Variable rated speed control in partial load operation of a wind turbine

A method for operating a wind turbine during partial load operation includes determining a power output of the wind turbine. The method also includes determining whether the power output is below a rated power of the wind turbine. If the power output is at the rated power, the method includes maintaining a speed set point of the wind turbine equal to a rated speed set point. However, if the power output is below the rated power, then the method includes varying, via a controller, the speed set point of the wind turbine as a function of a torque of the wind turbine in a non-monotonic torque-speed relationship.

FIELD

The present invention relates generally to wind turbines, and more particularly, to systems and methods for controlling a wind turbine in partial load operation using a variable rated speed set point.

BACKGROUND

Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and a blade root of the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.

At low wind speeds, there is insufficient torque exerted by the wind on the rotor blades to make them rotate. However, as the wind speed increases, the rotor of the wind turbine begins to rotate and generate electrical power. The wind speed at which the wind turbine first starts to rotate and generate power is generally referred to as the cut-in wind speed. As the wind speed rises above the cut-in wind speed, the level of electrical power rises rapidly until the power output reaches the limit that the electrical generator of the wind turbine is capable of, which is generally referred to as the rated power output. Similarly, the wind speed at which the rated power is reached is generally referred to as the rated wind speed. At wind speeds above the rated wind speed, the wind turbine is designed to limit the power output to the rated power. To avoid damage to the wind turbine, a braking system is typically employed when the wind speed reaches a cut-out wind speed. Thus, for conventional operation, the rated wind speed is a constant value. In other words, when the rotor reaches the rated power from an increase in wind speed, it maintains that value as winds continue to increase.

Typically, the wind turbine operates such that it reaches a rated rotor speed at a wind speed at or below the rated wind speed. In the upper partial load region of operation, defined on a torque-speed curve as the portion at rated speed and increasing torque to rated power, the wind turbine experiences lower performance due to operating away from its optimal tip speed ratio (TSR). Such operation introduces the potential for reduced aerodynamic efficiency and the need to mitigate that potential. Increasing the rotor speed allows the wind turbine to maintain optimum TSR operation up to a higher wind speed; however, the system is electrically, mechanically, and/or thermally limited such that it cannot maintain the higher generator speed at rated power levels.

Accordingly, a system and method that addresses the aforementioned problems would be welcomed in the technology. For example, a system and method that incorporates a variable rated speed set point in partial load operation of the wind turbine would be advantageous.

BRIEF DESCRIPTION

In one aspect, the present subject matter is directed to a method for operating a wind turbine during partial load operation. The method includes determining a power output of the wind turbine. The method also includes determining whether the power output is below a rated power of the wind turbine. If the power output is at the rated power, the method includes maintaining a speed set point of the wind turbine equal to a rated speed set point. However, if the power output is below the rated power and an operational space exists below system constraints of the wind turbine, then the method includes varying, via the controller, the speed of the wind turbine based on a non-monotonic torque-speed relationship.

In another embodiment, the step of varying the speed of the wind turbine based on the non-monotonic torque-speed relationship may include operating to an increased speed set point of the wind turbine above the rated speed set point at rated power until at least one of the system constraints are reached. Further, the step of varying the speed of the wind turbine based on the non-monotonic torque-speed relationship may include decreasing the speed of the wind turbine as a function of the torque after reaching the increased speed set point of the wind turbine above the rated speed set point until rated power is reached. More specifically, in such embodiments, the method may include decreasing the speed of the wind turbine back to the rated speed set point after at least one of the system constraints are reached.

In further embodiments, the method may include dynamically calculating the increased speed set point as a function of the torque or power of the wind turbine. More specifically, in certain embodiments, the step of dynamically calculating the increased speed set point as a function of the torque or the power of the wind turbine may include receiving, via a turbine controller, the electrical constraints of one or more components from a converter controller of the wind turbine in real-time, calculating a plurality of intermediate speed set points as a function of the torque of the wind turbine for the electrical constraints, and selecting one of the plurality of intermediate speed set points to be the speed set point.

In alternative embodiments, the method may include determining the increased speed set point based on the torque or the power via one or more look-up tables.

In several embodiments, the method may further include increasing one or more over-speed condition margins of the wind turbine in response to varying the speed set point of the wind turbine as a function of a torque of the wind turbine.

