Method and controller for controlling grid-connected power converters

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 power converters connected to a grid, and more particularly to a method and controller for controlling power converters using virtual impedance. [Background technology]

[0002] In power transmission networks, when it is necessary to interconnect DC and AC networks, DC power is converted to AC power. 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 conversions from AC to DC or DC to AC.

[0003] When converting DC power to AC power, for example, at the interface between a DC transmission line and the grid, the power converter 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 active and reactive power exchanged with the grid, achieving a relatively constant active and reactive power exchange over a sub-transient to transient timescale (e.g., between approximately 10ms and 150ms). The power converter uses a current reference for the active component of the current to achieve the desired active power output. Power converters operating in GFL mode include the ability to manage voltage and / or reactive power in a way that provides commands for the reactive component of the current. Wide-bandwidth current regulators develop the voltage commands that the power converter applies to the grid so that the actual current closely follows the commands. Therefore, power converters operating in GFL mode provide current-source characteristics over a sub-transient to transient timescale.

[0005] Alternatively, a power converter operating in GFM mode provides voltage source characteristics on a sub-transient to transient timescale, where the voltage phase angle and magnitude are controlled to remain mostly static, thereby achieving the adjustment function required by the grid. This configuration allows current to flow according to grid requirements, and the converter contributes to establishing a voltage and frequency for the grid.

[0006] Unlike power converters operating in GFL mode, power converters operating in GFM mode regulate the AC voltage and frequency of the power converter instead of AC current on a sub-transient time scale (e.g., less than 150 ms). However, in the event of a grid fault or other serious grid disturbance (hereinafter referred to as "grid event"), the power converter may experience an overload of current, power, or energy because it regulates voltage rather than current, power, or energy. This overload may trip the power converter operating in GFM mode offline, or, in the worst-case scenario, damage the semiconductor devices within the power converter, rendering it permanently inoperable.

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

[0008] [Patent Document 1] U.S. Patent Application Publication No. 2023 / 369865 [Overview of the project]

[0009] Considering these considerations and the new grid requirements, it is desirable to develop a method that allows power converters to maintain the behavior of the GFM to some extent while operating at (but not exceeding) the hardware limits.

[0010] According to a first aspect, a method for controlling a power converter connected to a grid is provided, 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 the determination that the first parameter exceeds the first threshold, the following steps are performed.The controller controls 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. The controller determines the output current of the power converter. In response to determining that the output current is at or close to an equilibrium state, the following steps are performed: The controller determines a virtual impedance for the power converter based on the output current of the power converter. The controller modifies the control system based on the virtual impedance.The controller controls the power converter in a first constrained grid-forming mode, comprising the modified control system determining the control data based on measured signals indicating 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 state may include determining that the output current is within a predefined threshold of an equilibrium state. The predefined threshold may be application-dependent.

[0012] In some embodiments, the reference signal includes 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 the determination 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; In response to determining that the updated output current is at or is close to an equilibrium condition, the following steps are performed.The controller determines an updated virtual impedance for the power converter based on the updated output current of the power converter. The controller further modifies the control system based on the updated virtual impedance. The controller controls 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 indicating 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 in which 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 there is no limit to the number of constrained grid-forming modes thereafter.

[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 circuit. 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, the 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 the 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, while 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 the Thevenin equivalent circuit of the power converter connected to the grid, based on 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, while 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 the new internal voltage phasor reference.

[0033] In some embodiments, the controller determines the virtual impedance and the fixed virtual voltage offset by calculating, while 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 for the virtual impedance 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.

[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, and thereby returning to control the power converter in the initial grid-forming mode.

[0035] In some embodiments, the 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 limiting 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 an ad or q current reference inputted into a Proportional Integral (PI) controller in a current controller has saturated, and / or the PR (Proportional Integral) controller of the current controller. Determining that the d or q current reference to be inputted, following an inverse reference frame transformation, has saturated in a Proportional Resonant (PR) controller.Determining that the 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 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 an ad or q current reference inputted into a Proportional Integral (PI) controller in a current controller has saturated, and / or determining that an ad or q current reference inputted to a Proportional Resonant (PR) controller in a current controller following 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 the 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 the modulation index has saturated that a modulation index has saturated). ;

[0038] According to a second embodiment, a controller for controlling a power converter connected to a grid is provided, 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 close to an equilibrium state, 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] Generally, 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, respond to determining that the updated output current is at or is close to an equilibrium condition, and 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 powerThe system is configured to further modify the control system based on the updated virtual impedance, control the power converter in a second constrained grid-forming mode, and 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 in which 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 there is no limit to the number of constrained grid-forming modes thereafter.

[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, which 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, while 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 decreasing in magnitude, and thereby return to control the power converter in the initial grid-forming mode.

[0057] In some embodiments, the 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 the current reference of a dq-frame current regulator using a commanded angle or a Phase Locked Loop (PLL) angle has saturated, and / or determining that the d or q current reference inputted into a Proportional Integral (PI) controller in a current controller has saturated, and / or determining that the 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 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 the modulation index has saturated a modulation index has saturated). ;

[0059] In some embodiments, the controller is configured to determine that the second parameter related to the controller and / or the power converter exceeds the second threshold, and includes one or more of the following: Determining that the current reference of a dq frame current regulator using a command angle or PLL (Phase Locked Loop) angle has saturated, and / or determining that the d or q current reference input to the PI (Proportional Integral) controller in the current controller has saturated, and / or determining that the d or q current reference input to the PR (Proportional Resonant) controller in the current controller following the inverse reference frame conversion has saturated; and / or determining that the d or q current reference input to the deadbeat controller in the current controller has saturated according to the inverse reference frame conversion, and / or determining that the phasor current limit has been reached, and / or determining that the active power limit has been reached, and / or determining that the energy limit has been reached, and / or determining that a component is pulse-dropped or blocking, and / or determining that the voltage reference has saturated, and / or determining that the voltage output has reached its limit, and / or determining that the 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 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, a power converter is provided 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, a computer program is provided which, when executed by a processor of a controller for controlling a power converter, causes the controller to perform the method of the first aspect.

