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 an electrical 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.

In addition, the reduction in the proportion of synchronous machines with respect to inverter-based resources, which determine the grid defining parameters voltage and frequency, have contributed to decreasing stability margins. The immediate consequence of the decreased stability margins is a grid collapse when subjected to voltage and frequency disturbances in the grid.

Accordingly, many existing inverter-based resources, such as doubly-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. Thus, turbine-controls with grid-following converters are designed to inject maximum power available from the wind, independent of generation/load balance in the grid. More specifically, <FIG> illustrates the basic elements of the main circuit and converter control structure for a grid-following doubly-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 doubly-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 type converters provide a voltage-source characteristic, where the angle and magnitude of the voltage are controlled to achieve the regulation functions needed by the grid. Thus, grid-forming converters participate in generation-load balance in a similar way as conventional generators based on synchronous machines. Therefore, wind turbines with grid-forming converter controls requires the turbine controls to manage both the power output from available wind together with the power demands of the grid. 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 Double-Feb Wind Turbine Generator. " Another example is <CIT>.

As an example, <FIG> illustrates a schematic diagram of one embodiment of a main circuit of a grid-forming system. As shown, the main circuit includes a power-electronic converter with connections on DC and AC sides. This converter receives gating commands from a controller that creates an AC voltage phasor Vcnv at an angle of Thvcnv. The angle is with respect to a reference phasor having a fixed frequency. The DC side is supplied with a device capable of generating or absorbing power for even a short duration. Such devices may include, for example, batteries, solar panels, rotating machines with a rectifier, or capacitors. In addition, as shown, the circuit includes an inductive impedance Xcnv connecting the converter to its point of interconnection, shown as the voltage Vt and angle ThVt in <FIG>. The electrical system behind the point of interconnect is shown as a Thevenin equivalent with impedance Zthev and voltage Vthev at angle ThVthev. This equivalent can be used to represent any circuit, including grid-connected and islanded circuits with loads. In practical situations, the impedance Zthev will be primarily inductive.

Still referring to <FIG>, the closed-loop portion of the main control receives feedback signals from the voltage and current at the point of interconnection. Additional inputs are received from higher-level controls (not shown). While <FIG> illustrates a single converter as an example, any grouping of equipment that can create an electrical equivalent of a controlled voltage Vcnv behind an impedance Xcnv can have the control schemes disclosed applied to achieve the same performance benefits.

Referring now to <FIG>, a control diagram for providing grid-forming control according to conventional construction is illustrated. As shown, a converter controller <NUM> receives references (e.g., Vref and Pref) and limits (e.g., VcmdLimits and PcmdLimits) from higher-level controls <NUM>. These high-level limits are on physical quantities of voltage, current, and power. The main regulators include a fast voltage regulator <NUM> and a slow power regulator <NUM>. These regulators <NUM>, <NUM> have final limits applied to the converter control commands for voltage magnitude (e.g., VcnvCmd) and angle (e.g., θPang and θPLL) to implement constraints on reactive- and real-components of current, respectively. Further, such limits are based upon a predetermined fixed value as a default, with closed-loop control to reduce the limits should current exceed limits.

Accordingly, the present disclosure is directed to systems and methods for controlling an inverter-based resource that is responsive to grid frequency excursions, while also allowing for maximizing power generation when grid-frequency conditions are normal.

In one aspect, the present disclosure is directed to a method for controlling an inverter-based resource having a power converter connected to an electrical grid. The method includes receiving, via a controller, a first power limit signal for the inverter-based resource from an external controller. The method also includes receiving, via a local power constraint module of the controller, a second power limit signal for the inverter-based resource. Further, the method includes determining, via the controller, a constrained power limit signal based on the first and second power limit signals. Moreover, the method includes applying, via the controller, a first frequency droop function to the constrained power limit signal. The method also includes determining, via a maximum power tracking algorithm of the controller, at least one of a power reference signal or a pitch reference signal for the inverter-based resource as a function of an output of the first frequency droop function and the constrained power limit signal. In addition, the method includes determining, via the controller, one or more control commands for the inverter-based resource based on at least one of the power reference signal or the pitch reference signal. As such, the method includes controlling, via the controller, the inverter-based resource based on the one or more control commands so as to support a grid frequency of the electrical grid within power available at the inverter-based resource.

