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, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is typically geared to a generator for producing electricity.

Wind turbines can be distinguished in two types: fixed speed and variable speed turbines. Conventionally, variable speed wind turbines are controlled as current sources connected to a power grid. In other words, the variable speed wind turbines rely on a grid frequency detected by a phase locked loop (PLL) as a reference and inject a specified amount of current into the grid. The conventional current source control of the wind turbines is based on the assumptions that the grid voltage waveforms are fundamental voltage waveforms with fixed frequency and magnitude and that the penetration of wind power into the grid is low enough so as to not cause disturbances to the grid voltage magnitude and frequency. Thus, the wind turbines simply inject the specified current into the grid based on the fundamental voltage waveforms. However, with the rapid growth of the wind power, wind power penetration into some grids has increased to the point where wind turbine generators have a significant impact on the grid voltage and frequency. When wind turbines are located in a weak grid, wind turbine power fluctuations may lead to an increase in magnitude and frequency variations in the grid voltage. These fluctuations may adversely affect the performance and stability of the PLL and wind turbine current control.

Furthermore, many existing renewable generation converters, such as double-fed wind turbine generators, operate in a "grid-following" mode. Grid-following type devices utilize fast current-regulation loops to control active and reactive power exchanged with the grid. More specifically, <FIG> illustrates the basic elements of the main circuit and converter control structure for a grid-following double-fed wind turbine generator. As shown, the active power reference to the converter is developed by the energy source regulator, e.g., the turbine control portion of a wind turbine. This is conveyed as a torque reference, which represents the lesser of the maximum attainable power from the energy source at that instant, or a curtailment command from a higher-level grid controller. The converter control then determines a current reference for the active component of current to achieve the desired torque. Accordingly, the double-fed wind turbine generator includes functions that manage the voltage and reactive power in a manner that results in a command for the reactive component of current. Wide-bandwidth current regulators then develop commands for voltage to be applied by the converters to the system, such that the actual currents closely track the commands.

Alternatively, grid-forming (GFM) inverter-based resources (IBR) act as a voltage source behind an impedance and provide a voltage-source characteristic, where the angle and magnitude of the voltage are controlled to achieve the regulation functions needed by the grid. In particular, with this structure, current will flow according to the demands of the grid, while the converter contributes to establishing a voltage and frequency for the grid. This characteristic is comparable to conventional generators based on a turbine driving a synchronous machine. Thus, a grid-forming source must include the following basic functions: (<NUM>) support grid voltage and frequency for any current flow within the rating of the equipment, both real and reactive; (<NUM>) prevent operation beyond equipment voltage or current capability by allowing grid voltage or frequency to change rather than disconnecting equipment (disconnection is allowed only when voltage or frequency are outside of bounds established by the grid entity); (<NUM>) remain stable for any grid configuration or load characteristic, including serving an isolated load or connected with other grid-forming sources, and switching between such configurations; (<NUM>) share total load of the grid among other grid-forming sources connected to the grid; (<NUM>) ride through grid disturbances, both major and minor, and (<NUM>) meet requirements (<NUM>)-(<NUM>) without requiring fast communication with other control systems existing in the grid, or externally-created logic signals related to grid configuration changes.

The basic control structure to achieve the above grid-forming objectives was developed and field-proven for battery systems in the early <NUM>'s (see e.g., <CIT> entitled "Battery Energy Storage Power Conditioning System"). Applications to full-converter wind generators and solar generators are disclosed in <CIT> entitled "System and Method for Control of a Grid Connected Power Generating System," and <CIT> entitled "Controller for controlling a power converter. " Applications to grid-forming control for a doubly-fed wind turbine generator are disclosed in <CIT> entitled "System and Method for Providing Grid-Forming Control for a Doubly-Feb Wind Turbine Generator.

In particular, as shown in <FIG>, the grid-forming voltage-source is realized on the stator voltage for implementing grid forming control for a double-fed wind turbine generator using the stator voltage regulator. More specifically, as shown, the stator voltage regulator <NUM> is configured to receive a higher level command (e.g., EI) for magnitude of the stator voltage and a higher level command (e.g., δIT) for angle of the stator voltage with respect to the phase-locked loop angle. Further, as shown, the stator voltage regulator <NUM> can then convert the voltage command(s) to a stator voltage command (e.g., VS_Cmd_xy) as shown at <NUM>. The stator voltage regulator <NUM> may then determine a magnetizing current feed forward signal (e.g., IM_FF_xy) as a function of the stator voltage command and a magnetizing admittance (e.g., jBmag <NUM>), which may correspond to a magnetizing susceptance. As such, the magnetizing current feed forward signal is configured to facilitate a rapid response of stator voltage to the stator voltage command.

In addition, as shown, the stator voltage regulator <NUM> may also receive a stator voltage feedback signal (e.g., VS_Fbk_xy) and, as shown at <NUM>, determine a difference between the stator voltage feedback signal and the stator voltage command. Thus, in an embodiment, as shown, the stator voltage regulator <NUM> may also determine a magnetizing current correction signal (e.g., IM_Corr_xy) via a proportional-integral regulator <NUM> Accordingly, as shown at <NUM>, the stator voltage regulator <NUM> can then add the magnetizing current feed forward signal (e.g., IM_FF_xy) to the magnetizing current correction signal (IM_Corr_xy) from the power regulator to determine the magnetizing current command (e.g., IM_Cmd_xy).