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

[0063] It will be understood that certain features of different embodiments of the present invention share the technical effects and advantages of corresponding features of other embodiments of the present invention. More specifically, controllers, power converters, computer programs, and non-transient computer-readable media share the technical effects and advantages of the methods of the present invention.

[0064] Furthermore, it should be understood that the use of terms such as "first" and "second" is merely for distinguishing similar features and, unless otherwise specified, is not intended to indicate the relative importance of one feature to others.

[0065] Within the scope of this application, the various aspects, embodiments, examples and alternatives, in particular their individual features, described in the preceding paragraphs, claims and / or the following description and drawings are expressly intended to be taken independently or in any combination. That is, all embodiments and all features of any embodiment can be combined in any way and / or in any combination, unless such features are incompatible. [Brief explanation of the drawing]

[0066] [Figure 1] This is a schematic diagram (not to scale) of a power system including a DC power supply, power converter, and grid. [Figure 2A] This is a schematic diagram (no scale) of a power converter and power system controller. [Figure 2B] This is a schematic diagram (not to scale) showing a power converter and a power system controller. [Figure 3] This is a schematic diagram (without scale) showing the electrical circuits of a normal power system and a simplified electrical circuit. [Figure 4] This is a schematic diagram (not to scale) of a method used to control a power converter according to embodiments of the present disclosure. [Figure 5] This is a schematic diagram (without scale) showing the electrical circuits of the power system during a grid event. [Figure 6] This is a schematic diagram (unscaled) of the Thevenin equivalent circuit including the virtual impedance of the power system during grid events. [Figure 7] This is a schematic diagram (without scale) of a phasor diagram showing the phase during the first constraint system formation mode. [Modes for carrying out the invention]

[0067] Figure 1 is a schematic (unscaled) diagram of a power system 100 including a power converter 204 connected between a DC power supply 202 and a grid 232. The grid 232 may be an AC grid 232. This diagram is not intended to be limited to representing a specific power system, or connection, or interconnection, and is further provided as a general example illustrating the operating principle of a power system useful for understanding the present invention. Accordingly, it will be understood that certain features in the diagram are shown connected to each other with a specific number of connections, but this is not intended to be limiting, but rather to illustrate the general connections between features / components. In connection therewith, the relative dimensions or distances between components recognized in the diagram are also not intended to be limiting. Accordingly, it will be understood that the power system 100 and the principles and features described herein can be applied to the interconnections constituting the controller 200 shown in Figure 2A, or to a power converter or network operating using the controller 200.

[0068] The power system 100 consists 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 and functions substantially as an inverter. The power converter 204 is also configured to convert AC power to DC power and functions essentially as a rectifier. The power converter 204 can represent a cluster of multiple power converters that are electrically close together and operate in a consistent manner with respect to grid events or control criteria. The power converter 204 consists of a single converter in the case of a unipolar system and includes two converters in the case of a bipolar system. The power converter 204 may also mean (represent) a plurality of 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 the DC power supply 202 via a first line 206. The DC power supply 202 is connected to the DC side 204a of the power converter 202 via a second line 208. The first and second lines 206 and 208 are illustrative and do not represent any particular type of connection or cable.

[0070] The power converter 204 is connected to the grid 232. The grid 232 is connected to the AC side 204b of the power converter 204.

[0071] The DC source 202 and / or grid 232 may be a power transmission system including a 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. Alternatively, the DC source 202 may consist 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 grid 232 may be of any size and may have varying electrical characteristics depending on the operating factors.

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

[0073] If the DC power supply 202 is a power storage device or a high-voltage direct current (HVDC) transmission line, power can be supplied to the DC power supply 202 from the grid 232. In this case, the grid 232 supplies AC power to the power converter 204 on the AC side 204b. The power converter 204 inverts the received AC power into DC power for the power supply 202. The DC power is transmitted from the DC side 204a to the power supply 202.

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

[0075] It will be understood that converters or power conversion means consist of many different technologies, such as voltage source converters (using, for example, insulated-gate bipolar transistor (IGBT) valves). Such converters are generally considered to use "power electronics." Power electronics converters include, for example, multilevel voltage source converters.

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

[0077] Furthermore, it will be understood that power system 100 can be used in conjunction with a three-phase power system. In a three-phase power system, three conductors supply alternating current power to consumers, each of which is the first, second, and third phases. Each of the first, second, and third phases typically has a voltage or current of equal magnitude, and these voltages or currents are out of phase with each other by 120°.

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

[0079] Figure 2A is a schematic diagram showing one embodiment of a controller 200 that may be used when carrying out the method described herein to control the power converter 204 shown in Figure 1 to operate in grid-forming (GFM) mode.

[0080] As shown in Figure 2A, the power system 100 includes a DC power supply 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 the power system 100, including the DC power supply 202, power converter 204, grid 232, and controller 200, is shown in Figure 2B.

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

[0082] The control data 230 may include a pattern of gate pulses that can be used to switch semiconductor devices within the power converter 204 to quickly obtain (e.g., in a time much shorter than 10 ms) 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 furthermore, the gate pulses may be used to achieve a specific reference signal 212 at either the poles of the power converter 204 or at a location adjacent to the poles of the power converter 204.

[0083] The reference signal 212 may be an internal voltage phasor reference, including a voltage magnitude reference and a phase angle reference. The reference signal 212 is supplied to the controller 200 by a system forming controller (not shown), which may be located outside the controller 200. The voltage magnitude reference and phase angle reference of the reference signal 212 may change over time based on the measurement signals 222 and 224, according to the control purpose of the system forming controller.

