Method and controller for controlling grid-connected power converter

Abstract

To provide a method and a controller for controlling a grid-connected power converter.SOLUTION: A method (400) includes, in response to determining that a first parameter associated with a controller (200) and / or a power converter (204) exceeds a first threshold, controlling the power converter in a first limiting mode (430) by limiting an output current (222) of the power converter (204) on the basis of a current and / or reference value, determining the output current (222) of the power converter (204), and determining (440) a virtual impedance of the power converter (204) on the basis of the output current (222) of the power converter (204) in response to determining that the output current (222) is at or approaching an equilibrium state, and modifying a control system (228) on the basis of the virtual impedance.SELECTED DRAWING: Figure 4

Description

[Technical field]

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

[0002] In power transmission networks, DC power is converted to AC power when it is necessary to interconnect DC and AC networks. In such power transmission networks, power conversion means, 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 perform the necessary conversion from AC to DC or from DC to AC.

[0003] When converting DC power to AC power, for example at the interface between a DC transmission line and a grid, power converters may be operated in grid-following mode (GFL) or grid-forming mode (GFM).

[0004] In GFL mode, the power converter utilizes fast current-regulation loops to control the real and reactive power exchanged with the grid, achieving a relatively constant real and reactive power exchange on a sub-transient to transient timescale (e.g., between about 10 ms and 150 ms). The power converter uses a current reference for the real component of the current to achieve the desired real power output. A power converter operating in GFL mode includes the capability to manage voltage and / or reactive power in a manner that results in a command for the reactive component of the current. Wide-bandwidth current regulators develop the command for the voltage that the power converter applies to the grid such that the actual current closely tracks the command. Thus, a power converter operating in GFL mode provides a current-source characteristic on a sub-transient to transient timescale.

[0005] Alternatively, a power converter operating in GFM mode provides a voltage source characteristic over sub-transient to transient timescales, where the voltage phase angle and magnitude are controlled to remain mostly static, providing the regulation function required by the grid. With this structure, current flows according to grid demand, and the converter contributes to establishing a voltage and frequency for the grid.

[0006] A power converter operating in GFM mode differs from a power converter operating in GFL mode in that in GFM mode, the power converter regulates the AC voltage and frequency instead of the AC current on sub-transient time scales (e.g., less than 150 ms). However, in the event of a fault event on the grid or other severe grid disturbance (referred to herein as a “grid event”), because the power converter regulates voltage instead of current, power, or energy, the power converter may experience a current, power, or energy overload. This overload may cause a power converter operating in GFM mode to trip offline or, in a worst-case scenario, may damage semiconductor devices in the power converter, rendering the power converter inoperable indefinitely.

[0007] When such grid events occur, the power converter needs to ensure self-protection, and current limiting is usually achieved by switching the control to GFL mode. However, under certain conditions, switching the control mode in this way may compromise the stability of the power converter and / or the stability of the overall power system. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] US Patent Application Publication No. 2023 / 369865 Summary of the Invention

[0009] Given these considerations and new grid requirements, it is desirable to develop methods that allow power converters to maintain some degree of GFM behavior while operating close to (but not beyond) their hardware limits.

[0010] According to a first aspect, there is provided a method for controlling a power converter connected to a grid, where the power converter is initially controlled in a grid-forming mode to output current at a nominal voltage, based on control data provided by a controller, the controller comprising a control system which determines the control data based on measured signals indicative of currents and / or voltages downstream from the power converter, and a reference signal. The method comprises determining, by the controller, that a first parameter related to the controller and / or the power converter exceeds a first threshold. In response to determining that the first parameter exceeds the first threshold, the following steps are performed:controlling, by the controller, the power converter in a first limiting mode, by limiting the power converter output current based on a present value and / or a reference value; determining, by the controller, the output current of the power converter; and in response to determining that the output current is at or is close to an equilibrium condition, the following steps are performed: determining, by the controller, a virtual impedance for the power converter based on the output current of the power converter; and modifying, by the controller, the control system based on the virtual impedance.controlling, by the controller, the power converter in a first constrained grid-forming mode, comprising the modified control system determining the control data based on measured signals indicative of currents and / or voltages downstream from the power converter, and a reference signal.

[0011] Determining that the output current is at or close to an equilibrium condition may comprise determining that the output current is within a predefined threshold of an equilibrium condition. The predefined threshold may be application dependant.

[0012] In some embodiments, the reference signal contains a voltage reference provided by a grid-forming controller.

[0013] In some embodiments, the controller comprises a grid-forming controller.

[0014] In some embodiments, determining the output current of the power converter comprises estimating the output current.

[0015] In some embodiments, determining the output current of the power converter comprises measuring the output current.

[0016] In some embodiments, determining the output current of the power converter comprises using a reference for the output current.

[0017] In some embodiments, the virtual impedance is a fixed or a static virtual impedance. This means that the determined value of the virtual impedance does not change during the constrained grid-forming mode.

[0018] In some embodiments, the method further includes determining, by the controller, that a second parameter related to the controller and / or the power converter exceeds a second threshold. In response to determining that the second parameter exceeds the second threshold, the following steps are performed: controlling, by the controller, the power converter in a second limiting mode, by limiting the power converter output current based on an updated present value and / or an updated reference value; determining, by the controller, an updated output current of the power converter; and in response to determining that the updated output current is at or is close to an equilibrium condition, the following steps are performed.determining, by the controller, an updated virtual impedance for 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-forming mode, comprising the further modified control system determining the control data based on the measured signals indicative of currents and / or voltages downstream from the power converter, and the reference signal.

[0019] By determining that a second parameter related to the controller and / or the power converter exceeds a second threshold, and subsequently controlling the power converter in a second constrained grid-forming mode, an iterative control process can be established wherein a parameter that exceeds a threshold causes the power converter to be controlled in a further constrained mode. Accordingly, there is no limit to the number of parameters that may be determined to exceed a threshold, and subsequently there is no limit to the number of constrained grid-forming modes.

[0020] In some embodiments, the control system is a cascaded control system including a voltage controller and a current controller. The voltage controller uses a voltage control system to produce first data, based on the reference signal and the measured signals, and outputs the first data to the current controller. The current controller uses a current control system to produce second data, based on the first data and the measured signals. 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 to be equal to the present value and / or the reference value, thereby limiting the power converter output current.

[0021] In some embodiments, setting the first data to be equal to a present value and / or a reference value comprises freezing, disabling or not permitting the modified control system from determining some or all of the first data.

[0022] In some embodiments, controlling the power converter in the first constrained grid-forming mode comprises unfreezing, enabling, or permitting the modified control system from determining some or all of the first data.

[0023] In some embodiments, the control system is a single-loop controller.

[0024] In some embodiments, the control system is a direct voltage controller. The controller comprises a limiting controller. The controller controls the power converter in the first limiting mode by the limiting controller restricting the output current of the power converter from increasing past the present value and / or the reference value, thereby limiting the power converter output current.

[0025] In some embodiments, the limiting controller restricts the output current of the power converter from increasing past the present value or the reference value by modifying, controlling, and / or restricting the control data.

[0026] In some embodiments, the control system includes a virtual admittance. The virtual admittance can replace a conventional controller.

[0027] In some embodiments, the current controller produces the second data relatively faster than the voltage controller produces the first data.

[0028] In some embodiments, determining, by the controller, the virtual impedance for the power converter further comprises determining, by the controller, the virtual impedance for the power converter further comprises determining, by the controller, the virtual impedance and a fixed virtual voltage offset for the power converter based on the output current of the power converter; and modifying, by the controller, the control system based on the virtual impedance further comprises modifying, by the controller, the control system based on the virtual impedance and the fixed virtual voltage offset.

[0029] In some embodiments, modifying the control system based on the virtual impedance comprises determining, by the controller, an adjustment term for 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.

[0030] In some embodiments, the implementation of the adjustment term includes a modification of the internal voltage phasor reference.

[0031] In some embodiments, the controller determines the virtual impedance by calculating, whilst the power converter is in the first limiting mode, a virtual voltage defined as a voltage difference between an internal voltage phasor reference for the power converter and a node voltage at a node between an output of the power converter and the grid, and analyzes a Thevenin equivalent circuit of the power converter connected to the grid, including the virtual voltage, the output current of the power converter in the equilibrium condition, and the virtual impedance, using circuit analysis methods to resolve the Thevenin equivalent circuit for the virtual impedance.

[0032] In some embodiments, the controller determines the virtual impedance by calculating, whilst the power converter is in the first limiting mode, a new internal voltage phasor reference and a virtual voltage defined as a voltage difference between the new internal voltage phasor reference for the power converter and a node voltage at a node between an output of the power converter and the grid, and using analytical methods to solve for the virtual impedance and new internal voltage phasor reference.

[0033] In some embodiments, the controller determines the virtual impedance and the fixed virtual voltage offset by calculating, whilst the power converter is in the first limiting mode, a virtual voltage defined as a voltage difference between an internal voltage phasor reference for the power converter and a node voltage at a node between an output of the power converter and the grid, and uses circuit analysis methods to resolve the Thevenin equivalent circuit of the power converter connected to the grid, including the virtual voltage, the output current of the power converter in the equilibrium condition, and the virtual impedance.

