Patent Description:
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 including one or more rotor blades. The rotor blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy 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.

Wind turbine generators and wind farms are typically designed to deliver constant active and reactive power to the utility grid with the delivered power being independent of system frequency. This is accomplished by decoupling the rotor inertia and speed from the grid using fast acting power electronics and controls. Due to increases in wind-farm size and penetration, some utilities are now requiring that wind-farm and wind-turbine controls provide enhanced capabilities such as frequency stabilization.

Conventional synchronous generators naturally respond to frequency disturbances due to the grid coupled rotating inertia and governor controls of such generators. Some utility operators require that wind turbines respond in a similar way to frequency disturbances. Specifically, a short duration power increase (for example, five or ten percent of rated power) may be needed when frequency dips below a certain threshold.

It is a well-known characteristic of utility systems that the grid frequency tends to decrease when the load exceeds the generation and to increase when the generation exceeds the load. Such decreases or increases may occur in a monotonic manner, an oscillating manner, or combinations thereof when the grid is subjected to a sudden change in the balance between generation and load. It is a consideration in the design of such a system that any method to achieve compensation of such decreases or increases should be one that does not cause unacceptable coupling between grid oscillatory modes and the wind turbine mechanical oscillatory modes.

Modern wind farms include the capability to curtail output power below the level available based on wind conditions. Utility grid operators sometimes require curtailment if the available grid power is not needed by the utility. Continuous curtailment may also be required by utility operators to provide an operating range for the wind farm to increase power output when frequency decreases.

Thus, as grid requirements continue to change, wind turbine power systems are continuously in need of being able to meet such requirements. Accordingly, the present disclosure is directed to systems and methods for controlling a wind turbine connected to a power grid that activates a predefined control scheme in response to a frequency drop in the power grid in order to the grid requirements thereof.

Various aspects and advantages of the invention will be set forth in part in the following description, or may be clear from the description, or may be learned through practice of the invention.

According to the invention, the present disclosure is directed to a method for controlling a wind turbine connected to a power grid, according to claim <NUM>.

The method includes monitoring a frequency of the power grid. In response to detecting a frequency event, such as a frequency drop or decrease, occurring in the power grid, the method includes activating a control scheme in order to meet one or more grid requirements of the power grid. The control scheme includes increasing a power output of the wind turbine to, at least, a pre-event measured grid power. Further, the method includes calculating a power correction factor for a power set point of the wind turbine as a function of, at least, the frequency event. Moreover, the method includes adjusting the power set point via the power correction factor such that the power output follows a predetermined trajectory. In addition, the control scheme includes controlling, via a turbine controller, the wind turbine based on the adjusted power set point for as long as the control scheme is activated.

According to the invention, the control scheme further includes applying a gain to the power correction factor to obtain an adjusted power correction factor. More specifically, in such embodiments, the gain may be determined as a function of one or more electrical or mechanical limits of the wind turbine.

Furthermore, the control scheme includes calculating a torque correction factor as a function of the adjusted power correction factor and a speed of the wind turbine, adding the torque correction factor to a torque set point as a feedforward term, and adding the adjusted power correction factor to the power set point. In such embodiments, the method may include imposing an above rated mode when the control scheme is activated such that a speed regulator governs a pitch angle and a power regulator governs the torque set point.

In further embodiments, the method may include switching a mode of operation when the control scheme is deactivated from the imposed above rated mode to an optimum operation condition or switching the mode of operation from the above rated mode of operation to a below rated mode of operation where the speed regulator is controlling the torque set point.

Thus, in certain embodiments, if the wind turbine is operating in the below rated mode of operation when the frequency event is detected, the method may include switching the mode to the above rated mode of operation when the control scheme is activated. In such embodiments, the method may also include switching the mode of operation back to the below rated mode of operation when the frequency event is over.

