Grid-forming control of inverter-based resource using virtual impedance

A method for providing grid-forming control of an inverter-based resource connected to an electrical grid includes providing, via a processor, at least one virtual impedance value of the inverter-based resource. The method also includes determining a voltage drop across the at least one virtual impedance value of the inverter-based resource using at least one current feedback signal, the voltage drop comprising a voltage magnitude and a voltage angle. Further, the method includes receiving one or more voltage or current signals of the inverter-based resource. Moreover, the method includes determining a control command for the inverter-based resource as a function of the voltage drop across the virtual impedance value(s) of the inverter-based resource and the one or more voltage or current signals.

FIELD

The present disclosure relates generally to inverter-based resources and, more particularly, to systems and methods for providing grid-forming control of an inverter-based resource using a virtual impedance.

BACKGROUND

Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, 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.1illustrates 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, the impedance of the GFM IBR is normally dictated by the hardware of the system, such as reactors, transformers, or rotating machine impedances. 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: (1) support grid voltage and frequency for any current flow within the rating of the equipment, both real and reactive; (2) 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); (3) 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; (4) share total load of the grid among other grid-forming sources connected to the grid; (5) ride through grid disturbances, both major and minor, and (6) meet requirements (1)-(5) 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 1990's (see e.g., U.S. Pat. No. 5,798,633 entitled “Battery Energy Storage Power Conditioning System”). Applications to full-converter wind generators and solar generators are disclosed in U.S. Pat. No. 7,804,184 entitled “System and Method for Control of a Grid Connected Power Generating System,” and U.S. Pat. No. 9,270,194 entitled “Controller for controlling a power converter.” Applications to grid-forming control for a doubly-fed wind turbine generator are disclosed in PCT/US2020/013787 entitled “System and Method for Providing Grid-Forming Control for a Doubly-Feb Wind Turbine Generator.”

In particular, a simple circuit of a full-conversion grid-forming inverter-based resource connected to a grid is shown inFIG.2, where the voltage E1and angle δ1reflect quantities synthesized by the grid-forming resource and Xtermis the reactance of the grid-forming resource. The steady-state power flow in the system is characterized by the following relationship:
PT=(E1Vthev/(Xterm+Xthev))*sin(δ1−θth)≅(E1Vthev/(Xterm+Xthev))*(δ1−θth)   Equation (1)

The power generated by the grid-forming resource depends on the external grid voltage (Vthev) and grid impedance (Xthev), which are generally unknown and changing. Therefore, for conventional systems, control of the grid-forming resource is practically realized by controlling the voltage source with respect to a locally measured voltage and angle (VTand θT). The active power equation can therefore be written as follows:
PT=(E1VT/Xterm)*sin(δ1T)≅(E1VT/Xterm)*δ1TEquation (2)
where δ1Treflects the difference between the grid-forming resource physical voltage angle and the locally measured angle. As such, the active power dynamics of the system are related to the impedance of the system as follows:
dPT/dδ1Tα1/XtermEquation (3)

Referring now toFIG.3, a schematic diagram for controlling active power and voltage for an inverter-based resource is illustrated. As shown, output E1reflects the desired converter voltage magnitude and output δ1Treflects the desired converter voltage angle with respect to a locally measured angle (θT). Accordingly, the active power output and voltage are controlled through manipulation of the converter voltage so that the resulting voltage drop across the internal reactance (Xterm) achieves the desired control objectives. This voltage drop is given by the following equation:
VT=E1−j*Xterm*ITEquation (4)

The Xterm, however, is dictated by the hardware of the power circuit and may include reactors and/or transformer impedance. Furthermore, with grid-forming control, current changes rapidly when there are grid disturbances. Therefore, for conventional systems, the control action is typically 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. As such, if the current exceed limits, the control responds rapidly to force the current to be within limits. However, 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, the system and method of the present disclosure are configured so that the effective impedance can be set as a parameter independent of the equipment physical characteristics. In particular, the present disclosure is directed to a system and method for creating a configurable virtual impedance in the GFM IBR to add flexibility in tuning the dynamics of the system.