The method may also include adjusting, via the controller, the power reference signal using a second frequency droop function before determining the one or more control commands.

In an embodiment, a frequency reference for at least one of the first and second frequency droop functions is a filtered version of a frequency feedback of the inverter-based resource. In such embodiments, a filter bandwidth of the first frequency droop function is higher than a filter bandwidth of the second frequency droop function.

In the aspect, the inverter-based resource includes a wind turbine power system having at least one generator. In such embodiments, the one or more control commands include at least one of a power signal for a converter controller of the power converter or a pitch command for a pitch system of the wind turbine power system.

In the aspect, the controller is a turbine controller or a converter controller of the wind turbine power system.

In the aspect, the method may include determining, via the local power constraint module of the controller, the second power limit signal for the inverter-based resource by determining, via the controller, a compensation for the power reference signal to account for frequency droop operation, determining, via the controller, a compensated output power reference signal based on the power reference signal, and determining a final compensated power reference based on the compensation for the power reference signal and the compensated output power reference signal.

In an embodiment, determining the compensation for the power reference signal to account for frequency droop operation may include receiving, via the controller, a frequency grid reference signal and a frequency grid feedback signal from the electrical grid, determining a difference between the frequency grid reference signal and the frequency grid feedback signal, and applying, via the controller, a second frequency droop function to the difference to determine the compensation for the power reference signal.

In further embodiments, determining the compensation for the power reference signal to account for frequency droop operation may include compensating, via a first filtered differential element of the controller, an output of the second frequency droop function to determine the compensated output power reference signal.

In yet another embodiment, determining the compensated output power reference signal based on the power reference signal may include filtering, via the controller, the power reference signal and compensating, via a second filtered differential element of the controller, the filtered power reference signal to determine the compensated output power reference signal.

In additional embodiments, the method may include applying a margin offset to the final compensated power reference.

In still further embodiments, the method may include generating, via the controller, a frequency reference signal based on the frequency grid feedback signal from the electrical grid, and sending, via the controller, the frequency reference signal to a converter controller of the power converter. In such embodiments, the frequency reference signal drives a converter droop to zero during steady state and the power setpoint limit constrains the power setpoint closer to actual power being generated by the inverter-based resource, thereby allowing the first frequency droop function to respond to power demands from the electrical grid.

In particular embodiments, generating the frequency reference signal may include filtering, via one or more filters of the controller, the frequency grid feedback signal from the electrical grid. In such embodiments, the one or more filters may include at least one of a first-order low-pass filter or a rolling-average low-pass filter.

In one aspect, the present disclosure is directed to a system for controlling a wind turbine power system having a grid-forming power converter connected to an electrical grid according to the method. The system includes a turbine controller comprising at least one processor, the at least one processor configured to perform a plurality of operations, including receiving a first power limit signal for the wind turbine power system from an external controller, receiving a second power limit signal for the wind turbine power system, determining a constrained power limit signal based on the first and second power limit signals, applying a first frequency droop function to the constrained power limit signal, determining a power reference signal for the wind turbine power system as a function of an output of the first frequency droop function and the constrained power limit signal, adjusting the power reference signal using a second frequency droop function, determining one or more control commands for the wind turbine power system based on the adjusted power reference signal, and controlling the wind turbine power system based on the one or more control commands so as to support a grid frequency of the electrical grid within power available at the wind turbine power system. It should be understood that the system may further include any of the additional features and/or steps described herein.

Conventional thermal power generation plants utilize frequency droop control functions to share system active power loads among generators connected to the system and to support system frequency. Frequency droop control is characterized by changing active power generation in proportion to deviation in grid frequency in a direction that supports the grid frequency. Modern wind turbines based on grid-following converter technology do not utilize frequency droop for the purposes of sharing load with other generators, but rely on other generators (typically thermal power plants based on synchronous machines) to manage the variations in generation/load in the system. Thus, the wind turbines are generally allowed to inject the maximum power available from the local wind resource independent of system loads. Some exceptions to this behavior include larger (abnormal) frequency events, in which the wind turbine power output may be curtailed to mitigate any large imbalances in system generation/load.