[0084] The grid-forming controller can generate a reference signal 212 using a grid-forming control algorithm. The grid-forming control algorithm may consist 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 one of the above variations. Regardless of the type of grid-forming algorithm, the grid-forming algorithm generally changes the phase angle reference under normal operating conditions to achieve autonomous synchronization with other grid-forming power converters and, in some cases, also achieves 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 adjustment component, also known as a "Q controller," which adjusts the magnitude of the output reactive power and / or local voltage at a nearby adjustment point to a desired setpoint. The reference setpoint for the grid formation algorithm may consist of fixed active and reactive power reference signals, or it may be time-varying, provided by another external controller or remote grid operator. Under conditions where grid 232 exhibits a relatively "stiff" grid or bulk power system, the grid formation algorithm typically achieves its objective (i.e., achieves active and reactive power references) with zero steady-state error, typically within 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 contain other grid formations or synchronous power converters and may contain any amount of total load, including possibly zero total load), the requirement of power balance between generation and load is often given higher priority than the requirement of adhering to active power and / or reactive power setpoints, and the grid formation control algorithm may be expected not to precisely achieve its active power and / or reactive power criteria in a steady state. In this case, the grid formation resource, e.g., power converter 204, may be expected to continue to synchronize with other grid formation resources that may be present in the island network, e.g., other power converters (settling into a stable steady-state equilibrium), and furthermore, to supply or absorb active power and / or reactive power in a manner that may not achieve the active power and reactive power criteria supplied to the grid formation resource, e.g., power converter 204, in order to meet the load present in the island.

[0087] The voltage magnitude and phase angle references, generated by the system formation algorithm and included in the reference signal 212, are used by the controller 200 to determine the control data 230, as will be detailed later. Under normal operating conditions, depending on the type of voltage control method employed within the controller 200, the control data 230 may generally be generated by the controller 200 to produce an AC voltage waveform at the converter poles (at least partially composed of the fundamental frequency voltage magnitude and phase angle) that differs from the fundamental frequency voltage magnitude and phase angle components included in the reference signal 212, both transiently and in the steady state.

[0088] The measured signals 222 and 224 indicate the current 222 and / or voltage 224 downstream of the power converter 204.

[0089] The controller 200 consists of a control system 228, which in this example is a cascaded control system 228. Alternatively, the controller 200 may consist of a single-loop voltage control 228B, which will be further described later. The cascaded control system 228 includes a voltage controller 214 and a current controller 216, as shown in Figure 2A.

[0090] The voltage controller 214 uses a voltage control system to generate first data 215 based on the reference signal 212 and measurement signals 222, 224, and outputs the first data 215 to the current controller 216. The phase angle component of the reference signal 212 may optionally be used in one or more reference frame transformations within the voltage controller 214. The current controller 216 uses a current control system to generate second data based on the first data 215 and measurement signals 222, 224. In some embodiments, the current controller 216 may also use the voltage magnitude and / or phase angle component of the reference signal 212 in one or more feedforward control functions and / or reference frame transformations included within the current controller 216. The controller 200 determines 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 produce a time-varying voltage waveform (including, at least in part, the magnitude and phase angle of the time-varying fundamental frequency voltage) at the poles of the power converter 204, which 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 change rapidly as needed to realize the magnitude and phase angle components of the reference signal 212 at an adjustment point that is electrically close but not directly at the poles of the power converter 204. This adjustment point can be located, for example, on the grid-side winding terminal of an AC transformer that couples the power converter 204 to the grid 232. The magnitude and phase angle components of the voltage of the reference signal 212 are expected to change over time in response to grid events, in accordance with the grid formation control objectives described above. Since the voltage control system of the 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 magnitude and phase angle components of the pole voltages of the power converter 204 at the fundamental frequency are expected to change more rapidly than the magnitude and phase angle components of the reference signal 212.

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

[0093] The single-loop voltage control system 228B operates as follows: The controller 200 generates control data 230 that produce time-varying voltages at the poles of the power converter 204 (including, at least in part, the magnitude and phase angle of the time-varying fundamental frequency voltage), which generally do not necessarily match the magnitude and / or phase angle components of the reference signal 212. Similar to the cascaded control system 228, the generated pole voltages of the power converter 204 are varied as needed to realize the magnitude and / or phase angle components of the reference signal 212 at adjustment points that are electrically close to the poles of the power converter 204 but not directly to the poles of the power converter 204.

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

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

[0096] Regardless of the type of voltage control system used, controller 200 can also use "nominal virtual impedance". To achieve this, the voltage magnitude and voltage angle references in the reference signal 212 are first modified by controller 200 to correspond to a time-varying phasor voltage drop proportional to the measured or predicted output current of power converter 204. Controller 200 can employ various means to calculate the phasor voltage drop of the nominal virtual impedance. For example, it can consist of the product of a complex value of the 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 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] If the controller 200 includes a cascade controller 228 or a single-loop controller 228B and nominal virtual impedance is used, the pole voltages of the power converter 204 are rapidly manipulated to achieve a voltage phasor equal to the voltage phasor obtained by subtracting the nominal virtual impedance voltage drop from the reference signal 212 (internal voltage phasor reference) at the time of adjustment.

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

[0099] The control data 230 is supplied to the power converter 204. The power converter 204 operates based on the control data 230, namely, converting DC power to AC power or AC power to DC power, as well as generating or absorbing reactive power. In this way, the control data 230 controls the power converter 204.

[0100] Although not shown in Figure 2A, the controller 200 may include memory and at least one processor. The memory may include computer-readable instructions that, when executed by at least one processor, cause the controller 200 to perform one or more of the methods described herein.

[0101] Although not shown in Figure 2A, the controller 200 may further include a transceiver device. The transceiver device may include a separate transmitter and receiver. The transceiver device can be used to communicate operationally with other components described herein, either directly using wired or wireless means, or via a further interface such as a network interface. The transceiver device can, 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 and a reference voltage.

[0102] At least one processor is capable of executing computer-readable instructions and / or performing 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 memory and transceivers.

[0103] Memory may be a computer-readable storage medium. For example, memory may include a non-volatile computer storage medium. For example, memory may include a hard disk drive, flash memory, etc.