BRIEF DESCRIPTION

In one aspect, the present disclosure is directed to a method for providing grid-forming control of an inverter-based resource connected to an electrical grid. The method includes providing, via a processor, at least one virtual impedance value of the inverter-based resource. The method also includes determining a voltage drop across the at least one virtual impedance value of the inverter-based resource using at least one current feedback signal. The voltage drop includes a voltage magnitude and a voltage angle. Further, the method includes receiving one or more voltage or current signals of the inverter-based resource. Moreover, the method includes determining a control signal for the inverter-based resource as a function of the voltage drop across the virtual impedance value(s) of the inverter-based resource and the one or more voltage or current signals.

In an embodiment, the virtual impedance value(s) may include, for example, an internal virtual impedance value at a node internal of the inverter-based resource or an external virtual impedance value at a node external of the inverter-based resource.

In one embodiment, for example, the virtual impedance value(s) may include the internal virtual impedance value. In such embodiments, the one or more voltage or current signals may include at least one of a virtual voltage magnitude command or a virtual voltage angle command behind the internal virtual impedance. Thus, in an embodiment, determining the control signal for the inverter-based resource as a function of the voltage drop across the at least one virtual impedance value of the inverter-based resource and the one or more voltage or current signals may include calculating a physical control command as a function of the virtual voltage magnitude command, the virtual voltage angle command, and the voltage drop. In another embodiment, calculating the physical control command for the inverter-based resource may include subtracting the voltage drop across the internal virtual impedance value from the virtual voltage command to obtain the physical control command for the inverter-based resource.

In further embodiments, the virtual impedance value(s) may include the external virtual impedance value. In such embodiments, the voltage or current signal(s) may include, for example, at least, a physical voltage feedback signal. Thus, in such embodiments, determining the control signal for the inverter-based resource as a function of the voltage drop across the virtual impedance value(s) of the inverter-based resource and the voltage or current signal(s) may include determining a remote, virtual voltage feedback signal as a function of the physical voltage feedback signal and the voltage drop.

In additional embodiments, determining the remote, virtual voltage feedback signal as a function of the physical voltage feedback signal and the voltage drop may include, for example, subtracting the voltage drop across the external virtual impedance value from the physical voltage feedback signal. Moreover, in an embodiment, determining the control signal for the inverter-based resource as a function of the voltage drop across the virtual impedance value(s) of the inverter-based resource and the voltage or current signal(s) may include calculating an angle input of the remote voltage feedback signal, providing the angle input to a phase-locked loop regulator of the inverter-based resource, and generating a phase-locked loop angle and a phase-locked loop frequency for the inverter-based resource based on the angle input.

In several embodiments, the inverter-based resource may be a wind turbine power system, a solar inverter, an energy storage system, a STATCOM, a hydro-power system, or an inverter-based system.

In another aspect, the present disclosure is directed to a method for providing grid-forming control of an inverter-based resource connected to an electrical grid. The method includes providing, via a processor, at least one virtual impedance value of the inverter-based resource. Further, the method includes implementing the virtual impedance value(s) into one or more control signals for the inverter-based resource so as to tune at least one of an active power output of the inverter-based resource for changes in an external network angle or a grid angle estimation through a phase-locked loop of the inverter-based resource for changes in the active power output of the inverter-based resource. It should be understood that the method may further include any of the additional features and/or steps described herein.

In yet another aspect, the present disclosure is directed to a system for providing grid-forming control of an inverter-based resource connected to an electrical grid. 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, providing at least one virtual impedance value of the inverter-based resource, and implementing the virtual impedance value(s) into one or more control signals for the inverter-based resource so as to tune at least one of an active power output of the inverter-based resource for changes in an external network angle or a grid angle estimation through a phase-locked loop of the inverter-based resource for changes in the active power output of the inverter-based resource. It should be understood that the system may further include any of the additional features described herein.

