Method and controller for controlling power converter connected to electrical network

Abstract

A method for controlling a power converter connected to a power grid is provided. The method includes determining, by the controller, that a first parameter associated with the controller and / or the power converter exceeds a first threshold. In response to determining that the first parameter exceeds the first threshold: controlling, by the controller, the power converter in a first limiting mode by limiting the power converter output current based on the current value and / or the reference value; and determining, by the controller, an output current of the power converter. In response to determining that the output current is at or near the equilibrium condition: determining, by the controller, a virtual impedance of the power converter based on the output current of the power converter; modifying, by the controller, the control system based on the virtual impedance; and controlling, by the controller, the power converter in a first constrained grid formation mode, including the modified control system determining control data based on a signal indicative of a measurement of current and / or voltage downstream of the power converter and a reference signal.

Description

Technical Field

[0001] The present invention relates to a method and a controller for controlling a power converter connected to a power grid, and more particularly, to a method and a controller for controlling the power converter by using a virtual impedance. Background Art

[0002] In a power transmission network, DC power is converted to AC power, where it is necessary to interconnect the DC and AC networks. In any such power transmission network, power conversion components (also known as converters, power converters, inverters, power inverters or power electronics based resources) are required at each interface between AC and DC power to achieve the required conversion from AC to DC or from DC to AC.

[0003] For example, when converting DC power to AC power at the interface between a DC transmission line and a grid, the power converter may operate in a grid following mode (GFL) or a grid forming mode (GFM).

[0004] In GFL mode, the power converter utilizes a fast current regulation loop to control the active and reactive power exchanged with the grid, thereby achieving relatively constant active and reactive power exchange in a sub-transient to transient time scale (e.g., between about 10ms and 150ms). The power converter uses a current reference for the active component of the current to achieve the desired active power output. The power converter operating in GFL mode includes the functionality of managing voltage and / or reactive power in a manner that generates commands for the reactive component of the current. Then, a wide bandwidth current regulator generates a command for the voltage applied by the power converter to the grid so that the actual current closely tracks the command. Therefore, the power converter operating in GFL mode provides a current source characteristic in a sub-transient to transient time scale.

[0005] Alternatively, a power converter operating in GFM mode provides a voltage source characteristic in the sub-transient to transient time scale, where the phase angle and amplitude of the voltage are controlled to remain mostly static in this time scale to achieve the regulation function required by the grid. With this structure, current will flow according to the needs of the grid, and the converter helps to establish the voltage and frequency for the grid.

[0006] Thus, a power converter operating in GFM mode differs from a power converter operating in GFL mode because the GFM mode regulates the AC voltage and frequency of the power converter, rather than the AC current, in a sub-transient time scale (e.g., <150 milliseconds). However, if a fault event or other severe grid disturbance (referred to herein as a "grid event") occurs in the grid, the power converter may experience a current, power, or energy overload because the power converter is regulating voltage rather than current, power, or energy. Such an overload may cause the power converter operating in GFM mode to trip offline, or in the worst case, damage the semiconductor devices within the power converter and render the power converter inoperable indefinitely.

[0007] During such grid events, self-protection of the power converter is ensured and current limiting is usually achieved via switching control to GFL mode. However, in certain circumstances, switching the control mode in this way can potentially jeopardize the stability of the power converter and the stability of the entire power system. Summary of the invention

[0008] In view of these considerations and emerging grid requirements, it is desirable to develop methods that allow power converters to maintain some semblance of GFM behavior while operating close to (but not exceeding) hardware limitations.

[0009] According to a first aspect, there is provided a method for controlling a power converter connected to a power grid, wherein the power converter is initially controlled in a grid formation mode to output a current at a nominal voltage based on control data provided by a controller, the controller comprising a control system, the control system determining the control data based on a signal indicating a measurement of a current and / or a voltage downstream of the power converter and a reference signal. The method comprises determining, by the controller, that a first parameter associated with the controller and / or the power converter exceeds a first threshold. In response to determining that the first parameter exceeds the first threshold: controlling, by the controller, the power converter in a first restricted mode by limiting the output current of the power converter based on a current value and / or a reference value; and determining, by the controller, the output current of the power converter. In response to determining that the output current is at or near a balanced condition: determining, by the controller, a virtual impedance of the power converter based on the output current of the power converter; modifying, by the controller, the control system based on the virtual impedance; and controlling, by the controller, the power converter in a first constrained grid formation mode, comprising the modified control system determining the control data based on a signal indicating a measurement of a current and / or a voltage downstream of the power converter and a reference signal.

[0010] Determining that the output current is at or near a balanced condition may include determining that the output current is within a predefined threshold of a balanced condition. The predefined threshold may be application dependent.

[0011] In some embodiments, the reference signal comprises a voltage reference provided by a grid forming controller.

[0012] In some embodiments, the controller comprises a grid forming controller.

[0013] In some embodiments, determining the output current of the power converter includes estimating the output current.

[0014] In some embodiments, determining the output current of the power converter includes measuring the output current.

[0015] In some embodiments, determining the output current of the power converter includes using a reference for the output current.

[0016] In some embodiments, the virtual impedance is a fixed or static virtual impedance. This means that the determined value of the virtual impedance does not change during the constrained grid forming mode.

[0017] In some embodiments, the method further includes determining, by the controller, that a second parameter associated with the controller and / or the power converter exceeds a second threshold. In response to determining that the second parameter exceeds the second threshold: controlling, by the controller, the power converter in a second limiting mode by limiting the power converter output current based on an updated current value and / or an updated reference value; and determining, by the controller, an updated output current of the power converter. In response to determining that the updated output current is at or near a balanced condition: determining, by the controller, an updated virtual impedance of the power converter based on the updated output current of the power converter; further modifying, by the controller, the control system based on the updated virtual impedance; and controlling, by the controller, the power converter in a second constrained grid formation mode, including the further modified control system determining the control data based on the measured signal indicating the current and / or voltage downstream of the power converter and the reference signal.

[0018] By determining that a second parameter associated with the controller and / or the power converter exceeds a second threshold value, and subsequently controlling the power converter in a second constrained grid formation mode, an iterative control process may be established, wherein the parameter exceeding the threshold value results in controlling the power converter in another constrained mode. Thus, there is no limit to the number of parameters that may be determined to exceed the threshold value, and therefore there is no limit to the number of constrained grid formation modes.

[0019] In some embodiments, the control system is a cascade control system including a voltage controller and a current controller. The voltage controller generates first data using a voltage control system based on the reference signal and the measured signal, and outputs the first data to the current controller. The current controller generates second data using a current control system based on the first data and the measured signal. The controller determines the control data based on the second data. The controller controls the power converter in the first limiting mode by setting the first data equal to the current value and / or the reference value, thereby limiting the output current of the power converter.

[0020] In some embodiments, setting the first data equal to the current value and / or the reference value comprises freezing, inhibiting or not allowing the modified control system to determine some or all of the first data.

[0021] In some embodiments, controlling the power converter in the first constrained grid formation mode includes unfreezing, enabling, or allowing a modified control system to determine some or all of the first data.

[0022] In some embodiments, the control system is a single loop controller.

[0023] In some embodiments, the control system is a DC voltage controller. The controller includes a limiting controller. The controller controls the power converter in the first limiting mode by limiting the output current of the power converter to increase beyond the current value and / or the reference value through the limiting controller, thereby limiting the output current of the power converter.

[0024] In some embodiments, the limit controller limits the output current of the power converter from increasing beyond the current value or the reference value by modifying, controlling and / or limiting the control data.

[0025] In some embodiments, the control system includes a virtual admittance. The virtual admittance can replace a conventional controller.

[0026] In some embodiments, the current controller generates the second data relatively faster than the voltage controller generates the first data.

[0027] In some embodiments, determining the virtual impedance of the power converter by the controller also includes determining the virtual impedance and a fixed virtual voltage offset of the power converter by the controller based on the output current of the power converter; and modifying the control system based on the virtual impedance by the controller also includes modifying the control system based on the virtual impedance and the fixed virtual voltage offset by the controller.

[0028] In some embodiments, modifying the control system based on the virtual impedance includes: determining, by the controller, an adjustment term of the control system by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current; and implementing, by the controller, the adjustment term into the control system.

[0029] In some embodiments, the implementing of the adjustment term comprises modification of the internal voltage phasor reference.

[0030] In some embodiments, the controller determines the virtual impedance by: calculating a virtual voltage when the power converter is in the first limiting mode, the virtual voltage being defined as a voltage difference between an internal voltage phasor reference of the power converter and a node voltage at a node between the output of the power converter and the grid; and solving the Thevenin equivalent circuit of the virtual impedance using a circuit analysis method based on a Thevenin equivalent circuit of the power converter connected to the grid, including the virtual voltage, the output current of the power converter under the balanced condition, and the virtual impedance.

[0031] In some embodiments, when the power converter is in the first limiting mode, the controller determines the virtual impedance by calculating a new internal voltage phasor reference and a virtual voltage, the virtual voltage being defined as a voltage difference between the new internal voltage phasor reference of the power converter and a node voltage at a node between the output of the power converter and the grid, and using an analytical method to solve for the virtual impedance and the new internal voltage phasor reference.

[0032] In some embodiments, when the power converter is in the first limiting mode, the controller determines the virtual impedance and the fixed virtual voltage offset by calculating a virtual voltage, wherein the virtual voltage is defined as the voltage difference between an internal voltage phasor reference of the power converter and a node voltage at a node between the output of the power converter and the power grid; and based on the Thevenin equivalent circuit of the power converter connected to the power grid, including the virtual voltage, the output current of the power converter under the balanced condition, and the virtual impedance, a circuit analysis method is used to solve the Thevenin equivalent circuit of the virtual impedance.

[0033] In some embodiments, the method further includes reducing, by the controller, the virtual impedance as a result of the voltage difference between the (new or original) internal voltage phasor reference of the power converter and the node voltage at the node decreasing in magnitude, and thereby returning to controlling the power converter in the initial grid formation mode.

[0034] In some embodiments, a time period from determining that the first parameter associated with the controller and / or the power converter exceeds the first threshold to the controller completing the step of controlling the power converter (204) in the first limiting mode is less than or equal to 50 ms.

[0035] In some embodiments, determining that the first parameter associated with the controller and / or the power converter exceeds the first threshold includes one or more of the following: determining that a current reference of a dq-system current regulator using a command angle or a phase-locked loop (PLL) angle has been saturated; and / or determining that a d or q current reference of a proportional-integral (PI) controller input into a current controller has been saturated; and / or determining that a d or q current reference of a proportional-resonant (PR) controller to be input into the current controller after an inverse reference frame transformation has been saturated; and / or determining that a d or q current reference of a deadbeat controller to be input into the current controller after an inverse reference frame transformation has been saturated; and / or determining that a phasor current limit has been reached; and / or determining that an active power limit has been reached; and / or determining that an energy limit has been reached; and / or determining that a component is pulsing down or blocking; and / or determining that a voltage reference has been saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has been saturated.

