Wind turbine load mitigation

A wind turbine blade assembly comprises a rotatable hub, a blade secured to the hub, and a pitch system. The pitch system is disposed to rotate the blade with respect to the hub about a pitch axis not extending through an aerodynamic centroid of the blade. The pitch system comprises a hydraulic cylinder pitch actuator and a relief valve. The hydraulic cylinder pitch actuator has first and second pressure chambers. The relief valve is configured to aerodynamically unload the blade by equilibrating the first and second pressure chambers in response to a pressure differential between the first and second pressure chambers exceeding a critical threshold corresponding to a pre-strain blade twist.

BACKGROUND

The present disclosure relates generally to wind turbines, and more particularly to a load mitigation system for wind turbine blades.

Modern wind turbines typically comprise a plurality of flexible blades extending from a rotor hub. Three-bladed hubs are particularly common among large scale wind turbines. Wind incident on turbine blades rotates the hub with respect to a generator nacelle mounted atop a tower. This nacelle rides a bearing track or rail atop the tower, allowing the nacelle to be yawed so that the hub and blades face incoming wind. Similarly, each blade is mounted to the hub via a bearing assembly that provides the freedom of rotational movement necessary for each blade to be pitched to regulate aerodynamic load and energy capture from the wind. The blades of large wind turbines are usually pitched using hydraulic actuators, although non-hydraulic actuators are fairly common for smaller turbines. Some wind turbines are capable of separately pitching each blade to a different angle, while others pitch all blades identically. Blade pitching is used to avoid premature damage or wear to wind turbine components, and to control wind turbine generator speeds and torques. In particular, blades are commonly pitched to feather for aerodynamic unloading both during prolonged periods of dangerously high wind speeds, and during rapid gusts.

Several methods exist for controlling wind turbine generator speeds and power absorption by pitching wind turbine blades to feather in response to transient gusts. Some wind turbines sense rotor torque, and pitch blades for aerodynamically unloading in response to high torque events. Other wind turbines control blade pitch based on rotor speed, or based on strain sensed via strain gauges in each (or at least one) blade. All of these mechanisms experience a time lag between the onset of a heavy blade load (e.g. from a rapid gust) and subsequent pitch-unloading in response to the heavy blade load. Some wind turbines use twist-bend coupled flexible blades that passively twist to alleviate aerodynamic load as blades deflect under high wind speeds.

SUMMARY

According to one embodiment of the present invention, a wind turbine blade assembly comprises a rotatable hub, a blade secured to the hub, and a pitch system. The pitch system is disposed to rotate the blade with respect to the hub about a pitch axis not extending through an aerodynamic centroid of the blade. The pitch system comprises a hydraulic cylinder pitch actuator and a relief valve. The hydraulic cylinder pitch actuator has first and second pressure chambers. The relief valve is configured to aerodynamically unload the blade by equilibrating the first and second pressure chambers in response to a pressure differential between the first and second pressure chambers exceeding a critical threshold corresponding to a pre-strain blade twist.

According to a second embodiment of the present invention, a method for mitigating aerodynamic loads on a wind turbine blade comprises mounting the blade rotatably to a hub about a pitch axis that does not pass through an aerodynamic centroid of the blade. The blade is pitched by controlling differential pressure between first and second pressure chambers of a hydraulic cylinder extending from the hub to the blade. Pressure between the first and second pressure chambers is equilibrated in response to the differential pressure exceeding a critical threshold corresponding to a pre-strain blade twist.

DETAILED DESCRIPTION

FIG. 1is a front view of wind turbine10, which comprises tower12, nacelle14, yaw bearing assembly16, hub18, blades20, and pitch bearing assemblies22. Wind turbine10is a power generating wind turbine that converts the mechanical energy of wind incident on blades20into electrical current. Tower12is a semi-rigid, elongated structure configured to elevate nacelle14. Nacelle14is a protective housing that encloses an electrical generator (not shown) and associated hardware for generating and conditioning electrical power from rotation of hub18. Nacelle14rides tower12via yaw bearing assembly16, which enables nacelle14to rotate 360° atop tower12. Yaw bearing assembly16may, for instance, comprise a system of rails, tracks, or roller bearings providing nacelle14with yaw freedom atop tower12. Nacelle14can, for instance, be pitched at any angle along yaw bearings16to face incident wind. Wind turbine10may include wind speed and direction monitoring sensors (not shown) to ascertain the most advantageous yaw facing of nacelle14, as well as yaw actuators (not shown) to rotate nacelle14into position. Nacelle14may include both rigid structural supports (formed, e.g., of cast steel) and lightweight protective housing components (formed, e.g., of fiberglass and/or sheet metal).

