WO2007132303A1

WO2007132303A1 - Wind turbine system with ac servo motor rotor blade pitch control, using super-capacitor energy storage

Info

Publication number: WO2007132303A1
Application number: PCT/IB2007/000820
Authority: WO
Inventors: Kevin L. Cousineau; William Erdman; Peter Wahl
Original assignee: Clipper Windpower Technology, Inc.
Priority date: 2006-05-13
Filing date: 2007-03-26
Publication date: 2007-11-22

Abstract

A wind turbine rotor blade load control. A blade pitch servo amplifier powered by three-phase electrical power drives an AC Motor that changes the pitch of the rotor blade. A blade length servo amplifier powered by three-phase electrical power drives an AC Motor that changes the length of the rotor blade. A supercapacitor energy storage provides emergency backup power to the Pitch servo amplifier in the event of a loss of the three-phase electrical power. The supercapacitor energy storage provides sufficient power to shut down the wind turbine by feathering the turbine blades. Upon return of three-phase electrical power the supercapacitor is recharged and turbine is re -started.

Description

WIND TURBINE SYSTEM WITH AC SERVO MOTOR ROTOR BLADE PITCH CONTROL, USING SUPER-CAPACITOR ENERGY STORAGE

CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to co-pending US Provisional Patent Application Number 60/704,845, which was filed on August 1, 2005, titled "Variable Speed Wind Turbine System With Individual Blade Pitch Control" and US Patent Application Number 11/084,640, which was filed on March 18, 2005, titled "Servo-Controlled Extender Mechanism For Extendable Rotor

Blades For Power Generating Wind And Ocean Current Turbines" , both of which are assigned to Clipper Windpower Technology, Inc., the assignee of this application, and are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to fluid-flow turbines, such as wind turbines under water current turbines, and to other prime movers, and more particularly to variable speed turbines employing torque control , extendible rotor blades and rotor blade pitch control for turbine speed and load control .

DESCRIPTION OF THE PRIOR ART In the past, wind turbines have been operated at constant speed. The torque produced by blades and main shaft determines the power delivered by such a wind turbine. The turbine is typically controlled by a pitch command signal, which is fed to a turbine blade pitch angle servo, referred to herein as a Pitch Control Unit or PCU. This servo controls the pitch of the rotor blades and therefore the speed of the wind turbine rotor. A second signal is sent to a Generator Control Unit or GCU. This signal controls the torque of the turbine by commanding a specific current from the generator. A combination of speed regulation by the Pitch System in combination with Torque control of the generator results in power regulation of the output power of the wind turbine.

US Patent 6,726,439 of Mikhail, et al "Retractable Rotor Blades For Power Generating Wind And Ocean Current Turbines And Means For Operating Below Set Rotor Torque Limits" describes a rotor system in which a rotor control adjusts power capture and loading of the rotor through extension and retraction of a radius of sweep of the rotor blade to increase and decrease the cross-sectional area (disk of rotation) of fluid flow swept by the rotor blades.

The mechanical torque (or thrust) delivered by the rotor is controlled such that the torque (or thrust) is limited to below a threshold value. This enables an extended rotor blade configuration to operate within adjustable torque and thrust load limits.

To alleviate the problems of power surges and mechanical loads with constant speed wind turbines, the wind power industry has been moving towards the use of variable speed wind turbines. A variable speed wind turbine is described in US Patent Number 7,072,110, granted May 9, 2006, assigned to Clipper Windpower, Inc.

Many modern wind turbines employ electric motors to control the pitch angle of the blades. Turbines with two or three blades employ individual motors and servo control systems for each blade. In addition, batteries are used to provide a means of pitching the blades during periods when there is a loss of AC power. Further, many of these systems are designed as the primary means of stopping or slowing the rotor during a power outage, turbulent wind conditions or other pre-defined emergency conditions.