In additional embodiments, the control scheme may include changing a speed set point of the wind turbine to a rated speed of the wind turbine via a bumpless transfer. More specifically, in one embodiment, while switching the mode back to the below rated mode of operation, the step of changing the speed set point of the wind turbine to the rated speed of the wind turbine via the bumpless transfer may include tracking, via at least one filter, a current speed of the wind turbine and gradually increasing the speed set point based on the tracking until the rated speed is reached so as to smoothly transition out of the control scheme.

In yet another embodiment, if the wind turbine is operating in the above rated mode of operation, the method may include transitioning out of the control scheme when the frequency event is over via a standard or normal operating process. For example, in one embodiment, the standard operating process may include increasing the adjusted power set point to a predetermined power set point.

In still further embodiments, the method may include disabling one or more control loops of the turbine controller for as long as the control scheme is activated, wherein disabling the one or more control loops prevents an additional power drop of the wind turbine.

In another aspect, the present disclosure is directed to a method for controlling a wind turbine connected to a power grid. The method includes monitoring a frequency of the power grid. In response to detecting a frequency event occurring in the power grid, the method includes activating a control scheme in order to meet one or more grid requirements of the power grid. The control scheme includes calculating a power correction factor for the power set point as a function of, at least, the frequency event. Further, the control scheme includes calculating a torque correction factor as a function of the power correction factor and a speed of the wind turbine. Moreover, the control scheme includes adjusting the power set point via the power correction factor. In addition, the control scheme includes adding the torque correction factor to the adjusted power set point as a feedforward term. Thus, the control scheme also includes controlling, via a turbine controller, the wind turbine based on the adjusted power set point for as long as the control scheme is activated. It should also be understood that the method may further include any of the additional features and/or steps as described herein.

In yet another aspect, the invention is directed to a stabilization system for a wind power generation system connected to a power grid, according to claim <NUM>. The stabilization system includes a deadband limiter for detecting when a signal is outside of a respective signal range, wherein the signal comprises a frequency. Further, the stabilization system includes a power shaper for providing a supplementary power correction factor as a function of the frequency. In addition, the system includes a torque shaper for initially increasing a power output of the wind turbine to a pre-event measured grid power and adjusting a power set point as a function of the supplementary power correction factor so as to temporarily boost the supplied power to the power grid in response to the signal being outside of the respective signal range. Moreover, the system includes a turbine controller for controlling the wind turbine based on the adjusted power set point for as long as the signal is outside of the respective signal range. It should also be understood that the system may further include any of the additional features as described herein.

More specifically, the stabilization system further includes a gain block for applying a gain to the power correction factor to obtain an adjusted power correction factor, the gain being determined as a function of one or more electrical or mechanical limits of the wind turbine.

Furthermore, the system includes a power regulator optimization module for calculating a torque correction factor as a function of the adjusted power correction factor and a speed of the wind turbine, adding the torque correction factor to a torque set point as a feedforward term, and adding the adjusted power correction factor to the power set point.

In additional embodiments, the system may include an operational mode selector for determining a mode of operation of the wind turbine. In such embodiments, the operational mode selector is configured to impose an above rated mode when the control scheme is activated such that a speed regulator is governing a pitch angle and a power regulator is governing the torque set point. In addition, the operational mode selector is further configured to switch a mode of operation when the control scheme is deactivated from the imposed above rated mode to an optimum operation condition or switch the mode of operation from the above rated mode of operation to a below rated mode of operation where the speed regulator is controlling the torque set point.

In one embodiment, if the wind turbine is operating in the below rated mode of operation when the frequency event is detected, the turbine controller switches the mode to the above rated mode of operation in response to the signal being outside of the respective signal range. In further embodiments, the turbine controller may also switch the mode back to the below rated mode of operation in response to the signal returning within the respective signal range.

In several embodiments, the stabilization system may also include a bumpless transfer module for changing a speed set point of the wind turbine to a rated speed of the wind turbine while switching the mode back to the below rated mode of operation. More specifically, the bumpless transfer module may include a low-pass filter for tracking a current speed of the wind turbine and gradually increasing the speed set point based on the tracking until the rated speed is reached so as to smoothly transition out of the control scheme.