[0034] In some embodiments, the method further comprises reducing, by the controller, the virtual impedance, as a result of the voltage difference between (new or original) the internal voltage phasor reference for the power converter and the node voltage at the node reducing in magnitude, and thereby returning to control the power converter in the initial grid-forming mode.

[0035] In some embodiments, a time period from determining that the first parameter related to 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 liming mode, is less than or equal to 50 ms.

[0036] In some embodiments, determining that the first parameter related to the controller and / or power converter exceeds the first threshold comprises one or more of: determining that a current reference of a dq-frame current regulator using commanded angle or Phase Locked Loop, PLL, angle has saturated, and / or determining that ad or q current reference inputted into a Proportional Integral, PI, controller in a current controller has saturated, and / or determining that a ... Determining that ad or q current reference to be inputted, following an inverse reference frame transformation, into a Proportional Resonant, PR,determining that a d or q current reference to be inputted, following an inverse reference frame transformation, into a deadbeat controller in a current controller 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 determining that a component is pulse dropping or blocking; and / or determining that a voltage reference has saturated; and / or determining that a voltage output has reached a limit. limit, and / or determining that a modulation index has saturated.

[0037] In some embodiments, determining that the second parameter related to the controller and / or power converter exceeds the second threshold comprises one or more of: determining that a current reference of a dq-frame current regulator using commanded angle or Phase Locked Loop, PLL, angle has saturated, and / or determining that ad or q current reference inputted into a Proportional Integral, PI, controller in a current controller has saturated, and / or determining that ad or q current reference inputted into a Proportional Resonant, PR, controller in a current controller following an inverse reference frame transformation has saturated. or q current reference to be inputted, following an inverse reference frame transformation, into a Proportional Resonant, PR, controller in a current controller has saturated);and / or determining that ad or q current reference to be inputted, following an inverse reference frame transformation, into a deadbeat controller in a current controller 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 determining that a component is pulse dropping or blocking; and / or determining that a voltage reference has saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has saturated. that a modulation index has saturated). ;

[0038] According to a second aspect, there is provided a controller for controlling a power converter connected to a grid, the controller comprising: a control system arranged to determine control data based on measured signals indicative of currents and / or voltages downstream from the power converter, and a reference signal. The controller is arranged to control the power converter in a grid-forming mode to output current at a nominal voltage, based on the control data. The controller is configured to determine that a first parameter related to the controller and / or the power converter exceeds a first threshold.Responsive to determining that the first parameter exceeds the first threshold: control the power converter in a first limiting mode, wherein the controller is configured to limit the power converter output current based on a present value and / or a reference value; and determine the output current of the power converter.Responsive to determining that the output current is at or is close to an equilibrium condition, the controller is configured to determine a virtual impedance for the power converter based on the output current of the power converter; modify the control system based on the virtual impedance; and control the power converter in a first constrained grid-forming mode, wherein the modified control system is configured to determine the control data based on measured signals of currents and / or voltages downstream from the power converter, and a reference signal.

[0039] In general, the controller tends to be configured to execute the methods described herein.

[0040] In some embodiments, determining the output current of the power converter comprises estimating the output current.

[0041] In some embodiments, determining the output current of the power converter comprises measuring the output current.

[0042] In some embodiments, determining the output current of the power converter comprises using a reference for the output current.

[0043] In some embodiments, the virtual impedance is a fixed or a static virtual impedance. This means that only one value for the virtual impedance is determined, and the determined value of the virtual impedance does not change during the constrained grid-forming mode.

[0044] In some embodiments, setting the first data to be equal to a present value or a reference value comprises freezing, disabling or not permitting the modified control system from determining some or all of the control data.

[0045] In some embodiments, controlling the power converter in the first constrained grid-forming mode comprises unfreezing, enabling, or permitting the modified control system from determining some or all of the first data.

[0046] In some embodiments, the controller is further configured to determine that a second parameter related to the controller and / or the power converter exceeds a second threshold, and in response to determining that the second parameter exceeds the second threshold: control the power converter in a second limiting mode, the controller is configured to limit the power converter output current based on an updated present value and / or an updated reference value, determine an updated output current of the power converter, and in response to determining that the updated output current is at or is close to an equilibrium condition, determine an updated virtual impedance of the power converter based on the updated output current of the power converter. impedance for the power converter based on the updated output current of the powerand further modifying the control system based on the updated virtual impedance and controlling the power converter in a second constrained grid-forming mode, and the further modified control system is configured to determine the control data based on the measured signals and the reference signal.

[0047] By determining that a second parameter related to the controller and / or the power converter exceeds a second threshold, and subsequently controlling the power converter in a second constrained grid-forming mode, an iterative control process can be established wherein a parameter that exceeds a threshold causes the power converter to be controlled in a further constrained mode. Accordingly, there is no limit to the number of parameters that may be determined to exceed a threshold, and subsequently there is no limit to the number of constrained grid-forming modes.

[0048] In some embodiments, the control system is a cascaded control system including a voltage controller and a current controller. The voltage controller is configured to use a voltage control system to produce first data, based on the reference signal and the measured signals, and output the first data to the current controller. The current controller is configured to use a current control system to produce second data, based on the first data and the measured signals. 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 to be equal to the present value or the reference value, thereby limiting the power converter output current.

[0049] In some embodiments, the control system is a single-loop.

[0050] In some embodiments, the control system is a direct voltage controller. The controller comprises a limiting controller. The controller is configured to control the power converter in the first limiting mode by the limiting controller being configured to restrict the output current of the power converter from increasing past the present value and / or the reference value, thereby limiting the power converter output current.

[0051] In some embodiments, the control system includes a virtual admittance. The virtual admittance can replace a conventional controller.

[0052] In some embodiments, the current controller produces the second data relatively faster than the voltage controller produces the first data. In some embodiments.

[0053] In some embodiments, the controller being configured to modify the control system based on the virtual impedance comprises the controller being configured to determine an adjustment term for 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 term into the control system.

[0054] In some embodiments, the controller is arranged to implement the adjustment term by modifying the internal voltage phasor reference.

[0055] In some embodiments, the controller is configured to determine the virtual impedance by calculating, whilst the power converter is in the first limiting mode, a virtual voltage defined as a voltage difference between an internal voltage phasor reference for the power converter and a node voltage at a node between an output of the power converter and the grid; and 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 in the equilibrium condition, and the virtual impedance, using circuit analysis methods to resolve the Thevenin equivalent circuit for the virtual impedance.

[0056] 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 for the power converter and the node voltage at the node reducing in magnitude, and thereby return to control the power converter in the initial grid-forming mode.

[0057] In some embodiments, a time period from determining that the first parameter related to 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 liming mode, is less than or equal to 50 ms.

[0058] In some embodiments, the controller being configured to determine that the first parameter related to the controller and / or power converter exceeds the first threshold comprises one or more of the following: determining that a current reference of a dq-frame current regulator using commanded angle or Phase Locked Loop, PLL, angle has saturated, and / or determining that ad or q current reference inputted into a Proportional Integral, PI, controller in a current controller has saturated, and / or determining that ad or q current reference to be inputted, following an inverse reference frame transformation, into a Proportional Resonant, PR, controller in a current controller has saturated;and / or determining that ad or q current reference to be inputted, following an inverse reference frame transformation, into a deadbeat controller in a current controller 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 determining that a component is pulse dropping or blocking; and / or determining that a voltage reference has saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has saturated. a modulation index has saturated). ;

[0059] 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, and includes one or more of the following: determining that a current reference of a dq frame 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 the current controller has saturated, and / or determining that a d or q current reference input to a Proportional Resonant (PR) controller in the current controller has saturated following an inverse reference frame transformation; and / or determining that a d or q current reference input to a deadbeat controller in the current controller has saturated following an inverse reference frame transformation; 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 sagging or blocking, and / or determining that a voltage reference has saturated, and / or determining that a voltage output has reached a limit, and / or determining that a modulation index has saturated (the controller being configured to determine that the second parameter related to the controller and / or power converter exceeds the second threshold comprises one or more more of: determining that a current reference of a dq-frame current regulator using commanded angle or Phase Locked Loop, PLL, angle has saturated;and / or determining that a d or q current reference inputted into a Proportional Integral, PI, controller in a current controller has saturated; and / or determining that a d or q current reference to be inputted, following an inverse reference frame transformation, into a Proportional Resonant, PR, controller in a current controller has saturated; and / or determining that a d or q current reference to be inputted, following an inverse reference frame transformation, into a deadbeat controller in a current controller 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 determining that a component is pulse dropping or blocking; and / or determining that a voltage reference has saturated; and / or determining that a voltage output has reached a limit; and / or determining that a modulation index has saturated)。;

[0060] 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 the controller of the second aspect.

[0061] 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 of the first aspect.

[0062] According to a fifth aspect, there is provided a non-transitory computer-readable storage medium comprising the computer program of the fourth aspect.

[0063] It will be appreciated that particular features of different aspects of the invention share the technical effects and advantages of corresponding features of other aspects of the invention, and more particularly, the controller, power converter, computer program, and non-transitory computer readable medium share the technical effects and advantages of the methods of the invention.