Operation during lightning conditions is a concern for such systems, especially since these systems are located inside the wind turbine hub, usually 50 to 300 feet or more above ground level and as such, become targets for electrical atmospheric discharge. Due to these concerns, DC motors are combined with batteries to assure proper operation of the pitch system, independent of the servo amplifier. The assumption is that the servo amplifier will fail due to lightning, but that the battery and DC motors are more robust in this regard and thus will continue to operate.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to an apparatus and method of controlling a wind turbine having a number of rotor blades. A servo amplifier powered by three-phase electrical power drives an AC Motor that changes the pitch of the rotor blade . A supercapacitor type energy storage provides emergency backup power to the servo amplifier in the event of a loss of the three-phase electrical power. The super-capacitor energy storage provides sufficient power to shut down the wind turbine by feathering the turbine blades . Upon return of three-phase electrical power, the supercapacitor is recharged and the turbine is re-started.

In accordance with an aspect of the invention, a blade length servo amplifier powered by three-phase electrical power drives an AC Motor that changes the length of the rotor blade. No energy storage is required for this system due to the slow operation speed of the blade extension and retraction. All emergency controls are handled through the pitch system, which will pitch the blades to their full feathered (90 Degree pitch angle) position and thus unload the rotor and slow the rotor to less than 1 RPM, preventing damage to the turbine. This can occur during any position of the extended blade as required by the safety system.

This invention describes a new approach to wind turbine electric pitch control systems, employing AC motors and energy storage provided by supercapacitors instead of DC motors and Lead Acid batteries for the storage. The use of an AC motor servo system is common to other industries, but not in the application of wind turbine blade pitch control due to safety issues surrounding lightning protection and power outage problems long associated with operation of modern wind turbines .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its mode of operation will be more fully understood from the following detailed description when taken with the appended drawings in which:

FIGURE 1 is a block diagram of a variable speed wind turbine in which the present invention is embodied;

FIGURE 2 is a block diagram of a prior art turbine blade pitch control;

FIGURE 3 is a perspective diagram of a turbine blade hub in which the in which the present invention is embodied; and,

FIGURE 4 is a block diagram of the turbine blade pitch control of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Refer to FIGURE 1, which is a block diagram of a variable- speed wind turbine apparatus in which the present invention is embodied. The basic components of the system are as follows. (1) A turbine drive train including a rotor hub-mounted pitch servo system 40, blade rotor including root blade 42 and extender blade 44, gearbox and a permanent magnet generator (PMG) 48, (2) generator rectifier/inverter unit 50; (3) a control system comprising a turbine control unit (TCU) ; generator control unit (GCU) 62, (4) a pad-mount transformer 52, and (5) SCADA interface 64 connecting the system to a utility grid.

The turbine comprises one or more rotor blades 42, 44 connected, via a rotor hub mounted pitch-angle servo, which is powered through slip rings via blade drive signal bus 74. The hub 40 is mechanically connected to a turbine main-shaft 46, which transmits the turbine's torque to a gearbox 48. There is a sensor for measuring turbine speed 54 on the low speed shaft, the output of which is the shaft speed 56. The turbine shaft is coupled via gearbox 48 and some suitable coupling device to, in this example, a permanent magnet or wound field synchronous generator. The generator electrical output is connected to block 50, which includes a rectifier, which converts the electrical power to DC voltage and current I (wind) on a DC bus. The DC bus is connected to wind turbine generator (WTG) inverter. The inverter regulates the DC current and by doing so, the generator torque is controlled. The inverter regulates this DC current by synchronizing to the grid and by supplying unity power factor current into the grid system. The control of the inverter (within block 50) is provided by a generator control unit (GCU) 62. The GCU takes inputs such as grid voltage, DC bus voltage, grid current load power demand I (demand in) from its own measurements and receives commands such as torque level from a Turbine Control unit (TCU) 60. The AC grid voltage measurement and current measurement are obtained from the output of block 50 and are used by the GCU for purposes of synchronizing the inverter to the AC grid. The converter takes all of its input voltage and current signals and converters these into pulse-width- modulated (PWM) signals, which tell a switch in the inverter 50 when to turn on and off. These switches are controlled in such a way as to maintain regulated AC output current in response to the current command supplied by the TCU. Line filters on the inverter output are used to reduce any harmonics that may have been generated by the inverter before passing power to a pad-mount transformer 52 on the utility grid.