In yet another embodiment, the turbine controller may disable a drivetrain damper of the wind turbine in response to the signal being outside of the respective signal range.

Various features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims.

In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention, which is defined by the claims.

Referring now to the drawings, <FIG> illustrates a wind turbine system <NUM> operable to generate electric power is illustrated. As shown, the wind turbine system <NUM> includes a hub <NUM> having multiple rotor blades <NUM> mounted thereto. The rotor blades <NUM> convert the mechanical energy of the wind into a rotational torque, which is further converted into electrical energy by the wind turbine system <NUM>. The wind turbine system <NUM> further includes a turbine portion <NUM> that is operable to convert the mechanical energy of the wind into a rotational torque and a power conversion system <NUM> that is operable to convert the rotational torque produced by the turbine portion <NUM> into electrical power. A drive train <NUM> is provided to couple the turbine portion <NUM> to the power conversion system <NUM>. The wind turbine power conversion system <NUM> typically comprises a doubly fed asynchronous generator with a power electronic converter for rotor field control or a synchronous generator for use with a full power electronic converter interface to collector system <NUM>.

The turbine portion <NUM> includes a turbine rotor low-speed shaft <NUM> that is coupled to the hub <NUM>. Rotational torque is transmitted from rotor low-speed shaft <NUM> to a generator shaft <NUM> via drive train <NUM>. In certain embodiments, such as the embodiment illustrated in <FIG>, drive train <NUM> includes a gear box <NUM> transmitting torque from low-speed shaft <NUM> to a high speed shaft <NUM>. A high speed shaft <NUM> is coupled to power conversion system shaft <NUM> with a coupling element <NUM>.

Power conversion system <NUM> is coupled to wind turbine controls <NUM>. Wind turbine controls <NUM> receive signals <NUM> from the power conversion system that are representative of the operating parameters of the system. Wind turbine controls <NUM>, in response, may generate control signals, for example a pitch signal <NUM> to change the pitch of blades <NUM> or a torque signal for the power conversion system. Wind turbine controls <NUM> are also coupled to a wind farm controller <NUM>.

Referring to <FIG>, an electrical power generation system <NUM> for generating electrical power is illustrated. For purposes of illustration, the electrical power generation system <NUM> includes a wind farm <NUM> electrically coupled to an electrical grid <NUM>. The electrical grid <NUM> is utilized to transfer electrical power from the wind farm <NUM> to electrical loads.

The wind farm <NUM> is provided to generate electrical power utilizing wind energy. The wind farm <NUM> includes wind turbines <NUM>, <NUM>, <NUM> (more generally referenced as "energy sources"), a collector system <NUM>, a transformer <NUM>, wind turbine controllers <NUM>, <NUM>, <NUM>, a measurement device <NUM>, and a wind farm controller <NUM>. It should be noted that a number of wind turbines utilized in the wind farm <NUM> can vary. For example, the number of wind turbines in the wind farm <NUM> can be greater than three wind turbines or less than or equal to three wind turbines.

The wind turbines <NUM>, <NUM>, <NUM> are provided to generate voltages and currents utilizing wind energy. The wind turbines <NUM>, <NUM>, <NUM> are operably controlled utilizing the wind turbine controllers <NUM>, <NUM>, <NUM>, respectively, which communicate with the wind turbines <NUM>, <NUM>, <NUM>, respectively.

The wind turbine controllers <NUM>, <NUM>, <NUM> are configured to generate command signals which control operation of the wind turbines <NUM>, <NUM>, <NUM>, respectively. Further, the wind turbine controllers <NUM>, <NUM>, <NUM> are provided to measure operational parameters associated with the wind turbines <NUM>, <NUM>, <NUM> respectively. The wind turbine controllers <NUM>, <NUM>, <NUM> operably communicate with the wind farm controller <NUM>.