[0036] In some embodiments, determining that the second parameter associated with the controller and / or the power converter exceeds the second threshold includes one or more of the following: determining that a current reference of a dq-system current regulator using a command angle or a phase-locked loop (PLL) angle has been saturated; and / or determining that a d or q current reference of a proportional-integral (PI) controller input to a current controller has been saturated; and / or determining that a d or q current reference of a proportional-resonant (PR) controller to be input to the current controller after an inverse reference frame transformation has been saturated; and / or determining that a d or q current reference of a deadbeat controller to be input to the current controller after an inverse reference frame transformation has been saturated; and / or determining that a phasor current limit has been reached; and / or determining that an active power limit has been reached; and / or determining that an energy limit has been reached; and / or determining that a component is pulsing down or blocking; and / or determining that a voltage reference has been saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has been saturated.

[0037] According to a second aspect, there is provided a controller for controlling a power converter connected to a power grid, the individual controller comprising: a control system arranged to determine control data based on a signal indicating a measurement of a current and / or a voltage downstream of the power converter and a reference signal. The controller is arranged to control the power converter to output a current at a nominal voltage in a grid formation mode based on the control data. The controller is configured to determine that a first parameter associated with the controller and / or the power converter exceeds a first threshold. In response to determining that the first parameter exceeds the first threshold: controlling the power converter in a first limiting mode, wherein the controller is configured to limit the power converter output current based on a current value and / or a reference value; and determining the output current of the power converter. In response to determining that the output current is at or near a balanced condition: the controller is configured to determine a virtual impedance of the power converter based on the output current of the power converter; modifying the control system based on the virtual impedance; and controlling the power converter in a first constrained grid formation mode, wherein the modified control system is configured to determine the control data based on a signal indicating a measurement of a current and / or a voltage downstream of the power converter and a reference signal.

[0038] Typically, the controller is intended to be configured to perform the methods described herein.

[0039] In some embodiments, determining the output current of the power converter includes estimating the output current.

[0040] In some embodiments, determining the output current of the power converter includes measuring the output current.

[0041] In some embodiments, determining the output current of the power converter includes using a reference for the output current.

[0042] In some embodiments, the virtual impedance is a fixed or static virtual impedance. This means that only one value of the virtual impedance is determined and the determined value of the virtual impedance does not change during the constrained grid formation mode.

[0043] In some embodiments, setting the first data equal to a current value or a reference value comprises freezing, inhibiting or not allowing the modified control system to determine some or all of the control data.

[0044] In some embodiments, controlling the power converter in the first constrained grid formation mode includes unfreezing, enabling, or allowing a modified control system to determine some or all of the first data.

[0045] In some embodiments, the controller is further configured to determine that a second parameter associated with the controller and / or the power converter exceeds a second threshold; in response to determining that the second parameter exceeds the second threshold: control the power converter in a second limiting mode, wherein the power converter output current is limited based on an updated current value and / or an updated reference value; and determine an updated output current of the power converter; in response to determining that the updated output current is at or near a balanced condition: determine an updated virtual impedance of the power converter based on the updated output current of the power converter; further modify the control system based on the updated virtual impedance; and control the power converter in a second constrained grid formation mode, wherein the further modified control system is configured to determine the control data based on the measured signal and the reference signal.

[0046] By determining that a second parameter associated with the controller and / or the power converter exceeds a second threshold value, and subsequently controlling the power converter in a second constrained grid formation mode, an iterative control process may be established, wherein the parameter exceeding the threshold value results in controlling the power converter in another constrained mode. Thus, there is no limit to the number of parameters that may be determined to exceed the threshold value, and therefore there is no limit to the number of constrained grid formation modes.

[0047] In some embodiments, the control system is a cascade control system including a voltage controller and a current controller. The voltage controller is configured to generate first data using a voltage control system based on the reference signal and the measured signal, and output the first data to the current controller. The current controller is configured to generate second data using a current control system based on the first data and the measured signal. The controller is configured to determine the control data based on the second data. The controller is further configured to control the power converter in the first limiting mode by setting the first data equal to the current value or the reference value, thereby limiting the power converter output current.

[0048] In some embodiments, the control system is a single loop.

[0049] In some embodiments, the control system is a DC voltage controller. The controller includes a limit controller. The controller is configured to control the power converter in the first limit mode by limiting the output current of the power converter to increase beyond the current value and / or the reference value, thereby limiting the output current of the power converter.

[0050] In some embodiments, the control system includes a virtual admittance. The virtual admittance can replace a conventional controller.

[0051] In some embodiments, the current controller generates the second data relatively faster than the voltage controller generates the first data.

[0052] In some embodiments, the controller is configured to modify the control system based on the virtual impedance, including: the controller is configured to determine an adjustment item of the control system by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current; and implement the adjustment item into the control system.

[0053] In some embodiments, the controller is arranged to implement the adjustment term by modifying the internal voltage phasor reference.

[0054] In some embodiments, the controller is configured to determine the virtual impedance by: calculating a virtual voltage when the power converter is in the first limiting mode, the virtual voltage being defined as a voltage difference between an internal voltage phasor reference of the power converter and a node voltage at a node between an output of the power converter and the grid; and solving the Thevenin equivalent circuit of the virtual impedance using a circuit analysis method based on a Thevenin equivalent circuit of the power converter connected to the grid, including the virtual voltage, the output current of the power converter under the balanced condition, and the virtual impedance.

[0055] In some embodiments, the controller is further configured to reduce the virtual impedance as a result of the voltage difference between the internal voltage phasor reference of the power converter and the node voltage at the node decreasing in magnitude, and thereby return to controlling the power converter in the initial grid formation mode.

[0056] In some embodiments, a time period from determining that the first parameter associated with the controller and / or the power converter exceeds the first threshold to the controller completing the step of controlling the power converter (204) in the first limiting mode is less than or equal to 50 ms.

[0057] In some embodiments, the controller is configured to determine that the first parameter associated with the controller and / or the power converter exceeds the first threshold, including one or more of the following: determining that a current reference of a dq-system current regulator using a command angle or a phase-locked loop (PLL) angle has been saturated; and / or determining that a d or q current reference of a proportional-integral (PI) controller input into a current controller has been saturated; and / or determining that a d or q current reference of a proportional-resonant (PR) controller to be input into the current controller after an inverse reference frame transformation has been saturated; and / or determining that a d or q current reference of a deadbeat controller to be input into the current controller after an inverse reference frame transformation has been saturated; and / or determining that a phasor current limit has been reached; and / or determining that an active power limit has been reached; and / or determining that an energy limit has been reached; and / or determining that a component is pulsing down or blocking; and / or determining that a voltage reference has been saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has been saturated.

[0058] In some embodiments, the controller is configured to determine that the second parameter associated with the controller and / or the power converter exceeds the second threshold, including one or more of the following: determining that a current reference of a dq-system current regulator using a command angle or a phase-locked loop (PLL) angle has been saturated; and / or determining that a d or q current reference of a proportional-integral (PI) controller input into a current controller has been saturated; and / or determining that a d or q current reference of a proportional-resonant (PR) controller to be input into the current controller after an inverse reference frame transformation has been saturated; and / or determining that a d or q current reference of a deadbeat controller to be input into the current controller after an inverse reference frame transformation has been saturated; and / or determining that a phasor current limit has been reached; and / or determining that an active power limit has been reached; and / or determining that an energy limit has been reached; and / or determining that a component is pulsing down or blocking; and / or determining that a voltage reference has been saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has been saturated.

[0059] According to a third aspect, there is provided a power converter comprising: a DC side for connection to a DC source; an AC side for connection to a grid; and a controller according to the second aspect.

[0060] According to a fourth aspect, there is provided a computer program comprising instructions which, when executed by a processor of a controller for controlling a power converter, cause the controller to perform the method according to the first aspect.

[0061] According to a fifth aspect, there is provided a non-transitory computer-readable storage medium comprising the computer program according to the fourth aspect.

[0062] It will be appreciated that specific features of different aspects of the invention share the technical effects and benefits of corresponding features of other aspects of the invention. More particularly, the controller, power converter, computer program and non-transitory computer readable medium share the technical effects and benefits of the method of the invention.

[0063] It will also be appreciated that the use of the terms "first," "second," etc., are merely intended to help distinguish similar features, and are not intended to indicate the relative importance of one feature to another, unless otherwise stated.

[0064] Within the scope of the present application, it is expressly intended that the various aspects, embodiments, examples and alternatives set forth in the preceding paragraphs and claims and / or the following description and drawings, and in particular the individual features therein, may be employed independently or in any combination. That is, all embodiments and all features of any embodiment may be combined in any manner and / or combination, unless these features are incompatible.

[0065] The present invention provides a technical solution as follows.

[0066] Technical solution 1. A method (400) for controlling a power converter (204) connected to a power grid (232), wherein, based on control data (230) provided by a controller (200), the power converter (204) is initially controlled (410) in a grid forming mode to output a current (222) at a nominal voltage, the controller (200) comprising a control system (228), the control system (228) determining the control data (230) based on a measured signal (222, 224) indicating a current and / or voltage downstream of the power converter (204) and a reference signal (212); the method comprising:

[0067] Determining (420), by the controller (200), that a first parameter associated with the controller (200) and / or the power converter (204) exceeds a first threshold;

[0068] In response to determining that the first parameter exceeds the first threshold:

[0069] Controlling the power converter in a first limiting mode (430) by the controller (200) by limiting an output current (222) of the power converter (204) based on a current value and / or a reference value;

[0070] determining, by the controller (200), the output current (222) of the power converter (204);

[0071] In response to determining that the output current (222) is at or near a balanced condition:

[0072] determining (440), by the controller (200), a virtual impedance of the power converter (204) based on the output current (222) of the power converter (204);

[0073] The controller (200) modifies the control system (228) based on the virtual impedance; and the controller (200) controls the power converter (204) in a first constrained grid forming mode (450), including the modified control system (228) determining the control data (230) based on a measured signal (222, 224) indicative of a current and / or voltage downstream of the power converter (204) and a reference signal (212).