It should be understood that the system constraint(s) may include mechanical constraints, electrical constraints, and/or thermal constraints of one or more components of the wind turbine. More specifically, in such embodiments, the mechanical constraints of the one or more components of the wind turbine may include loads determined by direct measurement, loads calculated based on internal models of the controller, loads calculated based on operational history of the wind turbine, loads calculated based on a wind resource, simulated loading profiles, or combinations thereof. Further, the electrical constraints of the one or more components of the wind turbine may include a grid condition, a reactive power demand, converter current margins, converter voltage margins, cable ampacity, internal or external power commands, a grid strength, ambient conditions, thermal margins, temperature, or similar.

In another aspect, the present disclosure is directed to a system for operating a wind turbine during partial load operation. The system includes a controller having one or more processors. The processor(s) are configured to perform one or more operations, including but not limited to providing a rated power for the wind turbine and comparing a power output of the wind turbine with the rated power. If the power output is at the rated power, the processor is configured to maintain a speed set point of the wind turbine equal to a rated speed set point. Alternatively, if the power output is below the rated power, the processor is configured to vary the speed of the wind turbine based on a non-monotonic torque-speed relationship. It should be understood that the system may also include any of the additional features described herein.

In yet another aspect, the present subject matter is directed to a method for operating a wind turbine during partial load operation. The method includes providing a rated power for the wind turbine. If a power output of the wind turbine is at the rated power, the method includes maintaining a speed of the wind turbine equal to a rated speed set point. In contrast, however, if the power output is below the rated power, the method includes operating the speed of the wind turbine based on a non-monotonic torque-speed relationship and system constraints of the wind turbine. It should be understood that the method may also include any of the additional steps and/or features described herein.

These and other features, aspects and advantages of the present invention will become better understood with reference the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the embodiments of the invention and, together with the description, serve to explain the principles of the invention.

DETAILED DESCRIPTION

Generally, the present disclosure is directed to systems and methods that utilize a variable speed set point that is introduced into the controller in an effort to improve the partial-load performance of a wind turbine that is speed-limited in its partial-load operation condition. Such control systems are particularly useful for wind turbines that operate using doubly-fed induction generators (DFIGs). For conventional wind turbines, the rated speed set point is a constant value. Thus, when the rotor first reaches the rated speed set point from an increase in wind speed, it maintains that value even as wind speeds continue to increase. In the present disclosure, however, the speed set point for the partial load operation condition is higher than the speed set point at rated power. As such, the present disclosure utilizes the existing system margins at below-rated power operation. Further, the new torque-speed curve is a non-monotonic curve that is defined by a new constraint that follows the electrical, mechanical, or thermal system capability curve in upper partial load.

As used herein, non-monotonic operation refers to a relationship between two operating conditions that is not continuously increasing or decreasing. More specifically, a non-monotonic speed-torque relationship is defined as when torque increases, the speed does not always increase as torque increases, rather at a certain point (e.g. an inflection point), speed will start to decrease as torque continues to increase. In contrast, monotonicity refers generally to the characteristic of a function with a first derivative that does not change sign, which is characteristic of standard torque-speed operation. Non-monotonicity is the converse, in which determining whether the dependent variable is decreasing depends on the value and direction of the independent variable.

The various embodiments of the system and method described herein provide numerous advantages not present in the prior art. For example, the controller change can be implemented using existing turbine software. Further, by increasing the rotor speed, the systems and methods of the present disclosure allow the wind turbine to maintain optimum tip speed ratio operation up to a higher wind speed so as to maintain the peak region of the power coefficient longer. Thus, the present disclosure expands the operational space of the wind turbine and increases power performance. In addition, the present disclosure improves stall margin for fouled or iced blades.

The wind turbine10may also include a wind turbine controller26centralized within the nacelle16. However, in other embodiments, the controller26may be located within any other component of the wind turbine10or at a location outside the wind turbine. Further, the controller26may be communicatively coupled to any number of the components of the wind turbine10in order to control the operation of such components and/or to implement a correction action. As such, the controller26may include a computer or other suitable processing unit. Thus, in several embodiments, the controller26may include suitable computer-readable instructions that, when implemented, configure the controller26to perform various different functions, such as receiving, transmitting and/or executing wind turbine control signals.