Furthermore, as shown at <NUM>, the stator voltage regulator <NUM> may determine the rotor current command(s) (e.g., IR_Cmd_xy) as a function of the magnetizing current command (e.g., IM_Cmd_xy) and a stator current feedback signal (e.g., IS_Fbk_xy). Thus, in an embodiment, the measured stator current signal may be fed into the rotor current command, as shown at <NUM>, so as to substantially decouple a stator responsive stator voltage from one or more grid characteristics. More specifically, in particular embodiments, as shown, the stator voltage regulator <NUM> may determine the rotor current command(s) by adding the magnetizing current command to the measured stator current feedback signal. In addition, as shown, a limiter <NUM> may place limits to the rotor current command as appropriate to respect equipment rating(s).

Further, <NPL> describes an adaptive backstepping based nonlinear control scheme incorporated with machine loss reduction and parameter uncertainties for grid-connected doubly fed induction generator (DFIG) driven wind energy conversion system (WECS). The proposed nonlinear controller is developed to stabilize both the grid and rotor side current control loops of direct-drive DFIG-based WECS, and incorporates the system uncertainty and nonlinearities while ensuring the stability of the drive system through Lyapunov stability criteria determined based on differential equations representing a dynamic model of an ideal three-phase, three windings DFIG configuration obtained from a state-space model in synchronous rotating reference frame, and depending on the magnetization inductance (more particular error magnetization inductances defined as differences between actual and estimated values for the magnetization inductance). At variable speed, the rotor side converter (RSC) regulates speed, flux and magnetization inductance of the machine, wherein the magnetization inductance is updated online to cope with variations according to an update rule.

In such systems, non-linear magnetizing characteristics of the generator can be determined by electrical tests. Modern double-fed wind turbine generators have a wide range of ratings and generator types used for applications. Additionally, identical generator designs practically exhibit different magnetizing characteristics due to nonuniformity in construction or materials. For these reasons, it is not practical to perform tests separately for each generator, nor is it practical to use the same characteristics for all generators.

Accordingly, systems and methods for estimating the magnetizing reactance automatically using the existing converter hardware and feedbacks would be beneficial for use in grid-forming converter control systems in double-fed wind-turbine generators.

In one aspect, the present disclosure provides to a method for controlling a wind turbine power system connected to an electrical grid according to claim <NUM>. The wind turbine power system has a double-fed wind turbine generator coupled to a power converter having a line-side converter and a rotor-side converter coupled together via a DC link. The method includes determining, via a controller, at least one non-linear magnetizing parameter of the double-fed wind turbine generator. The method also includes developing, via the controller, a model of the at least one non-linear magnetizing parameter of the double-fed wind turbine generator. Further, the method includes using, via the controller, the model in a stator voltage regulator of the double-fed wind turbine generator to provide grid-forming control of the double-fed wind turbine generator.

In an embodiment, the non-linear magnetizing parameter(s) is a magnetizing reactance.

Determining the non-linear magnetizing parameter(s) of the double-fed wind turbine generator may include (a) providing a plurality of data arrays comprising, at least, a data array of operating data points and a data array of non-linear magnetizing parameter data points, (b) enabling rotor control of the double-fed wind turbine generator with a stator switch open, (c) setting an operating set point of the double-fed wind turbine generator equal to a first operating data point in the data array of operating data points, (d) controlling the rotor-side converter to the operating set point and frequency for a time period, (e) collecting current and voltage feedbacks for the time period, and (f) calculating the at least one non-linear magnetizing parameter based on the current and voltage feedbacks for the time period.

In further embodiments, the method may also include (g) storing the operating set point and the at least one non-linear magnetizing parameter together in a data array, (h) setting the operating set point of the double-fed wind turbine generator equal to a remainder of the operating data points in the data array for subsequent time periods, and (i) repeating steps (d) through (g) for each of the operating data points in the data array for the subsequent time period.

In additional embodiments, determining the non-linear magnetizing parameter(s) of the double-fed wind turbine generator may include averaging the collected current and voltage feedbacks for the time period to remove noise and calculating the non-linear magnetizing parameter(s) based on the averaged current and voltage feedbacks for the time period.

In several embodiments, the data array of operating data points may include, for example, flux data points or stator voltage data points.

In particular embodiments, the data array of operating data points may include, at least, a range of expected operating data points that the double-fed wind turbine generator is expected to operate during normal operation. In another embodiment, the data array of operating data points may include one or more additional data points to capture operating data points beyond the normal operation to estimate one or more characteristics of the double-fed wind turbine generator for at least one of abnormal conditions or temporary conditions.

In certain embodiments, developing the model of the non-linear magnetizing parameter(s) of the double-fed wind turbine generator may include creating a saturation curve of the at least one non-linear magnetizing parameter versus the operating set point. For example, in an embodiment, the saturation curve may include a piecewise-linear curve fit.

In an embodiment, the method may include determining the non-linear magnetizing parameter(s) automatically using existing converter hardware and feedbacks.