DETAILED DESCRIPTION

In general, the present disclosure is directed to systems and method for providing grid-forming control for inverter-based resources that configure the controls so that an effective impedance can be set as a parameter independent of the equipment physical characteristics. 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. In one embodiment, effective impedance can be a fixed value determined by studies of the application scenario or may be a variable, e.g. as determined by a control logic that adapts to measured grid conditions. In addition, in one implementation, a larger effective impedance can be used to reduce the extreme nonlinearity associated with the rapid rise into the current limiting region, e.g. during a grid fault. Thus, upon fault clearing, the larger virtual impedance allows for inrush current to be within limits. After the grid fault, the virtual impedance may then be lowered as grid voltage recovers so that the converter contributes to supporting the grid while operating within its linear region. In another implementation, a lower effective impedance can be used to improve the support provided to the grid for milder events.

The wind turbine10may also include a wind turbine controller26centralized within the nacelle16. However, in other embodiments, the controller26may be located within any other component of the wind turbine10or at a location outside the wind turbine10. Further, the controller26may be communicatively coupled to any number of the components of the wind turbine10in order to control the operation of such components and/or implement a corrective or control action. As such, the controller26may include a computer or other suitable processing unit. Thus, in several embodiments, the controller26may include suitable computer-readable instructions that, when implemented, configure the controller26to perform various different functions, such as receiving, transmitting and/or executing wind turbine control signals. Accordingly, the controller26may 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 turbine10.

Referring now toFIG.5, a simplified, internal view of one embodiment of the nacelle16of the wind turbine10shown inFIG.1is illustrated. As shown, a generator24may be disposed within the nacelle16and supported atop a bedplate46. In general, the generator24may be coupled to the rotor18for producing electrical power from the rotational energy generated by the rotor18. For example, as shown in the illustrated embodiment, the rotor18may include a rotor shaft34coupled to the hub20for rotation therewith. The rotor shaft34may, in turn, be rotatably coupled to a generator shaft36of the generator24through a gearbox38. As is generally understood, the rotor shaft34may provide a low speed, high torque input to the gearbox38in response to rotation of the rotor blades22and the hub20. The gearbox38may then be configured to convert the low speed, high torque input to a high speed, low torque output to drive the generator shaft36and, thus, the generator24.

The wind turbine10may also one or more pitch drive mechanisms32communicatively coupled to the wind turbine controller26, with each pitch adjustment mechanism(s)32being configured to rotate a pitch bearing40and thus the individual rotor blade(s)22about its respective pitch axis28. In addition, as shown, the wind turbine10may include one or more yaw drive mechanisms42configured to change the angle of the nacelle16relative to the wind (e.g., by engaging a yaw bearing44of the wind turbine10that is arranged between the nacelle16and the tower12of the wind turbine10).

In addition, the wind turbine10may also include one or more sensors66,68for monitoring various wind conditions of the wind turbine10. For example, the incoming wind direction30, wind speed, or any other suitable wind condition near of the wind turbine10may be measured, such as through use of a suitable weather sensor66. 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 sensors68may be utilized to measure additional operating parameters of the wind turbine10, such as voltage, current, vibration, etc. as described herein.

Referring now toFIG.6, a schematic diagram of one embodiment of a wind turbine power system100is illustrated in accordance with aspects of the present disclosure. Although the present disclosure will generally be described herein with reference to the system100shown inFIG.6, 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 ofFIG.6and as mentioned, the rotor18of the wind turbine10(FIG.4) may, optionally, be coupled to the gearbox38, which is, in turn, coupled to a generator102, which may be a doubly fed induction generator (DFIG). As shown, the DFIG102may be connected to a stator bus104. Further, as shown, a power converter106may be connected to the DFIG102via a rotor bus108, and to the stator bus104via a line side bus110. As such, the stator bus104may provide an output multiphase power (e.g. three-phase power) from a stator of the DFIG102, and the rotor bus108may provide an output multiphase power (e.g. three-phase power) from a rotor of the DFIG102. The power converter106may also include a rotor side converter (RSC)112and a line side converter (LSC)114. The DFIG102is coupled via the rotor bus108to the rotor side converter112. Additionally, the RSC112is coupled to the LSC114via a DC link116across which is a DC link capacitor118. The LSC114is, in turn, coupled to the line side bus110.