As shown in the above-referenced patent 7, 042, 110 the TCU 60 and GCU 62 work together in a multiple generator system to stage the generators, when the turbine is operating at less than full power rating. The controller brings each generator of the plurality of synchronous generators in the turbine online sequentially in the event of low energy conditions of the source of energy (wind, water, etc.) to improve system efficiency at low power. The controller may optionally alternate the sequence in which the controller shifts the order in which the generators are brought online such that each generator receives substantially similar utilization. The TCU 60 receives sensor information provided by sensor inputs 58 such as turbine speed, blade pitch angle, tower acceleration (vibration) , nacelle acceleration (nacelle vibration) , wind speed, wind direction, wind turbulence, nacelle position, AC line parameters, DC bus voltage, generator voltage, power output, and other fault related sensors. The TCU 60 has control of the principle actuators on the turbine; the generators via the GCU 62, the pitch unit (PCU) 66 and the Blade Extension Control Unit (ECU) 68. The TCU 60 performs a complicated, coordinated control function for both of these elements, and does so in a way, which maximizes the energy capture of the turbine while minimizing the machine's mechanical loads. Finally, the TCU 60 also controls a yaw system, which works to keep the turbine always pointed into the wind. The TCU 60 is also in communication with the turbine's SCADA system 64 in order to provide and receive sensor and status information.

The Turbine Control Unit (TCU) sends the proper generator torque required as a signal to the Generator Control Unit (GCU) . This signal is based on the rotor speed and required torque at that speed, based on either a table or an algorithm. The converter modifies the torque command to help with gearbox damping, by employing notch filters within the torque command issued to the insulated gate bipolar transistor (IGBT) switches . In high winds the turbine remains at a constant average output power through a constant torque command from TCU and a constant speed command to the PCU.

The control system governs the variable rotor radius (via blade extension/retraction) , the pitch of the rotor blades, and the rotational rate of said rotor. The TCU 60 determines a pitch angle for the blades by means of an algorithm or lookup tables. A blade pitch command 70 is sent from the TCU 60 to the Blade Pitch Control Unit (PCU) 66 which generates blade rotation drive signals Dl, D2 , D3 , which pass over bus 74 to each of three servo motors that turn their respective blades.

The TCU 60 also determines the desired position of the extendable/ retractable blade extensions 44 by means of an algorithm or lookup tables . An extension command is 72 sent by the TCU 60 to the Blade Extension Control Unit (ECU) 68 which generates blade extension drive signals El, E2 , E3 , which pass over bus 74 to each of three servo motors that extend/retract their respective blade extensions.

Refer to FIGURE 3, which is a perspective diagram of a turbine blade hub in which the in which the present invention is embodied. The hub is used to attach three blades to the hub. The hub itself is attached to the slow speed input shaft of the turbine's gearbox. There are three pitch motor drives, one for each blade. Two pitch drives 72 of the three drives are illustrated in FIGURE 3. Each pitch motor drives a small gearbox connected to each individual blade bearing, through an inner gear arrangement for each blade, as shown. The system is capable of rotating the blade in pitch through 360 degrees, though most travel is limited to 90 degrees, from the operating pitch angle through the full-feathered pitch position of 90 degrees. Two limit switches 76 are used on each blade gear to assure that each limit position is reached, without over run, or even during failure of a single switch. Each blade has its own blade bearings 78, pitch motor drive gearbox72 and battery back-up unit 74. A single enclosure 70 houses the pitch control processor, which is a pitch control unit. A typical prior art wind turbine electric pitch system is shown in FIGURE 2.

With the Safety System powered (no safety issues with the turbine) , Relay Kl is in its Servo Position (down) and commands from the wind turbine controller are accepted into the PID control loop for blade pitch position control. The servo amplifier drives the DC motor into its proper position as measured by the motor's position sensor. Any error of this position is corrected until the motor resides at the proper position as required for turbine operations.

