Centralized control strategy for grid-connected converters

Abstract

In this application, a universal control (UniCon) scheme for grid-connected converters is provided that allows operation in grid-aligned, grid-stabilized, and grid-forming modes automatically. The converter does not require information about the grid or the connected sources and loads. The converter can configure itself where it operates based on local measurements. The converter can operate in a wide range of commonly encountered steady-state, transient, and fault conditions. UniCon provides a universal control strategy for converters to the grid, allowing operation in different modes including dispatch in grid-connected mode and automatic load sharing in islanded or microgrid modes. Under transient conditions, the converter provides inertial support and improves damping to stabilize and reduce disturbances. Multiple converters on a system do not require detailed system knowledge or low-latency communication for fast regulation, but use communication for slow regulation and system-level optimization when available.

Description

[Technical Field]

[0001] (Cross-reference of related applications) This application claims priority under U.S. Provisional Application No. 63 / 147,630, filed on 9 February 2021, which is incorporated herein by reference in its entirety as it is shown below.

[0002] (Technical field) This invention relates, in general terms, to power electronics and their control. More specifically, it relates to apparatus and methods for centralized control of grid-connected converters. [Background technology]

[0003] Grids are rapidly shifting from systems powered entirely by synchronous generators to systems incorporating numerous converters powered by distributed energy resources. At lower levels of converter adoption, it is reasonable to assume a robust grid and control converters with grid-aligned strategies. As converter adoption increases, the same converters may need to operate in grid-connected mode occasionally and grid-island mode otherwise, making control difficult when converters need to change operating modes. Furthermore, converters behave very differently from generators, especially during transitions and failures, often requiring significant changes to control strategies in these two distinct modes. Distinguishing between these two modes can also be challenging. Real-time control and coordination communication can, in principle, help mitigate this problem, but it requires low-latency communication and precise system knowledge (which is extremely difficult in geographically distributed and constantly changing systems).

[0004] Examples of major problems that can arise with a high degree of inverter-based resource (IBR) deployment include insufficient inertia, converter interaction, reduced system stability, and an increased likelihood of cascading failures. Converter control often had to deal with resonance, transient overloads leading to current limiting, and uncontrollable behavior in the rest of the system. Major failures and network changes can cause system transients that may lead to circuit breaks or vibrations, posing problems for protection systems. Ultimately, the relative size of the controlled converter in relation to the system it connects to may be unknown. This lack of knowledge poses a problem for converter controller design, which often relies on knowledge of the system and focuses on optimizing converter operation rather than system operation, using standard techniques. [Overview of the project]

[0005] This specification provides a universal control (UniCon) scheme for grid-connected converters. One objective of the UniCon scheme is to control converters so that they can transition from the current synchronous generator-dominated grid to an inverter-based resource (IBR)-dominated grid.

[0006] In some embodiments, the UniCon scheme automatically enables operation in grid-attached mode, grid-stabilized mode, and grid-forming mode. In some embodiments, the converter can determine its own operating point based on local measurements, without requiring information about the grid or connected power and loads. In some embodiments, the converter can operate over a wide range of steady-state, transient, and fault conditions that are typically encountered. In some embodiments, the converter control strategy can operate with a combination of synchronous generators and grid-attached or grid-forming converters connected to the grid, and can operate with robust or fragile grids or microgrids. In some embodiments, converter control is based on the assumption that many (hundreds) converters are connected to the grid and all converters work together to achieve system objectives. In some embodiments, UniCon provides a universal control strategy for converters on the grid, enabling operation in different modes, including power supply in grid-attached mode and automatic load sharing in island mode or microgrid mode. In some embodiments, under transient conditions, the converter provides inertial support and improves damping to stabilize and reduce disturbances. In some embodiments, multiple converters on the system do not require detailed system knowledge or low-latency communication for high-speed adjustment, but utilize communication for low-speed adjustment and system-level optimization if available. The proposed control strategy has several different elements. All of these elements must function in an integrated manner to achieve the desired overall system-level operation.

[0007] In one embodiment, the converter controller is configured to control the converter in grid formation mode without relying on measured values ​​of the converter terminal voltage frequency.

[0008] The converter controller may be configured to synchronize the converter with another grid-forming converter, microgrid, and / or grid.

[0009] The converter controller may be configured to control the converter in island mode after it has been disconnected from another grid-forming converter, or from a network including a microgrid or grid.

[0010] The converter controller may be configured to synchronize the converter with another gridforming converter, microgrid, and / or grid having different frequencies and phases but not exceeding the ratings of the gridforming converter.

[0011] The converter controller may be configured to control the converter under conditions of high frequency change.

[0012] The converter controller may be configured to control the converter as the power generation / load changes.

[0013] The converter controller may be configured to dynamically manage active and reactive power during transient events.