[0074] Technical solution 2. The method according to technical solution 1 further comprises:

[0075] Determining (460), by the controller (200), that a second parameter associated with the controller (200) and / or the power converter (204) exceeds a second threshold;

[0076] In response to determining that the second parameter exceeds the second threshold:

[0077] controlling the power converter in a second limiting mode by the controller (200) by limiting an output current (222) of the power converter (204) based on an updated current value and / or an updated reference value;

[0078] determining, by the controller (200), an updated output current (222) of the power converter (204);

[0079] In response to determining that the updated output current (222) is at or near an equilibrium condition:

[0080] determining, by the controller (200), an updated virtual impedance of the power converter (204) based on the updated output current (222) of the power converter (204);

[0081] further modifying, by the controller (200), the control system (228) based on the updated virtual impedance; and

[0082] Controlling the power converter (204) in a second constrained grid forming mode (450) by the controller (200) includes the further modified control system (228) determining the control data (230) based on the measured signals (222, 224) indicative of current and / or voltage downstream of the power converter and the reference signal (212).

[0083] Technical Solution 3. The method according to Technical Solution 1 or 2, wherein the control system (228) is a cascade control system (228) including a voltage controller (214) and a current controller (216); wherein

[0084] The voltage controller (214) generates first data (215) using a voltage control system based on the reference signal (212) and the measured signal (222, 224), and outputs the first data (215) to the current controller (216);

[0085] The current controller (216) generates second data using a current control system based on the first data (215) and the measured signals (222, 224);

[0086] The controller (200) determines the control data (230) based on the second data; and

[0087] The controller (200) controls the power converter (204) in the first limiting mode (430) by setting the first data (215) equal to the current value and / or the reference value, thereby limiting the output current (222) of the power converter (204).

[0088] Technical solution 4. The method according to technical solution 1 or 2, wherein

[0089] The control system (228B) is a DC voltage controller (214B);

[0090] The controller (200) includes a limit controller (217); and

[0091] The controller (200) controls the power converter (204) in the first limiting mode (430) by limiting the output current (222) of the power converter (204) to increase beyond the current value or the reference value through the limiting controller (217), thereby limiting the output current (222) of the power converter (204).

[0092] Technical solution 5. A method according to any of the above technical solutions, wherein

[0093] Determining (440) the virtual impedance of the power converter (204) by the controller (200) further comprises determining, by the controller (200), the virtual impedance and a fixed virtual voltage offset of the power converter (204) based on the output current (222) of the power converter (204); and

[0094] The modifying, by the controller (200), the control system (228) based on the virtual impedance further includes modifying, by the controller (200), the control system (228) based on the virtual impedance and the fixed virtual voltage offset.

[0095] Technical solution 6. The method according to any of the preceding technical solutions, wherein modifying the control system (228) based on the virtual impedance comprises:

[0096] determining, by the controller (200), an adjustment term of the control system (228) by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current (222); and

[0097] The adjustment item is implemented into the control system (228) by the controller (200).

[0098] Technical solution 7. The method according to any of the above technical solutions, wherein the controller (200) determines the virtual impedance in the following manner:

[0099] calculating a virtual voltage when the power converter (204) is in the first limiting mode (430), the virtual voltage being defined as the voltage difference between an internal voltage phasor reference of the power converter (204) and a node voltage at a node (224) between an output of the power converter (204) and the grid (232); and

[0100] Based on a Thevenin equivalent circuit of the power converter (204) connected to the power grid (232), including the virtual voltage, the output current of the power converter under the equilibrium condition, and the virtual impedance, a circuit analysis method is used to solve the Thevenin equivalent circuit of the virtual impedance.

[0101] Technical solution 8. The method according to technical solution 7 further comprises:

[0102] As a result of the voltage difference between the internal voltage phasor reference of the power converter (204) and the node voltage at the node (224) decreasing in magnitude, the controller (200) reduces (490) the virtual impedance and thereby returns to controlling the power converter (204) in the initial grid forming mode (410).

[0103] Technical Solution 9. A method according to any of the aforementioned technical solutions, wherein the time period from determining that the first parameter related to the controller (200) and / or the power converter (204) exceeds the first threshold to the controller completing the step of controlling the power converter (204) in the first limiting mode is less than or equal to 50ms.

[0104] Technical solution 10. The method according to any of the preceding technical solutions, wherein the determining that the first parameter related to the controller (200) and / or the power converter (204) exceeds the first threshold value comprises one or more of the following:

[0105] Determining that a current reference of a dq-coordinate current regulator using a command angle or a phase-locked loop (PLL) angle has saturated; and / or

[0106] Determining that a d or q current reference input to a proportional-integral (PI) controller in a current controller (216) has saturated; and / or

[0107] determining that a d or q current reference of a proportional resonant PR controller to be input into a current controller (216) after an inverse reference frame transformation has saturated; and / or

[0108] determining that a deadbeat controller d or q current reference to be input to a current controller (216) after an inverse reference frame transformation has saturated; and / or

[0109] determining that a phasor current limit has been reached; and / or

[0110] determining that an active power limit has been reached; and / or

[0111] determining that an energy limit has been reached; and / or

[0112] Determine that the component is pulsing or blocking; and / or

[0113] determining that the voltage reference has saturated; and / or

[0114] Determining that the voltage output has reached a limit; and / or

[0115] Determine if the modulation index is saturated.

[0116] Technical solution 11. A controller (200) for controlling a power converter (204) connected to a power grid (232), the controller (200) comprising:

[0117] a control system (228) arranged to determine control data (230) based on signals (222, 224) indicative of measurements of current and / or voltage downstream of the power converter (204) and a reference signal (212);

[0118] wherein the controller (200) is arranged to control the power converter (204) to output current (222) at a nominal voltage in a grid forming mode based on the control data (230); and

[0119] Wherein, the controller is configured to:

[0120] Determining that a first parameter associated with the controller (200) and / or the power converter (204) exceeds a first threshold;

[0121] In response to determining that the first parameter exceeds the first threshold:

[0122] controlling the power converter in a first limiting mode (430), wherein the controller (200) is configured to limit an output current (222) of the power converter (204) based on a current value and / or a reference value;

[0123] determining the output current (222) of the power converter (204);

[0124] In response to determining that the output current (222) is at or near a balanced condition:

[0125] determining a virtual impedance of the power converter (204) based on the output current (222) of the power converter (204);

[0126] modifying the control system based on the virtual impedance (228); and

[0127] The power converter (204) is controlled in a first constrained grid forming mode (450), wherein the modified control system (228) is configured to determine the control data (230) based on measured signals (222, 224) of current and / or voltage downstream of the power converter and a reference signal (212).

[0128] Technical Solution 12. The controller according to Technical Solution 11, wherein the controller is further configured as:

[0129] determining that a second parameter associated with the controller (200) and / or the power converter (204) exceeds a second threshold;

[0130] In response to determining that the second parameter exceeds the second threshold:

[0131] controlling the power converter in a second limiting mode, wherein the controller is configured to limit an output current (222) of the power converter (204) based on an updated current value and / or an updated reference value;

[0132] determining an updated output current (222) of the power converter (204);

[0133] In response to determining that the updated output current (222) is at or near an equilibrium condition:

[0134] determining an updated virtual impedance of the power converter (204) based on the updated output current (222) of the power converter (204);

[0135] further modifying the control system based on the updated virtual impedance (228); and

[0136] The power converter (204) is controlled in a second constrained grid forming mode (450), wherein the further modified control system (228) is configured to determine the control data (230) based on the measured signals (222, 224) and the reference signal (212).

[0137] Technical Solution 13. A controller according to Technical Solution 11 or 12, wherein the control system (228) is a cascade control system (228) including a voltage controller (214) and a current controller (216); wherein

[0138] The voltage controller (214) is configured to generate first data (215) using a voltage control system based on the reference signal (212) and the measured signal (222, 224), and is further configured to output the first data (215) to the current controller (216);

[0139] The current controller (216) is configured to generate second data using a current control system based on the first data (215) and the measured signal (222, 224);

[0140] The controller (200) is further configured to determine the control data (230) based on the second data; and

[0141] The controller (200) is further configured to control the power converter (204) in the first limiting mode (430) by setting the first data (215) equal to the current value or the reference value, thereby limiting the output current (222) of the power converter (204).

[0142] Technical solution 14. A controller according to any one of technical solutions 11 to 13, wherein

[0143] The controller is configured to modify the control system (228) based on the virtual impedance, including the controller being configured to determine an adjustment term of the control system (228) by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current (222), and implementing the adjustment term into the control system (228).

[0144] Technical solution 15. A power converter, comprising:

[0145] A DC side for connection to a DC source;

[0146] an AC side for connection to a grid; and

[0147] A controller (200) according to any one of technical solutions 11 to 14. BRIEF DESCRIPTION OF THE DRAWINGS

[0148] Figure 1 is a schematic illustration (not to scale) of a power system including a DC power source, a power converter and a power grid;

[0149] Figure 2A is a schematic illustration (not to scale) of a power converter and a controller of a power system;

[0150] Figure 2B is a schematic illustration (not to scale) of a power converter and a controller of a power system;

[0151] Figure 3 is a schematic illustration (not to scale) of the electrical circuits of the power system and a simplified electrical circuit during normal conditions;

[0152] Figure 4 is a schematic illustration (not to scale) of a method for controlling a power converter according to an embodiment of the present disclosure;

[0153] Figure 5 is a schematic illustration (not to scale) of an electrical circuit of a power system during a grid event;

[0154] Figure 6 is a schematic illustration (not to scale) of a Thevenin equivalent circuit including a virtual impedance of a power system during a grid event; and

[0155] Figure 7 is a schematic illustration (not to scale) of a phasor diagram showing phasors during a first constrained grid formation mode. DETAILED DESCRIPTION

[0156] Figure 11 is a schematic illustration (not to scale) of a power system 100 including a power converter 204 connected between a DC source 202 and an electrical grid 232. The electrical grid 232 may be an AC electrical grid 232. The illustration is not intended to be limited to representing a particular power system, or connection, or interconnection, but is also provided as a general example of the operating principles of a power system useful for understanding the present invention. Thus, while specific features in the illustration are shown as being connected to one another by a particular number of connections, it will be understood that this is not intended to be limiting, but rather is intended to illustrate the general connections between features / components. Relatedly, the relative sizes or distances between components understood in the illustration are also not intended to be limiting. Thus, it will be understood that the principles and features in the power system 100 and the principles and features discussed herein may be applied to a variety of power systems including Figure 2A The interconnection of the controller 200 shown in , or a power converter or network operated using the controller 200 .

[0157] The power system 100 includes a first inverter-based resource 204 (also referred to as a power converter). The power converter 204 is configured to convert DC power into AC power, thereby essentially acting as an inverter. The power converter 204 is also configured to convert AC power into DC power, thereby essentially acting as a rectifier. The power converter 204 can represent a cluster of multiple power converters, which can be close in electrical proximity and operate in a consistent manner relative to grid events or control references. The power converter 204 can include a single converter in the case of a unipolar system, or two converters in the case of a bipolar system. The power converter 204 can represent multiple converter stations arranged into a multi-terminal power transmission system. In this example, the power converter 204 includes a DC side 204a and an AC side 204b.