[0064] It will also be understood that the use of terms such as "first" and "second" is merely to distinguish between similar features and is not intended to indicate the relative importance of one feature over another, unless specifically specified.

[0065] Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives described in the preceding paragraphs, claims and / or the following description and drawings, in particular their individual features, may be taken independently or in any combination, i.e. all embodiments and all features of any embodiment may be combined in any way and / or combination, except where such features are incompatible. [Brief description of the drawings]

[0066] [Figure 1] FIG. 1 is a schematic diagram (not to scale) of a power system including a DC power source, a power converter, and a grid. [Figure 2A] FIG. 1 is a schematic diagram (not to scale) of a power converter and controller of a power system. [Figure 2B] FIG. 1 is a schematic diagram (not to scale) showing a power converter and a controller of a power system. [Diagram 3] FIG. 1 is a schematic diagram (not to scale) showing the electrical circuit of a power system under normal conditions and a simplified electrical circuit. [Figure 4] 1 is a schematic diagram (not to scale) of a method used to control a power converter in accordance with an embodiment of the present disclosure. [Diagram 5] FIG. 1 is a schematic diagram (not to scale) showing the electrical circuit of a power system during a grid event. [Figure 6] FIG. 1 is a schematic (not to scale) of a Thevenin equivalent circuit including virtual impedances of a power system during a grid event. [Figure 7] FIG. 2 is a schematic (not to scale) of a phasor diagram showing phases during a first constraint system formation mode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0067] FIG. 1 is a schematic diagram (not to scale) of a power system 100 including a power converter 204 connected between a DC power source 202 and a grid 232. The grid 232 may be an AC grid 232. The diagram is not intended to be limited to depicting a particular power system, or connections, or interconnections, and is further provided as a general example illustrating principles of operation of power systems useful in understanding the present invention. It will therefore be understood that while certain features in the diagram are shown connected to each other with a particular number of connections, this is also not intended to be limiting, but rather to illustrate the general connections between the features / components. Relatedly, the relative dimensions or distances between components identified in the diagram are also not intended to be limiting. It will therefore be understood that the power system 100 and the principles and features described herein may be applied to the interconnections making up the controller 200 shown in FIG. 2A, or to power converters or networks operating using the controller 200.

[0068] The power system 100 is comprised of a first inverter-based resource 204 (also known as a power converter). The power converter 204 is configured to convert DC power to AC power, essentially functioning as an inverter. The power converter 204 is also configured to convert AC power to DC power, essentially functioning as a rectifier. The power converter 204 can represent a cluster of multiple power converters that are electrically close and operate in a coherent manner with respect to grid events or control criteria. The power converter 204 is comprised of a single converter in the case of a monopolar system, and includes two converters in the case of a bipole system. The power converter 204 may represent multiple converter stations arranged as a multi-terminal power transmission system. In this example, the power converter 204 includes a DC side 204a and an AC side 204b.

[0069] The power converter 204 is connected to a DC power source 202 via a first line 206. The DC power source 202 is connected to a DC side 204a of the power converter 202 via a second line 208. The first and second lines 206, 208 are exemplary and do not represent any particular type of connection or cable.

[0070] 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.

[0071] The DC source 202 and / or the grid 232 may be a power transmission system including power generation apparatus, transmission apparatus, distribution apparatus, and electrical loads. The DC source 202 may include renewable power generation resources such as wind-power generation resources, solar-power generation resources, battery generation resources, supercapacitor generation resources, and bio-power generation resources. The DC source 202 may alternatively be composed of a network of such resources. The grid 232 may be a consumer network. As a non-limiting 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 depending on operational factors.

[0072] The operation of the power system 100 can be generally described as follows: A DC source 202 provides DC power on DC side 204a to a power converter 204. The power converter 204 converts the received DC power to AC power for a grid 232. The AC power is transmitted to the grid 232 from AC side 204b, for example, for consumption. In a particular example, the power converter 204 may be located in an offshore wind farm or may be located onshore.

[0073] If the DC power source 202 is a power 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 to the power converter 204 on the AC side 204b. The power converter 204 inverts the received AC power to DC power for the power source 202. The DC power is transmitted to the power source 202 from the DC side 204a.

[0074] It will be appreciated that various additional electrical components may be located at any particular location or with any particular function / part of the example power system 100. These may include switches, transformers, resistors, reactors, surge arrestors, doubly-fed induction generators, harmonic filters, and other components known in the art. These additional electrical components may be in series or parallel with the first and / or second lines 206, 208 of the power system 100. For example, the second line 208 may be comprised of three additional electrical components, including a first cable connected between the poles of the AC side 204b and a first set of windings of the transformer, a transformer, and a second cable connected between a second set of windings of the transformer and the grid 232. In another embodiment, the AC side 204b may be connected to a first winding set of a doubly-fed induction machine and the grid 232 may be connected to a second winding set of the doubly-fed induction machine.

[0075] It will be appreciated that converters or power conversion means may be constructed in many different technologies, such as voltage source converters (e.g., using insulated gate bipolar transistor (IGBT) valves). Such converters are generally considered to use "power electronics." Power electronic converters include, for example, multi-level voltage source converters.

[0076] It will be appreciated that cables used as electrical transmission media may include the following non-limiting examples of crosslinked polyethylene (XLPE) and / or mass impregnated (MI) insulated cables: Such cables may include a conductor (such as copper or aluminum) surrounded by an insulation layer. The dimensions of the cable and its associated layers may vary depending on the particular application, particularly the operating voltage requirements. The cable may further include reinforcement or "armouring" for applications such as undersea installations. The cable may further include a sheath / screen that is grounded at one or more points.

[0077] It will be further understood that the power system 100 can be used with a three-phase power system in which three conductors deliver respective first, second and third phase AC power to consumers. Each of the first, second and third phases typically has an equal magnitude voltage or current, which are 120° out of phase with each other.

[0078] In a three-phase power system, phase currents and voltages can be represented by three single-phase components: a positive sequence component; a negative sequence component; and a zero-sequence component. It is the positive sequence component that rotates in phase according to the power system. Therefore, in an ideal scenario, only positive sequence voltages / currents will be present. It should be understood that an imbalance in the magnitude or phase angle of voltages or currents between the first, second, and third phases of a three-phase system can give rise to negative or zero-sequence components. Such an imbalance can be caused, for example, by a fault in the grid 232 or a change in grid conditions (referred to herein as a grid event).

[0079] FIG. 2A is a schematic diagram illustrating one embodiment of a controller 200 that may be used in implementing the methods described herein to control the power converter 204 shown in FIG. 1 to operate in a grid-forming (GFM) mode.

[0080] As shown in Figure 2A, the power system 100 includes a DC power source 202, a power converter 204, a grid 232, and a controller 200. The controller 200 is connected to the power converter 204. Another example of a power system 100 including a DC power source 202, a power converter 204, a grid 232, and a controller 200 is shown in Figure 2B.

[0081] The controller 200 receives the reference signal 212 and the measurement signals 222, 224. The controller 200 outputs control data 230 to the power converter 204.

[0082] The control data 230 may include a pattern of gating pulses that may be used to switch semiconductor devices within the power converter 204 to quickly (e.g., in much less than 10 ms) obtain a desired AC voltage waveform (including, at least in part, the magnitude and phase angle of the fundamental frequency voltage) at the poles of the power converter 204, and further, the gating pulses may be used to achieve a particular reference signal 212 either at the poles of the power converter 204 or at a location proximate to the poles of the power converter 204.

[0083] The reference signal 212 may be an internal voltage phasor reference that includes a voltage magnitude reference and a phase angle reference. The reference signal 212 is provided to the controller 200 by a grid formation controller (not shown), which may be external to the controller 200. The voltage magnitude and phase angle references of the reference signal 212 may vary over time based on the measurement signals 222, 224 and according to a control objective of the grid formation controller.

[0084] The grid-forming controller may generate the reference signal 212 using a grid-forming control algorithm. The grid-forming control algorithm may consist of any one of a virtual synchronous machine-based grid-forming algorithm (also known as an 'inertial power regulator'), a droop-control-based grid-forming algorithm, a virtual oscillator control (VOC) based grid-forming algorithm, an isochronous (fixed-frequency) grid-forming control algorithm, or any variation of the above. Regardless of the type of grid-forming algorithm, the grid-forming algorithm generally varies the phase angle reference under normal operating conditions to achieve autonomous synchronization with other grid-forming power converters and potentially also the desired active power output.

[0085] The grid formation control algorithm allows the grid formation controller to autonomously achieve the aforementioned objections using only local voltage and current measurements. The grid formation control algorithm may also include a reactive power regulation component, also known as a "Q controller," which regulates the output reactive power and / or local voltage magnitude at nearby regulation points to a desired set point. The reference set points for the grid formation algorithm may consist of fixed active and reactive power reference signals or may be time-varying provided by another external controller or remote grid operator. Under conditions in which the grid 232 exhibits a relatively "stiff" grid or bulk power system, the grid formation algorithm typically achieves its objective (i.e., achieve active and reactive power references) with zero steady-state error with a typical settling time of at least 100 ms to about a few seconds.