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

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

[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 Figure 3, the power system 100 in normal conditions can be described using the first circuit diagram 310 and the second circuit diagram 320. When operating in GFM mode, in normal conditions, the power converter 204 is controlled to output power to the grid 232 at the nominal voltage. Figure 3 shows the Thevenin voltage phasor V of the grid 232. g , converter voltage phasor V at the pole of power converter 204 c The initial internal voltage phasor reference V of power converter 204 cv i (i.e., the output of the inertial power regulator and the "Q controller"), the Thevenin impedance Z of grid 232 g , power converter 204 converter impedance Zc is shown. The converter impedance Zc of the connection of the power converter 204 to the grid 232 (i.e., grid connection) (e.g., due to 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 connection point (regulation point) of the power converter 204 and the grid 232 p is shown, and the output current I of the power converter 204 p 222 is shown. The second circuit diagram 320 simplifies the first circuit diagram 310. The second circuit diagram 320 is simplified because, under normal operating conditions, the coupling voltage V p is approximately equal to the initial internal voltage phasor reference V cv i and thus, although the converter impedance Z c physically still exists, it is considered to have a negligible effect on the behavior of the system at the timescales of interest (sub-transient to transient) and is thus removed from the circuit diagram. Continuing to refer to FIG. 3, the power converter 204, from an electrical characteristic perspective, includes the converter impedance Z c and the converter voltage source 301 (generating the converter voltage phasor V c ). This is because, as described above, the power converter 204 includes the converter voltage source 301 since the power converter 204 is controlled in the GFM mode.

[0107] The grid 232, from an electrical characteristic perspective, includes the Thevenin impedance Z g and the grid voltage source 303 (Thevenin voltage phasor V gThis includes generating ( ). As mentioned above, grid 232 is a consumer network and may have any number of power plants connected to grid 232 that supply electrical energy. Therefore, grid 232 includes both a consumer network and power plants. The consumer network has a Thevenin impedance Z g It can be simplified to this. The power plant uses a Thevenin voltage phasor V g It can be simplified to a grid voltage source 303 that is responsible for supplying and maintaining the voltage. Generally, for the scale of consumer networks and power plants, the Thevenin impedance Z is actually g and / or Thevenin voltage phasor V g It is impossible to know, measure, or determine it.

[0108] When a consumer network fails, the Thevenin impedance Z g When a change occurs and a failure occurs in the power plant, the Thevenin voltage phasor V g A change occurs. Any of these failures will result in a failure in grid 232. However, under normal conditions, grid 232 does not have such failures.

[0109] If a failure occurs in grid 232, or in any other serious grid failure referred to herein as a grid event, the power converter 204 may experience a current, power, or energy overload because it regulates voltage but not current, power, or energy. For a power converter controlled in normal GFM mode, this overload tends to trip the power converter offline or, in the worst-case scenario, damage the semiconductor devices within the power converter, rendering it permanently inoperable.

[0110] When such grid events occur, self-protection of the power converter must be ensured, and current limiting is typically achieved by switching the control to GFL mode. However, switching the control mode in this manner can jeopardize the stability of the power converter and the overall power system. Controller 200 tends to eliminate or mitigate this by controlling the power converter 204 in accordance with the method disclosed in Figure 4.

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

[0112] Subsequently, a grid event occurs at grid 232. In this example, the grid event causes the Thevenin voltage phasor V g The first parameter decreases. The controller 200 monitors the first parameter and determines 420 that the first parameter related to the controller 200 and / or the power converter 204 exceeds the first threshold. Thus, the first parameter exceeds the first threshold as a result of a grid event.

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

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

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

[0116] For example, in one embodiment where the output current is a variable being limited, the first threshold may be defined as the perimeter or boundary of a two-dimensional shape that defines the permissible region in which the output current of a resource, defined as a scalar complex value or a "space vector," may exist (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., Figure 7), a rectangle, or any other shape on the two-dimensional complex plane, and the first threshold is somewhat concentric with a somewhat larger shape bordered by the perimeter, which is a second threshold defining the actual physical limits.

[0117] In one embodiment, the shape bounded by the first threshold is a circle with a radius equal to 0.95, corresponding to a threshold of 0.95 percent of the rated current, where the magnitude of the current per unit is calculated as a function of the d-axis current and q-axis current using conventional methods such as the Euclidean norm, 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 the first limiting mode 430 by limiting the output current 222 of the power converter 204 based on the current value or a reference value.

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

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

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

[0122] The magnitude and phase angle of the current are expected to remain relatively unchanged for the remainder of the period spent in the first limiting mode 430, and if no further grid events occur during the period spent in the first limiting 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, in order to limit the output current (222) of the power converter (204), a portion of the control systems 228, 228B is disabled from determining the value of the control data 230 based on the measurement signals 222, 224 and the reference signal 212.

[0124] The current value is the value of a current reference contained in the controller 200 when it is determined that the first parameter has exceeded the first threshold. In this way, the current reference is effectively "frozen," meaning that the current reference remains at the same value as when it was determined that the first parameter had exceeded 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 specific 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 consist of, for example, a system-forming voltage angle reference included in the reference signal 212, or may be derived alternatively from a PLL, frequency-locked loop (FFL), second-order generalized integrator (SOGI), or other means.

[0126] In the first limiting mode 430, the current reference contained within the controller 200 is set to a current value or a reference value, so any higher-level controllers that may otherwise affect the control data 230 or the value of the current reference (e.g., the voltage controller 214, or the system-forming controller that generates the reference signal 212) can also be optionally “frozen,” meaning their control behavior can be modified so that their outputs are locked to a specific value.

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

[0128] If the target higher-level controller has one or more integrators, "freezing" the controller may include 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 lineage formation algorithm that generates the reference signal 212. This is because the lineage formation algorithm tends to have a slow bandwidth, and therefore the response time is relatively slow on a sub-transient time scale.

[0130] Therefore, in the first limiting mode 430, the initial internal voltage phasor reference V of the power converter 204 cv i It is expected that this will not change abruptly, regardless of changes in measurement signals 222 and 224.

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

[0132] As described above, the grid event is the Thevenin voltage phasor V of grid 232. g This causes a change in the Thevenin voltage phasor V of grid 232. g Initial internal voltage phasor reference V with changes in static or frozen cv i The coupling voltage V p This causes a change. Therefore, the second circuit diagram 320, as described in relation to Figure 3, is not valid for the first limiting mode 430.