Nacelle14carries hub18, a central support that anchors blades20. Although hub18is depicted with three blades20, configurations with two blades, or with four or more blades, are also possible. Wind incident on blades20rotates hub18with respect to nacelle14, driving the electrical generator therein. Blades20are elongated airfoil elements with aerodynamic centroids located aft of respective pitch axes Ap(discussed below). Blades20may, for instance, be composite airfoils formed primarily of fiberglass. Some embodiments of blades20may be hollow, and some embodiments of blades20may be formed from a plurality of overlapping laminated layers.

Each blade20is pitched on pitch bearing assembly22about pitch axis Apas described in greater detail below with respect toFIG. 2. Blade20can be pitched to reduce rotation speed of hub18and/or aerodynamic load on blades20, thereby limiting or avoiding wear or damage to wind turbine10. Pitch bearing assembly22may, for instance, comprise a plurality of cylindrical or tapered roller bearings, ball bearings, or abutting tracks or rails configured to allow blade20to rotate about pitch axis Ap. In the depicted embodiment, a neutral bending axis Anis angled at offset angle Θ relative to pitch axis Ap, such that the aerodynamic centroid of each blade20is offset from its pitch axis Ap, causing direct facing aerodynamic loads on blades20(e.g. from wind into the page, inFIG. 1) to exert a pitch torque on blades20. More generally, blade20may take any appropriate shape such that the aerodynamic centroid of blade20lies aft of pitch axis Ap. As described in greater detail below, resulting pitch torque is used to trigger feathering of blades20to mitigate transient aerodynamic loads, thereby avoiding turbine overspeed conditions and/or excessive mechanical strains along blades20.

FIG. 2is a mixed schematic and simplified cross-sectional view of wind turbine10through section line2-2ofFIG. 1.FIG. 2illustrates nacelle14, hub18, blades20, pitch bearing assemblies22, rotor24, hydraulic cylinder26, pins28and30, first pressure chamber32, piston33, second pressure chamber34, first pressure line36, second pressure line38, high pressure supply40, pitch control valve42, supply connection44, pitch controller46, relief valve48, pressure sensor50, and relief controller52.FIG. 2depicts two broken-away blades20as shown inFIG. 1, as well as a third blade shown in phantom, extending out of the page.

As discussed above with respect toFIG. 1, hub18is a rotating support structure that carries blades20, and attaches rotatably to nacelle14. Rotor24is a rotating component such as a shaft or hub situated at least partially within nacelle14. Rotor24may, for instance, be a permanent magnet generator rotor, or a driveshaft leading to a permanent magnet generator rotor or gearbox. Hub18supports each blade20via a corresponding pitch bearing assembly22disposed to allow blade20to rotate about pitch axis Ap. Hydraulic cylinder26is secured to blade20via pin28at a location off of pitch axis Ap, and to hub18via pin30. Pins28and30may, for instance, be trunnion pins disposed to allow hydraulic cylinder26to rotate with respect to hub18as the pitch of blade20changes. Hydraulic cylinder26is a bi-directional linear hydraulic motor with first and second pressure chambers32and34separated by piston33. Hydraulic cylinder26may, for instance, use oil as a hydraulic pressure fluid.

Blade20is pitched by varying fluid pressures within first pressure chamber32and second pressure chamber34. Fluid pressure in first and second pressure chambers32and34drives piston33to an equilibrium position determined by hydraulic pressure in each pressure chamber and the twisting moments applied to the blade as a result of airflow. This position of piston33determines the overall extension of hydraulic cylinder26, and thereby the pitch angle of blade20. Higher pressure in first pressure chamber32than in second pressure chamber34draws in piston33, exerting a torque on blade20that pitches blade20to feather. Conversely, higher pressure in second pressure chamber34than in first pressure chamber32drives piston33out, pitching blade20into the wind.