The RSC112and the LSC114may 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 converter106may be coupled to a converter controller120in order to control the operation of the rotor side converter112and/or the line side converter114as described herein. It should be noted that the converter controller120may be configured as an interface between the power converter106and the turbine controller26and may include any number of control devices.

In typical configurations, various line contactors and circuit breakers including, for example, a grid breaker122may also be included for isolating the various components as necessary for normal operation of the DFIG102during connection to and disconnection from a load, such as the electrical grid124. For example, a system circuit breaker126may couple a system bus128to a transformer130, which may be coupled to the electrical grid124via the grid breaker122. In alternative embodiments, fuses may replace some or all of the circuit breakers.

In operation, alternating current power generated at the DFIG102by rotating the rotor18is provided to the electrical grid124via dual paths defined by the stator bus104and the rotor bus108. On the rotor bus side108, sinusoidal multi-phase (e.g. three-phase) alternating current (AC) power is provided to the power converter106. The rotor side power converter112converts the AC power provided from the rotor bus108into direct current (DC) power and provides the DC power to the DC link116. As is generally understood, switching elements (e.g. IGBTs) used in the bridge circuits of the rotor side power converter112may be modulated to convert the AC power provided from the rotor bus108into DC power suitable for the DC link116.

In addition, the line side converter114converts the DC power on the DC link116into AC output power suitable for the electrical grid124. In particular, switching elements (e.g. IGBTs) used in bridge circuits of the line side power converter114can be modulated to convert the DC power on the DC link116into AC power on the line side bus110. The AC power from the power converter106can be combined with the power from the stator of DFIG102to provide multi-phase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the electrical grid124(e.g. 50 Hz or 60 Hz).

Additionally, various circuit breakers and switches, such as grid breaker122, system breaker126, stator sync switch132, converter breaker134, and line contactor136may be included in the wind turbine power system100to connect or disconnect corresponding buses, for example, when current flow is excessive and may damage components of the wind turbine power system100or for other operational considerations. Additional protection components may also be included in the wind turbine power system100.

Moreover, the power converter106may receive control signals from, for instance, the local control system176via the converter controller120. The control signals may be based, among other things, on sensed states or operating characteristics of the wind turbine power system100. Typically, the control signals provide for control of the operation of the power converter106. For example, feedback in the form of a sensed speed of the DFIG102may be used to control the conversion of the output power from the rotor bus108to 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)120,26to control the power converter106, 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 converter106also compensates or adjusts the frequency of the three-phase power from the rotor for changes, for example, in the wind speed at the hub20and the rotor blades22. 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 converter106, and specifically, the bi-directional characteristics of the LSC114and RSC112, facilitate feeding back at least some of the generated electrical power into generator rotor. More specifically, electrical power may be transmitted from the stator bus104to the line side bus110and subsequently through the line contactor136and into the power converter106, specifically the LSC114which acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into the DC link116. The capacitor118facilitates 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 RSC112that 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 controller120. The converted AC power is transmitted from the RSC112via the rotor bus108to the generator rotor. In this manner, generator reactive power control is facilitated by controlling rotor current and voltage.

Referring now toFIG.7, the wind turbine power system100described herein may be part of a wind farm150. As shown, the wind farm150may include a plurality of wind turbines152, including the wind turbine10described above, and an overall farm-level controller156. For example, as shown in the illustrated embodiment, the wind farm150includes twelve wind turbines, including wind turbine10. However, in other embodiments, the wind farm150may 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 turbines152are communicatively coupled to the farm-level controller156, e.g. through a wired connection, such as by connecting the turbine controller26through suitable communicative links154(e.g., a suitable cable). Alternatively, the turbine controllers may be communicatively coupled to the farm-level controller156through a wireless connection, such as by using any suitable wireless communications protocol known in the art. In further embodiments, the farm-level controller156is configured to send and receive control signals to and from the various wind turbines152, such as for example, distributing real and/or reactive power demands across the wind turbines152of the wind farm150.