During a loss of power, the safety system relay Kl will drop out (return to up) and the pitch motors will begin operating using the battery supply. This causes the motors to pitch the turbine blades from their operating pitch position to a "feathered" or 90 degree position. In this position the turbine is "safe" as the RPM of the rotor is greatly reduced to less than ^XA RPM under even the highest of wind conditions. Once the fault is cleared, or power is returned, the safety loop may be reset and operation of the Servo Amplifier can began again, allowing the turbine controller to command the blade to the required positions for proper operation of the turbine . The system described is highly immune to lightning discharge, due to the nature of the DC motor and their associated batteries. Usually these are battery systems with voltages from 108 to 216 VDC and controlled by a simple contactor (Kl) , whereas the servo amplifier is usually built using low voltage, solid state components, more easily damaged by high voltage lightning discharges.

Although reliable, the nature of DC motors and batteries have caused problems with these conventional pitch systems. First, the batteries are a source of maintenance requirements. In some cases batteries only last 1 or 2 years, and the cost associated with their replacement includes turbine down time during this period and a loss of electrical production by the wind turbine itself. Second, as turbine grow in size and complexity, the requirements of the pitch system grow also, including torque, acceleration and pitch rate. Today, most DC motor pitch systems have reached their torque, acceleration and pitch rate limits, yet the demands on these systems continue to rise.

The battery systems, ranging in voltages from about 100 to over 300, require many cells in series. Each connection is prone to problems as are the effects of charging such long strings of cells. Some manufacturers charge only small groups of cells, but in all cases, temperature compensation, battery- voltage monitoring and state-of-charge indication is required for operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Large, multi-blade, multi-megawatt , wind turbines introduce new requirements for the pitch system. Individual blade pitch systems become necessary as the blade diameter grows to help prevent problems associated with differences in wind velocity across such a large radius during operation.

Active pitch control, to reduce tower loads, are also required for many applications. These requirements, together with the increased mass and size of the blades drive the pitch systems toward high performance AC motors instead of DC motors . In addition to increased torque, speed and acceleration, servo driven AC motors are easily available as "Commercial Off The Shelf" products whereas DC motors for turbine applications are all custom designed and built.

Energy storage requirements increase nearly directly with the increased requirements for the pitch system. Larger batteries mean more weight, more cells, and more maintenance problems. Supercapacitors , now relatively common, are capable of providing the quantity of energy storage needed during power outages for pitching the blades, and can do so with higher reliability than batteries. Supercapacitors are capable being charged and discharged over 500,000 cycles compared to only about 1,000 cycles for the best lead acid batteries. Supercapacitors are easy to charge, require no temperature compensation, and operate down to minus 40 degrees without a large change in their capacity.

In accordance with the present invention, a new technology wind turbine pitch system, employing an AC motor and AC motor servo amplifier (drive) along with Supercapacitor energy storage is shown in FIGURE 4. The logic of FIGURE 4 is located in the Blade Pitch Control Unit 66 shown in FIGURE 1. Similar logic, with the exception of the supercapacitor storage, is located in the Blade Extension Control Unit 68 shown in FIGURE 1. The blade extension system is a slowly moving system and does not require back up power during a power outage .

As with the DC motor pitch system, a single electric motor is used to drive the blade pitch gearbox which in turn drives the blade pitch bearing and moves the blade into its required position. Also like the DC motor pitch system, a motor mounted, multi-turn, absolute position encoder is used to determine the actual pitch angle of the blade. Three-phase AC power is obtained from the turbine nacelle through a set of slip rings to the rotating hub.

The three-phase power from the slip-ring assembly is coupled to charge an energy storage device, which supplies power to the servo amplifier under the control of the Pitch Control Processor. The energy storage device may be a storage device made up of electrochemical double layer capacitors, known as supercapacitors, ultracapacitors, or hybrid energy storage devices, for example a combination of a battery and a super capacitor. The energy storage device is preferably a supercapacitor because of its tolerance of extreme temperatures. An advanced battery system such as a lithium-thionyl-chloride system may be used because of its long life and its ability to function in very extreme temperatures. The power from the slip-ring assembly is used to charge the supercapacitor, which is used as a backup for blade pitch control functions in the event of a loss of power. Once the power returns the supercapacitor may be switched out .

The level of capacitor charge is directly proportional to its voltage and is monitored by the Pitch Processor Control via the DC Bus Voltage Sense input line. The Pitch Processor Control regulates the charging system by switching between "charge" and "operate" during initial application of power to the three phase, switched, rectifier system. The Pitch Processor allows a second method of battery charging, through the Regeneration Resistor control line. This is usually turned on when the servo motor is being driven into a generator mode by the weight or counter balance of the blade during certain pitch activity.