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

[0134] In the context of this disclosure, the term “faulted” does not necessarily refer to a specific grid short-circuit or open-circuit condition, but is used in a broader sense to describe any grid event in which grid-forming resources may experience an exceedance of a parameter (e.g., a current limit threshold). A non-exhaustive list of such grid events is given below. symmetric or asymmetric line-to-ground short circuits or line-to-line short circuits when islanded or grid-tied; symmetric or asymmetric line openings when islanded or grid-tied; sag or swell of power system voltage magnitude when grid-tied; load step or jump of power system phase angle when grid-tied; unexpected control actions taken by neighboring resources; unexpected losses of neighboring resources when islanded or grid-tied; large steps in constant-power load, constant current load, constant impedance load, or machine load; intentional or unintentional islanding events or abrupt changes in grid strength; intentional or unintentional synchronization events; or combinations of the aforementioned grid events when islanded or grid-tied, such that multiple events occur simultaneously or sequentially.

[0135] Reference current I ref f This can be determined by components within the power system 100 relating to one or more of the following: limits on the magnitude of positive or negative sequence currents, limits on active or reactive power transfer, limits on energy transfer to or from a DC power source, and / or limits on any other quantity of interest.

[0136] First data 215 or reference current I ref fAssuming that the control system 228 or current controller 216 is stable, the output current 222 does not change because it is set to its current value (i.e., frozen). 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 tends to converge toward equilibrium, which can generally be indicated by the magnitude of the current controller error signal (or the magnitude of multiple current controller error signals if multiple current control channels exist) decreasing to a value equal to or close to zero. In the first limiting mode 430, the controller 200 thus effectively controls the power converter 204 as a current source 501.

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

[0138] However, the first limiting mode 430 can be stable under strong grid conditions where PLL instability is not expected. Under such conditions, it may be even more advantageous to use the PLL as the reference frame for a current reference, since the d-channel and q-channel components of the current reference can be selected to quickly and accurately achieve the active current setpoint and / or reactive current setpoint over an undefined period. Thus, energy transfer between the DC power supply 202 and the grid 232 tends to be easier to adjust.

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

[0140] However, operation in the first limiting mode 430 still does not constitute a conventional GFM because the voltage magnitude is not regulated, and this drawback can lead to unexpected results. Therefore, to reduce the risk of undesirable operation, it is generally intended that the converter 204 does not remain in the first limiting mode 430 for approximately 50-100ms or more, ideally 30ms or more.

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

[0142] Referring again to Figure 4, when the power converter 204 is controlled in the first limiting mode 430 (for example, current limiting mode), the controller 200 controls the output current I of the power converter 204. p Determine 222. Output current I p In response to ,222 reaching an equilibrium state, the controller 200 outputs current I p Based on 222 and the output voltage 224, the virtual impedance 440 of the power converter 204 is determined.

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

[0144] If the modified internal voltage phasor reference has not yet been determined for steady-state operation, the controller 200 sets the internal voltage phasor reference V of the power converter 204 while the power converter 204 is in the first limiting mode 430. cv And the node voltage (fault coupling voltage) V at node 224 (e.g., common coupling point) between power converter 204 and grid 232 p f Virtual voltage V acts as a voltage difference between and . v The virtual impedance 440 is determined by calculating the virtual voltage V of the power converter 204 in the balanced state. Next, the controller 200 determines the virtual voltage V of the power converter 204 in the balanced state. v , virtual impedance Z cv , and fault output current I p f The Thevenin equivalent circuit 600 shown in Figure 6 of the power converter 204 connected to grid 232, including 222, is generated. Next, the controller 200 uses circuit analysis to generate a virtual impedance Z cvThis solves the Thevenin equivalent circuit 600.

[0145] Since the Thevenin equivalent circuit 600 is a simple circuit diagram, it can be solved using Ohm's law, as shown in Equation 1. Z cv =(V cv -V p f ) / I ref f+ formula 1

[0146] Controller 200 has a virtual impedance Z cv Once determined, the controller 200 determines the adjustment term of the control system 228 and modifies the control system 228 to include the adjustment term, thereby reducing the virtual impedance Z cv The control system 228 is modified based on the following: If a fixed virtual impedance has already been adopted under the initial system formation operating mode 410, the fixed virtual impedance is the virtual impedance Z cv It can be replaced with this.

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

[0148] Z cv By calculating in this way, the voltage V during the transition from the first restriction mode 430 to the first constraint system formation mode 450 is calculated. p f It tends to be guaranteed that the fault output current I in the equilibrium state does not change. p f However, the adjustment term is selected so that it is the same in the first restriction mode 430 and the first constraint system formation mode 450 (fault output current Ip f I remain after the transition ref f+ (Because it is equivalent to ). Therefore, the methods of the present disclosure tend to achieve a seamless and nearly instantaneous transition back to a (constrained) phylogenetic mode.

[0149] Equation 1 shows perhaps the simplest approach to determining a fixed virtual impedance that enables a seamless transition from the first constraint mode 430 to the constraint system formation mode 450, but this is not the only possible approach. By applying another calculation method that utilizes the sine and cosine theorems, it is possible to determine another virtual impedance that can achieve a seamless transition while satisfying various purposes. This alternative calculation method also requires determining a second virtual voltage consisting of a static or fixed virtual voltage offset, which is the internal voltage phasor reference V cv This is added to the first virtual voltage. This second virtual voltage is applied along with the first (current-dependent) virtual voltage offset, which is related to the calculated virtual impedance. The second virtual voltage is applied to the angle ∠V cv , its size |V cv | or V cv This can be considered a step change in the internal voltage phasor reference, which includes changes in both magnitude and angle.

[0150] Additional objectives that can be met using alternative methods may include one or more of the following: forcing a specific magnitude of virtual impedance, forcing a desired ratio of reactance to resistance within virtual impedance, and / or forcing specific values ​​of the magnitude and / or phase angle of the internal voltage phasor. It should be noted that some objectives are mutually exclusive, and furthermore, the pursuit of individual objectives may have significant implications for the system's behavior and / or performance in constrained GFM modes.