First pressure chamber32of hydraulic cylinder26receives hydraulic pressure fluid from first pressure line36, while second pressure chamber34receives hydraulic pressure fluid from second pressure line38. Hydraulic pressure fluid is ultimately provided by high pressure supply40, a pressurized fluid reservoir, but is split between first and second pressure lines36and38by pitch control valve42. Pitch control valve42receives and returns hydraulic pressure fluid from high pressure supply40through supply connection44, which may for instance be a double fluid line with a supply line and a return line connecting pitch control valve42to high pressure supply40. Pitch control valve42regulates pressure to both first and second pressure lines36and38, and can, for instance, be a dual aperture valve that controls relative pressure by separately varying orifice size of apertures to each pressure line, thereby varying corresponding permanent pressure drop from supply connection44to first and second pressure lines36and38. In other embodiments, pitch control valve42may be a split valve with a fixed total output aperture size configurably split between first pressure line36and second pressure line38. Although pitch control valve42is depicted as a single valve, some embodiments of wind turbine10may comprise a plurality of separate pitch control valves42, such as one pitch control valve each for first pressure line36and second pressure line38.

Pitch control valve42is actuated according to instructions from pitch controller46. Pitch controller46may, for instance, be a logic-capable device or subcomponent of a logic-capable device such as a microcomputer or microprocessor configured to control wind turbine10so as to handle varying wind speeds, power demand levels, generator conditions, and other factors. Pitch controller46may also be responsive to direct user input, e.g. to shut down wind turbine10by fully feathering blades20.

Transient loads from gusts are mitigated by relief valve48, independently of pitch control valve42. Relief valve48is a switch valve capable of switching between a closed normal operation state Vnwherein first and second pressure lines36and38are disconnected, and an open relief state Vrwherein first and second pressure lines36and38are fluidly connected. In some embodiments, the relief state of relief valve48may also interrupt first and second fluid lines36and38to fluidly disconnect first and second pressure chambers32and34from pitch control valve42. While relief valve48is in normal operation state Vn, the pitch of blade20is controlled by pitch control valve42in response to pitch controller46, as discussed above. While relief valve48is in relief state Vr, pressure rapidly equilibrates between first and second pressure chambers32and34, pitching blade20to feather as a result of the applied aerodynamic loads and in a direction to aerodynamically unload the blade20.

As discussed above, the aerodynamic centroid of blade20is offset from pitch axis Ap, such that aerodynamic loads on blade20place a pitch torque τ on blade20. Pitch torque τ tends to drive blade20towards feather under transient aerodynamic loads from gusting winds, reducing pressure in first pressure chamber32and increasing pressure in second pressure chamber34. Relief valve48ordinarily operates in normal operation state Vn, switching to relief state Vronly when differential pressure ΔP between first and second pressure chambers32and34(and correspondingly between first and second pressure lines36and38) exceeds a critical threshold ΔPTcorresponding to a threshold pitch torque τT. Threshold pitch torque τTis a safe torque threshold below which blade20is unlikely to experience damaging strain. When subjected to heavy winds, blade20takes time to deflect under aerodynamic loads to the point of strain. By reactively equilibrating pressures in first and second pressure chambers32and34in response to any differential pressure ΔP corresponding to pitch torque τ>τT, relief valve48is able to mitigate sudden aerodynamic loads before damaging strain can occur. Particular values of τTand ΔPTmay vary depending on the construction and materials of blade20.

In the depicted embodiment, differential pressure ΔP is sensed by pressure sensor50. Pressure sensor50may, for instance, be an array of two or more absolute pressure electronic sensors disposed along pressure lines36and38, or a direct differential pressure electronic sensor such as a double diaphragm seal sensor. Also in the depicted embodiment, relief controller52controls relief valve48according to a sensor output of pressure sensor50, switching relief valve48from normal operation state Vnto relief state Vrwhenever differential pressure ΔP>ΔPT. Relief controller52may, for instance, be a microcomputer or microprocessor-based logic-capable device. In some embodiments, relief controller52may evaluate sensed pressure based on a critical threshold ΔPTthat varies as a function of pitch control parameters received from pitch controller46, or supplied to both pitch controller46and relief controller52. Alternatively, relief valve48may be actuated directly in response to a sensor output of pressure sensor50. In still other embodiments of wind turbine10, relief valve48may be a passively pressure-responsive mechanical valve such as a spring valve that opens automatically whenever pressure in second pressure line38exceeds pressure in first pressure line36by at least ΔPT. In various embodiments, relief valve48may return to normal operation state Vnas soon as differential pressure ΔP drops below critical threshold ΔPT, or may not return to normal operation state Vnuntil differential pressure ΔP drops below a separate return threshold ΔPR<ΔPT.