The collector system <NUM> is electrically coupled to the wind turbines <NUM>, <NUM>, <NUM> and routes voltages and currents from each of the turbines to the power transformer <NUM>. The power transformer <NUM> receives the voltages and currents from the wind turbines <NUM>, <NUM>, <NUM> and outputs a voltage and a current having desired characteristics onto the electrical grid <NUM>. For example, the power transformer <NUM> can output a voltage having a desired amplitude and a current having a desired amplitude onto the electrical grid <NUM>.

According to the invention, the measurement device <NUM> is electrically coupled to a point of interconnection <NUM> between the transformer <NUM> and the electrical grid <NUM>. The measurement device <NUM> is configured to measure electrical parameters associated with the electrical grid. For example, the measurement device <NUM> is configured to measure a voltage level (VPOI) at the point of interconnection <NUM>, a real power level (Pn) at the point of interconnection <NUM>, and a frequency level (Fn) at the point of interconnection <NUM>. It should be noted that the measurement device <NUM> can measure parameters on either side of the transformer <NUM> or at individual turbines.

The wind farm controller <NUM> is provided to control operation of the wind turbines <NUM>, <NUM>, <NUM> based on measured or estimated parameter values at the point of interconnection <NUM> associated with either the wind farm <NUM> or the electrical grid <NUM>. The wind farm controller <NUM> is configured to generate command messages that are received by the wind turbine controllers <NUM>, <NUM>, <NUM> for controlling operation of the wind turbines <NUM>, <NUM>, <NUM>, respectively. In addition, the wind turbines <NUM>, <NUM>, <NUM> can be operated based on various operational modes of operation.

Referring now to <FIG>, a schematic block diagram of a control system according to the present disclosure is illustrated. As shown, the control system includes a stabilization system <NUM> for a power generation system <NUM> (such as one of the wind turbines <NUM>, <NUM>, <NUM>) connected to a power grid <NUM>. Further, as shown, the stabilization system <NUM> includes an input frequency washout <NUM> configured for tracking slow variations in grid frequency and used for calculating frequency deviations around a center point. In addition, the stabilization system <NUM> includes a deadband limiter <NUM> configured for detecting when a signal <NUM> from the input frequency washout <NUM> is outside of a signal range. The signal <NUM> may include any appropriate signal. For example, as shown, the signal <NUM> corresponds to system frequency. Further, the signal may be obtained either by direct measurement of the respective signal or by measurement of another signal and computations to obtain the respective signal. Thus, in response to detecting a frequency event, such as a frequency drop or decrease that is outside of the signal range, the stabilization system <NUM> is configured to activate a control scheme in order to meet one or more grid requirements of the power grid <NUM> for as long as the signal <NUM> is outside of the signal range.

In such embodiments, the deadband limiter <NUM> is configured to limit the frequency deviation signal between frequency threshold values determined by the application. Utility grid frequency typically has a nominal value equal to either <NUM> Hertz (Hz) or <NUM>. However, the frequency may drift somewhat such that the center point is at a different value such as <NUM> rather than <NUM>, for example. As such, the input frequency washout <NUM> is used to find the actual frequency rather than the nominal value. Typically, the frequency is measured at a substation of the wind farm <NUM>, but measurement at that location is not required.

Further, the frequency deadband limiter <NUM> is used to limit the response of the stabilization system <NUM> to sufficiently large events. The frequency will always vary somewhat due to dithering that occurs when loads come on and off the system. For example, load variation typically affects frequency by about <NUM>, depending on the system. As such, the stabilization system <NUM> is useful for more significant events that occur when a sudden difference is present between load and generation. Such significant events may include a utility system losing a large generator or a transmission line tripping. In one example, the deadband is set to a predetermined value, such as plus or minus <NUM> off the center point frequency. The selection of this limit is typically based on factors such as the location and nature of the power generation system and the variability of frequency center points. The stabilization system <NUM> may have a variable limit that is set by the end user after taking into account such factors.