[0151] Without loss of generality, assume that the controller already has a fixed "steady state" virtual impedance Z with known real and imaginary components. CVSS Assume that this persists in both the initial GFM mode and the constrained GFM mode (410 and 450 respectively). This virtual impedance persists in both the initial GFM mode and the constrained GFM mode (410 and 450 respectively). V CVSS At the end of the limiting mode 430, is the product of Z CVSS and I p (or the product of Z CVSS and an I such as the current reference signal included in the first data 215, or a proxy (alternative indicator) of I p or an estimated value of I p ), representing the virtual voltage drop across the virtual impedance. Also, to achieve a seamless transition from the limiting mode 430 to the constrained GFM mode 450, an additional "current limit" virtual impedance Z CVL is added in series with Z CVSS , and in the constrained GFM mode 450, assume that Z CV = Z CVL +Z CVSS (optionally, |Z CVSS | may be equal to zero).

[0152] As an example of an alternative method for determining the fixed Z cv (and optionally Z cvL ) required for seamless transition, perform the following calculations to determine the required magnitude of the additional current limit virtual impedance |Z cvL | and the required internal voltage phasor reference angle ∠V cv . This calculation is the required reactance-to-resistance ratio of the current limit virtual impedance r XR = (imag(Z cvL )) / (real(Z cvL )) )) (or, if known, the angle ∠Z of the equivalent current limit virtual impedance cvL) and the desired magnitude of the internal voltage phasor |V cv This 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 as follows: XR =(imag(Z cvL )) / (real(Z cvL )) and current-limiting virtual impedance |Z cvL Given a desired magnitude of |V|, the required magnitude of the internal voltage phasor necessary to achieve a seamless transition from current limiting mode 430 to constraint system formation mode 450 is |V|. CV | and the required internal voltage phasor reference angle ∠V cv This will be decided. 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 previous discussion, it was assumed that the virtual impedance consisted of non-salient reactive and resistive elements. That is, the reactance within the virtual impedance is assumed to be the same magnitude in the d-channel and q-channel, and the resistance within the virtual impedance is assumed to be the same magnitude in the d-channel and q-channel. However, in steady state and constrained GFM modes, it is also possible to apply a significant virtual impedance for the purpose of current limiting. When the virtual impedance is significant, the reactance within the virtual impedance may have different values ​​in the d-channel and q-channel. The same applies to the resistance within the virtual impedance.

[0155] As mentioned above, the phase angle component of the reference signal 212 can be used as the reference frame for the first data 215 in the first limiting mode 430, or a PLL, FFL, or SOGI can be employed for this purpose. Using the same reference frame, an orientation-dependent voltage drop across different d- and q-channel components of reactance and resistance elements in the virtual impedance can be calculated.

[0156] Applying a significant virtual impedance to current limiting in constrained GFM mode has the potential advantage that the voltage drop induced by the magnitude of the current limiting impedance is concentrated almost entirely (or entirely) on the specific channel where the overload occurs. The presence of orthogonal low-impedance channels may increase operational flexibility and improve regulated current supply capability in constrained GFM mode. Similar to the case of non-salient current limiting impedance, analytical techniques can be used to calculate the d-channel and q-channel components of the salient entry actance and / or resistance, enabling a seamless transition from the first limiting mode 430 to constrained GFM mode 450.

[0157] In the preceding discussion, it can be assumed that controller 200 operates exclusively with respect to positive sequence quantities, and method 400 is applied to positive sequence quantities. However, it should be noted that controller 200 and control method 400 can also be applied independently to negative sequences, and controller 200 may represent a controller that provides negative sequence voltage control functionality, including cascaded negative sequence voltage control, single-loop negative sequence voltage control, or direct negative sequence voltage control. In this case, since it is desirable that the negative sequence voltage is zero, the reference signal 212 is typically zero (however, a non-zero negative sequence voltage reference can also be implemented). With respect to the modes in Figure 4, the modes and mode transitions function in negative sequence implementations as they do in positive sequence implementations. For example, if the measured negative sequence current 222 or some substitute for this current (e.g., current reference or estimated current) exceeds the negative sequence threshold, the first limiting mode 430 can be entered. Similar to the case of positive sequence applications, a negative sequence virtual impedance can be determined to enable a seamless transition to the constrained GFM mode of the negative sequence controller. Assuming that both positive and negative sequence controllers exist, the overall control strategy shown in Figure 4 can optionally be applied simultaneously to both positive and negative sequences (with identical or different control parameters) or to only one of the sequences.

[0158] In the first constraint grid formation mode 450, the power converter 204 can adjust its output voltage, thereby allowing it to continue supporting the grid 232. Signals and controllers that were frozen or disabled in the limit mode 430 are reactivated in the constraint GFM mode 450 and, if necessary, reinitialized to values ​​that do not disrupt system balance during the transition from limit mode 430 to constraint GFM mode 450. The controller 200 controls the power converter 204 in the first constraint grid formation mode 450 within 50 ms of controlling the power converter 204 in limit mode 430. This tends to reduce or minimize the time the power converter 204 is in GFL mode, thereby allowing the power converter 204 to quickly return to GFM mode, on an electromechanical timescale, where it can continue supporting the grid 232.

[0159] Referring to Figure 7, the phasor figure 700 shows the simulation results for the first constraint system formation mode 450, with the internal voltage phasor reference V cv With the first phasor 701, the fault output current I of the power converter 204 in the balanced state is... p f This is 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 This is designated as the fourth phasor 704, and the fault Thevenin voltage phasor V of grid 232 g f This is shown as the fifth phasor 705. The phasor diagram 700 also includes the X-axis 710 and the Y-axis 720, each with units of voltage or current (per unit). It also shows the first threshold as the first limit 706.

[0160] In the example shown in Figure 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. Therefore, the first parameter is the second phasor 702, and the first threshold is the first limit 706.

