, therefore increasing turbine availability over those turbines that do not have DC link pitch control capability. 
     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.