Referring now toFIG. 2, a simplified, internal view of one embodiment of the nacelle16of the wind turbine10shown inFIG. 1is illustrated. As shown, the generator24may be coupled to the rotor18for producing electrical power from the rotational energy generated by the rotor18. For example, as shown in the illustrated embodiment, the rotor18may include a rotor shaft34coupled to the hub20for rotation therewith. The rotor shaft34may, in turn, be rotatably coupled to a generator shaft36of the generator24through a gearbox38. As is generally understood, the rotor shaft34may provide a low speed, high torque input to the gearbox38in response to rotation of the rotor blades22and the hub20. The gearbox38may then be configured to convert the low speed, high torque input to a high speed, low torque output to drive the generator shaft36and, thus, the generator24.

Each rotor blade22may also include a pitch adjustment mechanism32configured to rotate each rotor blade22about its pitch axis28. Further, each pitch adjustment mechanism32may include a pitch drive motor40(e.g., any suitable electric, hydraulic, or pneumatic motor), a pitch drive gearbox42, and a pitch drive pinion44. In such embodiments, the pitch drive motor40may be coupled to the pitch drive gearbox42so that the pitch drive motor40imparts mechanical force to the pitch drive gearbox42. Similarly, the pitch drive gearbox42may be coupled to the pitch drive pinion44for rotation therewith. The pitch drive pinion44may, in turn, be in rotational engagement with a pitch bearing46coupled between the hub20and a corresponding rotor blade22such that rotation of the pitch drive pinion44causes rotation of the pitch bearing46. Thus, in such embodiments, rotation of the pitch drive motor40drives the pitch drive gearbox42and the pitch drive pinion44, thereby rotating the pitch bearing46and the rotor blade22about the pitch axis28. In further embodiments, the wind turbine10may employ direct drive pitch or a separate pitch drive systems including hydraulics. Similarly, the wind turbine10may include one or more yaw drive mechanisms66communicatively coupled to the controller26, with each yaw drive mechanism(s)66being configured to change the angle of the nacelle16relative to the wind (e.g., by engaging a yaw bearing68of the wind turbine10).

Still referring toFIG. 2, the wind turbine10may also include one or more sensors48,50,52for measuring operating and/or loading conditions of the wind turbine10. For example, in various embodiments, the sensors may include blade sensors48for measuring a pitch angle of one of the rotor blades22or for measuring a loading acting on one of the rotor blades22; generator sensors50for monitoring the generator24(e.g. torque, speed, acceleration and/or the power output); and/or various wind sensors52for measuring various wind parameters, such as wind speed, wind peaks, wind turbulence, wind shear, changes in wind direction, air density, or similar. Further, the sensors may be located near the ground of the wind turbine10, on the nacelle16, or on a meteorological mast of the wind turbine10. It should also be understood that any other number or type of sensors may be employed and at any location. For example, the sensors may be Micro Inertial Measurement Units (MIMUs), strain gauges, accelerometers, pressure sensors, angle of attack sensors, vibration sensors, Light Detecting and Ranging (LIDAR) sensors, camera systems, fiber optic systems, anemometers, wind vanes, Sonic Detection and Ranging (SODAR) sensors, infra lasers, radiometers, pitot tubes, rawinsondes, other optical sensors, and/or any other suitable sensors. It should be appreciated that, as used herein, the term “monitor” and variations thereof indicates that the various sensors may be configured to provide a direct measurement of the parameters being monitored or an indirect measurement of such parameters. Thus, the sensors may, for example, be used to generate signals relating to the parameter being monitored, which can then be utilized by the controller26to determine the actual parameter.

Referring now toFIG. 3, a block diagram of one embodiment of the controller26according to the present disclosure is illustrated. As shown inFIG. 3, the controller26may include one or more processor(s)58and associated memory device(s)60configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and the like and storing relevant data as disclosed herein). Additionally, the controller26may also include a communications module62to facilitate communications between the controller26and the various components of the wind turbine10. Further, the communications module62may include a sensor interface64(e.g., one or more analog-to-digital converters) to permit signals transmitted from the sensors48,50,52to be converted into signals that can be understood and processed by the processors58. It should be appreciated that the sensors48,50,52may be communicatively coupled to the communications module62using any suitable means. For example, as shown in FIG.3, the sensors48,50,52are coupled to the sensor interface64via a wired connection. However, in other embodiments, the sensors48,50,52may be coupled to the sensor interface64via a wireless connection, such as by using any suitable wireless communications protocol known in the art.