[0014] The converter controller may be configured to synchronize the reference voltage phase of the converter's reference voltage waveform with the terminal voltage phase of the converter's terminal voltage waveform in accordance with the phase jump of the terminal voltage phase.

[0015] The converter controller may be configured to black start the microgrid containing the converter.

[0016] The converter controller may be configured to manage active and reactive power sharing over a wide range of short-circuit ratios and / or X / R (reactance / resistance) ratios. The converter controller may also be configured to control a first grid formation converter to synchronize it with a second grid formation converter.

[0017] In another embodiment, the converter controller may include a phase jump module, an adaptive inertia module, and a virtual impedance module.

[0018] The converter controller may be configured to control the grid formation converter and synchronize it with the microgrid and / or grid.

[0019] The phase jump module may be configured to set the converter's reference voltage phase when the converter's terminal voltage waveform suddenly becomes out of sync with the converter's reference voltage waveform.

[0020] The phase jump module may be configured to set the reference voltage phase to the phase of the terminal voltage waveform by zero inertia response.

[0021] The phase jump module may be configured to synchronize the reference voltage waveform with the terminal voltage waveform without changing the voltage amplitude of the reference voltage waveform.

[0022] The phase jump module may be configured to non-linearly adjust the reference voltage phase.

[0023] The phase jump module may be configured to jump the reference voltage phase to the opposite quadrant in order to prevent the reference voltage waveform and the terminal voltage waveform from moving synchronously in opposite directions.

[0024] A phase jump module can be used when 1) the voltage difference between the terminal voltage waveform and the reference voltage waveform is higher than the voltage threshold, or 2) the current flowing through the converter's filter inductor is greater than the current threshold.

[0025] The phase jump module a) corrects the phase angle jump by detecting the voltage difference (EV) between the reference voltage waveform and the terminal voltage waveform. t (b) The determination is made at least partially based on the current flowing through the filter inductor of the converter and the terminal voltage waveform, and (b) the reference voltage phase can be set so that a phase angle jump and the product of the angular frequency of the reference voltage waveform and the sampling time are added to the reference voltage phase.

[0026] The phase angle jump is a function of the voltage difference (f), (i.e., f(EV)) t Function (f) may be proportional to a constant, a linear function, or a nonlinear function. Function (f) may be proportional to the hyperbolic tangent (tanh(EV). t It may include )).

[0027] A frequency loop including an adaptive inertia module can be configured to provide an output phase angle (θ(t)). The phase angle jump is θ jump Equivalent to, here,

number

[0028] The phase jump module may be configured to repeat steps a) to b) until the phase jump module is removed.

[0029] The phase jump module can be removed in any of the following cases: 1) the voltage difference falls below the voltage threshold and the current flowing through the filter inductor falls below the current threshold; 2) the reference voltage phase adjustment is performed a predetermined number of times; 3) a predetermined amount of time has elapsed since the phase jump module was put into use; or 4) steps a) to b) are performed a predetermined number of times.

[0030] The converter controller may further include a droop control module configured to set the reference power of the converter.

[0031] The droop control module may be configured to set the converter's reference power based at least in part on the inverse correlation between the converter's active power and frequency.

[0032] The droop control module can be located within a feedback loop with the adaptive inertia module.

[0033] The difference between the output of the droop control module and the terminal power may be provided as the input to the adaptive inertia module.

[0034] The sum of the output of the adaptive inertia module and the reference angular frequency of the reference voltage waveform may be provided as the input to the droop control module.

[0035] The adaptive inertia module may be configured to set the converter's inertia constant based at least in part on the power difference between the converter's terminal power and the converter's reference power.

[0036] The adaptive inertia module may be configured to synchronize the internal frequency and phase of a reference voltage waveform with the terminal frequency and phase of the terminal voltage waveform, without requiring the measurement of the terminal voltage waveform's frequency.

[0037] The adaptive inertia module may be configured to set the inertia constant based only on 1) the measured power of the converter, and / or 2) the difference between the reference power and the measured power, and / or 3) the terminal voltage amplitude of the terminal voltage waveform. The adaptive inertia module may simply be configured to set the inertia constant based on any one of the three metrics described above, any combination of any two of the three metrics described above, or all three of the three metrics described above.

[0038] The constant of inertia may be at least partially based on the nonlinear function of the power difference.

[0039] The inertia constant may be at least partially based on a polynomial function of the power difference.

[0040] The attenuation coefficient of the converter may be based at least partially on the power difference.

[0041] The damping coefficient may be at least partially based on the nonlinear function of the power difference.

[0042] The damping coefficient of the converter may be proportional to the inverse square root of the constant of inertia.

[0043] The virtual impedance module makes it possible to dynamically vary the effective output impedance of the converter.

[0044] A virtual impedance module may include a virtual impedance configured to increase in impedance in response to an increase in current.

[0045] The virtual impedance may include a virtual resistor configured to increase resistance using a nonlinear function in response to an increase in current.