[0158] The power converter 204 is connected to the DC source 202 via a first line 206. The DC source 202 is connected to the DC side 204a of the power converter 202 via a second line 208. The first and second lines 206, 208 are illustrative and do not represent any particular type of connection or cabling.

[0159] The power converter 204 is connected to a grid 232. The grid 232 is connected to the AC side 204b of the power converter 204.

[0160] The DC source 202 and / or the grid 232 may be an electric power transmission system, including power generation equipment, transmission equipment, distribution equipment, and electric loads. The DC source 202 may include a renewable power generation resource, such as a wind power generation resource, a solar power generation resource, a battery generation resource, a supercapacitor generation resource, a bio-power generation resource, etc. Alternatively, the DC source 202 may include a network of such resources. The grid 232 may be a consumer network. As a non-limiting example, for example, the DC source 202 may be a power generation network, and the grid 232 may be a consumer network. The DC source 202 and / or the grid 232 may be of any size and may have varying electrical characteristics due to operational factors.

[0161] The operation of the power system 100 can be generally described as follows. The DC power source 202 provides DC power to the power converter 204 of the DC side 204a. The power converter 204 converts the received DC power into AC power for the grid 232. For example, the AC power is transmitted from the AC side 204b to the grid 232 for consumption. In a specific example, the power converter 204 can be located within an offshore wind farm or can be located on shore.

[0162] In the case where the DC power source 202 is an energy storage device or a high voltage direct current (HVDC) transmission line, power can flow from the grid 232 to the DC power source 202. In this case, the grid 232 provides AC power at the AC side 204b to the power converter 204. The power converter 204 converts the received AC power into DC power for the power source 202. The DC power is transmitted from the DC side 204a to the power source 202.

[0163] It will be appreciated that in the example power system 100, various additional electrical components may be located in any particular location or have any particular features / components. These may include switches, transformers, resistors, reactors, surge arresters, doubly fed induction generators, harmonic filters, and other components known in the art. These additional electrical components may be connected in series or in parallel with the first and / or second lines 206, 208 in the power system 100. For example, the second line 208 may include three additional electrical components, including: a first cable connected between a pole of the AC side 204b and a first set of windings of a transformer; a transformer; and a second cable connected between a second set of windings of the transformer and a power grid 232. In another example, the AC side 204b may be connected to a first set of windings of a doubly fed induction machine, and the power grid 232 may be connected to a second set of windings of the doubly fed induction machine.

[0164] It will be appreciated that the converter or power conversion component may comprise a variety of different technologies, such as a voltage source converter (e.g. using insulated gate bipolar transistor (IGBT) valves). Such converters are generally considered to use 'power electronics'. For example, a power electronic converter may comprise a multi-level voltage source converter.

[0165] It will be appreciated that cables used as a power transmission medium may include the following non-limiting examples of cross-linked polyethylene (XLPE) and / or heavily impregnated (MI) insulated cables. Such cables may include a conductor (e.g., copper or aluminum) surrounded by an insulating layer. The dimensions of the cable and its associated layers may vary depending on the specific application (and in particular the operating voltage requirements). In applications such as subsea installations, the cable may also include reinforcement or 'armoring'. The cable may also include a sheath / shield that is grounded at one or more locations.

[0166] In addition, it will be understood that the power system 100 can be used with a three-phase power system. In a three-phase power system, three conductors supply the consumer with respective first, second and third phase AC power. Each of the first, second and third phases will typically have a voltage or current of equal magnitude, which are 120° out of phase with each other.

[0167] In a three-phase power system, the phase current and voltage can be represented by three single-phase components: a positive sequence component; a negative sequence component; and a zero sequence component. It is a positive sequence component that rotates in phase with the power system. Therefore, in an ideal case, there will be only positive sequence voltage / current. It will be understood that an imbalance in the magnitude or phase angle of the voltage or current between the first, second, and third phases of the three-phase system may cause a negative sequence or zero sequence component. This imbalance may be caused by, for example, a fault in the power grid 232 or a change in the power grid condition (referred to herein as a power grid event).

[0168] Figure 2A is a schematic diagram showing an embodiment of a controller 200 that can be used to implement the methods described herein to control Figure 1 The power converter 204 shown in FIG. 1 is operating in a grid forming (GFM) mode.

[0169] like Figure 2A As shown in FIG. 1 , the power system 100 includes a DC source 202, a power converter 204, a power grid 232, and a controller 200. The controller 200 is connected to the power converter 204. Figure 2B Another example of a power system 100 including a DC source 202 , a power converter 204 , a grid 232 , and a controller 200 is shown in FIG.

[0170] The controller 200 receives the reference signal 212 and the measured signals 222 , 224 . The controller 200 outputs control data 230 to the power converter 204 .

[0171] The control data 230 may include a pattern of gate pulses that can be used to switch semiconductor devices within the power converter 204 in such a manner so as to obtain a desired AC voltage waveform (consisting at least in part of a fundamental frequency voltage amplitude and a phase angle) very quickly (e.g., in much less than 10 ms) at the power converter 204, and further, the gate pulses can be used to implement a specific reference signal 212 at a pole of the power converter 204 or at a position very close to the pole of the power converter 204.

[0172] The reference signal 212 may be an internal voltage phasor reference, including a voltage magnitude reference and a phase angle reference. The reference signal 212 is provided to the controller 200 by a grid forming controller (not shown) that may be external to the controller 200. The voltage magnitude reference and the phase angle reference of the reference signal 212 may vary over time based on the measured signals 222, 224 and according to the control objectives of the grid forming controller.

[0173] The grid forming controller may use a grid forming control algorithm to generate the reference signal 212. The grid forming control algorithm may consist of any of the following: a grid forming algorithm based on a virtual synchronous machine (also known as an 'inertia power regulator'), a grid forming algorithm based on droop control, a grid forming algorithm based on a virtual oscillator control (VOC), an isochronous (fixed frequency) grid forming control algorithm, or any variation of the above algorithms. Regardless of the type of grid forming algorithm, the grid forming algorithm will typically change the phase angle reference under normal operating conditions to achieve autonomous synchronization with other grid forming power converters and possibly also achieve the desired active power output.

[0174] The grid formation control algorithm enables the grid formation controller to autonomously achieve the aforementioned objectives using only local voltage and current measurements. The grid formation control algorithm may also include a reactive power regulation component, also referred to as a 'Q controller', which regulates the output reactive power and / or the local voltage amplitude at a nearby regulation point to a desired set point. The reference set points of the grid formation algorithm may consist of active power and reactive power reference signals, which may be fixed or, alternatively, may be time-varying and provided by another external controller or remote grid operator. Under conditions where the grid 232 represents a relatively 'rigid' grid or bulk power system, the grid formation algorithm will typically achieve the objectives (i.e., achieve the active and reactive power references) with zero steady-state error, with typical settling times of at least 100 ms to about several seconds.

[0175] In some cases, such as in an island condition (where the grid 232 may not contain other grid forming or synchronous generating power converters, and may contain any amount of total load, potentially including zero total load), the requirement for power balance between generation and load is generally given higher priority than any requirement to adhere to active and / or reactive power set points, and the grid forming control algorithm may be expected to not precisely achieve its active and / or reactive power references in steady state. In such cases, the grid forming resource (e.g., power converter 204) may be expected to continue to synchronize (settle in a stable steady-state balance) with any other grid forming resource (e.g., other power converters that may be present in the islanded network), and in addition, to provide or absorb active and / or reactive power in a manner that may not achieve the active and reactive power references provided to the grid forming resource (e.g., power converter 204) in order to satisfy the loads present in the island.

[0176] The voltage magnitude reference and phase angle reference generated by the grid forming algorithm and included in the reference signal 212 are used by the controller 200 to determine the control data 230, as discussed in more detail later below. Under normal operating conditions, depending on the type of voltage control method employed within the controller 200, the control data 230 may be generated by the controller 200 to produce an AC voltage waveform (composed at least in part of a fundamental frequency voltage magnitude and phase angle) at the poles of the converter, which AC voltage waveform may generally differ from the fundamental frequency voltage magnitude and phase angle components included in the reference signal 212 in both transient and steady state.

[0177] Measured signals 222 , 224 are indicative of a current 222 and / or a voltage 224 downstream of power converter 204 .

[0178] The controller 200 includes a control system 228, which in this example is a cascade control system 228. The controller 200 may alternatively include a single loop voltage control 228B, which is discussed further below. Figure 2A As shown in , the cascade control system 228 includes a voltage controller 214 and a current controller 216 .

[0179] The voltage controller 214 generates first data 215 based on the reference signal 212 and the measured signals 222, 224 using a voltage control system, and outputs the first data 215 to the current controller 216. The phase angle component of the reference signal 212 can optionally be used for one or more reference frame transformations within the voltage controller 214. The current controller 216 generates second data based on the first data 215 and the measured signals 222, 224 using a current control system. In some embodiments, the current controller 216 can also use the voltage amplitude and / or phase angle component of the reference signal 212 in one or more feed-forward control features and / or reference frame transformations included in the current controller 216. The controller 200 determines the control data 230 based on the second data.

[0180] Cascade control system 228 operates as follows. Controller 200 generates control data 230 that produces a time-varying voltage waveform (composed at least in part of a time-varying fundamental frequency voltage amplitude and phase angle) at the poles of power converter 204 that typically does not necessarily match the amplitude and / or phase angle components of reference signal 212. Rather, under normal circumstances, the generated power converter 204 pole voltage will vary rapidly as necessary to achieve the amplitude and phase angle components of reference signal 212 at a regulation point that is electrically close to, but not directly at, the poles of power converter 204. The regulation point may be located, for example, at the grid-side winding terminals of an AC transformer coupling power converter 204 to grid 232. It is expected that the voltage amplitude and phase angle components of reference signal 212 will vary in time in response to grid events according to the above-described grid formation control objectives. Because the voltage control system of voltage controller 214 has a much higher control bandwidth (e.g., 7 to 10 times higher) than the control bandwidth of the grid formation control algorithm provided by the grid controller, it is expected that the voltage amplitude and phase angle components of the fundamental frequency of the power converter 204 pole voltage will change much faster than the voltage amplitude and phase angle components of reference signal 212.

[0181] The controller 200 may alternatively include Figure 2B The single-loop voltage control system 228B shown in FIG. The single-loop voltage control system 228B includes a single-loop controller 214B that generates control data 230 based on the reference signal 212 and the measured signals 222 , 224 .