As used herein, the term “processor” refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, a graphics processing unit (GPUs), and/or other programmable circuits now known or later developed. Additionally, the memory device(s)60may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)60may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)58, configure the controller26to perform various functions as described herein.

Referring now toFIG. 4, a flow diagram of one embodiment of a method100for operating a wind turbine (e.g. the wind turbine10ofFIG. 1) during partial load operation is illustrated. As used herein, “partial load operation” generally refers to operation of the wind turbine10below rated power. In other words, during partial load operation, the wind turbine10is producing less than rated power. Thus, as shown at102, the method100includes determining a power output of the wind turbine10. As shown at104, the method100includes determining whether the power output is below the rated power of the wind turbine10. If the power output is at the rated power, as shown at106, the method100includes maintaining a speed of the wind turbine equal to a rated speed set point.

However, if the power output is below the rated power, as shown at108, the method100also includes determining whether a torque set point of the wind turbine10is at or above a system constraint of the wind turbine10. If the torque set point is below system constraint(s), as shown at110, the method100may include targeting the speed set point to a maximum value and maintaining optimal torque as a function of the speed. If the torque set point is at or above system constraints, however, as shown at112, then the method100includes varying (e.g. via the controller26) the speed of the wind turbine based on a non-monotonic torque-speed relationship. For example, in one embodiment, the controller26may be configured to increase the speed of the wind turbine10above the rated speed set point as a function of the torque until at least one of mechanical constraints, electrical constraints, or thermal constraints of one or more components of the wind turbine10are reached. More specifically, the mechanical constraints of the one or more components of the wind turbine10may include loads determined by direct measurement, loads calculated based on internal models of the controller, loads calculated based on operational history of the wind turbine, loads calculated based on a wind resource, simulated loading profiles, or combinations thereof. Further, the electrical constraints of the one or more components of the wind turbine10may include a grid condition, a reactive power demand, converter current margins, converter voltage margins, cable ampacity, internal or external power commands, a grid strength, ambient conditions, thermal margins, temperature, or similar.

Referring now toFIG. 5, a graph illustrating a non-monotonic torque-speed curve according to the present disclosure that follows the electrical and/or mechanical system limits is illustrated. Further, a monotonic baseline torque-speed curve is represented by reference character70, whereas the new non-monotonic torque-speed curve is represented as72. As shown in Region I, the generator speed in the rated power operation of the wind turbine10remains the same when this new curve is applied and the higher rated speed set point is applied only at below rated power. As shown in Region II, however, the new control method permits an increase in the rated speed set point in partial load operation of the wind turbine10, e.g. as shown from rated speed set point74to increased speed set point76, a constant speed as shown along line76, and a subsequent decrease in the speed as shown from the increased speed set point76back the rated speed set point74. As such, in certain embodiments, the wind turbine10is configured to follow the non-monotonic control curve72according to the newly defined limits as a function of torque.

The applicability of the non-monotonic torque-speed curve72is determined by the torque achieved in the generator and the power converter of the wind turbine10(not shown). For example, for conventional systems, there is a single variable speed region that increases torque at a defined slope from a minimum speed to a maximum speed. The slope is defined by maintaining an optimal tip speed ratio. In contrast, as shown inFIG. 5, the new torque-speed curve72first increases the rated speed set point (i.e. from74to76).

After the speed of the wind turbine10reaches the increased speed set point76, the non-monotonic torque-speed curve72maintains the maximum operational speed until one or more system limits are reached. (FIG. 5). The controller26may then decrease the speed of the wind turbine10as a function of the torque from the increased speed set point76of the wind turbine10. For example, in such embodiments, the controller26may decrease the speed of the wind turbine10back to the rated speed set point70after at least one of the mechanical constraints, the electrical constraints, or the thermal constraints of one or more components of the wind turbine10are reached. More specifically, as shown inFIG. 5, once the non-monotonic torque-speed curve72reaches the higher rated speed set point76in partial load, the controller26is configured to apply an additional constraint on the torque-speed curve72at torque levels above the torque achieved when the system limit(s) is first reached. As such, the additional constraint may follow the system capability curve78. It should be further understood that system capability curve78may shift according to the environmental and/or grid conditions of the wind turbine10and takes into account electrical, mechanical, and/or thermal constraints of the wind turbine10.