In further embodiments, using the model in the stator voltage regulator of the double-fed wind turbine generator to provide grid-forming control of the double-fed wind turbine generator may include calculating an expected magnetizing reactance at a desired flux level using a stator flux command with slopes and y-intercepts of the saturation curve, calculating a magnetizing current feed forward signal based on the expected magnetizing reactance, calculating a magnetizing current command signal based on the magnetizing current feed forward signal and a magnetizing current correction signal, and calculating a rotor current command signals using the magnetizing current command signal and a stator current feedback signals.

In another aspect, the present disclosure provides a system for controlling a wind turbine power system connected to an electrical grid according to claim <NUM>. The wind turbine power system has a double-fed wind turbine generator coupled to a power converter having a line-side converter and a rotor-side converter coupled together via a DC link. The system includes a controller having at least one processor. The processor(s) is configured to perform a plurality of operations, including but not limited to determining at least one non-linear magnetizing reactance of the double-fed wind turbine generator, developing a model of the at least one non-linear magnetizing reactance of the double-fed wind turbine generator, and using the model in a stator voltage regulator of the double-fed wind turbine generator to provide grid-forming control of a double-fed wind turbine generator.

It should be understood that the system may further include any of the additional features described herein.

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 or of the invention as defined by the appended claims.

In general, the present disclosure is directed to systems and method for providing grid-forming control for a double-fed wind-turbine generator. More particularly, in certain embodiments, grid-Forming control in double-fed wind-turbine generators can be achieved by controlling current through the magnetizing branch in the generator, thereby producing a voltage drop across the magnetizing impedance. A fixed control setting representing the magnetizing reactance can be used in the control, but this will result in errors in the voltage that is realized by the controller. The errors will change based on operating conditions due to the non-linear characteristics of the real magnetizing impedance. Accordingly, the desired voltage of the grid-forming system can be more accurately controlled if the non-linear magnetizing characteristic(s), such as the non-linear magnetizing reactance, is included as part of the control scheme. For example, in an embodiment, the non-linear magnetizing characteristic(s) can be estimated by controlling the rotor converter of the double-fed generator before the stator switch is closed to begin operation. By sweeping through a range of pre-defined operating points and taking measurements at each point, a model of the saturation curve can be constructed. This model can then be used for the grid-forming controls as further described herein.

Referring now to the drawings, <FIG> illustrates a perspective view of one embodiment of a wind turbine <NUM> according to the present disclosure. As shown, the wind turbine <NUM> generally includes a tower <NUM> extending from a support surface <NUM>, a nacelle <NUM> mounted on the tower <NUM>, and a rotor <NUM> coupled to the nacelle <NUM>. The rotor <NUM> includes a rotatable hub <NUM> and at least one rotor blade <NUM> coupled to and extending outwardly from the hub <NUM>. For example, in the illustrated embodiment, the rotor <NUM> includes three rotor blades <NUM>. However, in an alternative embodiment, the rotor <NUM> may include more or less than three rotor blades <NUM>. Each rotor blade <NUM> may be spaced about the hub <NUM> to facilitate rotating the rotor <NUM> to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub <NUM> may be rotatably coupled to an electric generator <NUM> (<FIG>) positioned within the nacelle <NUM> to permit electrical energy to be produced.

The wind turbine <NUM> may also include a turbine controller <NUM> centralized within the nacelle <NUM>. However, in other embodiments, the controller <NUM> may be located within any other component of the wind turbine <NUM> or at a location outside the wind turbine <NUM>. Further, the controller <NUM> may be communicatively coupled to any number of the components of the wind turbine <NUM> in order to control the operation of such components and/or implement a corrective or control action. As such, the controller <NUM> may include a computer or other suitable processing unit. Thus, in several embodiments, the controller <NUM> may include suitable computer-readable instructions that, when implemented, configure the controller <NUM> to perform various different functions, such as receiving, transmitting and/or executing wind turbine control signals. Accordingly, the controller <NUM> may generally be configured to control the various operating modes (e.g., start-up or shut-down sequences), de-rating or up-rating the wind turbine, and/or individual components of the wind turbine <NUM>.

Referring now to <FIG>, a simplified, internal view of one embodiment of the nacelle <NUM> of the wind turbine <NUM> shown in <FIG> is illustrated. As shown, a generator <NUM> may be disposed within the nacelle <NUM> and supported atop a bedplate <NUM>. In general, the generator <NUM> may be coupled to the rotor <NUM> for producing electrical power from the rotational energy generated by the rotor <NUM>. For example, as shown in the illustrated embodiment, the rotor <NUM> may include a rotor shaft <NUM> coupled to the hub <NUM> for rotation therewith. The rotor shaft <NUM> may, in turn, be rotatably coupled to a generator shaft <NUM> of the generator <NUM> through a gearbox <NUM>. As is generally understood, the rotor shaft <NUM> may provide a low speed, high torque input to the gearbox <NUM> in response to rotation of the rotor blades <NUM> and the hub <NUM>. The gearbox <NUM> may then be configured to convert the low speed, high torque input to a high speed, low torque output to drive the generator shaft <NUM> and, thus, the generator <NUM>.

The wind turbine <NUM> may also one or more pitch drive mechanisms <NUM> communicatively coupled to the turbine controller <NUM>, with each pitch drive mechanisms(s) <NUM> being configured to rotate a pitch bearing <NUM> and thus the individual rotor blade(s) <NUM> about its respective pitch axis <NUM>. In addition, as shown, the wind turbine <NUM> may include one or more yaw drive mechanisms <NUM> configured to change the angle of the nacelle <NUM> relative to the wind (e.g., by engaging a yaw bearing <NUM> of the wind turbine <NUM> that is arranged between the nacelle <NUM> and the tower <NUM> of the wind turbine <NUM>).