Relief valve48allows blade20to rapidly pitch to mitigate aerodynamic loads from sudden gusts, thereby unloading blade20before significant strain can occur. Relief valve48thus reduces wear and risk of damage to blade20and other components of wind turbine10. Relief valve48can operate entirely independently of pitch control valve42and pitch controller46, alleviating dangerous aerodynamic loads without any change in the pitch settings determined by pitch control valve42and pitch controller46. Relief valve48is thus a modular component than can be added to existing hydraulic blade pitch systems to provide load mitigation without interacting with existing pitch control components. Alternatively, relief valve48and pitch control valve52can operate together to provide redundant, fail-safe load mitigation via through pitch control valve on top of the faster load shedding provided by relief valve48.

A wind turbine blade assembly comprising a rotatable hub, a blade secured to the hub, and a pitch system. The blade has an aerodynamic centroid. The pitch system is disposed to rotate the blade with respect to the hub about a pitch axis not extending through the aerodynamic centroid. The pitch system comprises: a hydraulic cylinder pitch actuator with first and second pressure chambers; and a relief valve configured to aerodynamically unload the blade by equilibrating the first and second pressure chambers in response to a pressure differential between the first and second pressure chambers exceeding a critical threshold corresponding to pre-strain blade twist.

The pitch system further comprises: a pressure source supplying pressure a pressurized fluid to the hydraulic cylinder pitch actuator; first and second pressure lines carrying the pressurized fluid from the pressure source to the first and second pressure chambers, respectively; and a control valve configured to pitch the blade by varying the pressure differential.

Equilibrating the first and second pressure chambers comprises connecting the first and second pressure lines.

Equilibrating the first and second pressure chambers further comprises interrupting the first and second pressure lines.

The relief valve passively equilibrates the first and second pressure chambers in response to the pressure differential exceeding the critical threshold.

The relief valve is a passive mechanical spring valve.

A pressure sensor disposed to sense the differential pressure, wherein the relief valve equilibrates the first and second pressure chambers in response to the sensed differential pressure exceeding the critical threshold.

A logic-capable relief controller configured to control the relief valve based on the sensed differential pressure.

The relief valve ceases to equilibrate the first and second pressure chambers in response to the sensed differential pressure falling below the critical threshold.

The relief valve ceases to equilibrate the first and second pressure chambers in response to the sensed differential pressure falling below a return threshold less than the critical threshold.

A method for mitigating aerodynamic loads on a wind turbine blade comprises: mounting the blade rotatably to a hub about a pitch axis that does not pass through an aerodynamic centroid of the blade; pitching the blade by controlling differential pressure between first and second pressure chambers of a hydraulic cylinder extending from the hub to the blade; and equilibrating pressure between the first and second pressure chambers in response to the differential pressure exceeding a critical threshold corresponding to a pre-strain blade twist.

Equilibrating pressure between the first and second pressure chambers comprises switching a relief valve state to fluidly connect the first and second pressure chambers.

The relief valve is configured to passively connect the first and second pressure chambers in response to the differential pressure exceeding the critical threshold.

The differential pressure is sensed with an electronic pressure sensor, and the relief valve is actuated based on the sensed differential pressure.

Actuating the relief valve based on the sensed differential pressure comprises opening the relief valve to equilibrate pressure between the first and second pressure chambers whenever the sensed differential pressure exceeds the critical threshold.

Actuating the relief valve based on the sensed differential pressure further comprises closing the relief valve to equilibrate pressure between the first and second pressure chambers whenever the sensed differential pressure drops below the critical threshold.

Actuating the relief valve based on the sensed differential pressure further comprises closing the relief valve to equilibrate pressure between the first and second pressure chambers whenever the sensed differential pressure drops below a return threshold below the critical threshold.