In particular embodiments, the controller26may dynamically calculate the increased speed set point76as a function of the torque, power, and/or other sensed inputs to the controller26. More specifically, in such embodiments, the controller26may receive one or more electrical constraints of one or more components from a converter controller of the wind turbine10in real-time. Thus, the controller26can use the electrical constraints to calculate a plurality of intermediate speed set points as a function of the torque of the wind turbine10. Further, the controller26may select one of the plurality of intermediate speed set points to be the speed set point and the process may be updated as the electrical constraints change. In alternative embodiments, the controller26may determine the increased speed set point76based on the torque or power of the wind turbine10via one or more look-up tables.

Advantages of the present disclosure can be further understood with respect to the graphs illustrated inFIGS. 6-9, which depict the operational characteristics of the wind turbine10for a baseline control method and the control method of the present disclosure. More specifically,FIG. 6illustrates a graph of rotor speed (y-axis) versus wind speed (x-axis);FIG. 7illustrates a graph tip speed ratio (TSR) (y-axis) versus wind speed (x-axis);FIG. 8illustrates a graph of the power coefficient Cp(y-axis) versus wind speed (x-axis), andFIG. 9illustrates a graph of electrical power versus (y-axis) versus wind speed (x-axis).

Referring particularly toFIG. 6, curve80represents the rotor speed for a conventional control scheme, whereas curve82represents the rotor speed according to the present disclosure. As shown, a higher rated speed set point86is achieved (as compared to the original rated speed set point84of conventional systems). Subsequently, the control scheme of the present disclosure follows the electromechanical constraint curve88to return the rated rotor speed set point86to its original rated speed set point84at rated power.

As shown inFIG. 7, a performance benefit of the present disclosure is further illustrated by the extension of the constant optimal tip speed ratio line92(as compared to baseline TSR line90) to higher wind speeds by achieving a higher rated rotor speed (as shown in Region I). The constant TSR line92follows the optimum TSR for the rotor blade22, thus achieving a higher power coefficient Cpas shown inFIG. 8. More specifically, as shown in Region I ofFIG. 8, the new power coefficient Cp94is extended as compared to the conventional power coefficient Cp93. Thus, as shown inFIG. 9, a performance increase97(i.e. the area between the baseline power output95and the new power output96) is achieved by operating the wind turbine10to follow its electrical constraint at below-rated power and above-rated speed.

In certain instances, increasing the speed as described above may cause an increase in the over-speed set points of the wind turbine protection system. As such, the present disclosure also provides for certain over-speed handling techniques. For example, as shown inFIG. 10, the new over-speed margins99can be maintained at their current ratios to the baseline rated speed set point74and increased proportionally to the higher rated speed set point76(as compared to the baseline over-speed margins98). In other words, as shown, the higher rated speed set point76is configured to have negative loads impacts on gravity-driven loads by increasing the number of cycles in the fatigue evaluation. In additional embodiments, the new over-speed margins can be dynamically calculated, e.g. using proportional scaling of the over-speed margins to intermediate speed set points, a constant increase in the margins, and/or alternative over-speed handling procedures.

Referring now toFIG. 11, a flow diagram of one embodiment of a method200for operating a wind turbine (e.g. the wind turbine10ofFIG. 1) during partial load operation is illustrated. As shown at202, the method200includes providing a rated power for the wind turbine10. As shown at204, the rated power is compared to the power output of the wind turbine10. As shown at206, if the power output of the wind turbine10is at the rated power, the method200includes maintaining a speed of the wind turbine10equal to a rated speed set point. In contrast, as shown at208, if the power output is below the rated power, the method200includes operating the speed of the wind turbine10based on a non-monotonic torque-speed relationship and system constraints (e.g. mechanical constraints, electrical constraints, and/or thermal constraints) of the wind turbine10.