In addition, the wind turbine <NUM> may also include one or more sensors <NUM>, <NUM> for monitoring various wind conditions of the wind turbine <NUM>. For example, the incoming wind direction <NUM>, wind speed, or any other suitable wind condition near of the wind turbine <NUM> may be measured, such as through use of a suitable weather sensor <NUM>. Suitable weather sensors may include, for example, Light Detection and Ranging ("LIDAR") devices, Sonic Detection and Ranging ("SODAR") devices, anemometers, wind vanes, barometers, radar devices (such as Doppler radar devices) or any other sensing device which can provide wind directional information now known or later developed in the art. Still further sensors <NUM> may be utilized to measure additional operating parameters of the wind turbine <NUM>, such as voltage, current, vibration, etc. as described herein.

Referring now to <FIG> and <FIG>, schematic diagrams of certain embodiments of a wind turbine power system <NUM> is illustrated in accordance with aspects of the present disclosure. In particular, <FIG> illustrates a schematic view of one embodiment of the wind turbine electrical power system <NUM> suitable for use with the wind turbine <NUM> shown in <FIG>, whereas <FIG> illustrates a simplified equivalent circuit of the wind turbine electrical power system <NUM>. Although the present disclosure will generally be described herein with reference to the wind turbine electrical power system <NUM> shown in <FIG> and <FIG>, those of ordinary skill in the art, using the disclosures provided herein, should understand that aspects of the present disclosure may also be applicable in other power generation systems, and, as mentioned above, that the invention is not limited to wind turbine systems.

In the embodiment of <FIG> and as mentioned, the rotor <NUM> of the wind turbine <NUM> (<FIG>) may, optionally, be coupled to the gearbox <NUM>, which is, in turn, coupled to a generator <NUM>, which may be a doubly fed induction generator (DFIG) as described herein. As shown in <FIG> and <FIG>, the DFIG <NUM> may be connected to a stator bus <NUM>. Further, as shown in <FIG> and <FIG>, a power converter <NUM> may be connected to the DFIG <NUM> via a rotor bus <NUM>, and to the stator bus <NUM> via a line side bus <NUM>. As such, the stator bus <NUM> may provide an output multiphase power (e.g., three-phase power) from a stator of the DFIG <NUM>, and the rotor bus <NUM> may provide an output multiphase power (e.g., three-phase power) from a rotor of the DFIG <NUM>. The power converter <NUM> may also include a rotor side converter (RSC) <NUM> and a line side converter (LSC) <NUM>. The DFIG <NUM> is coupled via the rotor bus <NUM> to the RSC <NUM>. Additionally, the RSC <NUM> is coupled to the LSC <NUM> via a DC link <NUM> across which is a DC link capacitor <NUM>. The LSC <NUM> is, in turn, coupled to the line side bus <NUM>. The power converter <NUM> may also include a dynamic brake <NUM> as shown in <FIG>.

The RSC <NUM> and the LSC <NUM> may be configured for normal operating mode in a three-phase, pulse width modulation (PWM) arrangement using one or more switching devices, such as insulated gate bipolar transistor (IGBT) switching elements. In addition, as shown in <FIG>, the power converter <NUM> may be coupled to a converter controller <NUM> in order to control the operation of the RSC <NUM> and/or the LSC <NUM> as described herein. It should be noted that the converter controller <NUM> may be configured as an interface between the power converter <NUM> and the turbine controller <NUM> and may include any number of control devices.

In typical configurations, various line contactors and circuit breakers including, for example, a grid breaker <NUM> may also be included for isolating the various components as necessary for normal operation of the DFIG <NUM> during connection to and disconnection from a load, such as the electrical grid <NUM>. For example, a system circuit breaker <NUM> may couple a system bus <NUM> to a transformer <NUM>, which may be coupled to the electrical grid <NUM> via the grid breaker <NUM>. In alternative embodiments, fuses may replace some or all of the circuit breakers.

In operation, alternating current power generated at the DFIG <NUM> by rotating the rotor <NUM> is provided to the electrical grid <NUM> via dual paths defined by the stator bus <NUM> and the rotor bus <NUM>. On the rotor-bus side, sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power is provided to the power converter <NUM>. The RSC <NUM> converts the AC power provided from the rotor bus <NUM> into direct current (DC) power and provides the DC power to the DC link <NUM>. As is generally understood, switching elements (e.g., IGBTs) used in the bridge circuits of the RSC <NUM> may be modulated to convert the AC power provided from the rotor bus <NUM> into DC power suitable for the DC link <NUM>.

In addition, the LSC <NUM> converts the DC power on the DC link <NUM> into AC output power suitable for the electrical grid <NUM>. In particular, switching elements (e.g., IGBTs) used in bridge circuits of the LSC <NUM> can be modulated to convert the DC power on the DC link <NUM> into AC power on the line side bus <NUM>. The AC power from the power converter <NUM> can be combined with the power from the stator of DFIG <NUM> to provide multi-phase power (e.g., three-phase power) having a frequency maintained substantially at the frequency of the electrical grid <NUM> (e.g., <NUM> or <NUM>).