[0086] Under certain circumstances, such as an islanded condition (where grid 232 may not include other grid-forming or synchronous generating power converters and may include any amount of total load (including potentially zero total load)), the requirement of power balance between generation and load is often given higher priority than the requirement to adhere to active and / or reactive power setpoints, and the grid-forming control algorithm may not be expected to accurately achieve its active and / or reactive power criteria at steady state. In this case, the grid-forming resource, e.g., power converter 204, may be expected to remain synchronized (settled into a stable steady-state equilibrium) with other grid-forming resources, e.g., other power converters, that may be present in the islanded network, and further, to provide or absorb active and / or reactive power in a manner that may not achieve the active and reactive power criteria provided to the grid-forming resource, e.g., power converter 204, to satisfy the loads present in the island.

[0087] The voltage magnitude and phase angle references generated by the grid formation algorithm and contained in reference signal 212 are used by controller 200 to determine control data 230, as described in more detail below. Under normal operating conditions, and depending on the type of voltage control methodology employed within controller 200, control data 230 may generally be generated by controller 200 to generate AC voltage waveforms (composed at least in part of the magnitude and phase angle of the fundamental frequency voltage) at the poles of the converter that differ from the magnitude and phase angle components of the fundamental frequency voltage contained in reference signal 212, both transiently and at steady state.

[0088] The measurement signals 222 , 224 are indicative of the current 222 and / or voltage 224 downstream of the power converter 204 .

[0089] The controller 200 is configured with a control system 228, which in this example is a cascade control system 228. The controller 200 may alternatively be configured with a single loop voltage control 228B, described further below. The cascade control system 228 includes a voltage controller 214 and a current controller 216, as shown in FIG. 2A.

[0090] The voltage controller 214 generates first data 215 based on the reference signal 212 and the measurement 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 may optionally be used for one or more reference frame transformations in the voltage controller 214. The current controller 216 generates second data using a current control system based on the first data 215 and the measurement signals 222, 224. The current controller 216 may also use the voltage magnitude and / or phase angle components of the reference signal 212 in one or more feedforward control functions and / or reference frame transformations included within the current controller 216 in some embodiments. The controller 200 determines the control data 230 based on the second data.

[0091] The cascade control system 228 operates as follows: The controller 200 generates control data 230 that generates a time-varying voltage waveform (including, at least in part, a time-varying fundamental frequency voltage magnitude and phase angle) at the poles of the power converter 204 that generally does not necessarily match the magnitude and / or phase angle components of the reference signal 212. Rather, under normal circumstances, the generated pole voltages of the power converter 204 are rapidly varied as necessary to achieve the magnitude and phase angle components of the reference signal 212 at a regulation point that is electrically close to, but not directly at, the poles of the power converter 204. This regulation point may be located, for example, on a grid-side winding terminal of an AC transformer that couples the power converter 204 to the grid 232. The voltage magnitude and phase angle components of the reference signal 212 are expected to vary over time in response to grid events in accordance with the grid formation control objectives discussed above. Because the voltage control system of voltage controller 214 has a much higher control bandwidth (e.g., 7 to 10 times) than the control bandwidth of the grid formation control algorithm provided by the grid controller, the voltage magnitude and phase angle components of the fundamental frequency pole voltage of power converter 204 are expected to change more rapidly than the voltage magnitude and phase angle components of reference signal 212.

[0092] 2B, the controller 200 may alternatively include a single-loop voltage control system 228B, which includes a single-loop controller 214B that generates control data 230 based on the reference signal 212 and the measurement signals 222, 224.

[0093] The single loop voltage control system 228B operates as follows: The controller 200 generates control data 230 that generally generates a time-varying voltage (including, at least in part, a time-varying fundamental frequency voltage magnitude and phase angle) at the poles of the power converter 204 that does not necessarily match the magnitude and / or phase angle components of the reference signal 212. As with the cascade control system 228, the generated power converter 204 pole voltages are varied, as needed, to achieve a magnitude and / or phase angle component of the reference signal 212 at a regulation point that is electrically proximate to, but not directly at, the poles of the power converter 204.

[0094] In an alternative example, the single-loop controller 214B may instead include a “direct voltage control” system in which the data 230 is derived more directly from the reference signal 212. For a direct voltage control system, under normal operating conditions, the control data 230 generated by the controller 200 results in the magnitude and phase angle of the fundamental frequency voltage at the poles of the power converter 204 closely matching (both transiently and at steady state) the magnitude and phase angle of the voltage contained in the reference signal 212.

[0095] When either the single-loop or direct voltage control system is employed, the single-loop controller 214B may further include current limit control functions that are temporarily activated during abnormal grid events, such as severe faults. When activated, these current limit control functions modify the control data 230 to effectively limit the current, or some component of the current, to a maximum value to allow continued operation of the converter hardware, thereby temporarily abandoning the nominal control objective of the controller 200.

[0096] Regardless of the type of voltage control system used, the controller 200 may also use a “nominal virtual impedance.” To accomplish this, the voltage magnitude and voltage angle references in the reference signal 212 are first modified by the controller 200 to correspond to a time-varying phasor voltage drop proportional to the measured or predicted output current of the power converter 204. The controller 200 may employ various means to calculate the phasor voltage drop of the nominal virtual impedance. 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).

[0097] When the controller 200 includes the cascade controller 228 or the single-loop controller 228B and a nominal virtual impedance is used, the pole voltages of the power converter 204 are rapidly manipulated to achieve, at the regulation time, a voltage phasor equal to the voltage phasor of the reference signal 212 (internal voltage phasor reference) minus the nominal virtual impedance voltage drop.

[0098] When the controller 200 includes a direct voltage controller, the time-varying fundamental frequency voltage magnitude and voltage phase angle at the poles of the power converter 204 include a voltage phasor equal to the reference signal 212 (the internal voltage phasor reference) minus the nominal virtual impedance voltage drop.

[0099] The control data 230 is provided to the power converter 204. The power converter 204 performs operations based on the control data 230, i.e., converting DC power to AC power or AC power to DC power, as well as generating or absorbing reactive power. In this manner, the control data 230 controls the power converter 204.

[0100] 2A, the controller 200 may include a memory and at least one processor. The memory may include computer-readable instructions that, when executed by the at least one processor, cause the controller 200 to perform one or more of the methods described herein.

[0101] Also, although not shown in FIG. 2A, the controller 200 may further include a transceiver device. The transceiver device may include a separate transmitter and receiver. The transceiver device may be used to operatively communicate with other components described herein directly using wired or wireless means or through a further interface such as a network interface. The transceiver device may, for example, send and receive control signals using a transceiver. The control signals may include or define electrical control parameters such as a reference current or a reference voltage.

[0102] The at least one processor may execute computer-readable instructions and / or perform logical operations. The at least one processor may be a microcontroller, microprocessor, central processing unit (CPU), field programmable gate array (FPGA), or similar programmable controller. The controller 200 may further include user input devices and / or output devices. The processor may be communicatively coupled to a memory and a transceiver.

[0103] 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 disk drive, a flash memory, etc.

[0104] Also, although not shown in Figure 2A, the controller 200 may further include user input and / or output device interfaces that allow for visual, auditory, and / or tactile input / output. Examples of such user input / output devices include, but are not limited to, interfaces to electronic displays, touch screens, keyboards, mice, speakers, and microphones.

[0105] As mentioned above, in the disclosed embodiment, controller 200 provides control data 230 to power converter 204. Under normal operating conditions, control data 230 controls power converter 204 in grid forming (GFM) mode. Aspects of the present disclosure provide improvements when power converter 204 is operational and a grid event is present, as will now be described.

[0106] The cascade control system 228 is used by the controller 200 to control the power converter 204 as a grid forming resource. Referring to FIG. 3, the power system 100 can be described under normal conditions using a first circuit diagram 310 and a second circuit diagram 320. When operating in GFM mode, under normal conditions, the power converter 204 is controlled to output power to the grid 232 at a nominal voltage. FIG. 3 shows the Thévenin voltage phasor V of the grid 232. g , the converter voltage phasor V at the poles of the power converter 204 c , the initial internal voltage phasor reference V of the power converter 204 cv i (i.e., the output of the inertial power regulator and the “Q controller”), the Thévenin impedance Z of the grid 232 g , the converter impedance Z of the power converter 204c , is shown. The converter impedance Zc of the coupling (i.e., grid connection) of the power converter 204 to the grid 232 (due to, for example, transformer 'leakage reactance(s)', filters, valve inductances, etc.; note that this does not include the nominal virtual impedance, discussed above), the coupling voltage V at the common coupling point (regulation point) of the power converter 204 and the grid 232, p , and the output current I p 222. The second circuit diagram 320 is a simplification of the first circuit diagram 310. The second circuit diagram 320 is simplified because, under normal operating conditions, the coupled voltage V p is the initial internal voltage phasor reference V cv i and therefore the converter impedance Z c Although still physically present, has been removed from the schematic as it is believed to have negligible effect on the system behavior on the timescales of interest (sub-transient to transient). Continuing with reference to FIG. 3, power converter 204, from an electrical characteristics perspective, has a converter impedance Z c and the converter voltage source 301 (converter voltage phasor V c This is because, as described above, power converter 204 is controlled in GFM mode and therefore includes converter voltage source 301.