[0182] The single-loop voltage control system 228B operates as follows. The controller 200 generates control data 230 that produces a time-varying voltage (at least partially composed of a time-varying fundamental frequency voltage amplitude and phase angle) at the pole of the power converter 204, which time-varying voltage generally does not necessarily match the amplitude and / or phase angle components of the reference signal 212. Similar to the case of the cascade control system 228, the generated power converter 204 pole voltage will vary as needed to achieve the amplitude and / or phase angle components of the reference signal 212 at a regulation point that is electrically close to the pole of the power converter 204, but not directly at the pole of the power converter 204.

[0183] In an alternative example, the single-loop controller 214B may instead comprise a 'direct voltage control' system, in which the data 230 is derived more directly from the reference signal 212. In the case of a direct voltage control system, under normal operating conditions, the control data 230 generated by the controller 200 results in a fundamental frequency voltage magnitude and phase angle at the poles of the power converter 204 that closely matches (both transiently and steadily) the voltage magnitude and phase angle contained in the reference signal 212.

[0184] In the case of a single-loop voltage control system or a direct voltage control system, the single-loop controller 214B may also include current limiting control features that are temporarily activated during abnormal grid events (e.g., severe faults). These current limiting control features modify the control data 230 when activated to effectively limit the current or a component of the current to a maximum value in order to allow continued operation of the converter hardware, resulting in a temporary abandonment of the nominal control target of the controller 200.

[0185] Regardless of the type of voltage control system used, the controller 200 may also use a 'nominal virtual impedance'. To achieve this, the voltage amplitude reference and the voltage angle reference within the reference signal 212 are first modified by the controller 200 to accommodate a time-varying phasor voltage drop that is proportional to the measured or expected output current of the power converter 204. The controller 200 may employ various means of calculating the nominal virtual impedance phasor voltage drop. For example, it may consist of the product of a complex-valued nominal virtual impedance and a time-varying current phasor measurement obtained using the measured current 222. Alternatively, the nominal virtual impedance voltage drop may be equal to the product of the nominal impedance and a current phasor reference derived from the second data 215. The nominal virtual impedance used in the calculation is fixed, i.e., it does not change in a smooth or discrete manner except in rare cases (e.g., to compensate for changes in grid strength).

[0186] Where the controller 200 comprises a cascade controller 228 or a single loop controller 228B and a nominal virtual impedance is used, the power converter 204 pole voltage will be quickly manipulated to achieve a voltage phasor equal to the reference signal 212 (internal voltage phasor reference) minus the nominal virtual impedance voltage drop at the regulation point.

[0187] Where controller 200 comprises a DC voltage controller, the time varying fundamental frequency voltage magnitude and voltage phase angle at the poles of power converter 204 will comprise a voltage phasor equal to reference signal 212 (internal voltage phasor reference) minus the nominal virtual impedance voltage drop.

[0188] The control data 230 is provided to the power converter 204. The power converter 204 operates based on the control data 230, i.e., performs conversion of DC power to AC power or conversion of AC power to DC power, and generation or absorption of reactive power. In this way, the control data 230 controls the power converter 204.

[0189] Although in Figure 2A 2. Although not shown, the controller 200 may include a memory and at least one processor. The memory may include computer-readable instructions that, when executed by at least one processor, cause the controller 200 to perform the method(s) described herein.

[0190] In addition, despite the Figure 2A Controller 200 may also include a transceiver device. The transceiver device may include a separate transmitter and receiver. The transceiver device may be used to communicate operably with other components described herein directly or via other interfaces such as a network interface using wired or wireless means. The transceiver device may, for example, use a transmitter and a receiver to send and receive control signals. The control signal may include or define an electrical control parameter, such as a reference current or a reference voltage.

[0191] At least one processor may be capable of executing computer readable instructions and / or performing logical operations. At least one processor may be a microcontroller, a microprocessor, a central processing unit (CPU), a field programmable gate array (FPGA), or a similar programmable controller. The controller 200 may also include a user input device and / or an output device. The processor may be communicatively coupled to a memory and a transceiver.

[0192] The memory may be a computer-readable storage medium. For example, the memory may include a non-volatile computer storage medium. For example, the memory may include a hard drive, a flash memory, etc.

[0193] In addition, despite the Figure 2A200 may additionally include a user input device interface and / or a user output device interface that may allow visual, auditory, and / or tactile input / output. Examples of such user input / output devices include, but are not limited to, interfaces for electronic displays, touch screens, keyboards, mice, speakers, and microphones.

[0194] As described above, in the disclosed example, controller 200 provides control data 230 to power converter 204. Under normal operating conditions, control data 230 controls power converter 204 in grid formation (GFM) mode. As will now be discussed, aspects of the present disclosure provide improvements when power converter 204 is operating and there is a grid event.

[0195] The controller 200 uses a cascade control system 228 to control the power converter 204 as a grid forming resource. Figure 3 , the first circuit diagram 310 and the second circuit diagram 320 may be used to illustrate the power source system 100 during normal conditions. When operating in the GFM mode, under normal conditions, the power converter 204 is controlled to output power to the grid 232 at a nominal voltage. Figure 3 The Thevenin voltage phasor V of the power grid 232 is shown as follows: g ; The converter voltage phasor V at the pole of the power converter 204 c ; The initial internal voltage phasor reference V of the power converter 204 cv i (ie, the output of the inertial power regulator and the 'Q controller'); Thevenin impedance Z of the grid 232 g ; The converter impedance Z of the power converter 204 coupled (ie, grid-connected) to the grid 232 c (e.g., due to transformer '(s) leakage reactance', filters, valve inductance, etc.; note that this does not include the nominal virtual impedance discussed above); the coupled voltage V at the common coupling point (regulation point) of the power converter 204 to the grid 232 p and the output current I of the power converter 204 p 222. The second circuit diagram 320 is a simplified version of the first circuit diagram 310. The second circuit diagram 320 is simplified because under normal operating conditions, the coupling voltage V p Approximately equal to the initial internal voltage phasor reference V cv i , and thus the converter impedance Z c Although still physically present, they can be considered to have a negligible effect on the behavior of the system on the time scales of interest (subtransient to transient), and are therefore removed from the circuit diagram. Figure 3 From the perspective of electrical characteristics, the power converter 204 includes a converter impedance Zc and a converter voltage source 301 (which generates a converter voltage phasor V c ). This is because, as described above, the power converter 204 is controlled in the GFM mode and therefore the power converter 204 includes the converter voltage source 301 .

[0196] When viewed from an electrical characteristics perspective, the electrical network 232 includes a Thevenin impedance Z g and the grid voltage source 303 (which generates the Thevenin voltage phasor V g ). As described above, the grid 232 may be a consumer network, and any number of power stations may be connected to the grid 232 and supply power to the grid 232. Therefore, the grid 232 includes both the consumer network and the power station. The consumer network can be simplified as the Thevenin impedance Z g The power station can be simplified as a grid voltage source 303, which is responsible for providing and maintaining the Thevenin voltage phasor V g Often, due to the size of consumer networks and power stations, it is practically impossible to know, measure or determine the Thevenin impedance Z g and / or Thevenin voltage phasor V g .

[0197] A fault in the consumer network will result in a Thevenin impedance Z g changes, and a fault in the power station will cause the Thevenin voltage phasor V g Any of these faults is a fault in the grid 232. However, during normal conditions, there is no such fault in the grid 232.

[0198] If a fault or other severe grid disturbance (referred to herein as a grid event) occurs in grid 232, power converter 204 will likely experience a current, power, or energy overload because power converter 204 is regulating voltage rather than current, power, or energy. For a power converter controlled in a typical GFM mode, such an overload tends to cause the power converter to trip offline, or in a worst-case scenario, tends to damage semiconductor devices within the power converter and render the power converter inoperable indefinitely.

[0199] During such grid events, self-protection of the power converter is ensured and current limiting is usually achieved by switching the control to GFL mode. However, switching the control mode in this way has the potential to jeopardize the stability of the power converter and the stability of the entire power system. The controller 200 tends to be able to achieve this by switching the control mode to GFL mode according to the current limiting principle. Figure 4 The method disclosed in controls the power converter 204 to eliminate or reduce this situation.

[0200] like Figure 4As shown in FIG. 4 , based on control data 230 provided by controller 200, power converter 204 is initially controlled in GFM mode 410 to output power at a nominal voltage. Control system 228 determines control data 230 based on measured signals 222, 224 and reference signal 212. Controller 200 monitors a first parameter associated with controller 200 and / or power converter 204 to determine whether the first parameter exceeds a first threshold. Under normal conditions, the first parameter does not exceed the first threshold, and power system 100 is as follows: Figure 3 The first and second circuit diagrams 310, 320 shown in FIG. 1 operate as depicted in FIG.

[0201] Then a grid event occurs in the grid 232. In this example, the grid event causes the Thevenin voltage phasor V g Controller 200 monitors a first parameter and determines 420 that a first parameter associated with controller 200 and / or power converter 204 exceeds a first threshold. Thus, the first parameter exceeds the first threshold as a result of the grid event.

[0202] Determining 420 that the first parameter has exceeded the first threshold may include one or more of the following: determining that a single d or q channel of a current reference of a positive-sequence dq-based current regulator has saturated (wherein the positive-sequence d-channel and q-channel currents are appropriately decomposed into their respective orthogonal channels relative to a positively rotating reference frame that rotates synchronously with a reference angle output from a grid formation algorithm, or relative to a positively rotating reference frame that rotates synchronously with a phase-locked loop PLL angle locked to a measured positive-sequence voltage); determining that a single d or q channel of a current reference of a negative ... negatively rotating reference frame that rotates at the same speed as a reference angle output from a grid formation algorithm, but in a negative (opposite) direction; rotating reference frame, or a negative rotating reference frame that rotates synchronously with the PLL angle locked to the measured negative sequence voltage, with the negative sequence d channel and q channel currents appropriately decomposed into their respective orthogonal channels); and / or determining that a proportional integral PI controller with a dq current reference in the current controller 216 has saturated; and / or determining that a proportional resonant PR controller with an abc current reference or an alpha-beta reference in the current controller 216 has saturated; and / or determining that a phasor current limit has been reached; and / or determining that a component is pulsing down or blocking; and / or determining that a voltage reference has been saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has been saturated.

[0203] The first threshold is specified as a quantity or a set of quantities that are close to the real physical operating limit of the resource and / or close to the internal or externally imposed operating limit. The first threshold can be tailored in a particular manner to meet grid code requirements, for example, to favor the production of active power over reactive power during transients (e.g., assuming that the dq0 reference frame rotates synchronously with the internal voltage angle command of the GFM controller, the threshold of the d-axis current is greater than the threshold of the q-axis current).