Additionally, as shown in <FIG>, various circuit breakers and switches, such as grid breaker <NUM>, system breaker <NUM>, stator sync switch <NUM>, converter breaker <NUM>, and line contactor <NUM> may be included in the wind turbine power system <NUM> to connect or disconnect corresponding buses, for example, when current flow is excessive and may damage components of the wind turbine power system <NUM> or for other operational considerations. Additional protection components may also be included in the wind turbine power system <NUM>.

Moreover, the power converter <NUM> may receive control signals from, for instance, the local control system <NUM> via the converter controller <NUM>. The control signals may be based, among other things, on sensed states or operating characteristics of the wind turbine power system <NUM>. Typically, the control signals provide for control of the operation of the power converter <NUM>. For example, feedback in the form of a sensed speed of the DFIG <NUM> may be used to control the conversion of the output power from the rotor bus <NUM> to maintain a proper and balanced multi-phase (e.g., three-phase) power supply. Other feedback from other sensors may also be used by the controller(s) <NUM>, <NUM> to control the power converter <NUM>, including, for example, stator and rotor bus voltages and current feedbacks. Using the various forms of feedback information, switching control signals (e.g., gate timing commands for IGBTs), stator synchronizing control signals, and circuit breaker signals may be generated.

The power converter <NUM> also compensates or adjusts the frequency of the three-phase power from the rotor for changes, for example, in the wind speed at the hub <NUM> and the rotor blades <NUM>. Therefore, mechanical and electrical rotor frequencies are decoupled and the electrical stator and rotor frequency matching is facilitated substantially independently of the mechanical rotor speed.

Under some states, the bi-directional characteristics of the power converter <NUM>, and specifically, the bi-directional characteristics of the LSC <NUM> and RSC <NUM>, facilitate feeding back at least some of the generated electrical power into generator rotor. More specifically, electrical power may be transmitted from the stator bus <NUM> to the line side bus <NUM> and subsequently through the line contactor <NUM> and into the power converter <NUM>, specifically the LSC <NUM> which acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into the DC link <NUM>. The DC link capacitor <NUM> facilitates mitigating DC link voltage amplitude variations by facilitating mitigation of a DC ripple sometimes associated with three-phase AC rectification.

The DC power is subsequently transmitted to the RSC <NUM> that converts the DC electrical power to a three-phase, sinusoidal AC electrical power by adjusting voltages, currents, and frequencies. This conversion is monitored and controlled via the converter controller <NUM>. The converted AC power is transmitted from the RSC <NUM> via the rotor bus <NUM> to the generator rotor. In this manner, generator reactive power control is facilitated by controlling rotor current and voltage.

Referring now to <FIG>, the wind turbine power system <NUM> described herein may be part of a wind farm <NUM>. As shown, the wind farm <NUM> may include a plurality of wind turbines <NUM>, including the wind turbine <NUM> described above, and an overall farm-level controller <NUM>. For example, as shown in the illustrated embodiment, the wind farm <NUM> includes twelve wind turbines, including wind turbine <NUM>. However, in other embodiments, the wind farm <NUM> may include any other number of wind turbines, such as less than twelve wind turbines or greater than twelve wind turbines. In one embodiment, the turbine controllers of the plurality of wind turbines <NUM> are communicatively coupled to the farm-level controller <NUM>, e.g., through a wired connection, such as by connecting the turbine controller <NUM> through suitable communicative links <NUM> (e.g., a suitable cable). Alternatively, the turbine controllers may be communicatively coupled to the farm-level controller <NUM> through a wireless connection, such as by using any suitable wireless communications protocol known in the art. In further embodiments, the farm-level controller <NUM> is configured to send and receive control signals to and from the various wind turbines <NUM>, such as for example, distributing real and/or reactive power demands across the wind turbines <NUM> of the wind farm <NUM>.

Referring now to <FIG>, a block diagram of one embodiment of suitable components that may be included within the controller (such as any one of the turbine controller <NUM>, the converter controller <NUM>, and/or the farm-level controller <NUM> described herein) in accordance with example aspects of the present disclosure is illustrated. As shown, the controller may include one or more processor(s) <NUM>, computer, or other suitable processing unit and associated memory device(s) <NUM> that may include suitable computer-readable instructions that, when implemented, configure the controller to perform various different functions, such as receiving, transmitting and/or executing wind turbine control signals (e.g., performing the methods, steps, calculations and the like disclosed herein).

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, and other programmable circuits. Additionally, the memory device(s) <NUM> may 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) <NUM> may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s) <NUM>, configure the controller to perform various functions as described herein. Additionally, the controller may also include a communications interface <NUM> to facilitate communications between the controller and the various components of the wind turbine <NUM>. An interface can include one or more circuits, terminals, pins, contacts, conductors, or other components for sending and receiving control signals. Moreover, the controller may include a sensor interface <NUM> (e.g., one or more analog-to-digital converters) to permit signals transmitted from the sensors <NUM>, <NUM> to be converted into signals that can be understood and processed by the processor(s) <NUM>.

Referring particularly to <FIG>, as mentioned, a one-line circuit diagram of the wind turbine power system <NUM> is illustrated. More particularly, as shown, the power (PT) generated by the wind turbine power system <NUM> is the sum of the power from the generator stator (PS) and the LSC <NUM> (PL), given by Equation (<NUM>) below: <MAT>.