Grid-forming converter technology responds to changes in system generation/load in a similar way as conventional (thermal) generation. Similar to conventional thermal generation, frequency droop is used in grid-forming converters to share loading among other parallel connected grid-forming resources. Unlike conventional power generation, however, the amount of power available from wind-turbines is less predictable due to variations in wind. The amount of support to system frequency in terms of active power is therefore constrained by local wind conditions. Thus, the present disclosure is directed systems and methods of power control of grid-forming inverter-based resources with frequency droop that are capable of supporting grid frequency while also respecting limitations in power availability from wind.

As used herein, inverter-based resources generally refer to electrical devices that can generate or absorb electric power through switching of power-electronic devices. Accordingly, inverter-based resource may include wind turbine generators, solar inverters, energy-storage systems, STATCOMs, or hydro-power systems. For example, in one embodiment, the inverter-based resource may be a wind turbine power system having a rotor-side converter, a line-side converter, and a doubly-fed induction generator (DFIG) connected to the electrical grid.

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 wind 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 wind turbine controller <NUM>, with each pitch adjustment mechanism(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>, a schematic diagram of one embodiment of a wind turbine power system <NUM> is illustrated in accordance with aspects of the present disclosure. Although the present disclosure will generally be described herein with reference to the wind turbine <NUM> shown in <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 shown, the DFIG <NUM> may be connected to a stator bus <NUM>. Further, as shown, 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 rotor-side converter <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 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, the power converter <NUM> may be coupled to a converter controller <NUM> in order to control the operation of the rotor-side converter <NUM> and/or the line-side converter <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 <NUM>, sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power is provided to the power converter <NUM>. The rotor-side converter <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 rotor-side converter <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 line-side converter <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 line-side converter <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 constrained 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, 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 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 converter controller <NUM>, the turbine 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 include 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 now to <FIG>, a system <NUM> for providing grid-forming control of a double-fed generator of a wind turbine according to the present disclosure is illustrated. In particular, <FIG> illustrates a schematic diagram of one embodiment of the system <NUM> according to the present disclosure, particularly illustrating a one-line diagram of the double-fed wind turbine generator <NUM> with a high-level control structure for grid-forming characteristics.

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>. More particularly, as shown, the line-side converter 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 line-side converter <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 unique control structure for controlling the rotor-side converter <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>.

More particularly, the system <NUM> includes an inner-loop current-regulator structure and a fast stator voltage regulator to convert voltage commands from the grid-forming controls to rotor current regulator commands. Thus, the system of the present disclosure provides control of the rotor voltage of the double-fed wind turbine generator <NUM> to meet a higher-level command for magnitude and angle of stator voltage. Such control must be relatively fast and insensitive to current flowing in the stator of the double-fed wind turbine generator <NUM>.

Furthermore, in an embodiment, the inertial power regulator <NUM> of this system <NUM> implements various functions, including (<NUM>) following the active power reference supplied by the turbine control, and (<NUM>) sharing power among other parallel connected resources. Following the active power reference supplied by the turbine control is practically achieved through modification of the angle command to the stator voltage control, whereas sharing the power among other parallel connected resources is practically achieved through a frequency droop.

Referring now to <FIG>, an expanded block diagram of the inertial power regulator <NUM> with frequency droop is provided. As shown, the frequency reference signal ωREF and the phase lock loop frequency signal ωPLL being constrained to generate the frequency error signal Eω. As can be seen, the ωPLL signal, which represents the actual frequency of the inverter output is subtracted from the ωREF signal in a summing junction <NUM> to generate the Eω error signal. The Eω error signal is provided to a frequency bias circuit having a first control loop including a conventional proportional plus integral regulator <NUM> and a deadband circuit <NUM>. The deadband circuit <NUM> provides some range of variation of the frequency error signal, for example, approximately <NUM>/<NUM> without any change of output signal. This limits response due to natural fluctuations of the power system frequency. The proportional plus integral regulator <NUM> converts the error signal to a conventional bias signal which is applied to a summing junction <NUM>.

A second loop includes a proportional droop circuit <NUM> which may be an amplifier with a fixed gain that receives the Eω error signal and provides an immediate compensation signal to the summing junction <NUM>, the compensation signal being added to the output signal from the proportional plus integral regulator <NUM>. The output of the summing junction <NUM> is a power offset signal which is coupled to a summing junction <NUM> whose other input is the power reference signal PREF. Accordingly, the frequency offset signal from summing junction <NUM> serves to modify the power reference signal PREF. The purpose of such modification is to adjust the power reference signal PREF as a function of frequency shifts. More particularly, the intent of the system is to attempt to hold the system output frequency constant so that if there is an error between the output frequency and the reference frequency, the power reference signal PREF is adjusted to compensate for the frequency error.