[0204] Alternatively, the first threshold may be selected to favor the generation of positive or negative sequence current or voltage.In a similar manner, the actual maximum allowed threshold of the component or resource is defined by the second threshold.

[0205] For example, in one embodiment, where output current is the variable to be limited, the first threshold may be defined as a perimeter or boundary of a two-dimensional shape that specifies the allowable region in which the output current of the resource (defined as a scalar complex-valued number or 'space vector') may reside (a space vector drawn from the origin of the complex plane will not be allowed to end up at a value outside the shape defined by the perimeter as the first threshold); the shape may be a circle (e.g., Figure 7 ), a rectangle, or any other shape on a two-dimensional complex plane where the first threshold is slightly concentric with a slightly larger shape bounded by a perimeter that is a second threshold, the second threshold defining an actual physical limit.

[0206] In one embodiment, the shape bounded by the threshold first threshold is a circle with a radius equal to 0.95, corresponding to a per unit current amplitude threshold of 0.95% of the rated current, where the current amplitude is calculated in a conventional manner as a function of the d-axis and q-axis currents, for example via the Euclidean norm, and the second threshold is a slightly larger concentric circle with a radius of 1.0 or 1.1.

[0207] In response to determining that the first parameter exceeds the first threshold 420 , controller 200 controls power converter 204 in a first limiting mode 430 by limiting power converter 204 output current 222 based on a current value or a reference value.

[0208] In the case where the controller 200 includes a cascade control system 228, the first limiting mode 430 is implemented by setting the first data 215 equal to its current value or reference value, thereby limiting the output current (222) of the power converter (204).

[0209] Where the controller 200 includes a single loop or direct control system 228B, the controller 200 also includes a limit controller 217 that limits the output current 222 from increasing beyond a current value or reference value, thereby limiting the output current (222) of the power converter (204).

[0210] In the example of the cascade control system 228, as a result of setting the first data 215 equal to its current value or reference value, the control data 230 will manipulate the fundamental frequency voltage waveform at the poles of the power converter 204 relatively quickly in a sub-transient time scale so that the output current 222 converges to the current value.

[0211] During the remainder of the duration spent in the first limiting mode 430 , the current magnitude and current phase angle will remain relatively unchanged, and absent any further grid events during the period spent in the first limiting mode 430 , the control data 230 will also reach a steady state and be expected to not change significantly.

[0212] Regardless of the type of control system 228, 228B used, in the first limiting mode 430, a portion of the control system 228, 228B is prohibited from determining the value of the control data 230 based on the measured signals 222, 224 and the reference signal 212 in order to limit the output current (222) of the power converter (204).

[0213] The current value is the current reference value contained in the controller 200 when the determination is made that the first parameter exceeds the first threshold. In this way, the current reference is effectively 'frozen', ie the current reference remains at the same value as when the determination is made that the first parameter exceeds the first threshold.

[0214] In some examples, the current reference is 'fixed' in terms of current magnitude and current phase angle only relative to a reference frame defined by a particular angle. The choice of angle used to define the reference frame depends on the design of the current regulator or transient current limiter, and may, for example, include a grid forming voltage angle reference contained in the reference signal 212, or may alternatively be derived from a PLL, a frequency locked loop (FFL), a second order generalized integrator (SOGI), or by other means.

[0215] In the first limiting mode 430, because the current reference contained in the controller 200 is set to a present value or a reference value, any higher-level controllers (e.g., the voltage controller 214 or a grid forming controller that generates the reference signal 212) that may have otherwise influenced the value of the control data 230 or the current reference may also optionally be 'frozen', i.e., their control behavior is modified so that their output is locked at a particular value.

[0216] Specifically, if voltage controller 214 is present, it may be desirable to freeze voltage controller 214 .

[0217] Where one or more integrators are present in any higher level controller of interest, 'freezing' of the controller may involve temporarily zeroing the inputs to those integrators, or any other control action having a similar effect.

[0218] It may not be necessary to freeze the grid formation algorithm that generates the reference signal 212. This is because the grid formation algorithm tends to have a slower bandwidth, which makes it have a relatively slow response time in the sub-transient time scale.

[0219] Therefore, in the first limiting mode 430, it is not desirable for the initial internal voltage phasor reference V cv i drastically changes, regardless of changes in the measured signals 222 , 224 .

[0220] Furthermore, in some examples, slow changes in reference signal 212 may potentially be beneficial because these changes may help drive power system 100 synchronization by shifting the reference frame of the fixed current phasor reference. However, since power converter 204 is expected to remain in first limiting mode 430 only briefly (e.g., less than 50 ms), the initial internal voltage phasor reference V cv i Not much will change during this time.

[0221] As described above, the grid event will cause the Thevenin voltage phasor V g The Thevenin voltage phasor V of the power grid 232 has g The fixed or frozen initial internal voltage phasor reference V cv i Will result in a coupled voltage V p Therefore, as regards Figure 3 As discussed, the second circuit diagram 320 would not be valid for the first limiting mode 430 .

[0222] A simplified circuit diagram illustrating the operation of power converter 204 and grid 232 during first limiting mode 430 is shown in FIG. Figure 5 The third circuit diagram 500 is shown in FIG. 500 . The third circuit diagram 500 shows: the fault Thevenin voltage phasor V of the power grid 232 g f ; The initial internal voltage phasor reference V of the power converter 204 cv i ; Thevenin impedance Z of the faulty grid 232 g f ; Fault coupling voltage V at the common coupling point (regulation point) of the power converter 204 to the grid 232 p f Output current I of power converter 204 during grid events p f 222; and the reference current I of the power converter 204 during the grid event reff+ .

[0223] In the context of the present disclosure, the term 'faulty' does not necessarily refer to a specific grid short or open circuit condition, but is used in a broader sense to refer to any grid event that may cause a grid forming resource to experience a parameter (e.g., a current limit threshold) violation. A non-exhaustive list of such grid events is provided as follows: symmetrical or asymmetrical line-to-ground or line-to-line short circuit when islanded or grid-connected, symmetrical or asymmetrical line open circuit when islanded or grid-connected, a drop or rise in voltage amplitude of the power grid when grid-connected, a load step or jump in the phase angle of the power grid when grid-connected, an unintended control action taken by a neighboring resource, an unexpected loss of a neighboring resource when islanded or grid-connected, a large step in constant power load, constant current load, constant impedance load, or machine load, an intentional or unintentional islanding event or a sudden change in grid strength, an intentional or unintentional synchronization event, or a combination of the above grid events (when islanded or grid-connected such that multiple events occur in a concurrent or sequential manner).

[0224] Reference current I ref f It may be determined by components in the power system 100 associated with one or more of limiting positive or negative sequence current magnitude, limiting active or reactive power transfer, limiting energy transfer to or from a DC source, and / or limiting any other quantity of interest.

[0225] Because the first data 215 or the reference current I ref f is set to a current value (ie, is frozen), so assuming that the control system 228 or the current controller 216 is stable, the output current 222 will not change. As a result, in the first limiting mode 430, during a grid event, the output current I of the power converter 204 is p f 222 will tend to converge toward equilibrium, which can generally be indicated by the magnitude of the current controller error signal (or multiple current controller error signal magnitudes in the case of more than one current control channel) decreasing to a value equal to or close to zero. In the first limiting mode 430, the controller 200 thus effectively controls the power converter 204 as a current source 501.

[0226] Controlling power converter 204 as a current source is similar to controlling power converter 204 in a grid tracking (GFL) mode. GFL mode tends to be undesirable for extended periods of time because controlling power converter 204 in GFL mode tends to prevent power converter 204 from providing grid 232 support functionality as a result of operating in GFM mode. Additionally, extended operation in GFL mode may also result in a loss of stability of power converter 204 and / or grid 232.

[0227] However, the first limiting mode 430 may be stable under strong grid conditions where the PLL is not expected to become unstable. In this case, it may be more advantageous to use the PLL for the reference frame of the current reference because in this case, the d-channel and q-channel components of the current reference may be selected so that they quickly and accurately achieve the active and / or reactive current set points over an indefinite period of time. As a result, the energy transfer between the DC source 202 and the grid 232 tends to be easier to regulate.

[0228] Alternatively, instead of a phase angle derived from a PLL, FFL, or SOGI, a phase angle component of the reference signal 212 may be used as a reference frame for the control data 230 in the limiting mode 430. In this mode, the phase angle component may be frozen, or may optionally be allowed to vary. If the phase angle reference within the reference signal 212 is not frozen during the first limiting mode 430, then the slowly varying phase angle component of the reference signal 212 may be used as a reference frame for the first data 215. This practice may be considered desirable because it may allow for some limited preservation of the ability of the power converter 204 to pursue the synchronous GFM control goal.

[0229] However, even so, operation in the first limiting mode 430 still does not constitute a GFM in the conventional sense, since the voltage amplitude is not regulated, and this drawback may lead to unintended consequences. Therefore, to reduce the risk of undesired behavior, it is generally intended that the converter 204 does not remain in the first limiting mode 430 for more than about 50 to 100 ms, and ideally no more than 30 ms.

[0230] Controller 200 typically completes the step of controlling power converter 204 in first limiting mode 430 (i.e., enabling power converter 204 to act as a current source) within a time period less than or equal to 50 ms from when a determination is made that the first parameter has exceeded the first threshold. As a result, in the electromagnetic time scale, output current I p 222 quickly reaches equilibrium (ie, steady state). This advantageously tends to prevent power converter 204 from tripping offline, since hardware-related limits will not be violated. For example, the current limiting implemented in this mode tends to prevent any semiconductor devices within power converter 204 from being damaged.

[0231] Reference again Figure 4 When controlling the power converter 204 in the first limiting mode 430 (eg, in the current limiting mode), the controller 200 determines the output current I of the power converter 204. p 222. In response to the output current I p 222 reaches the equilibrium condition, the controller 200 is based on the output current Ip 222 and output voltage 224 are used to determine a virtual impedance 440 of power converter 204 .

[0232] In some embodiments, instead of using the measured signals 222, 224, the controller 200 may alternatively use a proxy for one or both output signals, such as a predicted current (i.e., the current phasor reference 215) or an estimated current (e.g., a current estimate obtainable via a system model or via an observer) in its determination of the virtual impedance 440. A modified internal voltage phasor reference may also be determined.

[0233] In the case where a modified internal voltage phasor reference has not yet been determined for steady-state operation, when the power converter 204 is in the first limiting mode 430 , the controller 200 calculates the virtual voltage V v To determine the virtual impedance 440, the virtual voltage V v is defined as the internal voltage phasor reference V cv and a node voltage (fault coupling voltage) V at a node 224 (eg, a point of common coupling) between the power converter 204 and the grid 232. p f The controller 200 then generates a Thevenin equivalent circuit 600 of the power converter 204 connected to the grid 232 (at Figure 6 ), which includes the virtual voltage V of the power converter 204 under balanced conditions v , virtual impedance Z cv and fault output current I p f 222. The controller 200 then uses circuit analysis to solve for the virtual impedance Z cv The Thevenin equivalent circuit of 600.