Referring now toFIG.8, a block diagram of one embodiment of suitable components that may be included within the controller (such as any one of the turbine controller26, the converter controller120, and/or the farm-level controller156described herein) in accordance with example aspects of the present disclosure is illustrated. As shown, the controller may include one or more processor(s)158, computer, or other suitable processing unit and associated memory device(s)160that 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).

Such memory device(s)160may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)158, configure the controller to perform various functions as described herein. Additionally, the controller may also include a communications interface162to facilitate communications between the controller and the various components of the wind turbine10. 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 interface164(e.g., one or more analog-to-digital converters) to permit signals transmitted from the sensors66,68to be converted into signals that can be understood and processed by the processor(s)58.

Referring now toFIG.9, a schematic diagram of one embodiment of a system200for 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 system200may include many of the same features ofFIG.6described herein, with components having the same reference characters representing like components. Further, as shown, the system200may include a control structure for controlling the line-side converter that is similar to the control structure shown inFIG.6.

Moreover, as shown, the line-side converter control structure may include a DC regulator212and a line current regulator214. The DC regulator212is configured to generate line-side current commands for the line current regulator214. The line current regulator214then generates line-side voltage commands for a modulator218. The modulator218also receives an output (e.g. a phase-locked loop angle) from a phase-locked loop216to generate one or more gate pulses for the line-side converter114. The phase-locked loop216typically generates its output using a voltage feedback signal.

Furthermore, as shown, the system200may also include a control structure for controlling the rotor-side converter112using grid-forming characteristics. In particular, as shown inFIG.9, the system200may include a stator voltage regulator206for providing such grid-forming characteristics. In addition, as shown, the system200may include a grid voltage/VAR regulator202, an inertial power regulator204, a rotor current regulator208, and a modulator210.

In an embodiment, the grid volt/VAR regulator202receives a voltage reference (e.g. VT_REF) from the farm-level controller156and generates a stator voltage magnitude command (e.g. VS_Mag_Cmd), whereas the inertial power regulator receives a power reference from the turbine controller26and generates a stator voltage angle command (e.g. VS_Angle_Cmd). More specifically, in an embodiment, as shown, the stator voltage regulator206determines 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 signal240of the double-fed generator120. It should be understood that the stator feedback current240is a strong indicator of the characteristics of the externally connected power system, i.e. the grid. Therefore, the stator feedback current240can 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 regulator206are further explained and described in PCT/US2020/013787 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.

Therefore, the present disclosure is directed to a system and method that configures the controls so that the effective impedance can be set as a parameter independent of the equipment physical characteristics. Referring now toFIG.10, a flow diagram of one embodiment of such a method300for providing grid-forming control of an inverter-based resource connected to an electrical grid using at least one virtual impedance according to the present disclosure is illustrated. It should be appreciated that the disclosed method300may be implemented with any suitable inverter-based resource having any suitable configuration. In several embodiments, for example, the inverter-based resource may be a wind turbine power system (e.g. having a full conversion power system or a dual-fed power conversion system as illustrated inFIG.9), a solar inverter, an energy storage system, a STATCOM, a hydro-power system, or any other inverter-based system. In addition, althoughFIG.10depicts 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 (302), the method300includes providing, via a processor, at least one virtual impedance value of the inverter-based resource. As used herein, a tunable “virtual” impedance value generally refers to impedance behavior that can be mimicked by a system, rather than the impedance being provided by a particular component (such as an inductor). Thus, the virtual or effective impedance can be a fixed value determined by studies of the application scenario. Alternatively, the virtual impedance may be a variable, e.g. as determined by a control logic that adapts to measured grid conditions. In one embodiment, as an example, a larger effective impedance can be used to reduce the extreme nonlinearity associated with the rapid rise into the current limiting region, e.g. during a grid fault. Thus, upon fault clearing, the larger virtual impedance allows for inrush current to be within limits. After the grid fault, the virtual impedance may then be lowered as grid voltage recovers so that the converter contributes to supporting the grid while operating within its linear region. In addition, in an embodiment, a lower effective impedance can be used to improve the support provided to the grid for milder events.