[0107] From the standpoint of electrical properties, grid 232 has a Thévenin impedance Z g and grid voltage source 303 (Thevenin voltage phasor V gAs mentioned above, the grid 232 is a consumer network and there may be any number of power plants connected to the grid 232 and supplying electrical energy. Thus, the grid 232 includes both a consumer network and power plants. The consumer network has a Thevenin impedance Z g The generating station can be simplified to the Thévenin voltage phasor V g 303, which is responsible for supplying and maintaining the grid voltage. Generally, due to the size of the consumer network and the power plant, in practice, the Thévenin impedance Z g and / or the Thévenin voltage phasor V g It is impossible to know, measure or determine.

[0108] When a fault occurs in the consumer network, the Thévenin impedance Z g When a change occurs in the Thévenin voltage phasor V g Any of these disturbances will result in a fault in grid 232. However, under normal conditions, grid 232 has no such faults.

[0109] In the event of a fault in grid 232 or other severe grid disturbance, referred to herein as a grid event, power converter 204 may experience a current, power, or energy overload because power converter 204 is regulating voltage and not current, power, or energy. For a power converter controlled in a normal GFM mode, this overload would tend to trip the power converter offline or, in a worst case scenario, damage semiconductor devices in the power converter, rendering the power converter inoperable indefinitely.

[0110] When such a grid event occurs, the power converter's self-protection must be ensured, and current limiting is usually achieved by switching the control to GFL mode. However, switching the control mode in such a manner may jeopardize the stability of the power converter and the stability of the overall power system. The controller 200 tends to eliminate or reduce this by controlling the power converter 204 according to the method disclosed in FIG. 4.

[0111] As shown in Figure 4, the power converter 204 is initially controlled in a GFM mode 410 to output power at a nominal voltage based on control data 230 provided by the controller 200. The control system 228 determines the control data 230 based on the measurement signals 222, 224 and the reference signal 212. The controller 200 monitors a first parameter associated with the controller 200 and / or the 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 the power system 100 is operating as depicted in the first and second circuit diagrams 310, 320 shown in Figure 3.

[0112] Then, a grid event occurs on grid 232. In this example, the grid event causes the Thévenin voltage phasor V g The controller 200 monitors the first parameter and determines 420 that the first parameter associated with the controller 200 and / or the power converter 204 exceeds a first threshold. Thus, the first parameter exceeds the first threshold as a result of the grid event.

[0113] Determining 420 that the first parameter has exceeded the first threshold may include one or more of the following: determining that an individual d or q channel of the current reference of the positive sequence dq frame current regulator has saturated (positive sequence d and q channel currents are properly resolved into their respective quadrature channels with respect to a positive rotating reference frame that rotates synchronously to a reference angle output from the phylogenetic algorithm or with respect to a positive rotating reference frame that rotates synchronously to a PLL (Phase Locked Loop) angle that is locked to the measured positive sequence voltage); determining that an individual d or q channel of the current reference of the negative sequence dq frame current regulator has saturated (negative sequence d and q channel currents are properly resolved into their respective quadrature channels with respect to a negative rotating reference frame that rotates at the same rate as the reference angle output from the phylogenetic algorithm but in the negative (opposite) direction or with respect to a negative rotating reference frame that rotates synchronously to a PLL angle that is locked to the measured negative sequence voltage); and and / or determining that a proportional-integral (PI) controller in the current controller 216 having a dq current reference has saturated, and / or determining that a proportional-resonant (PR) controller in the current controller 216 having an abc current reference or an α-β reference has saturated, and / or determining that a phasor current limit has been reached, and / or determining that a component is pulse sagging or blocking, and / or determining that the voltage reference has saturated, and / or determining that the voltage output has reached a limit, and / or determining that the modulation index has saturated.

[0114] The first threshold is specified as a quantity or set of quantities that are close to the true physical operating limits of the resource and / or close to inherent or externally imposed operating limits. The first threshold may be created in a specific manner to meet grid code requirements, e.g., to favor active and reactive power generation during transients (e.g., the threshold is greater for d-axis currents than q-axis currents, assuming a dq0 reference frame that rotates synchronously with the GFM controller's internal voltage angle command).

[0115] Alternatively, the first threshold may be selected to favor the generation of positive or negative sequence currents or voltages. Similarly, the actual maximum allowable threshold of a component or resource is defined by the second threshold.

[0116] For example, in one embodiment where output current is the variable to be limited, the first threshold may be defined as the perimeter or boundary of a two-dimensional shape that defines the allowable region in which the output current of the resource, defined as a scalar complex value or "space vector," may lie (a space vector drawn from the origin of the complex plane is not allowed to terminate at a value outside the shape defined by the perimeter, which is the first threshold); this shape may be a circle (e.g., the shape may be a circle (e.g., FIG. 7), a rectangle, or any other shape on the two-dimensional complex plane, with the first threshold being somewhat concentric with a somewhat larger shape bordered by the perimeter, which is the second threshold, that defines the actual physical limit.

[0117] In one embodiment, the shape bounded by the first threshold is a circle with a radius equal to 0.95, corresponding to a per unit current magnitude threshold of 0.95 percent of the rated current, calculated in a conventional manner, such as via the Euclidean norm, as a function of the d-axis and q-axis currents, and the second threshold is a slightly larger concentric circle with a radius of 1.0 or 1.1.

[0118] In response to determining that the first parameter exceeds the first threshold 420, the controller 200 controls the power converter 204 in a first limiting mode 430 by limiting the output current 222 of the power converter 204 based on a current value or a reference value.

[0119] When the controller 200 configures the cascade control system 228, the first limiting mode 430 is achieved by setting the first data 215 equal to its current or reference value, thereby limiting the output current (222) of the power converter (204).

[0120] If the controller 200 configures a single loop or direct control system 228B, the controller 200 further configures a limit controller 217 to limit the output current 222 from increasing beyond a present or reference value, thereby limiting the output current (222) of the power converter (204).

[0121] In the example cascade control system 228, setting the first data 215 equal to its current or reference value results in the control data 230 manipulating the fundamental frequency voltage waveform at the pole of the power converter 204 relatively quickly on a sub-transient timescale such that the output current 222 converges to the current value.

[0122] The current magnitude and current phase angle remain relatively unchanged for the remainder of the period spent in the first limit mode 430, and, absent any further grid events during the period spent in the first limit mode 430, the control data 230 is also expected to reach a steady state and remain substantially unchanged.

[0123] 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 disabled from determining a value of the control data 230 based on the measurement signals 222, 224 and the reference signal 212 to limit the output current (222) of the power converter (204).

[0124] The current value is the value of a current reference contained in the controller 200 when the determination was made that the first parameter exceeded the first threshold. In this manner, the current reference is effectively "frozen," i.e., the current reference remains at the same value it was at when the first parameter was determined to exceed the first threshold.

[0125] In some examples, the current reference is "fixed" in terms of current magnitude and current phase angle only with respect 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 be comprised, for example, of a grid-formed voltage angle reference included in the reference signal 212 or may alternatively be derived from a PLL, frequency-locked loop (FFL), second-order generalized integrator (SOGI), or other means.

[0126] In the first limit mode 430, the current reference contained within the controller 200 is set to a current or reference value, so that any higher level controllers (e.g., the voltage controller 214, or the grid controller generating the reference signal 212) that may have otherwise affected the value of the control data 230 or the current reference may also optionally be "frozen", i.e., have their control actions altered such that their outputs are locked to particular values.

[0127] In particular, if a voltage controller 214 is present, it may be desirable to freeze the voltage controller 214.

[0128] If the higher-level controller in question has one or more integrators, "freezing" the controller may involve temporarily disabling the inputs to these integrators, or other control actions that have a similar effect.

[0129] It may not be necessary to freeze the phylogenetic algorithm that generates the reference signal 212 because phylogenetic algorithms tend to have slow bandwidths and therefore relatively slow response times on sub-transient time scales.

[0130] Thus, in the first limiting mode 430, the initial internal voltage phasor reference V of the power converter 204 cv i is not expected to change rapidly, regardless of changes in the measurement signals 222, 224.

[0131] Additionally, in some examples, slow changes in reference signal 212 may be potentially beneficial because these changes may help drive power system 100 toward synchronization by shifting the frame of reference of the fixed current phasor reference. However, because power converter 204 is expected to remain in first limit mode 430 only for a short period of time (e.g., less than 50 ms), the initial internal voltage phasor reference V cv i does not change significantly during this time.

[0132] As described above, a grid event is a Thévenin voltage phasor V g This causes a change in the Thévenin voltage phasor V of grid 232. g Static or frozen initial internal voltage phasor reference V with changes in cv i is the combined voltage V p Therefore, the second circuit diagram 320 as described in relation to FIG.

[0133] A simplified electrical diagram illustrating the operation of the power converter 204 and the grid 232 during the first limit mode 430 is shown in FIG. 5 as a third circuit diagram 500. The third circuit diagram 500 shows: g f ; initial internal voltage phasor reference V of power converter 204 cv i ;Faulted Thevenin impedance Z of grid 232 g f Faulty coupling voltage V at the common coupling point (coordination point) of the power converter 204 to the grid 232 p f ; the output current I of the power converter 204 during a grid event p f , 222; and the reference current I of the power converter 204 during a grid event. ref f+ This shows that.