Still referring to <FIG>, as shown, the stabilization system <NUM> may also include a power shaper <NUM> configured for generating a supplementary power correction factor <NUM> as a function of the frequency in response to the signal <NUM> being outside of the signal range. As used herein, the power correction factor <NUM> may include any linear or non-linear term. In additional embodiments, the power correction factor <NUM> may be calculated in real-time or may be predetermined static factor pre-programed in the turbine controller <NUM>. Moreover, as shown, the stabilization system <NUM> includes a limit controller <NUM> configured to prevent the adjustment signal from causing the energy source of the power generation system to operate outside of at least one operating constraint. Further, as shown, the stabilization system <NUM> includes a turbine controller <NUM> for controlling the power generation system <NUM> based on an adjustment signal <NUM> for as long as the signal <NUM> is outside of the respective signal range.

As such, the power shaper <NUM> provides a signal for the turbine controller <NUM> to transiently boost power while staying within energy source operating constraints and limiting coupling between grid oscillatory modes and energy source mechanical modes. Although the word "power" is used, torque could alternatively be used for the same effect, and power as used herein is meant to encompass torque. As such, the system may be set up to be based on power throughout, torque throughout, or a combination of power and torque. For example, in one embodiment, the control scheme is completed for a power response, but implementation is such that the wind turbine obtains a torque command that corresponds to the desired power control.

Mechanical oscillations are a common concern in power systems. Such systems may be characterized by an elemental rotating system having two inertias connected by a spring. The spring can be a physical shaft or the behavior of generators interconnected by a transmission network. More specifically, a wind turbine has some inertia in the wind turbine hub end and some inertia in the wind turbine generator with the shaft between them being susceptible to spring type motion and oscillatory modes. Additionally, other generators on the utility system have respective inertias that may result in oscillatory modes. Such oscillatory modes may perturb the utility grid frequency.

Accordingly, the power shaper <NUM> is configured to shape a pulse in response to the frequency event and decrease the frequency disturbance magnitude in the power generation system. Although the power shaper <NUM> may be made to address either positive or negative frequency events, the shaper <NUM> will be particularly useful in embodiments addressing negative frequency events because less alternatives (other than curtailed power operation) exist to momentarily increase power above nominal.

The limit controller <NUM> is used to prevent turbine over and under speed operation. Most <NUM> wind turbines have a predetermined speed range, such as for example from about <NUM> rotations per minute (rpm) to about <NUM> rpm. The goal when the operating constraint is generator rotor speed is to ensure that no request is processed for additional power when the generator is close to the wind turbine cut-in speed limit and that no control signal for reducing power is processed when the generator is close to the wind turbine cut-out speed limit. In one embodiment, the limit controller <NUM> includes a power limiter <NUM> configured to change the bounds of integrator <NUM> in response to the wind turbine speed (i.e. the generator speed or the rotor speed). The generator speed is just one example of an operating constraint and other constraints may be used in addition or alternatively, as discussed below.

More specifically, in certain embodiments, the adjustment signal <NUM> may include any combination of a power command, a torque command, and/or a speed command. The adjustment signal <NUM> is typically further constrained so as to limit oscillatory coupling. If the grid frequency is oscillatory but at a level smaller than the deadband, no adjustment signal is generated. If the grid is oscillatory and larger than the deadband, then the shaping characteristic prevents the compounding of the oscillation. The shape of the power adjustment signal can be controlled by gain and ramp values that will typically vary with application, due to differing utility requirements and responses.

Referring still to <FIG>, the illustrated stabilization system <NUM> further includes an output frequency washout <NUM> configured to drive an adjustment signal ΔP to zero. The output frequency washout <NUM> is shown in the illustrated position for purposes of example only and may be present in any appropriate control block. For example, in another embodiment, the washout function may be included within control loop <NUM>. In addition, the stabilization system <NUM> may include a limiter <NUM> configured to modify an output of the output frequency washout <NUM>. Thus, as shown, the output of the limiter <NUM> may be referred to herein as the supplementary power correction factor <NUM>.