For example, as shown inFIG.11, an equivalent circuit350of a full-conversion inverter-based resource with a virtual impedance is illustrated. In particular, as shown, the components indicated in phantom are representative of “virtual” components in that, such components are not actual hardware of the inverter-based resource, but rather, are provided or mimicked using software of the resource. Moreover, as shown in the illustrated embodiment, the virtual impedance value(s) may include, for example, an internal virtual impedance value XPat a node internal of the inverter-based resource and/or an external virtual impedance value XGat a node external of the inverter-based resource. Thus, in certain embodiments, two virtual impedances may be implemented, each with a certain purpose as related to the active power dynamics of the system. For example, in an embodiment, the internal virtual impedance may allow for tuning an active power output of the inverter-based resource for changes in external network angle. In another embodiment, the external virtual impedance may allow for tuning of a grid angle estimation through the phase-locked loop for changes in active power output of the grid-forming resource. In such embodiments, the multiple degrees of freedom allow for configuration and tuning of active power dynamics for grid-forming converter controls with various hardware types as well as various types of external networks.

In addition, as shown inFIG.11, the voltage magnitude and angle used to control the active power and voltage of the grid-forming inverter-based resource are the synthesized voltage magnitude EPand angle δPbehind the internal virtual impedance XP. In such embodiments, this voltage reflects an artificial node internal to the inverter-based resource. Moreover, the voltage magnitude and angle used as the reference for the controls are referred to inFIG.11as VGand δG. This voltage reflects a node further into the external network outside of the inverter-based resource. The voltages E1∠δ1and VT∠δTrepresent the physical converter voltage and angle and grid-forming terminal voltage and angle, respectively. The equivalent impedance of this system represents the series combination of the hardware impedance XT(which is associated with a current ITand PTflow therethrough), the internal virtual impedances XP, and the external virtual impedance XG, given by Equation (5) below:
Xterm=XT+XP+XGEquation (5)

The converter voltage can be calculated from the synthesized voltage and virtual impedance by Equation (6) below:
E1=EP−j*XP*ITEquation (6)

The remote voltage can be calculated from the terminal voltage by Equation (7) below:
VG=VT−j*XG*ITEquation (7)

Referring back toFIG.10, as shown at (304), the method300also includes determining a voltage drop across the at least one virtual impedance value of the inverter-based resource using at least one current feedback signal. In such embodiments, the voltage drop includes a voltage magnitude and a voltage angle. As shown at (306), the method300includes receiving one or more voltage or current signals of the inverter-based resource. In addition, as shown at (308), the method300includes determining at least one control signal for the inverter-based resource as a function of the voltage drop across the virtual impedance value(s) of the inverter-based resource and the one or more voltage or current signals. Such method steps can be better understood with respect to the control diagrams ofFIGS.12and13, which are described and explained in more detail below.

Referring particularly toFIG.12, a schematic diagram of one embodiment of a control system400for implementation of a full-conversion inverter-based resource with the internal virtual impedance value being used is illustrated. Thus, as shown, the voltage or current signals (e.g. the inputs of the system400) may include at least one of a synthesized voltage magnitude EPor a physical voltage angle δPG. More specifically, in an embodiment, the synthesized voltage magnitude EPor a physical voltage angle δPGmay be the outputs from the grid volt/VAR regulator202and the inertial power regulator204(FIG.9), respectively. Thus, synthesized values, as used herein, generally refer to parameters generating by an outside control loop of the inverter-based resource, and are thus not measurable, but rather, are typically simulated in the controls. Accordingly, as shown at402ofFIG.12, the system400may determine or calculate calculating a physical voltage command (e.g. EP_Cmd_xy) as a function of the synthesized voltage magnitude EP and the physical voltage angle δPG. More specifically, as shown, the system400may calculate the physical voltage command using Equation (8) below:
EP_Cmd_xy=EP*exp(jδPG)  Equation (8)

In addition, as shown at404, the system400is configured to determine the voltage drop across the internal virtual impedance value XPof the inverter-based resource using current feedback signal IT_Fbk_xy. Thus, as shown at406, the voltage drop can be subtracted from the physical voltage command to determine the voltage command (e.g. E1_Cmd_xy) for the inverter-based resource.