[0234] Because the Thevenin equivalent circuit 600 is a simple circuit diagram, Ohm's law can be used to solve the Thevenin equivalent circuit 600, as shown in Equation 1.

[0235]

[0236] Once the controller 200 has determined the virtual impedance Z cv , the controller 200 determines the adjustment term of the control system 228 and modifies the control system 228 to include the adjustment term based on the virtual impedance Z cv Modify the control system 228. In the case where a fixed virtual impedance has been adopted in the initial grid formation operation mode 410, the virtual impedance Z cv Instead of a fixed virtual impedance.

[0237] The controller then controls the power converter 204 in the first constrained grid forming mode 450 , wherein the modified control system 228 determines the control data 230 based on the measured signals 222 , 224 of current and / or voltage downstream of the power converter 204 and the reference signal 212 .

[0238] Calculate Z in this way cv Tends to guarantee voltage will not change in the transition from the first limiting mode 430 to the first constrained grid formation mode 450. This is because the adjustment term is selected so that the fault output current under balanced conditions is the same in the first limiting mode 430 and in the first constrained grid forming mode 450 (because the fault output current will still be equal to the current after the transition ). Thus, the method of the present disclosure tends to achieve a seamless and near instantaneous transition back to (constrained) grid forming mode.

[0239] While Equation 1 illustrates the simplest method of determining a fixed virtual impedance that enables a seamless transition from the first limiting mode 430 to the constrained grid forming mode 450, it is not the only possible method. Alternative calculation methods utilizing the sine and cosine laws may be applied to determine a different virtual impedance that also enables a seamless transition while meeting various objectives. The alternative calculation method may require also determining a second virtual voltage consisting of a static or fixed virtual voltage offset that will be added to the internal voltage phasor reference V CV This second virtual voltage will be applied together with the first (current dependent) virtual voltage offset associated with the calculated virtual impedance. The second virtual voltage can be considered as a step change of the internal voltage phasor reference, including its angle ∠V CV , its amplitude |V CV | or V CV The amplitude and angle of the change.

[0240] Additional goals that may be met using the alternative approach may include one or more of: enforcing a specific magnitude of a virtual impedance; enforcing a desired ratio of reactance to resistance within the virtual impedance; and / or enforcing specific values ​​of internal voltage phasor magnitudes and / or phase angles. It should be noted that some goals are mutually exclusive, and furthermore, pursuit of any single goal may have a significant impact on the behavior and / or performance of the system in the constrained GFM mode.

[0241] Without loss of generality it is assumed that the controller has adopted a fixed 'steady-state' virtual impedance Z with known real and reactive components CVSS , which is expected to hold in both the initial and constrained GFM modes (410 and 450, respectively). Let ZCVSS Indicates that at the end of restriction mode 430, Z CVSS and I p The product of (or by Z CVSS and I p , or a product of a proxy of I p The virtual voltage drop across the virtual impedance is given by the estimated value of . It is also assumed that the voltage drop will be only in the constrained GFM mode 450 with Z CVSS Add additional 'current limiting' virtual impedance Z in series CVL , to achieve a seamless transition from the restricted mode 430 to the constrained GFM mode 450, so that in the constrained GFM mode 450 Z CV =Z CVL +Z CVSS . (Optionally, |Z CVSS | can be equal to zero).

[0242] As the fixed Z required to determine the seamless transition CV (and it must be Z CVL ) value, given the desired reactance to resistance ratio of the current limiting virtual impedance (or, if known, the equivalent angle ∠Z of the current limiting virtual impedance CVL ) and the desired amplitude V of the given internal voltage phasor CV |Z, the following calculation can be performed to determine the required magnitude of the additional current limiting virtual impedance required to achieve a seamless transition CVL | and the required internal voltage phasor reference angle ∠V CV :

[0243] ∠Z CVL =tan -1 (r XR )

[0244] γ=π-∠(V p +V CVSS )+∠I p +∠Z CVL

[0245] B=|V p +V CVSS |

[0246] C=|V cv |

[0247]

[0248]

[0249]

[0250] ∠V CV =∠(V p +V CVSS )-α

[0251] As another example, given the desired reactance to resistance ratio of the current limiting virtual impedance And given the desired magnitude of the current limiting virtual impedance |Z CVL |, the following calculation determines the required magnitude V of the internal voltage phasor required to achieve a seamless transition from the current limiting mode 430 to the constrained grid forming mode 450 CV | and the required internal voltage phasor reference angle ∠V CV :

[0252] A=|Z CVL ||I p |

[0253] B=|V p +V CVSS |

[0254] ∠Z CVL =tan -1 (r XR )

[0255] γ=π-∠(V p +V CVSS )+∠I p +∠Z CVL

[0256] C=[A 2 +B 2 -2ABcos(γ)] 0.5

[0257]

[0258] |V cv |=C

[0259] ∠V CV =∠(V p +V CVSS )-α

[0260] In the above discussion, the virtual impedance is considered to be composed of non-significant reactance and resistance elements. That is, any reactance within the virtual impedance is considered to have the same magnitude in the d channel and the q channel, and any resistance within the virtual impedance is considered to have the same magnitude in the d channel and the q channel. However, for the purpose of current limiting, a significant virtual impedance can also be applied in steady state and in constrained GFM mode. If the virtual impedance is significant, the reactance within the virtual impedance may have a different value in the d channel than in the q channel. The same may be true for any resistance within the virtual impedance.

[0261] It is mentioned above that the phase angle component of the reference signal 212 may be used as a reference frame for the first data 215 in the first limiting mode 430, or a PLL, FFL or SOGI etc. may be used for this purpose. The same reference frame may be used to calculate the 'orientation dependent' voltage drops across the different d-channel and q-channel components of the reactive and resistive elements within the virtual impedance.

[0262] The application of a significant virtual impedance to current limiting in the constrained GFM mode has a potential advantage because the voltage drop caused by the current limiting impedance magnitude can be concentrated mostly (or entirely) in the specific channel being overloaded. Due to the presence of orthogonal low impedance channels, greater operational flexibility and enhanced stable current supply capabilities may be possible in the constrained GFM mode. As in the case of the non-significant current limiting impedance, analytical techniques can be used to calculate the d-channel and q-channel components of the significant reactance and / or resistance, which allows a seamless transition from the first limiting mode 430 to the constrained GFM mode 450.

[0263] In the foregoing discussion, it may be assumed that the controller 200 operates exclusively on positive sequence quantities and that the method 400 is applied to positive sequence quantities. However, it will be noted that the controller 200 and the control method 400 may also be independently applied to negative sequence, where the controller 200 represents a controller providing negative sequence voltage control functionality, which may include cascaded negative sequence voltage control, single loop negative sequence voltage control, or direct negative sequence voltage control. In this case, the reference signal 212 is typically zero because a zero negative sequence voltage is desired (however, a non-zero negative sequence voltage reference may also be implemented). With respect to Figure 4 The modes, modes, and mode transitions in the negative sequence implementation function in the same manner as the modes and mode transitions in the positive sequence implementation. For example, when the measured negative sequence current 222 or some proxy for that current (e.g., a current reference or estimated current) exceeds a negative sequence threshold, the first restricted mode 430 may be entered. As in the case of a positive sequence application, a negative sequence virtual impedance may be determined that will allow the negative sequence controller to seamlessly transition to the constrained GFM mode. Assuming both positive and negative sequence controllers are present, Figure 4The overall control strategy shown in can optionally be applied to both positive and negative sequences (with the same or different control parameters), or to only one of the sequences.

[0264] In the first constrained grid formation mode 450, the power converter 204 is able to regulate the power converter 204 output voltage and thereby continue to support the grid 232. Signals and controllers that have been frozen or disabled in the restricted mode 430 are reactivated in the constrained GFM mode 450 and, if necessary, reinitialized to values ​​that do not disturb the system balance during the transition from the restricted mode 430 to the constrained GFM mode 450. The controller 200 controls the power converter 204 in the first constrained grid formation mode 450 within 50 ms of controlling the power converter 204 in the restricted mode 430. This tends to reduce or minimize the amount of time that the power converter 204 is in the GFL mode, which tends to allow the power converter 204 to quickly return to the GFM mode within an electromechanical time scale, whereby the power converter 204 can continue to support the grid 232.

[0265] refer to Figure 7 In the simulation results of the first constrained grid formation mode 450, the phasor diagram 700 shows the internal voltage phasor reference V as the first phasor 701. cv , the fault output current I of the power converter 204 in a balanced condition as the second phase quantity 702 p f , as the node voltage (fault coupling voltage) V of the third phase 703 p f , as the virtual voltage V of the fourth phase 704 v and the fault Thevenin voltage phasor V of the power grid 232 as the fifth phasor 705 g f The phasor diagram 700 also includes an x-axis 710 and a y-axis 720 , each having units of voltage or current (per unit). A first threshold is also shown as a first limit 706 .

[0266] exist Figure 7 In the example shown, the controller 200 determines that the first parameter exceeds the first threshold by determining that the phasor current has reached the phasor current limit. Thus, the first parameter is the second phasor 702 and the first threshold is the first limit 706.

[0267] As in Figure 7 As can be seen in FIG. 1 , the fifth phase quantity 705 is less than 1 p.u., which indicates that there has been a grid event on the grid 232. The fault output current I of the power converter 204 is p f, as shown by the second phasor 702, is at the first limit 706. The fourth phasor 704 is the difference between the first phasor 701 and the third phasor 703. Therefore, the power converter 204 is controlled in the first constrained grid forming mode 450, wherein the fault output current I of the power converter 204 is p f is maintained in a balanced condition (as shown by the second phasor 702), as used to determine the virtual impedance Z cv The virtual voltage V v As a result, the virtual impedance Z cv Used to determine adjustment items of the control system 228.

[0268] In one embodiment, the adjustment term may include a fourth phasor 704 (virtual voltage V v ), which can be defined as the impedance phasor Z cv and the measured current phase I p f In another embodiment, the adjustment term may alternatively include the impedance phasor Z contained in the first data 215 cv In one embodiment, the adjustment term may be applied as a virtual voltage added to the internal voltage phasor reference 212, which modifies the reference provided to the voltage controller 214 (or 214B in the case of a direct voltage regulation or single loop voltage regulation controller) so that the voltage controller properly implements the virtual voltage drop when it performs its control actions.