More specifically, as shown in the embodiment of <FIG>, the power shaper <NUM> includes a gain block <NUM> and a control loop <NUM> used to provide a shaping response which is faster upon initiation and slower upon recovery (in other words "fast up, slow down"). The gain of block <NUM> may vary and will typically be set according to location and system requirements.

Within the control loop <NUM>, the gain block <NUM> is used to control the rate of signal adjustment for the integrator <NUM>. The gain of block <NUM> is typically a fixed value designed to control the response of the control loop <NUM>. Thus, the frequency is quickly compensated for upon an event, but the power adjustment after the event recovers slowly. In addition, the gain block <NUM> defines the pulse shape and may be varied based on system needs or conditions. For example, in one embodiment, at least one of the parameter settings for the gain block <NUM> is configured for having a variable value in response to at least one of a utility condition, a utility command, generator speed, and air density.

Referring still to <FIG>, as shown, the stabilization system <NUM> further includes a torque shaper <NUM> configured to modify the supplementary power correction factor <NUM> before being used as an input for the turbine controller <NUM>. More specifically, as shown, the torque shaper <NUM> receives the supplementary power correction factor <NUM> and applies a tunable gain to the signal via gain block <NUM> to obtain an adjusted power set point <NUM>. In certain embodiments, the gain may be a function of one or more electrical and/or mechanical system capabilities as well as limits requested from the utility/grid operator. The adjusted power set point <NUM> can then be further modified via one or more modules <NUM>, <NUM>, <NUM>, <NUM> within the torque shaper <NUM>, which are discussed in more detail below.

More specifically, <FIG> illustrates various schematic block diagrams of the various modules <NUM>, <NUM>, <NUM>, <NUM> of the torque shaper <NUM> illustrated in <FIG>. For example, as shown in <FIG>, one of the modules of the torque shaper <NUM> may include an operational mode selector module <NUM>. In certain embodiments, the operational mode selector module <NUM> allows the turbine controller <NUM> to assign appropriate regulation functions for the wind turbine <NUM>. More specifically, as shown in <FIG>, the mode selector module <NUM> allows the turbine controller <NUM> to assign appropriate regulation functions (e.g. torque set point <NUM> and angle set point <NUM>) for the pitch drive system <NUM> and the converter controller <NUM> when the control scheme is activated (as shown at block <NUM> of <FIG>). For example, as shown in <FIG>, the pitch controller <NUM> is operating in a speed regulation mode and the converter controller <NUM> is operating in power regulation mode. As such, the control scheme, when activated, is configured to optimize operation of the turbine controller <NUM> such that the power converter operates to regulate power while the pitch system will regulate the speed during the frequency event.

More specifically, as shown in <FIG> and <FIG>, the power regulator <NUM> of the turbine controller <NUM> may generate the torque set point <NUM> using a proportional integral controller <NUM>. Further, as shown, the proportional integral controller <NUM> receives a difference between the power set point of the power generation system <NUM> and the power feedback from the grid <NUM>. The output of the proportional integral controller <NUM> may then be fed into a ramp rate limiter <NUM> that generates the torque set point <NUM> for the converter controller <NUM> (<FIG>). Further, as shown in <FIG>, the torque set point <NUM> from the power regulator <NUM> may be used as an input to the power regulator optimizer module <NUM>, which is further discussed below.

Referring now to <FIG>, one of the modules of the torque shaper <NUM> may include a power set point sequence module <NUM>. As such, the power set point sequence module <NUM> assists to initialize the power set point regulator so as to minimize the error with respect to power feedback, thereby creating a bumpless transfer of the power set point. More specifically, as shown, when the control scheme is activated (as indicated by line <NUM> changing from zero to one in <FIG>), the power set point sequencing block <NUM> receives the power feedback <NUM> from the grid <NUM> and sets the power command <NUM> to a pre-event measured grid power (i.e. a measured power of the electrical grid <NUM> before the frequency event occurs). As such, the torque shaper <NUM> is configured to adjust the power set point as a function of the supplementary power correction factor <NUM> so as to temporarily boost the supplied power to the power grid <NUM> in response to the signal being outside of the respective signal range.