Referring particularly toFIG.13, a schematic diagram of one embodiment of a control system500for implementation of a full-conversion inverter-based resource with the external virtual impedance value being used is illustrated. Thus, as shown, the voltage or current signals (e.g. the inputs of the system500) may include, for example, at least, a physical voltage feedback signal VT_Fbk_xy. In addition, as shown at502, the system500is configured to determine the voltage drop across the external virtual impedance value XGof the inverter-based resource using current feedback signal IT_Fbk_xy. Thus, as shown at504, the voltage drop can be subtracted from the physical voltage feedback signal VT_Fbk_xy to determine a remote voltage feedback signal VG_Fbk_xy for the inverter-based resource.

Thus, in such embodiments, as shown at506, the system500may then calculate an angle input θGof the remote voltage feedback signal and provide the angle input θGto a phase-locked loop regulator508of the inverter-based resource. Accordingly, as shown, the phase-locked loop regulator508is configured to generate a phase-locked loop angle θPLLand a phase-locked loop frequency ωPLLfor the inverter-based resource based on the angle input.

Referring now toFIG.14, a flow diagram of another embodiment of a method600for providing grid-forming control of an inverter-based resource connected to an electrical grid using at least one virtual impedance according to the present disclosure is illustrated. It should be appreciated that the disclosed method600may be implemented with any suitable inverter-based resource having any suitable configuration. In addition, althoughFIG.14depicts 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 (602), the method600includes providing, via a processor, at least one virtual impedance value of the inverter-based resource. As shown at (604), the method600includes implementing the virtual impedance value(s) into control signals for the inverter-based resource so as to tune at least one of an active power output of the inverter-based resource for changes in an external network angle or a grid angle estimation through a phase-locked loop of the inverter-based resource for changes in the active power output of the inverter-based resource.

Clause 1. A method for providing grid-forming control of an inverter-based resource connected to an electrical grid, the method comprising:

providing, via a processor, at least one virtual impedance value of the inverter-based resource;

determining a voltage drop across the at least one virtual impedance value of the inverter-based resource using at least one current feedback signal, the voltage drop comprising a voltage magnitude and a voltage angle;

receiving one or more voltage or current signals of the inverter-based resource; and,

determining a control signal for the inverter-based resource as a function of the voltage drop across the at least one virtual impedance value of the inverter-based resource and the one or more voltage or current signals.

Clause 2. The method of clause 1, wherein the at least one virtual impedance value comprises at least one of an internal virtual impedance value at a node internal of the inverter-based resource or an external virtual impedance value at a node external of the inverter-based resource.

Clause 3. The method of clause 2, wherein the at least one virtual impedance value comprises the internal virtual impedance value, and wherein the one or more voltage or current signals comprise at least one of a virtual voltage magnitude command or a virtual voltage angle command behind the internal virtual impedance.

Clause 4. The method of clause 3, wherein determining the control signal for the inverter-based resource as a function of the voltage drop across the at least one virtual impedance value of the inverter-based resource and the one or more voltage or current signals further comprises:

calculating a physical control command as a function of the virtual voltage magnitude command, the virtual voltage angle command, and the voltage drop.

Clause 5. The method of clause 4, wherein calculating the physical control command for the inverter-based resource further comprises:

subtracting the voltage drop across the internal virtual impedance value from the virtual voltage command to obtain the physical control command for the inverter-based resource.

Clause 6. The method of clause 2, wherein the at least one virtual impedance value comprises the external virtual impedance value, and wherein the one or more voltage or current signals comprises, at least, a physical voltage feedback signal.