Thus, as shown, the proportional droop circuit <NUM> modifies the power reference PREF from the turbine control by adding a droop term (i.e., the output from <NUM>) determined by the difference between the frequency reference and the actual frequency. Under normal conditions, the grid frequency is close to nominal and the droop term is zero. When there is an imbalance in generation and load, the grid frequency may deviate from nominal and the droop term will cause the power converter <NUM> to generate power different from the power reference PREF from the turbine control. The impact of this power deviation on the turbine control may be unintended changes in speed of the drivetrain, potentially leading to trips of the wind turbine <NUM>.

Still referring to <FIG>, the power regulator <NUM> also introduces an inertial regulator <NUM> which modifies the power error signal to simulate the inertia of synchronous machines. More particularly, the inertial regulator <NUM> prevents sudden frequency changes or power changes which can cause transient torques to be generated by the motors coupled to the inverter output if sudden changes in the inverter output are experienced. The inertial regulator <NUM> may include a conventional electronic circuit having the characteristics of a filtered differential element in that its output signal gradually increases in response to an increase in the input signal.

If the power reference signal is modified by the frequency bias circuit, the resultant signal identified as PORD is developed at an output terminal of the summing junction <NUM> and applied to a summation circuit <NUM> where the commanded power or ordered power is compared to the measured output power PB of the system. Note here that the signal PB represents the real power developed at the output of the inverter. The output signal from the summation circuit <NUM> represents the power error signal which is applied to the inertial regulator <NUM>. The signal developed by the inertial regulator as described above represents the desired frequency ω<NUM> of the internal voltage E<NUM> and, if the frequency is properly tracking, will be the same as the frequency ωPLL. In this regard, the signal ω<NUM> developed at the output of the inertial regulator <NUM> is summed in a summing junction <NUM> with the ωPLL signal. Any difference between the phase lock loop frequency and the signal ω<NUM> results in an error signal which is applied to a filtered differential element <NUM> to develop the δIT signal. In such embodiments, the filtered differential element <NUM> may be a conventional type of filtered differential element whose output signal δIT is an angle offset which can be summed with the output signal from the phase lock loop to generate the output signal θ<NUM>.

Referring now to <FIG>, a simplified, block diagram of the main inputs and outputs of the turbine controller <NUM> is provided. In an embodiment, the primary objective of the turbine controller <NUM> is to maximize power generated by the generator <NUM> based on available power from the wind and within the power constraint imposed by the power setpoint limit PwrSet. Typically, the turbine controller <NUM> achieves this objective by regulating the speed and active power of the generator <NUM>. Thus, the turbine controller <NUM> utilizes maximum power-point tracking algorithms to determine a power reference to the converter controller <NUM> and a pitch command to the pitch control to realize these control objectives.

Under normal grid conditions, the power setpoint (PwrSet) is set to nominal power rating of the generator <NUM>. The turbine controller <NUM> adjusts pitch and converter power references to maximize the power output within the power setpoint. Therefore, actual power may deviate significantly from the setpoint based on wind conditions, but generally stays below the power setpoint. Under curtailed conditions, the power setpoint is reduced below nominal power rating, but the controls continue to operate the same way but are constrained to a lower power. Note that the power setpoint may also be interpreted as a power limit, as the controller is allowed to produce as much power as possible within this constraint.

The changes include three main elements: (<NUM>) a local constraint combined with the power setpoint limit to obtain a constrained power limit, (<NUM>) Frequency reference supplied to grid-forming converter (ωREF) based on the filtered grid frequency feedback, and (<NUM>) frequency droop (Pdrp) adjustment to the constrained power setpoint limit. The first frequency droop function adjusts the constrained power setpoint limit to support the grid frequency, like conventional droop control functions in other types of generators.

Thus, certain modifications to the turbine and converter controller <NUM> and methods of operating same are provided by the present disclosure. In particular, and referring now to <FIG> and <FIG>, a method <NUM> and system <NUM> for controlling an inverter-based resource having a grid-forming power converter connected to an electrical grid according to the present disclosure are provided.