A temporary power boost may be obtained by temporarily absorbing energy from the energy source. For wind turbines, the additional energy is available from the turbine inertia and from excess wind. If desired, other forms of energy storage, besides inertia (such as battery storage) can also be used. In one embodiment, for example, the power may be increased by five to ten percent for up to ten seconds. Providing a transient response has several benefits in addition to grid stability, including, for example, use of stored energy that would not otherwise be available at the turbine output without the need to build in an operating margin (e.g., by curtailed mode operation).

Referring now to <FIG>, another one of the modules of the torque shaper <NUM> may include a power regulator optimization module <NUM>. More specifically, as shown in <FIG>, the power regulator optimization module <NUM> is configured to calculate a torque correction factor <NUM> as a function of the adjusted power correction factor <NUM> and a speed <NUM> of the power generation system <NUM>. In such embodiments, the torque correction factor <NUM> can be added to the torque set point <NUM> as a feedforward term to obtain an adjusted torque set point <NUM>. In other words, as shown, the adjusted power correction factor <NUM> is fed to the power regulator optimization module <NUM> of the turbine controller <NUM> in two places: to the power regulator closed loop to ensure that the power regulator is optimized and as a feedforward term for quick response. If the power generation system <NUM> is operating in the below rated mode of operation when the frequency event is detected, the turbine controller <NUM> is configured to switch the mode to the above rated mode of operation in response to the signal <NUM> being outside of the respective signal range. In further embodiments, the turbine controller <NUM> may also switch the mode back to the below rated mode of operation in response to the signal returning within the respective signal range.

Referring now to <FIG> and <FIG>, still another one of the modules of the torque shaper <NUM> may include a bumpless transfer module <NUM>. For example, as shown in <FIG>, when the control scheme is active <NUM>, the module <NUM> is configured to optimize operation of the system <NUM>, however, oftentimes, there are multiple control loops active during operation thereof that can reduce the power output of the system <NUM>. For example, in one embodiment, one of the control loops may include the drivetrain damper of the power generation system <NUM>, which may reduce the power output of the system <NUM>. Accordingly, in certain embodiments, the bumpless transfer module <NUM> is configured to disable the drivetrain damper of the wind turbine <NUM> in response to the signal <NUM> being outside of the respective signal range to prevent a power reduction. Thus, as shown at <NUM> of <FIG>, the module <NUM> is configured to determine when the frequency event is over, and then provide a drivetrain damper bumpless transfer <NUM>. For example, in certain embodiments, the module <NUM> implements the bumpless transfer <NUM> via a low-pass filter. In addition, referring back to <FIG>, the output of the bumpless transfer module <NUM> may be fed through a torque damper <NUM> before being provided to the turbine controller <NUM>.

In addition, as shown in <FIG> and <FIG>, as the control scheme exits (or is deactivated) and if the mode of the power generation system <NUM> is the below rated mode, then the speed set point <NUM> of the system <NUM> may be initialized or changed to a rated speed via bumpless transfer as shown at block <NUM>. Similar to <FIG>, as shown at <NUM> and <FIG>, the speed set point <NUM> may track the turbine speed <NUM>, e.g. via a low-pass filter. Thus, the module <NUM> is configured to gradually increase the speed set point <NUM> based on the tracking until the rated speed is reached so as to smoothly transition out of the control scheme. It should be understood that the speed set point <NUM> may be ramped back to the rated speed via a fixed or variable rate.