Further, the power from the LSC <NUM> (PL) can be approximated by assuming all the power from the rotor (PR) of the DFIG <NUM> passes to the LSC <NUM>, as given in Equation (<NUM>) below: <MAT> wherein the slip is defined by the relationship of Equation (<NUM>) provided below: <MAT> wherein ωelec is the electrical frequency of the wind turbine power system <NUM>, and ωrot is the rotor speed of the rotor <NUM> of the wind turbine power system <NUM>.

Thus, in an embodiment, by combining the aforementioned relationships, the ratio of the stator power (PS) to the total power (PT) can be expressed using Equation (<NUM>) below: <MAT>.

Still further relationships illustrated in <FIG>, such as voltage (V), current (I), and impedance (X), etc., will be described in more detail herein.

Furthermore, as shown in <FIG>, the magnetizing reactance (Xm) of the DFIG <NUM> varies significantly with flux due to the non-linear permeability of the materials used in the construction of the generator iron core. Other factors that determine the magnetizing reactance are the stator and rotor winding turns, dimensions of stator and rotor, and material used in the construction. While these factors vary among different generator ratings and designs, the non-linear characteristic of the magnetizing reactance generally follows a characteristic curve as shown in <FIG>.

Referring now to <FIG>, a schematic diagram of one embodiment of a system <NUM> for providing grid-forming control of a double-fed generator of a wind turbine according to the present disclosure is illustrated. More specifically, as shown, the system <NUM> may include many of the same features of <FIG> described herein, with components having the same reference characters representing like components. Further, as shown, the system <NUM> may include a control structure for controlling the line-side converter that is similar to the control structure shown in <FIG>.

Moreover, as shown, the LSC control structure may include a DC regulator <NUM> and a line current regulator <NUM>. The DC regulator <NUM> is configured to generate line-side current commands for the line current regulator <NUM>. The line current regulator <NUM> then generates line-side voltage commands for a modulator <NUM>. The modulator <NUM> also receives an output (e.g., a phase-locked loop angle) from a phase-locked loop <NUM> to generate one or more gate pulses for the LSC <NUM>. The phase-locked loop <NUM> typically generates its output using a voltage feedback signal.

Furthermore, as shown, the system <NUM> may also include a control structure for controlling the RSC <NUM> using grid-forming characteristics. In particular, as shown in <FIG>, the system <NUM> may include a stator voltage regulator <NUM> for providing such grid-forming characteristics. In addition, as shown, the system <NUM> may include a grid voltage/VAR regulator <NUM>, an inertial power regulator <NUM>, a rotor current regulator <NUM>, and a modulator <NUM>.

In an embodiment, the grid voltage/VAR regulator <NUM> receives a voltage reference (e.g., VT_REF) from the farm-level controller <NUM> and generates a stator voltage magnitude command (e.g., VS_Mag_Cmd), whereas the inertial power regulator receives a power reference from the turbine controller <NUM> and generates a stator voltage angle command (e.g., VS_Angle_Cmd). More specifically, in an embodiment, as shown, the stator voltage regulator <NUM> determines one or more rotor current commands (e.g., IRCmdy and IRCmdx) as a function of the stator voltage magnitude command, the stator voltage angle command, and/or a stator current feedback signal <NUM> of the DFIG <NUM>. It should be understood that the stator feedback current <NUM> is a strong indicator of the characteristics of the externally connected power system, i.e., the grid. Therefore, the stator feedback current <NUM> can be used as a feedback signal to decouple the response of stator voltage to variations to the nature of the grid. Further details relating to the stator voltage regulator <NUM> are further explained and described in <CIT> entitled "System and Method for Providing Grid-Forming Control for a Doubly-Feb Wind Turbine Generator," which is incorporated herein by reference in its entirety.

As mentioned, with grid-forming control, current changes rapidly when there are grid disturbances. Further, the control action is gradual to restore the steady-state operating conditions commanded by higher-level controls. The amount of current change is inversely related to the total impedance of the circuit. However, if the current exceeds limits, then the control responds rapidly to force the current to be within limits. This drastic nonlinearity can cause chaotic behavior when applied to a grid consisting of many other similar systems. Alternatively, if the current change is too small, then the grid-forming system will not contribute as much as it could to support the grid.

Thus, <FIG> generally describe methods <NUM>, <NUM> and a system <NUM> for providing grid-forming control of a double-fed wind turbine generator, such as DFIG <NUM> according to the present disclosure. Referring particularly to <FIG>, a flow diagram of one embodiment of a method <NUM> method for controlling a wind turbine power system connected to an electrical grid, such as wind turbine power system <NUM>, according to the present disclosure is illustrated. It should be appreciated that the disclosed method <NUM> may be implemented with any suitable double-fed wind turbine generator having any suitable configuration. In addition, although <FIG> depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure.