Referring particularly to <FIG>, a flow diagram of one embodiment of a method <NUM> and schematic diagrams of a system <NUM> for controlling an inverter-based resource having a grid-forming power converter connected to an electrical grid according to the present disclosure are provided. Thus, the method <NUM> of the present disclosure can be better understood with reference to the system <NUM> illustrated in <FIG> and <FIG>, which illustrates the modifications to the inverter-based resource controller <NUM> for grid-forming applications. As mentioned, the modifications include, at least, (<NUM>) a local constraint combined with the power setpoint limit to obtain a constrained power limit, (<NUM>) Frequency reference supplied to grid-forming converter (ωREF) based on the filtered grid frequency feedback, and (<NUM>) frequency droop (Pdrp) adjustment to the constrained power setpoint limit.

Accordingly, in such embodiments, the frequency droop function adjusts the power setpoint limit to support the grid frequency, like conventional droop control functions in other types of generators. Moreover, as shown, the frequency reference supplied to the converter controller <NUM> is based on a filtered version of the grid frequency feedback. Thus, this newly supplied frequency reference is configured to drive the converter droop to zero during steady state, allowing the frequency droop in the turbine controller <NUM> to respond to the power demands from the electrical grid. In addition, the new constrained power limit follows closer to actual power being generated by the generator <NUM>, thereby allowing the droop function to adjust power away from the present output based on the grid frequency.

Referring particularly to <FIG>, in an embodiment, for example, the inverter-based resource may be a wind turbine power system having at least one power converter coupled to a generator. In general, the method <NUM> is described herein with reference to the wind turbine power system <NUM> of <FIG>. However, it should be appreciated that the disclosed method <NUM> may be implemented with any other suitable power generation systems having any other suitable configurations. Further, the method <NUM> may be implemented using the turbine controller <NUM>, the converter controller <NUM>, or any other suitable control device or combinations thereof. 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, constrained, and/or adapted in various ways without deviating from the scope of the present disclosure.

As shown at (<NUM>), the method <NUM> includes receiving a first power limit signal for the inverter-based resource from an external controller. For example, as shown in <FIG>, the turbine controller <NUM> receives the first power limit Power Limit_1 from the farm-level controller <NUM>. Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes receiving, via a local power constraint module of the controller, a second power limit signal for the inverter-based resource. For example, as shown in <FIG>, the turbine controller <NUM> may include a local power constraint module <NUM> that generates the second power limit Power_Limit_2 for the turbine controller <NUM>.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes determining, via the controller, a constrained power limit signal based on the first and second power limit signals. For example, as shown in <FIG>, the minimum module <NUM> is configured to determine the constrained power limit signal <NUM> as a function of the first and second power limit signals.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes applying, via the controller, a first frequency droop function to the constrained power limit signal <NUM>. For example, as shown in <FIG>, the turbine controller <NUM> may include the first frequency droop function <NUM> that can be applied to the constrained power limit signal <NUM>. In such embodiments, the first frequency droop function <NUM> generally includes one or more parameter settings defining the amount of power change from deviation in grid frequency.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes determining, via a maximum power tracking algorithm of the controller, at least one of a power reference signal or a pitch reference signal for the inverter-based resource as a function of an output of the first frequency droop function <NUM> and the constrained power limit signal <NUM>. For example, as shown in <FIG>, the turbine controller <NUM> may include the maximum power tracking algorithm <NUM> that can determine the power reference signal Power_ref for the converter controller <NUM> using the output <NUM> of the first frequency droop function <NUM> and the constrained power limit signal <NUM>.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes adjusting the power reference signal using a second frequency droop function <NUM>. For example, as shown in <FIG>, the converter controller <NUM> may include a power regulator module <NUM> that receives the power reference signal Power_ref and an output of the second frequency droop function <NUM> that can be used to adjust the power reference signal.

Moreover, as shown in <FIG>, a frequency reference (e.g., FrqRef1, FrqRef1) for at least one of the first and second frequency droop functions <NUM>, <NUM> may be a filtered version of a frequency feedback (e.g., FrqFbk1, FrqFbk2) of the inverter-based resource. Furthermore, in an embodiment, a filter bandwidth of the first frequency droop function <NUM> may be lower than a filter bandwidth of the second frequency droop function <NUM>.