The three-phase AC power is rectified by a three-phase bridge rectifier D1-D6. Three of the six rectifiers (Dl, D3 and D5) are Silicon Controlled Rectifers (SCR' s) used to control the charge current for the Supper Capacitor energy storage bank connected in parallel with the Servo Amplifier DC bus. Since the supercapacitors represent a short circuit, when discharged, a current limit circuit is required during charging. The SCR' s along with DC current measurement (Current Sensor) is used in a standard "off the shelf" arrangement of error amplifier and gate driver Ga, Gb, Gc, boards to control this charging sequence. For a standard application the charge current would be set 50 or 75 amperes. Charge time would depend upon the size of the capacitor bank. With a bank of 26 Farad and a charge current of 75 amperes the charge time to 250 volts will be less than 2 minutes.

Standard DC bus capacitors are also used for high frequency bypass and high-speed current control requirements of the servo amplifier due to their lower Internal Resistance and location on top of the DC bus, where the Supercapacitors are located in an external enclosure, with one enclosure mounted at each blade root, as shown in FIGURE 3.

Overall system control is provided by the Pitch Control Processor, an embedded or PLC type control system used to control the Supercapacitor charging circuit, regeneration control (R-gen Control) of the AC Motor, overall motor position control (Motro Angle) , and Servo Motor command and alarm monitoring. In addition the Pitch Control Processor communicates with the Turbine Control Unit (Turbine Controller to Pitch Controller Communication) via the Slip Ring Communications Interface to obtain the proper pitch angle required for operation and to communicate all motor and controller status and alarm states. For safety purposes the Servo Amplifier can be turned on and used to operate the AC motor, directly from the Safety Loop Interface line, without input from the Pitch Control Processor. This meets basic safety requirements for design certification under IEC rules for wind turbines, because the Pitch Control Processor is not part of this safety loop. Similarly, if the Pitch Processor fails, a watch dog timer output will open the safety loop and by doing so the motor will be driven by the servo amplifier, using the energy stored in the DC bus by the supercapacitors, to move the blades to their emergency feathered or 90 degree pitch position.

Blade Pitch Control Operation

Assuming that the safety loop is "closed" and that there is no need for an "emergency feather" command, the Pitch Control Processor will communicate with the Turbine Control Unit processor indicating the absence of alarms and the closure of its safety system. If power has just been applied to the turbine, the Pitch Control Processor will communicate that its DC bus is charging and will not release the servo amplifier for operation via the Mode Control line until these capacitors are charged. Once charged, the Turbine Control Unit processor will command a pitch setting that allows the blades of the turbine to pitch to an angle that will enable the turbine to rotate. Depending upon the actual turbine this may be an intermediate value of, for example, 40 degrees. This is normally accomplished at a relatively slow pitch rate of 1 or 2 degrees per second.

Once at 40 degrees, the turbine will be allowed to stay at this pitch angle until reaching a specified speed. A value of 300 RPM is common. Once exceeding this value the turbine is pitched to its operating pitch set point of somewhere near zero degrees. At this point the rotor will begin to accelerate and the turbine will begin to generate power when proper speed is achieved.

During high winds, the turbine unit will command a pitch set point that is less than zero degrees in order to control speed or power output of the turbine. This is usually a speed regulation system and usually operates at a 20 or 30 Hz rate. The blades will be given new pitch position and pitch velocity- commands 20 or 30 times per second in order to maintain the proper speed of the rotor.

If the turbine faults for a standard condition, the turbine control system will initiate a "normal" shut down. This means that it will command the Pitch Processor to continuously pitch the blades from their current operation position toward their feathered 90-degree position at a slow 1 to 4 degrees per second. At some point the generator will be disconnected from the grid and once the turbine reaches its 90-degree position it will be only rotating at a very slow (less than 1 RPM) speed at the hub.