Clause 7. The method of clause 6, wherein determining the control signal for the inverter-based resource as a function of the voltage drop across the at least one virtual impedance value of the inverter-based resource and the one or more voltage or current signals further comprises:

determining a remote, virtual voltage feedback signal as a function of the physical voltage feedback signal and the voltage drop.

Clause 8. The method of clause 7, wherein determining the remote, virtual voltage feedback signal as a function of the physical voltage feedback signal and the voltage drop further comprises:

subtracting the voltage drop across the external virtual impedance value from the physical voltage feedback signal.

Clause 9. The method of clause 7, wherein determining the control signal for the inverter-based resource as a function of the voltage drop across the at least one virtual impedance value of the inverter-based resource and the one or more voltage or current signals further comprises:

calculating an angle input of the remote voltage feedback signal;

providing the angle input to a phase-locked loop regulator of the inverter-based resource; and

generating a phase-locked loop angle and a phase-locked loop frequency for the inverter-based resource based on the angle input.

Clause 10. The method of any of the preceding clauses, wherein the inverter-based resource comprises at least one of a wind turbine power system, a solar inverter, an energy storage system, a STATCOM, or a hydro-power system.

Clause 11. A method for providing grid-forming control of an inverter-based resource connected to an electrical grid, the method comprising: providing, via a processor, at least one virtual impedance value of the inverter-based resource; and,

implementing the at least one virtual impedance value into one or more control signals for the inverter-based resource so as to tune at least one of an active power output of the inverter-based resource for changes in an external network angle or a grid angle estimation through a phase-locked loop of the inverter-based resource for changes in the active power output of the inverter-based resource.

Clause 12. The method of clause 11, wherein the at least one virtual impedance value comprises at least one of an internal virtual impedance value at a node internal of the inverter-based resource or an external virtual impedance value at a node external of the inverter-based resource.

Clause 13. The method of clauses 11-12, wherein the inverter-based resource comprises at least one of a wind turbine power system, a solar inverter, an energy storage system, a STATCOM, or a hydro-power system.

Clause 14. A system for providing grid-forming control of an inverter-based resource connected to an electrical grid, the system comprising:

a controller comprising at least one processor, the at least one processor configured to perform a plurality of operations, the plurality of operations comprising:

providing at least one virtual impedance value of the inverter-based resource; and,

implementing the at least one virtual impedance value into one or more control signals for the inverter-based resource so as to tune at least one of an active power output of the inverter-based resource for changes in an external network angle or a grid angle estimation through a phase-locked loop of the inverter-based resource for changes in the active power output of the inverter-based resource.

Clause 15. The system of clause 14, wherein the at least one virtual impedance value comprises at least one of an internal virtual impedance value at a node internal of the inverter-based resource or an external virtual impedance value at a node external of the inverter-based resource.

Clause 16. The system of clause 15, further comprising determining a voltage drop across the at least one virtual impedance value of the inverter-based resource using at least one current feedback signal.

Clause 17. The system of clause 16, wherein the at least one virtual impedance value comprises the internal virtual impedance value, wherein implementing the at least one virtual impedance value into the one or more control signals for the inverter-based resource further comprises:

calculating a physical control command as a function of the virtual voltage magnitude command, the virtual voltage angle command, and the voltage drop.

Clause 18. The system of clause 17, wherein the at least one virtual impedance value comprises the external virtual impedance value, wherein implementing the at least one virtual impedance value into the one or more control signals for the inverter-based resource further comprises:

determining a remote, virtual voltage feedback signal as a function of a physical voltage feedback signal and the voltage drop.

Clause 19. The system of clause 18, wherein determining the remote, virtual voltage feedback signal as a function of the physical voltage feedback signal and the voltage drop further comprises:

subtracting the voltage drop across the external virtual impedance value from the physical voltage feedback signal.

Clause 20. The system of clauses 18-19, wherein implementing the at least one virtual impedance value into the one or more control signals for the inverter-based resource further comprises:

calculating an angle input of the remote voltage feedback signal;

providing the angle input to a phase-locked loop regulator of the inverter-based resource; and

generating a phase-locked loop angle and a phase-locked loop frequency for the inverter-based resource based on the angle input.