As shown at (<NUM>), the method <NUM> includes determining, via a controller, at least one non-linear magnetizing parameter of the DFIG <NUM>. In an embodiment, the non-linear magnetizing parameter(s) may be a magnetizing reactance Xm. For example, in an embodiment, various tests can be performed on the DFIG <NUM> to measure the non-linear magnetizing characteristics. In one embodiment, such tests may include a no-load test. In such embodiments, this test may include running the DFIG <NUM> with zero slip and exciting the DFIG <NUM> through the stator winding. Measurements for a range of applied stator voltage levels while measuring stator current can be used to estimate the saturation curve. Furthermore, in such embodiments, the magnetizing reactance can be estimated using voltage and current measurements using Equation (<NUM>) below: <MAT> where the rotor current is assumed to be zero at zero slip and the flux can be calculated from the stator voltage using Equation (<NUM>) below: <MAT> where ωelec is the frequency of the voltage and Equations (<NUM>) and (<NUM>) reflect per-unit quantities.

Referring particularly to <FIG>, as an example, a flow diagram of one embodiment of a method <NUM> for estimating the non-linear magnetizing characteristic(s) is illustrated. As shown, the non-linear magnetizing characteristic(s) can be estimated by controlling the rotor converter of the DFIG <NUM> before the stator switch is closed to begin operation. All quantities are in per-unit. More particularly, as shown, the method <NUM> begins at <NUM> with (a) providing a plurality of data arrays including, for example, at least, a data array of operating data points and a data array of non-linear magnetizing parameter data points. In several embodiments, the data array of operating data points may include, for example, flux data points or stator voltage data points. For example, in one embodiment, the method <NUM> may include programming the controller with pre-defined arrays of test points for flux, given by Equation (<NUM>) below: <MAT>.

Moreover, in particular embodiments, the data array of operating data points may include, at least, a range of expected operating data points that the DFIG <NUM> is expected to operate during normal operation. In another embodiment, the data array of operating data points may include one or more additional data points to capture operating data points beyond the normal operation to estimate one or more characteristics of the DFIG <NUM> for at least one of abnormal conditions or temporary conditions.

In addition, as shown at (<NUM>), the method <NUM> may also include enabling rotor control of the DFIG <NUM> with the stator sync switch <NUM> open. As shown at (<NUM>), the method <NUM> includes (c) setting an operating set point of the DFIG <NUM> equal to a first operating data point in the data array of operating data points. As shown at (<NUM>), the method <NUM> includes (d) controlling the rotor-side converter to the operating set point and frequency for a time period (e.g., such as the desired flux and frequency).

The method <NUM> may also include (e) collecting current and voltage feedbacks for the time period. More particularly, as shown at (<NUM>), the method <NUM> may include averaging the collected current and voltage feedbacks for the time period to remove noise and calculating the non-linear magnetizing parameter(s) based on the averaged current and voltage feedbacks for the time period. Thus, as shown at (<NUM>), the method <NUM> includes (f) calculating the non-linear magnetizing parameter(s) based on the current and voltage feedbacks for the time period. For example, in an embodiment, the averaged values can then be used to calculate the magnetizing reactance using Equation (<NUM>) below: <MAT> where average flux values are calculated based on the stator voltage feedbacks using Equation (<NUM>) below: <MAT>.

In addition, as shown at (<NUM>), the method <NUM> may also include (g) storing the operating set point and the non-linear magnetizing parameter(s) together in a data array. For example, in an embodiment, the averaged values of flux and current may be stored together with a calculated value of Xm. This calculated value for Xm may also be stored in an array as presented in Equation (<NUM>) below: <MAT>.

The method <NUM> also includes (h) setting the operating set point of the DFIG <NUM> equal to a remainder of the operating data points in the data array for subsequent time periods. Thus, as shown at (<NUM>), the method <NUM> includes determining whether the method <NUM> has gone through each of the operating data points in the data array. More specifically, the method <NUM> includes (i) repeating any of the method steps described herein for each of the operating data points in the data array for subsequent time periods. If not, the method <NUM> continues as shown via arrow <NUM> repeating the steps need to complete the data array. Once all of the operating data points have been tested, the method <NUM> proceeds to step (<NUM>).

In certain embodiments, the data collected from the aforementioned test may be post-processed and analyzed separately after the testing is carried out to determine the generator magnetizing characteristics. Modern double-fed wind turbines have a wide range of ratings and generator types used for applications. Additionally, identical generator designs practically exhibit different magnetizing characteristics due to nonuniformity in construction or materials. For these reasons, it is not practical to perform these tests separately for each generator, nor is it practical to use the same curve for all generators. Therefore, the methods described herein for estimating the magnetizing reactance automatically use the existing converter hardware and feedbacks for use in grid-forming converter control systems in the DFIG <NUM>.

Referring to <FIG> and <FIG>, as shown at (<NUM>), the method <NUM> also includes developing, via the controller, a model of the non-linear magnetizing parameter(s) of the DFIG <NUM>. For example, in certain embodiments, developing the model of the non-linear magnetizing parameter(s) of the DFIG <NUM> may include creating a saturation curve <NUM> of the non-linear magnetizing parameter(s) versus the operating set point. For example, in an embodiment, the saturation curve <NUM> may include a piecewise-linear curve fit. More particularly, in an embodiment, the method <NUM> may include collecting the measurements at a range of stator voltage levels allows for construction of the non-linear magnetizing parameter(s) (e.g., the magnetizing reactance Xm) versus flux curve similar to that shown in <FIG>. Moreover, as shown at (<NUM>) of <FIG>, the method <NUM> may include fitting the operating data points to a curve fit, e.g., using an interpolation algorithm.