More specifically, as shown in <FIG>, various components of the power regulator module <NUM> described herein are illustrated. As shown, the power regulator module <NUM> is configured to determine the second power limit signal Power_Limit_2 for the inverter-based resource by determining a compensation for the power reference signal Power_Ref to account for frequency droop operation and determining a compensated output power reference signal PrefCmpO based on the power reference signal Power_Ref. In such embodiments, determining the compensation for the power reference signal Power_Ref to account for frequency droop operation may include receiving a frequency grid reference signal ωgridRef and a frequency grid feedback signal ωgridFbk from the electrical grid, determining a difference between the frequency grid reference signal ωgridRef and the frequency grid feedback signal ωgridFbk via summing junction, and applying a frequency droop function to the difference to determine the compensation for the power reference signal Pdrpcmp.

In another embodiment, as shown, determining the compensation for the power reference signal Pdrpcmp to account for frequency droop operation may also include compensating, via a first filtered differential element, an output of a frequency droop function. Further, in an embodiment, as shown, the compensated output power reference signal PrefCmpO may be determined based on the power reference signal Power_Ref by filtering the power reference signal Power_Ref via one or more filters and compensating, via a second filtered differential element, the filtered power reference signal to determine the compensated output power reference signal PrefCmpO.

Thus, as shown at summing junction, the compensation for the power reference signal Pdrpcmp and the compensated output power reference signal PrefCmpO may be summed together to determine a final compensated power reference PrefCmp1. Accordingly, as shown at summing junction, a margin offset PrefMrg may be added to the to the final compensated power reference PrefCmp1 to help prevent unintended limiting of the power setpoint PwrSet due to wind-speed changes. The output of summing junction is thus the second power limit, Power_Limit_2.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes determining one or more control commands for the inverter-based resource based on at least one of the adjusted power reference signal or the pitch reference signal. For example, in an embodiment, as shown in <FIG>, the control command(s) may include a power regulating signal (e.g., PregACTUATOR) for a converter controller <NUM> or a pitch command for a pitch system of the wind turbine power system <NUM>. Thus, in such embodiments, the output <NUM> of the power regulator module <NUM> may correspond to the control command(s). Referring back to <FIG>, as shown at (<NUM>), the method <NUM> includes controlling the inverter-based resource based on the one or more control commands so as to support a grid frequency of the electrical grid within power available at the inverter-based resource.

Claim 1:
A method (<NUM>) for controlling an inverter-based resource (<NUM>) having a power converter (<NUM>) connected to an electrical grid, the inverter-based resource comprising a wind turbine power system (<NUM>) having at least one doubly-fed induction generator (<NUM>), DFIG, connectable to the electrical grid, the wind turbine power system (<NUM>) being part of a wind farm (<NUM>) comprising a farm-level controller (<NUM>, <NUM>), the method is characterised by comprising:
receiving (<NUM>), via a controller (<NUM>, <NUM>) comprising at least one of a turbine controller (<NUM>) and a converter controller (<NUM>) of the wind turbine power system (<NUM>), a first power limit signal (Power_Limit_1) for the inverter-based resource from the farm-level controller (<NUM>, <NUM>);
receiving (<NUM>), via a local power constraint module (<NUM>) of the controller (<NUM>, <NUM>), a second power limit signal (Power_Limit_2) for the inverter-based resource;
determining (<NUM>), via the controller (<NUM>, <NUM>), a constrained power limit signal (<NUM>) based on the first and second power limit signals (Power_Limit_1, Power_Limit_2);
applying (<NUM>), via the controller (<NUM>, <NUM>), a first frequency droop function (<NUM>) to the constrained power limit signal (<NUM>);
determining (<NUM>), via a maximum power tracking algorithm of the controller (<NUM>, <NUM>), at least one of a power reference signal (Power_ref) and a pitch reference signal for the inverter-based resource as a function of an output of the first frequency droop function (<NUM>) and the constrained power limit signal (<NUM>);
determining (<NUM>), via the controller (<NUM>, <NUM>), one or more control commands (PregACTUATOR) for the inverter-based resource based on at least one of the power reference signal (POWER_Ref) and the pitch reference signal; and
controlling (<NUM>), via the controller (<NUM>, <NUM>), the inverter-based resource based on the one or more control commands (PregACTUATOR) so as to support a grid frequency of the electrical grid within power available at the inverter-based resource (<NUM>).