[0134] In the context of this disclosure, the term "faulted" does not necessarily refer to a specific grid short or open circuit condition, but is used in a broader sense to denote any grid event in which a grid-forming resource may experience a parameter (e.g., a current limit threshold) being exceeded. A non-exhaustive list of such grid events is provided below. Symmetric or asymmetric line-to-ground or line-to-line short circuits when islanded or grid-tied; symmetric or asymmetric line opens when islanded or grid-tied; a sag or swell in the power system voltage magnitude when grid-tied; a load step or jump in the power system phase angle when grid-tied; unexpected control actions taken by a neighboring resource; an unexpected loss of a neighboring resource when islanded or grid-tied; a large step in a 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-tied such that multiple events occur simultaneously or consecutively.

[0135] Reference current I ref f may be determined by components within power system 100 related to one or more of limitations on the magnitude of positive or negative sequence currents, limitations on active or reactive power transfer, limitations on energy transfer to or from DC sources, and / or limitations on any other quantity of interest.

[0136] The first data 215 or reference current I ref fis set to a present value (i.e., frozen), the output current 222 does not change, assuming the control system 228 or current controller 216 is stable. As a result, in the first limiting mode 430, the output current I of the power converter 204 during a grid event p f , 222 tend to converge toward equilibrium, which can generally be indicated by a reduction in the magnitude of the current controller error signal (or the magnitudes of the multiple current controller error signals, if multiple current control channels are present) to a value equal to or close to zero. In the first limit mode 430, the controller 200 thus effectively controls the power converter 204 as a current source 501.

[0137] Controlling power converter 204 as a current source is similar to controlling power converter 204 in a grid-following (GFL) mode. Controlling power converter 204 in a GFL mode tends to be undesirable for long periods of time because controlling power converter 204 in a GFL mode tends to prevent power converter 204 from providing the grid 232 support functions that result from operating in a GFM mode. Furthermore, extended operation in a GFL mode can also lead to loss of stability of power converter 204 and / or grid 232.

[0138] However, the first limiting mode 430 can be stable under strong grid conditions where the PLL is not expected to become unstable. Under such conditions, it may be more advantageous to use a PLL in a current reference frame, where the d-channel and q-channel components of the current reference may be selected to quickly and accurately achieve active and / or reactive current setpoints over an indefinite period of time. Thus, the energy transfer between the DC power source 202 and the grid 232 tends to be easier to regulate.

[0139] Alternatively, instead of the phase angle derived from the PLL, FFL, or SOGI, the phase angle component of the reference signal 212 may be used as the reference frame for the control data 230 in the limit mode 430. The phase angle component may be frozen in this mode or may be optionally allowed to vary. If the phase angle reference in the reference signal 212 is not frozen during the first limit mode 430, the slowly varying phase angle component of the reference signal 212 may be used as the reference frame for the first data 215. Such an implementation may be desirable as it may allow for some limited preservation of the power converter 204's ability to pursue the GFM control objective of synchronization.

[0140] However, operation in the first limit mode 430 still does not constitute a GFM in the traditional sense since the voltage magnitude is not regulated, and this drawback can lead to unexpected results. Therefore, to reduce the risk of undesired operation, it is typically intended that the converter 204 not remain in the first limit mode 430 for more than about 50-100 ms, and ideally not for more than 30 ms.

[0141] The controller 200 typically completes the step of controlling the power converter 204 in the first limit mode 430 (i.e., allowing the power converter 204 to function as a current source) within 50 ms or less from the time when it is determined that the first parameter exceeds the first threshold. As a result, the output current I p , 222, there is rapid achievement of equilibrium (i.e., steady state) on the electromagnetic time scales. This advantageously tends to prevent power converter 204 from tripping offline, since hardware-related limitations are not violated. For example, the current limit achieved in this mode tends to prevent damage to any of the semiconductor devices in power converter 204.

[0142] Referring again to FIG. 4, when the power converter 204 is controlled in a first limiting mode 430 (e.g., a current limiting mode), the controller 200 controls the output current I of the power converter 204. p ,222. Determine the output current I p In response to the output current I p , 222 and the output voltage 224 to determine a virtual impedance 440 of the power converter 204 .

[0143] In some embodiments, instead of using the measurement signals 222, 224, the controller 200 may alternatively use a proxy for one or both output signals, such as the expected current (i.e., the current phasor reference 215), or an estimated current (e.g., a current estimate that may be obtained via a model of the system or via an observer) in determining the virtual impedance 440. A modified internal voltage phasor reference may also be determined.

[0144] If a revised internal voltage phasor reference has not yet been determined for steady-state operation, controller 200 may further determine the internal voltage phasor reference V of power converter 204 while power converter 204 is in first limit mode 430. cv and a node voltage (fault coupling voltage) V at a node 224 (e.g., a common coupling point) between the power converter 204 and the grid 232. p f A virtual voltage V is defined as the voltage difference between v The controller 200 then determines the virtual impedance 440 by calculating the virtual voltage V of the power converter 204 at equilibrium. v , the virtual impedance Z cv , and the fault output current I p f , 222. The controller 200 then uses circuit analysis to generate a virtual impedance Z cv2. Solve the Thevenin equivalent circuit 600.

[0145] 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. Z cv =(V cv -V p f ) / I ref f+ formula 1

[0146] The controller 200 generates a virtual impedance Z cv Upon determining, the controller 200 determines tuning terms for the control system 228 and modifies the control system 228 to include the tuning terms, thereby adjusting the virtual impedance Z cv If a fixed virtual impedance is already employed under the initial system configuration operating mode 410, the fixed virtual impedance is adjusted based on the virtual impedance Z cv can be replaced by.

[0147] The controller then controls the power converter 204 in a first constrained grid formation mode 450, and the modified control system 228 determines the control data 230 based on the measurement signals 222, 224 of the current and / or voltage downstream of the power converter 204 and the reference signal 212.

[0148] Z cv By calculating in this way, the voltage V p f This tends to ensure that the fault output current I p f is selected to be the same in the first restricted mode 430 and the first constrained system formation mode 450 (the fault output current Ip f Even after the transition, I ref f+ Thus, the disclosed method tends to achieve a seamless and near-instantaneous transition back to the (constrained) phylogenetic mode.

[0149] Equation 1 shows perhaps the simplest approach to determine a fixed virtual impedance that provides a seamless transition from the first limited mode 430 to the constrained grid formation mode 450, but it is not the only possible approach. An alternative calculation method utilizing the sine and cosine theorems can be applied to determine another virtual impedance that can achieve a seamless transition while still meeting various objectives. This alternative calculation method also requires the determination of a second virtual voltage, consisting of a static or fixed virtual voltage offset, which is referred to as the internal voltage phasor reference V. cv This second virtual voltage is applied with a first (current-dependent) virtual voltage offset related to the calculated virtual impedance. The second virtual voltage is subtended by an angle ∠V cv , its magnitude |V cv | or V cv The internal voltage phasor reference can be thought of as a step change in the internal voltage phasor reference, which involves a change in both magnitude and angle of

[0150] Additional objectives that can be met using alternative methods may include one or more of enforcing a particular magnitude of the virtual impedance, enforcing a desired ratio of reactance to resistance in the virtual impedance, and / or enforcing particular values ​​of the magnitude and / or phase angle of the internal voltage phasors. It should be noted that some objectives are mutually exclusive and, further, the pursuit of individual objectives may have a significant effect on the behavior and / or performance of the system in the constrained GFM mode.

[0151] Without loss of generality, the controller already calculates a fixed "steady-state" virtual impedance Z with known real and reactive components. CVSS Assume that the virtual impedance is used in both the initial and constrained GFM modes (410 and 450, respectively). Assume that this virtual impedance is used in both the initial and constrained GFM modes (410 and 450, respectively). V CVSS When the restricted mode 430 ends, Z CVSS and I p (or Z CVSS and a current reference signal, such as I p or I p , which represents a virtual voltage drop across a virtual impedance given by the estimated current limiting voltage (VV) of the GFM-constrained mode 450. Also, to achieve a seamless transition from the limited mode 430 to the constrained GFM mode 450, an additional “current limiting” virtual impedance Z CVL Z CVSS In series with the constraint GFM mode 450, Z CV =Z CVL +Z CVSS (Optionally, |Z CVSS | may be equal to zero).