If the turbine faults for an emergency condition (such as an operator pressing an emergency stop button) the Safety Loop Interface line will force the servo amplifier to operate the pitch motor and ignore commands from the Pitch Control Processor. The servo amplifier will pitch the motor at a higher rate than the normal shut down or about 7.5 degrees per second all the way until the motor is stopped by the limit switches 76, mounted on the hub (See FIGURE 3) at the 90- degree position. Two limit switches are used on each blade to assure that this position is reached, without over run, or even during failure of a single switch.

This system is superior to the conventional DC motor/Lead Acid battery pitch system used on most wind turbines today. The use of supercapacitors, and an AC servo system and motors results in a simpler, more reliable system with high capabilities for torque and pitch rate and acceleration. To prevent problems associated with lightning damage, the servo amplifier is shielded within its own steel enclosure, with all of its input and output wires shielded along with wires going to the motor. In addition, the steel hub of the wind turbine acts like a faraday cage to help prevent damage from nearby lightning due to Electro Magnetic Pulse or induced voltages from the field of the lightning bolt.

The AC servo amplifier is a higher voltage device as compared to the Pitch Control Processor, giving it more protection during lightning discharges. Overall, adequate protection can be provided, and with the safety system driving the servo system directly, even greater protection than a standard AC motor drive system is offered.

Calculation of Energy Storage Requirements Most modern wind turbine employ motors rated at 2.5 to 10 kW. At 250 VDC a 10 kW motor would be expect to draw 40 amperes continuous. The peak draw will be greater and the actual draw during emergency operation will also be somewhat greater. For energy storage calculations, assume a 50 ampere continuous current draw during the time the turbine is required to feather from 0 to 90 degrees. The pitch rate associated with emergency feathering is usually anywhere from 2.5 to 10 degrees per second depending upon the type of turbine. Assume a pitch rate of 7 degrees per second for the calculations .

Today there are several manufacturers of high performance supercapacitors that can provide capacity greater than 2,500 farads at 2.3 to 2.7 volts each. Working with a standard 2,600 Farad, 2.7-volt capacitor, and placing 100 of these in series results in a capacitor with a value of:

Total Capacitance = Individual Capacitance/# of Capacitors in Series. This equals 26 farad with a voltage rating of 2.7 volt and a peak voltage rating of 300 volts DC.

The time available at a constant current draw of 50 amperes is dependent upon how far the capacitors can be discharged. When operating with these types of systems it is apparent that a voltage drop from 250 to 150 would be acceptable while still providing a reasonable pitch rate performance. Fortunately if the pitch rate varies, the highest pitch rate will occur during start of the emergency feather, when it is most needed, and as the blades pitch farther back toward their feather position, the rate will probably slow down. Assume that the overall, average, integrated pitch rate will be about 5 degrees per second or that the turbine will require 18 seconds to pitch from zero to 90 degrees.

Based on a 100 volt drop in DC bus voltage, the time available during that drop will be: t = Capacitance X Delta V/Current = 26 x 100/50 = 52 seconds .

Clearly the required energy storage for a 26-farad capacitor is more than sufficient to assure proper emergency feather of the turbine blades. Although a smaller capacitor may be used, the change in capacitance due to aging and temperature effects must be considered. These can account for as much as a 30% change in capacitance. Under those worst-case conditions the 26-Farad capacitor will only act like an 18- Farad capacitor and the total time available for discharge over a 100 volt delta will be: t = 18 X 100/50 = 36 seconds.

This is still twice the time needed, so clearly a smaller capacitor may be used in this application.

These calculations are used to show how one would size a supercapacitor storage bank for operation with this type of system.

Blade Extension Control Operation

The logic of FIGURE 4 is located in the Blade Pitch Control Unit 66 shown in FIGURE 1. Similar logic is located in the Blade Extension Control Unit 68 shown in FIGURE 1, except that there is no need for emergency operation of this extension system, and therefore no need for the super capacitor energy storage system for its control during emergency shut-down conditions.

The Blade Extension Control Unit 68 (FIGURE 1) will communicate with the Turbine Control Unit (TCU) 60 indicating the absence of alarms and the closure of its safety system. If power has just been applied to the turbine, the Blade Extension Control Unit will communicate that it is ready to accept commands from the TCU, as long as no faults are present. During high winds, the Turbine Control Unit will command that the blades be retracted.