[0161] As can be seen from Figure 7, the fifth phasor 705 is less than 1 p.u., which indicates that a grid event occurred on grid 232. The fault output current I of the power converter 204, shown as the second phasor 702. p f The first limit value is 706. The fourth phasor 704 is the difference between the first phasor 701 and the third phasor 703. In this way, the power converter 204 is controlled in the first constraint system formation mode 450, and the fault output current I of the power converter 204 is controlled. p f The virtual impedance Z used to determine the adjustment term of the control system 228 is cv Virtual voltage V used to determine v As a result, the power converter 204 is maintained in equilibrium (as shown by the second phasor 702) (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 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 This can include an impedance phasor Z. cv The measured current phasor I p f It can be defined as the product of the two. In another embodiment, the adjustment term is instead the impedance phasor Z cv The adjustment term may include the product of the current phasor reference included in the first data 215. In one embodiment, the adjustment term 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 can further provide a smooth transition from the first constraint grid formation mode 450 back to the initial grid formation mode 410. Prior to this smooth transition, the grid event is resolved and the current I p fAs it decreases, the third phasor 703 returns towards the first phasor 701. The fourth phasor 704 (virtual voltage V v Since ) is the difference between the first phasor 701 and the third phasor 703, the fourth phasor 704 will consequently decrease in size as grid events are eliminated.

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

[0165] This is advantageous because it allows the power converter 204 to smoothly return to normal grid formation control without disrupting the grid 232 as a result of controlling the power converter 204. Furthermore, the power converter 204 tends to maintain a certain form of grid formation control to support the power converter 204 during grid events.

[0166] In some embodiments, the controller 200 can determine that a second parameter related to the controller 200 and / or the power converter 204 has exceeded a second threshold.

[0167] In response to determining that the second parameter exceeds the second threshold, the controller controls the power converter 204 in a second limiting mode by limiting the output current 222 of the power converter 204 based on the updated current value or 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 updated reference value). As a result, the control data 230 is expected to rapidly manipulate the fundamental frequency voltage waveform at the poles of the power converter 204 to match the measured current 222 to the updated current value or updated reference value. If no further grid events occur during the period spent in the second limiting mode, the control data 230 is also expected to reach a steady state and remain substantially unchanged. Optionally, the control system 228 may disable determining the value of the control data 230 based on the measured signals 222, 224 and the reference signal 212. The controller 200 then determines the updated output current 222 of the power converter 204.

[0168] In response to the updated output current 222 reaching a state of equilibrium, the controller 200 determines the 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 done in the same manner as described above.

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

[0170] Determining that the second parameter exceeds the second threshold and taking corresponding action tends to be useful, for example, when another failure occurs following the failure that caused the grid event, or when adjustments are made in another part of grid 232 as a result of the grid event. Thus, subsequent grid events may occur following the initial grid event.

[0171] In some embodiments, the control system 228 is a cascaded control system 228 including a voltage controller 214 and a current controller 216. The voltage controller 214 uses a voltage control system to generate first data 215 based on a reference signal 212 and measurement signals 222, 224, and outputs the first data 215 to the current controller 216. The current controller 216 uses a current control system to generate second data based on the first data 215 and measurement signals 222, 224.

[0172] The current controller 216 can 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 the current value 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 virtual impedance or updated virtual impedance may include modifying the voltage control system of the voltage controller 214.

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

[0177] The methods and controllers of this disclosure tend to maintain the maximum possible grid formation control because the use of grid-following mode is kept to the shortest possible duration. The controllers and methods generally tend to provide stable operation during fault recovery and further tend to prevent loss of synchronization between grid formation resources (i.e., power converter 204) and the bulk power grid (i.e., grid 232). The controllers and methods tend to maximize the current capacity of the power converter 204 and enable prioritization of active vs. reactive current, angle vs. voltage, and positive vs. negative sequence through the determination of adjustment terms.

[0178] The precise implementation of the first parameter can be determined by the user, and therefore the controller and method tend to offer a degree of flexibility and adjustability as desired (for example, the controller and method can be configured to prioritize grid formation operation or to allow for extension of current limiting). [Explanation of symbols]

[0179] 100: Power System 200: Controller 202: DC Power Supply 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: Limiting Controller 222: Measurement Signal, Measurement Current 224: Measurement Signal, Measurement 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: Limiting Mode 450: Constraint System Formation Mode 490: Adjustment Term 500: Third circuit diagram 501: Current source 600: Thevenin equivalent circuit 700: Phasor diagram 701: First phasor 702: Second phasor 703: Third phasor 704: Fourth phasor 705: Fifth phasor 706: First limit

Claims

1. A method (400) for controlling a power converter (204) connected to a grid (232), wherein the power converter (204) is initially controlled (410) in grid-forming mode to output a current (222) at a nominal voltage based on control data (230) provided by a controller (200), the controller (200) includes a control system (228) that determines the control data (230) based on measurement signals (222, 224) indicating the current and / or voltage downstream of the power converter (204) and a reference signal (212), the method is: The controller (200) determines (420) that a first parameter related to the controller (200) and / or the power converter (204) has exceeded a first threshold, In response to the determination that the first parameter has exceeded the first threshold, 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 the current value and / or a reference value. The controller (200) determines the output current (222) of the power converter (204), In response to determining that the output current (222) is in equilibrium or close to equilibrium, The controller (200) determines the virtual impedance of the power converter (204) based on the output current (222) of the power converter (204) (440), The controller (200) modifies the control system (228) based on the virtual impedance, A method comprising the steps of controlling a power converter (204) in a first constraint system formation mode (450) by a controller (200), wherein a modified control system (228) determines control data (230) based on measurement signals (222, 224) indicating current and / or voltage downstream of the power converter (204) and a reference signal (212).

2. The controller (200) determines (460) that a second parameter related to the controller (200) and / or the power converter (204) has exceeded a second threshold, In response to the determination that the second parameter has exceeded the second threshold, The controller (200) controls the power converter in a second limiting mode by limiting the output current (222) of the power converter (204) based on the updated current value and / or updated reference value. The controller (200) determines the updated output current (222) of the power converter (204), In response to the determination that the updated output current (222) is in equilibrium or close to equilibrium, The controller (200) determines the updated virtual impedance of the power converter (204) based on the updated output current (222) of the power converter (204), The controller (200) further modifies the control system (228) based on the updated virtual impedance. The method according to claim 1, comprising the step of controlling a power converter (204) in a second constraint system formation mode (450) by a controller (200), wherein a further modified control system (228) determines control data (230) based on measurement signals (222, 224) indicating current and / or voltage downstream of the power converter and a reference signal (212).