In another embodiment, if the power generation system <NUM> is operating in the above rated mode of operation, the bumpless transfer module <NUM> may transition out of the control scheme when the frequency event is over via a standard or normal operating process. For example, in one embodiment, the standard operating process may include increasing the adjusted power set point to a predetermined power set point.

Additionally, although wind turbines are illustrated as the energy sources, the concepts disclosed herein are believed to be applicable to any non-conventional energy sources with several other examples including battery energy storage, microturbines, and/or fuel cells. In wind turbine embodiments, for example, the power obtained from the stabilization system <NUM> is not supported by the wind, so the turbine will slow down to provide the power from the spinning inertia. In such embodiments, it is desirable to hold the adjustment signal long enough to obtain a desired power pulse before allowing the system to recover while operating under energy source constraints. In one specific example, the energy source includes a generator with a constraint on the rotation speed of the generator.

In addition to speed constraints, other operating constraints may include, for example, constraints such as turbine torque (magnitude and time) constraints, ramp rate constraints, and blade pitch operating constraints. Torque constraints are typically set based on turbine design (that is, by how much and for how long a turbine can withstand exceeding its rated operating point). The output frequency washout <NUM> can be used to build in protections for such operating constraints.

Various options exist for distributed and system (or "central") level control. In one embodiment, the deadband limiter <NUM>, the power shaper <NUM>, the limit controller <NUM>, and/or the torque shaper <NUM> are embodied in a power generation system controller <NUM> (<FIG>). In another embodiment, the deadband limiter <NUM>, the power shaper <NUM>, the limit controller <NUM>, and/or the torque shaper <NUM> are embodied in a controller <NUM> of an energy source <NUM> (<FIG>). As another alternative, a separate controller (not shown) may be coupled to either the system controller <NUM> or the source controller <NUM>, or the various control sub-units/functions may be spread among several controllers.

Likewise, frequency estimation may be centralized or distributed. In other words, a frequency signal may be obtained by any desired means with several examples including: measurements at the energy source, measurements at a substation point <NUM> (<FIG>), measurements at the utility connection, or information from the utility. Typically, measurements are obtained at a substation because power fluctuations will tend to modulate apparent frequency (defined as the rate of change of voltage angle) differently at each turbine.

Claim 1:
A method for controlling a wind turbine (<NUM>, <NUM>, <NUM>) of a wind farm (<NUM>) connected to a power grid (<NUM>) at a point of interconnection (<NUM>), the power grid (<NUM>) being utilized to transfer electrical power from the wind farm (<NUM>) to electrical loads, the method comprising:
monitoring a frequency of the power grid (<NUM>);
measuring a real power at the point of interconnection (<NUM>) connecting the wind farm (<NUM>) to the power grid (<NUM>) as a pre-event measured grid power;
in response to detecting a frequency event occurring in the power grid (<NUM>), activating a control scheme in order to meet one or more grid requirements of the power grid (<NUM>), the control scheme comprising:
increasing a power output of the wind turbine (<NUM>, <NUM>, <NUM>) to, at least, the pre-event measured grid power at the point of interconnection (<NUM>);
calculating a power correction factor for a power set point of the wind turbine (<NUM>, <NUM>, <NUM>) as a function of, at least, the frequency event;
adjusting the power set point via the power correction factor such that the power output follows a predetermined trajectory;
controlling, via a turbine controller (<NUM>, <NUM>, <NUM>), the wind turbine (<NUM>, <NUM>, <NUM>) based on the adjusted power set point for as long as the control scheme is activated;
applying a gain to the power correction factor to obtain an adjusted power correction factor, the gain being determined as a function of one or more electrical or mechanical limits of the wind turbine (<NUM>, <NUM>, <NUM>); and
calculating a torque correction factor (<NUM>) as a function of the adjusted power correction factor and a speed of the wind turbine (<NUM>, <NUM>, <NUM>);
adding the torque correction factor to a torque set point as a feedforward term; and,
adding the adjusted power correction factor to the power set point in the power regulator (<NUM>) closed loop.