[0152] Fixed Z required for seamless transition cv (and Z if necessary) cvL As an example of an alternative method for determining |Z cvL The required magnitude of | and the required internal voltage phasor reference angle ∠V cv This calculation determines the required reactance-to-resistance ratio of the current-limiting virtual impedance. r XR =(imag(Z cvL )) / (real(Z cvL )) (or, if known, the angle ∠Z of the equivalent current-limiting virtual impedance cvL) and the desired magnitude of the internal voltage phasor |V cv Can be executed if | is given. ∠Z cvL =tan -1 (r XR ) γ=π-∠(V p +V CVSS )+∠I p +∠Z cvL B=|V p +V CVSS | C=|V CV | α=π-γ-sin -1 (B(sinγ) / C) A = C(sinα) / (sinγ) |Z cvL |=A / (|I p |) ∠V cv =∠(V p +V CVSS )-α

[0153] As another example, the desired reactance-to-resistance ratio r of the current-limiting virtual impedance can be calculated using the following calculation: XR =(imag(Z cvL )) / (real(Z cvL )) and the current limiting virtual impedance |Z cvL The required magnitude of the internal voltage phasor |V required to achieve a seamless transition from current limit mode 430 to constrained grid formation mode 450 given the desired magnitude of |V CV | and the required internal voltage phasor reference angle ∠V cv is determined. A=|Z cvL ||I p | B=|V p +V CVSS | ∠Z cvL =tan -1 (r XR ) γ=π-∠(V p+V CVSS )+∠I p +∠Z cvL C=[A 2 +B 2 -2ABcos(γ)] 0.5 α=π-γ-sin -1 (B(sinγ) / C) |V CV |=C ∠V cv =∠(V p +V CVSS )-α

[0154] In the above discussion, the virtual impedance is considered to be composed of non-salient reactive and resistive elements. That is, the reactance in the virtual impedance is considered to have the same magnitude in the d-channel and q-channel, and the resistance in the virtual impedance is considered to have the same magnitude in the d-channel and q-channel. However, it is also possible to apply a significant virtual impedance for the purpose of current limiting in steady-state and constrained GFM modes. When the virtual impedance is significant, the reactance in the virtual impedance may have different values ​​in the d-channel and q-channel. The same is true for the resistance in the virtual impedance.

[0155] It was mentioned above that the phase angle component of the reference signal 212 may be used as the reference frame of the first data 215 in the first limit mode 430, or a PLL, FFL, or SOGI, etc. may be employed for this purpose. The same reference frame may be used to calculate the "orientation-dependent" voltage drops across different d- and q- channel components of the reactive and resistive elements in the virtual impedance.

[0156] Applying a salient virtual impedance to current limiting in the constrained GFM mode has the potential advantage that the voltage drop induced by the magnitude of the current limiting impedance is concentrated mostly (or entirely) in the particular channel where the overload is occurring. The presence of an orthogonal low-impedance channel may allow for greater operational flexibility and improved regulated current delivery capability in the constrained GFM mode. As with the non-salient current limiting impedance, analytical techniques can be used to calculate the d-channel and q-channel components of the salient reactance and / or resistance that allow for a seamless transition from the first limiting mode 430 to the constrained GFM mode 450.

[0157] In the preceding 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 should be noted that the controller 200 and the control method 400 may also be independently applied to negative sequence quantities, with the controller 200 representing a controller providing negative sequence voltage control functionality, including cascaded negative sequence voltage control, single-loop negative sequence voltage control, or direct negative sequence voltage control, as the case may be. In this case, the reference signal 212 is typically zero since it is desired that the negative sequence voltage is zero (however, a non-zero negative sequence voltage reference may also be implemented). With respect to the modes of FIG. 4, the modes and mode transitions function in a negative sequence implementation as well as in a positive sequence implementation. For example, a first limit mode 430 may be entered when the measured negative sequence current 222 or some proxy for this current (e.g., a current reference or estimated current) exceeds a negative sequence threshold. As with the positive-sequence application, a negative-sequence virtual impedance can be determined that allows for a seamless transition to the constrained GFM mode of the negative-sequence controller. Assuming that both positive-sequence and negative-sequence controllers are present, the overall control strategy shown in Figure 4 can optionally be applied simultaneously to both the positive and negative sequences (with the same or different control parameters) or to only one sequence.

[0158] In the first constrained grid formation mode 450, the power converter 204 can adjust its output voltage, thereby continuing to support the grid 232. Signals and controllers that were frozen or disabled in the limit mode 430 are reactivated in the constrained GFM mode 450 and, if necessary, reinitialized with values ​​that do not disturb the system balance during the transition from the limit 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 limit mode 430. This tends to reduce or minimize the time that the power converter 204 is in the GFL mode, which tends to allow the power converter 204 to quickly return, on an electromechanical time scale, to the GFM mode where the power converter 204 can continue to support the grid 232.

[0159] Referring to FIG. 7, a phasor diagram 700 shows the internal voltage phasor reference V cv is the first phasor 701, the fault output current I p f as the second phasor 702, and the node voltage (fault coupling voltage) V p f The third phasor 703 is the node voltage (faulted coupling voltage) V, and the fourth phasor 704 is the virtual voltage V. v is the fourth phasor 704, and the fault Thevenin voltage phasor V of the grid 232 is g f is shown as a fifth phasor 705. Phasor diagram 700 also includes an x-axis 710 and a y-axis 720, each having units per voltage or current. It also shows a first threshold as a first limit 706.

[0160] 7, the controller 200 determines that the first parameter has exceeded 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.

[0161] As can be seen from FIG. 7, the fifth phasor 705 is less than 1 p.u., indicating that a grid event has occurred on the grid 232. The fault output current I of the power converter 204, shown as the second phasor 702, p f is at the first limit value 706. The fourth phasor 704 is the difference between the first phasor 701 and the third phasor 703. In this manner, the power converter 204 is controlled in the first constrained grid formation mode 450, and the fault output current I p f is the virtual impedance Z used to determine the tuning terms of the control system 228. cv The virtual voltage V used to determine v The power converter 204 is thus controlled in the first constrained grid-forming mode 450, wherein the faulted output current Ipf of the power converter 204 is maintained in the equilibrium condition (as shown by the second phasor 702), as a result of the virtual voltage Vv used to determine the virtual impedance Zcv, which is used to determine the adjustment term for the control system 228.

[0162] In one embodiment, the adjustment term is the fourth phasor 704 (virtual voltage V v ) which can be expressed as the impedance phasor Z cv and the measured current phasor I p f In another embodiment, the adjustment term can instead be defined as the product of the impedance phasor Z cv and a current phasor reference included in the first data 215. The adjustment term, in one embodiment, could be applied as a virtual voltage that is summed with the internal voltage phasor reference 212, which modifies the reference provided to the voltage controller 214 (or, in the case that a direct voltage regulation or single-loop voltage regulation controller is used, 214B) so that the voltage controller appropriately realizes the virtual voltage drop as it performs its control action.

[0163] The controller 200 may further provide a smooth transition from the first constrained grid mode 450 back to the initial grid mode 410. Prior to this smooth transition, the grid event must be resolved and the current I p fWhen the virtual voltage V decreases, the third phasor 703 moves back toward the first phasor 701. The fourth phasor 704 (virtual voltage V v Because Φ(ω) is the difference between the first phasor 701 and the third phasor 703, the fourth phasor 704 results in a smaller magnitude as grid events are cancelled.

[0164] After the grid event is released, the virtual voltage V v Once Z becomes small enough that the system is determined to be in equilibrium, action can be taken to initiate a smooth and controlled transition from the first constrained grid formation mode 450 to the initial grid formation mode 410. Therefore, the virtual impedance Z added by the controller 200 for current limiting is cv and the associated regulation terms converge towards zero. Thus, during a controlled transition, the controller 200 can further reduce the regulation term 490 of the modified control system 228 beyond the reduction that already occurred when the grid event cleared, eventually reducing the correction term to zero, thereby returning to controlling the power converter 204 in the grid formation mode 410.

[0165] This advantageously tends to allow power converter 204 to smoothly return to normal grid formation control without disrupting grid 232 as a result of controlling power converter 204. Additionally, power converter 204 tends to be able to maintain some form of grid formation control to support power converter 204 during a grid event.

[0166] In some embodiments, the controller 200 may determine 460 that a second parameter associated with the controller 200 and / or the power converter 204 exceeds a second threshold.

[0167] In response to determining that the second parameter exceeds the second threshold, the controller controls the power converter 204 in the second limiting mode by limiting the output current 222 of the power converter 204 based on the updated current value or the updated reference value (or, if a direct voltage regulation controller or a single-loop voltage regulation controller is used, the limit 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 rapidly 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. If there are no further grid events during the period spent in the second limiting mode, the control data 230 is also expected to reach a steady state and not change substantially. Optionally, the control system 228 can disable determining a value of the control data 230 based on the measurement signals 222, 224 and the reference signal 212. The controller 200 then determines an updated output current 222 of the power converter 204.

[0168] In response to the updated output current 222 reaching an equilibrium state, 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 is performed in a similar manner as described above.

[0169] Thereafter, the controller 200 further controls the power converter 204 in a second constrained grid formation mode 450 by the modified control system 228 determining the control data 230 based on the measurement signals 222 , 224 and the reference signal 212 .

[0170] Determining that the second parameter has exceeded a second threshold and taking corresponding action tends to be useful, for example, when a fault occurs following the fault that caused the grid event, or when an adjustment occurs in another portion of the grid 232 as a result of a grid event. Thus, subsequent grid events may occur following an initial grid event.

[0171] 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 uses the voltage control system to generate first data 215 based on the reference signal 212 and the measurement signals 222, 224 and outputs the first data 215 to the current controller 216. The current controller 216 uses the current control system to generate second data based on the first data 215 and the measurement signals 222, 224.