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 measurement signals (222, 224), and outputs the first data (215) to the current controller (216). The current controller (216) generates second data using the current control system based on the first data (215) and measurement signals (222, 224). The controller (200) determines the control data (230) based on the second data. The method according to claim 1, wherein the controller (200) controls the power converter (204) in a first limiting mode (430) by setting a first data (215) to be equal to a current value and / or a reference value, thereby limiting the 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), The method according to claim 1, wherein 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) so ​​as not to increase beyond a current value or a reference value, thereby limiting the output current (222) of the power converter (204).

5. The controller (200) determining the virtual impedance for the power converter (204) (440) further includes the controller (200) determining the virtual impedance and fixed virtual voltage offset for the power converter (204) based on the output current (222) of the power converter (204), The method according to claim 1, wherein the controller (200) modifies the control system (228) based on virtual impedance, further comprising the controller (200) modifying the control system (228) based on virtual impedance and a fixed virtual voltage offset.

6. Modifying the control system (228) based on virtual impedance is The controller (200) determines the adjustment term of the control system (228) by calculating a virtual voltage or virtual current as a function of virtual impedance and output current (222), The method according to claim 1, comprising the step of implementing adjustment terms in a control system (228) using a controller (200).

7. The controller (200) determines the virtual impedance of the power converter (204). While the power converter (204) is in a first limiting mode (430), the steps include calculating a virtual voltage defined as the voltage difference between the internal voltage phasor reference of the power converter (204) and the node voltage at the node (224) between the output of the power converter (204) and the grid (232), Based on the Thevenin equivalent circuit of a power converter (204) connected to a grid (232), the steps include analyzing the Thevenin equivalent circuit of the virtual impedance using a circuit analysis method, including the virtual voltage, the output current of the power converter in a balanced state, and the virtual impedance. The method according to claim 1, including the method described in claim 1.

8. The method according to claim 7, further comprising the step of reducing the virtual impedance (490) as a result of the controller (200) reducing the 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 be controlled in the initial grid formation mode (410).

9. The method according to claim 1, wherein the time from when it is determined that a first parameter related to the controller (200) and / or the power converter (204) exceeds a first threshold until the controller completes the step of controlling the power converter (204) in a first limiting mode is 50 ms or less.

10. Determining that a first parameter related to the controller (200) and / or power converter (204) exceeds a first threshold means Determining that the current reference of a dq-frame current regulator using a command angle or a PLL (Phase Locked Loop) angle has saturated, and / or The current controller (216) determines that the d or q current reference input to the proportional-integral (PI) controller has saturated, and / or The proportional resonant (PR) controller of the current controller (216) determines that the d or q current reference input after the inverse reference frame transformation has saturated, and / or The deadbeat controller of the current controller (216) determines that the d or q current reference input after the inverse reference frame conversion is saturated, and / or Determining that the phase current limit has been reached, and / or Determining that the active power limit has been reached, and / or Determining that energy limits have been reached, and / or Determining that a component is experiencing pulse drop or blocking, and / or Determining that the voltage reference is saturated, and / or Determining that the voltage output has reached its limit, and / or To determine that the modulation index has saturated, The method according to claim 1, comprising one or more of the above.

11. A controller (200) for controlling a power converter (204) connected to a grid (232), Includes a control system (228) configured to determine control data (230) based on measurement signals (222, 224) indicating the current and / or voltage downstream of the power converter (204), and a reference signal (212), The controller (200) is prepared to control the power converter (204) in grid formation mode to output a current (222) at a nominal voltage based on the control data (230). The controller is It is determined that a first parameter related to the controller (200) and / or power converter (204) exceeds a first threshold, In response to the determination that the first parameter has exceeded the first threshold, The power converter is controlled in a first limiting mode (430), and the controller (200) is configured to limit the output current (222) of the power converter (204) based on a current value and / or a reference value. Determine the output current (222) of the power converter (204), In response to determining that the output current (222) is in equilibrium or close to equilibrium, Based on the output current (222) of the power converter (204), the virtual impedance of the power converter (204) is determined. The control system (228) is modified based on the virtual impedance. A controller that controls a power converter (204) in a first constraint system formation mode (450), and a modified control system (228) is configured to determine 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, It is determined that a second parameter related to the controller (200) and / or power converter (204) exceeds a second threshold, In response to the determination that the second parameter has exceeded the second threshold, The power converter is controlled in a second limiting mode, and the controller is configured to limit the output current (222) of the power converter (204) based on an updated current value and / or an updated reference value. Determine the updated output current (222) of the power converter (204), In response to determining that the updated output current (222) is in equilibrium or close to equilibrium, Based on the updated output current (222) of the power converter (204), the updated virtual impedance of the power converter (204) is determined. The control system (228) is further modified based on the updated virtual impedance. The power converter (204) is controlled in a second constraint system formation mode (450), and a further modified control system (228) determines the control data (230) based on the measurement signals (222, 224) and the reference signal (212). The controller according to claim 11, configured as follows.

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 a reference signal (212) and 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 using a current control system based on the first data (215) and measurement signals (222, 224). The controller (200) is further configured to determine control data (230) based on the second data. The controller according to claim 11, further configured to control the power converter (204) in a first limiting mode (430) by setting a first data (215) to be equal to a current value or a reference value, thereby limiting the output current (222) of the power converter (204).

14. The controller according to claim 11, wherein the controller is configured to modify the control system (228) based on virtual impedance, and the controller is configured to determine an adjustment term for the control system (228) by calculating a virtual voltage or virtual current as a function of the virtual impedance and output current (222), and to implement the adjustment term in the control system (228).

15. It is a power converter, The DC side for connecting to a DC power supply, The AC side for connecting to the grid, A controller (200) according to any one of claims 11 to 14, Power converters, including...

Images