If the turbine faults for a standard condition, the turbine control unit will initiate a "normal" shut down. The blades will be retracted and the TCU will command the Pitch Processor to continuously pitch the blades from their current operation position toward their feathered 90-degree position at a slow 1 to 4 degrees per second. At some point the generator will be disconnected from the grid and once the turbine reaches its 90-degree position it will be only- rotating at a very slow (less than 1 RPM) speed at the hub.

If the turbine faults for an emergency condition (such as an operator pressing an emergency stop button) the Safety Loop Interface line will force the servo amplifier to operate the pitch motor. If the TCU is still active during this emergency, the TCU will command the extender servo to retract the blade, simultaneously. The servo amplifier will pitch the motor at a higher rate than the normal shut down or about 7.5 degrees per second all the way until the motor is stopped by the limit switches, mounted on the hub (See FIGURE 3) at the 90-degree position. Two limit switches are used on each blade to assure that this position is reached, without over run, or even during failure of a single switch. The blade pitch extender will operate at a fixed rate at all times, for normal operation and emergency operation, if the TCU is still active. If the TCU is inactive, then the extender will remain in its final position at the time of power failure.

It will be understood by those skilled in the art that for systems that have both blade pitch control and blade extension control, some common circuitry can be combined for use by both controls. For example, a common charging circuit, a common supercapacitor storage, a single control processor for both pitch control and blade extension control, and other common circuitry. It will also be understood by those skilled in the art that for systems that have both blade pitch control and blade extension control, back-up power may be provided for both control systems . In the event of AC power loss or other emergency situations, back-up power should be used to feather the blades to immediately shut down the turbine as described in this specification. A blade length servo amplifier powered by three-phase electrical power that drives an AC Motor can be used to change the length of the rotor blade by extending or retracting the blade extension. Separate or common supercapacitor energy storage can provide backup power to blade extension servo amplifiers in the event of a loss of the three-phase electrical power. The supercapacitor energy storage provides sufficient power to immediately shut down the wind turbine by feathering the blades by changing the pitch. Subsequently, the turbine blade length can then be reduced without AC power by retracting the blade extension to protect the blade extension from high wind turbulence or lightening strikes .

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the scope of the invention.

Claims

C L A I M S

1. An apparatus for controlling a wind turbine having at least one rotor blade comprising: an AC motor coupled to the blade to change the pitch of said rotor blade; a servo amplifier powered by rectified three-phase electrical grid power, the servo aplifier supplying three phase AC power that drives said AC motor; and, a supercapacitor type energy storage for providing emergency backup power to said servo amplifier in the event of a loss of the three-phase electrical grid power.

2. The apparatus of claim 1 wherein said super-capacitor energy storage is designed to provide sufficient power to shut down the wind turbine by feathering said rotor blade.

3. The apparatus of claim 1 or 2 wherein said super- capacitor is connected to a rectifier assembly which rectifies three-phase electrical grid power for charging the supercapacitor .

4. The apparatus of claim 3 wherein said three-phase electrical grid power is provided by a slip-ring assembly coupled to a rotatable hub, the hub being coupled to the rotor blade.

5. The apparatur of claim 3 wherein said rectifier assembly is coupled both for charging the supercapacitor and for powering the servo amplifier.

6. The apparatus of one of claims 1 to 5 wherein upon return of three-phase electrical power, the supercapacitor is recharged and the turbine is re-started.

7. The apparatus of claim 1 wherein said super-capacitor energy storage is designed to provide both sufficient power to shut down the wind turbine by feathering said rotor blade as well as excess power to power the servo amplifier during short-term grid power interruptions.

8. The apparatus of claim 7 wherein a circuit monitors the charge or the super-capacitor and controls the servo amplifier to remain in operating mode during grid power interruptions as long as the super capacitor does not discharge below a charge reserve threshold required to feather the rotor blade.

9. The apparatus of claim 2 wherein a circuit monitors the charge or the super-capacitor and controls the servo amplifier to shut down by feathering the rotor blade if the charge of the super-capacitor drops below a charge reserve threshold.