[0269] The controller 200 can also provide a smooth transition from the first constrained grid formation mode 450 back to the initial grid formation mode 410. Prior to this smooth transition, as the grid event clears and the current I p f The third phase 703 returns to the first phase 701. v ) is the difference between the first phasor 701 and the third phasor 703, so as the grid event clears, the fourth phasor 704 will therefore become smaller in magnitude.

[0270] After the grid event is cleared, once the virtual voltage V v If the current limit is small enough and the system is determined to be in balance, action can be taken to initiate a smooth controlled transition from the first constrained grid formation mode 450 to the initial grid formation mode 410. Thus, the virtual impedance Z added by the controller 200 for current limiting is cvThe components of and the associated adjustment terms will converge toward zero. Therefore, during the controlled transition, the controller 200 may further reduce the adjustment term 490 in the modified control system 228 beyond the reduction that has occurred when the grid event was cleared, eventually reducing the modification to zero, and thereby returning to controlling the power converter 204 in the grid forming mode 410.

[0271] This advantageously tends to allow power converter 204 to smoothly return to normal grid forming control without causing any disruption to grid 232 due to controlling power converter 204. Additionally, power converter 204 tends to be able to maintain some form of grid forming control to support power converter 204 during a grid event.

[0272] In some embodiments, controller 200 may determine 460 that a second parameter associated with controller 200 and / or power converter 204 exceeds a second threshold.

[0273] In response to determining that the second parameter exceeds the second threshold, the controller controls the power converter 204 in a second limiting mode by limiting the power converter 204 output current 222 based on the updated current value or the updated reference value (or, in the case of using a direct voltage regulation or a single-loop voltage regulation controller, the limiting controller 217 limits the output current 222 to the updated current value or the updated reference value). As a result, the control data 230 is expected to quickly manipulate the fundamental frequency voltage waveform at the pole of the power converter 204 so that the measured current 222 conforms to the updated current value or the updated reference value. During the time period spent in the second limiting mode, without further grid events, the control data 230 will also reach a steady state and is not expected to change significantly. Optionally, the control system 228 can be prohibited from determining the value of the control data 230 based on the measured signals 222, 224 and the reference signal 212. The controller 200 then determines the updated output current 222 of the power converter 204.

[0274] In response to the updated output current 222 reaching the equilibrium condition, the controller 200 determines an updated virtual impedance of the power converter 204 based on the updated output current 222 of the power converter 204, and further modifies the control system 228 based on the updated virtual impedance. Determining the updated virtual impedance and further modifying the control system 228 based on the updated virtual impedance are accomplished in the same manner as described above.

[0275] The controller 200 then controls the power converter 204 in the second constrained grid forming mode 450 by determining the control data 230 based on the measured signals 222 , 224 and the reference signal 212 via the further modified control system 228 .

[0276] For example, determining that the second parameter has exceeded the second threshold and taking responsive action may tend to be useful when the fault that caused the grid event is followed by another fault or an adjustment in another portion of the grid 232 as a result of the grid event. Thus, there may be subsequent grid events after an initial grid event.

[0277] In some embodiments, the control system 228 is a cascade control system 228 including a voltage controller 214 and a current controller 216. The voltage controller 214 generates first data 215 based on the reference signal 212 and the measured signals 222, 224 using the voltage control system, and outputs the first data 215 to the current controller 216. The current controller 216 generates second data based on the first data 215 and the measured signals 222, 224 using the current control system.

[0278] The current controller 216 may generate the second data relatively faster than the voltage controller 214 generates the first data 215 .

[0279] The controller 200 determines the control data 230 based on the second data.

[0280] As described above, the controller 200 controls the power converter 204 in the first limiting mode 430 by setting the first data 215 to be equal to the current value or the reference value, thereby limiting the output current 222 of the power converter 204 .

[0281] Modifications to the control system 228 based on the virtual impedance or the updated virtual impedance may include modifying the voltage control system of the voltage controller 214 .

[0282] In some embodiments, in response to controller 200 controlling power converter 204 in first limiting mode 430 , controller 200 may revert the control method to initial grid forming mode 410 if the duration for which the first parameter exceeds the first threshold is less than a defined minimum duration.

[0283] The method and controller of the present disclosure tend to be able to maintain grid forming control to the greatest extent possible, because the use of the grid following mode is kept as brief as possible. The controller and method generally tend to provide stable behavior during fault recovery and further prevent the loss of synchronization between the grid forming resource (i.e., power converter 204) and the bulk grid (i.e., grid 232). The controller and method tend to maximize the current capability of the power converter 204 and allow for prioritization of active and reactive current, angle and voltage, positive sequence and negative sequence by determining the adjustment terms.

[0284] The exact implementation of the first parameter may be determined by the user, and thus the controller and method are intended to provide some flexibility and tunability as desired (eg, the controller and method may be configured to prioritize grid forming behavior, or allow for extended current limiting, etc.).

Claims

1. A method (400) for controlling a power converter (204) connected to a power grid (232), wherein: Initially controlling (410) the power converter (204) to output current (222) at a nominal voltage in a grid forming mode based on control data (230) provided by a controller (200), the controller (200) comprising a control system (228) that determines the control data (230) based on measured signals (222, 224) indicative of current and / or voltage downstream of the power converter (204) and a reference signal (212); the method comprising: Determining (420), by the controller (200), that a first parameter associated with the controller (200) and / or the power converter (204) exceeds a first threshold; In response to determining that the first parameter exceeds the first threshold: Controlling the power converter in a first limiting mode (430) by the controller (200) by limiting an output current (222) of the power converter (204) based on a current value and / or a reference value; determining, by the controller (200), the output current (222) of the power converter (204); In response to determining that the output current (222) is at or near a balanced condition: determining (440), by the controller (200), a virtual impedance of the power converter (204) based on the output current (222) of the power converter (204); modifying, by the controller (200), the control system (228) based on the virtual impedance; and Controlling the power converter (204) in a first constrained grid forming mode (450) by the controller (200) includes the modified control system (228) determining the control data (230) based on signals (222, 224) indicative of measurements of current and / or voltage downstream of the power converter (204) and a reference signal (212).

2. The method according to claim 1, further comprising: Determining (460), by the controller (200), that a second parameter associated with the controller (200) and / or the power converter (204) exceeds a second threshold; In response to determining that the second parameter exceeds the second threshold: controlling the power converter in a second limiting mode by the controller (200) by limiting an output current (222) of the power converter (204) based on an updated current value and / or an updated reference value; determining, by the controller (200), an updated output current (222) of the power converter (204); In response to determining that the updated output current (222) is at or near an equilibrium condition: determining, by the controller (200), an updated virtual impedance of the power converter (204) based on the updated output current (222) of the power converter (204); further modifying, by the controller (200), the control system (228) based on the updated virtual impedance; as well as Controlling the power converter (204) in a second constrained grid forming mode (450) by the controller (200) includes the further modified control system (228) determining the control data (230) based on the measured signals (222, 224) indicative of current and / or voltage downstream of the power converter and the reference signal (212).

3. The method according to claim 1 or 2, wherein: The control system (228) is a cascade control system (228) including a voltage controller (214) and a current controller (216); wherein The voltage controller (214) generates first data (215) using a voltage control system based on the reference signal (212) and the measured signal (222, 224), and outputs the first data (215) to the current controller (216); The current controller (216) generates second data using a current control system based on the first data (215) and the measured signals (222, 224); The controller (200) determines the control data (230) based on the second data; and The controller (200) controls the power converter (204) in the first limiting mode (430) by setting the first data (215) equal to the current value and / or the reference value, thereby limiting the output current (222) of the power converter (204).

4. The method according to claim 1 or 2, wherein The control system (228B) is a DC voltage controller (214B); The controller (200) includes a limit controller (217); and The controller (200) controls the power converter (204) in the first limiting mode (430) by limiting the output current (222) of the power converter (204) to increase beyond the current value or the reference value through the limiting controller (217), thereby limiting the output current (222) of the power converter (204).

5. A method according to any preceding claim, wherein Determining (440) the virtual impedance of the power converter (204) by the controller (200) further comprises determining, by the controller (200), the virtual impedance and a fixed virtual voltage offset of the power converter (204) based on the output current (222) of the power converter (204); and The modifying, by the controller (200), the control system (228) based on the virtual impedance further includes modifying, by the controller (200), the control system (228) based on the virtual impedance and the fixed virtual voltage offset.

6. A method according to any preceding claim, wherein: The modifying the control system (228) based on the virtual impedance includes: determining, by the controller (200), an adjustment term of the control system (228) by calculating a virtual voltage or a virtual current as a function of the virtual impedance and the output current (222); and The adjustment item is implemented into the control system (228) by the controller (200).

7. A method according to any preceding claim, wherein: The controller (200) determines the virtual impedance in the following manner: calculating a virtual voltage when the power converter (204) is in the first limiting mode (430), the virtual voltage being defined as a voltage difference between an internal voltage phasor reference of the power converter (204) and a node voltage at a node (224) between an output of the power converter (204) and the grid (232); as well as Based on a Thevenin equivalent circuit of the power converter (204) connected to the power grid (232), including the virtual voltage, the output current of the power converter under the equilibrium condition, and the virtual impedance, a circuit analysis method is used to solve the Thevenin equivalent circuit of the virtual impedance.

8. The method according to claim 7, further comprising: As a result of the voltage difference between the internal voltage phasor reference of the power converter (204) and the node voltage at the node (224) decreasing in magnitude, the controller (200) reduces (490) the virtual impedance and thereby returns to controlling the power converter (204) in the initial grid forming mode (410).

9. A method according to any preceding claim, wherein: A time period from determining that the first parameter associated with the controller (200) and / or the power converter (204) exceeds the first threshold to the controller completing the step of controlling the power converter (204) in the first limiting mode is less than or equal to 50 ms.

10. A method according to any preceding claim, wherein: The determining that the first parameter associated with the controller (200) and / or the power converter (204) exceeds the first threshold value comprises one or more of the following: Determining that a current reference of a dq-coordinate current regulator using a command angle or a phase-locked loop (PLL) angle has saturated; and / or Determining that a d or q current reference input to a proportional-integral (PI) controller in a current controller (216) has saturated; and / or determining that a d or q current reference of a proportional resonant PR controller to be input into a current controller (216) after an inverse reference frame transformation has saturated; and / or determining that a deadbeat controller d or q current reference to be input to a current controller (216) after an inverse reference frame transformation has saturated; and / or determining that a phasor current limit has been reached; and / or determining that an active power limit has been reached; and / or determining that an energy limit has been reached; and / or Determine that the component is pulsing or blocking; and / or determining that the voltage reference has saturated; and / or Determining that the voltage output has reached a limit; and / or Determine if the modulation index is saturated.

Images