[0172] The current controller 216 may generate the second data relatively faster than the voltage controller 214 generates the first data 215 .

[0173] The controller 200 determines the control data 230 based on the second data.

[0174] The controller 200 controls the power converter 204 in the first limiting mode 430 by setting the first data 215 to be equal to a current or reference value, thereby limiting the output current 222 of the power converter 204 as described above.

[0175] Modifying the control system 228 based on the virtual impedance or the updated virtual impedance may include modifying a voltage control system of the voltage controller 214 .

[0176] In some embodiments, in response to the controller 200 controlling the power converter 204 in the first limiting mode 430, the controller may revert the control method to the initial grid formation mode 410 if the duration that the first parameter exceeds the first threshold is less than a defined minimum duration.

[0177] The disclosed method and controller tend to maintain maximum possible grid formation control since the use of grid following mode is kept as short as possible. The controller and method generally provide stable operation during fault restoration and also tend to prevent loss of synchronization between the grid formation resources (i.e., power converter 204) and the bulk power grid (i.e., grid 232). The controller and method tend to maximize the current capability of the power converter 204 and allow prioritization of active vs. reactive current, angle vs. voltage, positive vs. negative sequence through the determination of regulation terms.

[0178] The exact implementation of the first parameter can be determined by the user, and thus the controllers and methods tend to provide a degree of flexibility and tunability as desired (e.g., the controllers and methods can be configured to prioritize grid formation operations, or to allow for extended current limits, etc.). [Explanation of symbols]

[0179] 100: Power system 200: Controller 202: DC power source 204: Power converter / first inverter based resource 204a: DC side 204b: AC side 206: First line 208: Second line 212: Reference signal 214: Voltage controller 214b: Single loop controller 215: First data, second data 216: Current controller 217: Limit controller 222: Measurement signal, measured current 224: Measurement signal, measured voltage 228: Cascade control system 228B: Single loop voltage control system 230: Control data 232: Grid 301: Converter voltage source 303: Grid voltage source 310: First circuit diagram 320: Second circuit diagram 410: Initial GFM mode 430: Limit mode 450: Constrained grid formation mode 490: Regulation term 500: 3rd circuit diagram 501: Current source 600: Thevenin equivalent circuit 700: Phasor diagram 701: 1st phasor 702: 2nd phasor 703: 3rd phasor 704: 4th phasor 705: 5th phasor 706: 1st limit

Claims

1. A method (400) for controlling a power converter (204) connected to a grid (232), the power converter (204) being initially controlled (410) in a grid formation mode to output a current (222) at a nominal voltage based on control data (230) provided by a controller (200), the controller (200) including a control system (228) that determines the control data (230) based on measurement signals (222, 224) indicative of a current and / or a 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 a first threshold, controlling the power converter in a first limiting mode (430) by the controller (200) 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), an output current (222) of the power converter (204); In response to determining that the output current (222) is at or near equilibrium, determining (440) a virtual impedance of the power converter (204) based on an output current (222) of the power converter (204) by the controller (200); modifying the control system (228) based on the virtual impedance by the controller (200); and controlling the power converter (204) in a first constrained grid formation mode (450) by the controller (200), wherein a modified control system (228) determines control data (230) based on measurement signals (222, 224) indicative of current and / or voltage downstream of the power converter (204) and a reference signal (212).

2. 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 a second threshold, controlling the power converter in a second limiting mode by the controller (200) limiting an output current (222) of the power converter (204) based on the updated current value and / or the 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 equilibrium, 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 the control system (228) based on the updated virtual impedance by the controller (200); 2. The method of claim 1, further comprising: controlling, by the controller, the power converter in a second constrained grid formation mode, wherein the modified control system determines control data based on measurement signals indicative of currents and / or voltages downstream of the power converter and a reference signal.

3. 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) using a voltage control system based on the reference signal (212) and the measurement signals (222, 224), 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 measurement signals (222, 224) using a current control system; The controller (200) determines control data (230) based on the second data; 2. The method of claim 1, wherein the controller (200) controls the power converter (204) in a first limiting mode (430) by setting the first data (215) to be equal to a current value and / or a reference value, thereby limiting an output current (222) of the power converter (204).

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

5. Determining (440) a virtual impedance for the power converter (204), by the controller (200), further includes determining, by the controller (200), a virtual impedance and a fixed virtual voltage offset for the power converter (204) based on the output current (222) of the power converter (204); 2. The method of claim 1, wherein modifying, by the controller, the control system based on the virtual impedance further comprises modifying, by the controller, the control system based on the virtual impedance and a fixed virtual voltage offset.

6. Modifying the control system (228) based on the virtual impedance; determining, by the controller (200), a regulation term for 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, by the controller (200), the adjustment term in the control system (228).

7. The controller (200) determining a virtual impedance of the power converter (204) comprises: calculating a virtual voltage, 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) while the power converter (204) is in a first limiting mode (430); Based on a Thevenin equivalent circuit of a power converter (204) connected to a grid (232), the Thevenin equivalent circuit includes a virtual voltage, an output current of the power converter at equilibrium, and a virtual impedance, and analyzes a Thevenin equivalent circuit of the virtual impedance using a circuit analysis method; The method of claim 1 , comprising:

8. 8. The method of claim 7, further comprising: decreasing (490) the virtual impedance as a result of a decrease in magnitude of the voltage difference between the internal voltage phasor reference of the power converter (204) and the node voltage of the node (224), thereby returning the power converter (204) to control in the initial grid forming mode (410).

9. 2. The method of claim 1, wherein a time from determining that a first parameter associated with the controller and / or the power converter exceeds a first threshold to the controller completing the step of controlling the power converter in a first limit mode is 50 ms or less.

10. Determining that a first parameter associated with the controller (200) and / or the power converter (204) exceeds a first threshold value; Determining that the current reference of a dq frame current regulator using a command angle or a Phase Locked Loop (PLL) angle has saturated; and / or determining that the d or q current reference input to the proportional-integral (PI) controller of the current controller (216) is saturated; and / or determining that the d or q current references input to the proportional-resonant (PR) controller of the current controller (216) after inverse reference frame transformation are saturated; and / or determining that the d or q current references input to the deadbeat controller of the current controller (216) after inverse reference frame transformation are saturated; and / or Determining that a phase 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 the component is dropping or blocking the pulse; and / or Determining that the voltage reference is saturated; and / or Determining that the voltage output has reached a limit; and / or determining that the modulation index is saturated; The method of claim 1 , comprising one or more of:

11. A controller (200) for controlling a power converter (204) connected to a grid (232), comprising: a control system (228) arranged to determine control data (230) based on measurement signals (222, 224) indicative of current and / or voltage downstream of the power converter (204) and on a reference signal (212); The controller (200) is arranged to control the power converter (204) in a grid forming mode to output a current (222) at a nominal voltage based on the control data (230); The controller is determining 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 a first threshold, controlling the power converter in a first limiting mode (430), the controller (200) being configured to limit an output current (222) of the power converter (204) based on a current value and / or a reference value; determining an output current (222) of the power converter (204); In response to determining that the output current (222) is at or near equilibrium, determining a virtual impedance of the power converter (204) based on an output current (222) of the power converter (204); Modifying the control system (228) based on the virtual impedance; A controller configured to control the power converter (204) in a first constrained grid formation mode (450), wherein a modified control system (228) determines control data (230) based on measurement signals (222, 224) of current and / or voltage downstream of the power converter and a reference signal (212).

12. The controller further determining 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 a second threshold, controlling the power converter in a second limiting mode, the controller being configured to limit an output current (222) of the power converter (204) based on the updated current value and / or the updated reference value; determining an updated output current (222) of the power converter (204); In response to determining that the updated output current (222) is at or near equilibrium, determining an updated virtual impedance of the power converter (204) based on the updated output current (222) of the power converter (204); Further modifying the control system (228) based on the updated virtual impedance; controlling the power converter (204) in a second constrained grid formation mode (450), further comprising a modified control system (228) determining said control data (230) based on the measurement signals (222, 224) and the reference signal (212); The controller of claim 11 configured to:

13. The control system (228) is a cascade control system (228) including a voltage controller (214) and a current controller (216); The voltage controller (214) is configured to generate first data (215) using a voltage control system based on the reference signal (212) and the measurement signals (222, 224), and the voltage controller (214) is further configured to output the first data (215) to the current controller (216); The current controller (216) is configured to generate second data based on the first data (215) and the measurement signals (222, 224) using the current control system; The controller (200) is further configured to determine control data (230) based on the second data; 12. The controller of claim 11, wherein the controller is further configured to control the power converter in a first limiting mode by setting the first data to be equal to a current or reference value, thereby limiting an output current of the power converter.

14. 12. The controller of claim 11, wherein the controller is configured to modify the control system (228) based on the virtual impedance, the controller is configured to determine an adjustment term for 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 to implement the adjustment term in the control system (228).

15. 1. A power converter comprising: a DC side for connecting to a DC power source; an AC side for connecting to the grid; A controller (200) according to any one of claims 11 to 14; a power converter.

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