For example, in particular embodiments, a piecewise-linear curve fit can be obtained by connecting two consecutive points on the curve. Each linear segment can be approximated by a straight line with slope (Slp) and y-intercept (Icpt), using for example, Equations (<NUM>) and (<NUM>) below: <MAT> <MAT>.

In addition, in an embodiment, a linear segment can be calculated for the entire data set, giving arrays of the slopes and y-intercepts as represented by Equations (<NUM>) and (<NUM>) below: <MAT> <MAT> where the length of such arrays is N = k-<NUM>. In such embodiments, this collection of slopes and y-intercepts calculated based on measured data can be used to model the magnetizing characteristic for the voltage regulator in the grid-forming control.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes using, via the controller, the model in a stator voltage regulator of the double-fed wind turbine generator to provide grid-forming control of the DFIG <NUM>. More particularly, in an embodiment, integration of the magnetizing characteristic model (e.g., the saturation curve <NUM>) into a grid forming stator voltage regulator <NUM> is shown in <FIG>. For example, as shown in <FIG>, the stator voltage regulator <NUM> is configured to receive a higher level command (e.g., EI) for magnitude of the stator voltage and a higher level command (e.g., δIT) for angle of the stator voltage with respect to the phase-locked loop angle. Further, as shown, the stator voltage commands are converted to flux commands using a signal indicating electrical frequency ωelec. Moreover, the signals drawn in bold represent two-dimensional complex phasors. Thus, as shown, the stator voltage regulator <NUM> is configured to convert the voltage command(s) to a stator flux command (e.g., ΦS_Cmd_xy) as shown at <NUM>. Moreover, as shown, the stator voltage regulator <NUM> may determine or otherwise calculate an expected magnetizing reactance Xmag at a desired flux level using the stator flux command with the slopes and the y-intercepts of the saturation curve <NUM>. Thus, as shown at <NUM>, the stator voltage regulator <NUM> may then determine a magnetizing current feed forward signal (e.g., IM_FF_xy) as a function of the stator flux command and a function of the magnetizing reactance Xmag (e.g., <NUM>/Xmag) from the saturation curve <NUM>. This implementation assumes that the stator leakage flux is negligible compared to the magnetizing flux. More specifically, the process may utilize Equations (<NUM>) through (<NUM>) below: <MAT> For every Nth linear segment, Xmag is evaluated using Equation (<NUM>) below: <MAT> Assuming the slopes are descending and the y-intercepts are ascending for any two consecutive linear segments, the final Xmag, which is used in the calculation of the magnetizing current feed forward signal, is estimated to be the minimum of all N Xmag calculations, for example, using Equation (<NUM>) below: <MAT>.

This process may be repeated at every execution step of the controller as needed. In addition, as shown, the stator voltage regulator <NUM> may also receive a stator voltage feedback signal (e.g., VS_Fbk_xy) and convert the stator voltage feedback signal into a flux feedback signal as shown at <NUM> using the electrical frequency ωelec. Thus, as shown at <NUM>, the stator voltage regulator <NUM> may then determine a difference between the flux feedback signal and the stator flux command. Further, as shown, the stator voltage regulator <NUM> may also determine a magnetizing current correction signal (e.g., IM_Corr_xy) via a proportional-integral regulator <NUM>. Accordingly, as shown at <NUM>, the stator voltage regulator <NUM> can then add the magnetizing current feed forward signal (e.g., IM_FF_xy) to the magnetizing current correction signal (IM_Corr_xy) from the power regulator to determine the magnetizing current command (e.g., IM_Cmd_xy).

Claim 1:
A method for controlling a wind turbine power system (<NUM>) connected to an electrical grid, the wind turbine power system (<NUM>) having a double-fed wind turbine generator (<NUM>) coupled to a power converter (<NUM>) having a line-side converter (<NUM>) and a rotor-side converter (<NUM>) coupled together via a DC link (<NUM>), the method comprising:
determining, via a controller (<NUM>), at least one non-linear magnetizing parameter (Xm) of the double-fed wind turbine generator (<NUM>), wherein determining the at least one non-linear magnetizing parameter (Xm) of the double-fed wind turbine generator (<NUM>) further comprises:
(a) providing a plurality of data arrays comprising, at least, a data array of operating data points and a data array of non-linear magnetizing parameter (Xm) data points;
(b) enabling rotor control of the double-fed wind turbine generator (<NUM>) with a stator switch open;
(c) setting an operating set point of the double-fed wind turbine generator (<NUM>) equal to a first operating data point in the data array of operating data points;
(d) controlling the rotor-side converter (<NUM>) to the operating set point and frequency for a time period;
(e) collecting current and voltage feedbacks (VS_Fbk_xy, IS_Fbk_xy) for the time period; and
(f) calculating the at least one non-linear magnetizing parameter (Xm) based on the current and voltage feedbacks (VS_Fbk_xy, IS_Fbk_xy) for the time period;
developing, via the controller (<NUM>), a model (<NUM>) of the at least one non-linear magnetizing parameter (Xm) of the double-fed wind turbine generator (<NUM>); and,
using, via the controller (<NUM>), the model (<NUM>) in a stator voltage regulator (<NUM>) of the double-fed wind turbine generator (<NUM>) to provide grid-forming control of the double-fed wind turbine generator (<NUM>).