[0046] The virtual impedance module may further include a certain virtual inductor.

[0047] The virtual impedance may include a virtual resistor and a virtual inductor. The inductance of the virtual inductor and the resistance of the virtual resistor may be configured to increase in proportion to the increase in current using an integrator.

[0048] A frequency loop including an adaptive inertia module may be configured to provide an output frequency. The output frequency can be kept constant in freeze mode when the terminal voltage is below a predetermined value for a predetermined period of time. The output frequency can exit freeze mode when the terminal voltage is above a predetermined value for a predetermined period of time.

[0049] In another embodiment, the converter controller may be configured to control the grid-forming converter without relying on measured frequencies of the terminal voltage waveforms of the grid-forming converter.

[0050] The converter controller may be configured to synchronize the grid formation converter with another grid formation converter, a microgrid, and / or a grid.

[0051] The converter controller may be configured to control the grid-forming converter in island mode after it has been disconnected from another grid-forming converter, or from a microgrid, or from the grid.

[0052] The converter controller may be configured to synchronize the grid-forming converter with another grid-forming converter, microgrid, and / or grid having different frequencies and phases but not exceeding the ratings of the grid-forming converter.

[0053] The converter controller may be configured to control the grid-forming converter under conditions of high frequency change rates.

[0054] The converter controller may be configured to control the grid formation converter under varying power generation / load conditions.

[0055] The converter controller may be configured to dynamically manage active and reactive power during transient events.

[0056] The converter controller may be configured to synchronize the reference voltage phase of the reference voltage waveform of the grid forming converter with the terminal voltage phase of the terminal voltage waveform of the grid forming converter, in accordance with the phase jump of the terminal voltage phase.

[0057] The converter controller may be configured to black-start the microgrid, which includes a grid formation converter.

[0058] The above and further embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the drawings, similar reference numerals indicate similar structural elements and features in various figures. The drawings are not necessarily to scale, but rather are intended to illustrate the principles of the present invention. The drawings are illustrative only and not intended to limit one or more implementations of the apparatus according to the present invention. [Brief explanation of the drawing]

[0059] [Figure 1] Figure 1 is a flowchart showing the control structure of a converter according to an embodiment of the present invention.

[0060] [Figure 2] Figure 2 is an electrical circuit diagram showing an example grid model according to an embodiment of the present invention.

[0061] [Figure 3] Figure 3 is a block diagram showing one embodiment of the UniCon strategy according to an aspect of the present invention.

[0062] [Figure 4A] Figure 4A is a conceptual diagram showing the movement of voltage phasors after interconnection in a conventional method based on current loops.

[0063] [Figure 4B] Figure 4B is a conceptual diagram showing the movement of voltage phasors after interconnection according to a UniCon scheme of an embodiment of the present invention.

[0064] [Figure 5A] Figure 5A is a conceptual diagram illustrating a simple phase jump to reduce transient phenomena when the converter and grid voltages are the same but have different phases.

[0065] [Figure 5B] Figure 5B is a conceptual diagram illustrating a phase jump and subsequent quadrant jump to reduce transient phenomena when the converter and grid voltages are the same but in different phases.

[0066] [Figure 6] Figure 6 is an electrical circuit diagram showing models of two grids powered by grid formation converters according to embodiments of the present invention.

[0067] [Figure 7A] Figure 7A is a flowchart of the algorithm that is activated to start the phase jump algorithm according to an embodiment of the present invention.

[0068] [Figure 7B] Figure 7B is a flowchart of the phase jump algorithm called by the algorithm shown in Figure 7A according to an embodiment of the present invention.

[0069] [Figure 8A] Figure 8A shows the simulation results when two converters are connected, and is a plot of the load voltage of the first converter. [Figure 8B]Figure 8B shows the simulation results when two converters are connected, and plots the load voltage of the second converter. [Figure 8C] Figure 8C shows the simulation results when two converters are connected, and is a plot of the current of the first converter. [Figure 8D] Figure 8D shows the simulation results when two converters are connected, and is a plot of the current of the second converter. [Figure 8E] Figure 8E shows the simulation results when two converters are connected, and is a plot of the internal voltages of the converters. [Figure 8F] Figure 8F shows the simulation results when two converters are connected, and plots the output power of the converters. [Figure 8G] Figure 8G shows the simulation results when two converters are connected, and is a plot of the frequency of the converter according to an embodiment of the present invention.

[0070] [Figure 9A] Figure 9A is a plot of the nonlinear inertia constant (J) according to an embodiment of the present invention.

[0071] [Figure 9B] Figure 9B is a plot of the nonlinear damping coefficient (D) according to an embodiment of the present invention.

[0072] [Figure 10A] Figure 10A is an electrical circuit diagram showing an example grid model according to an embodiment of the present invention.

[0073] [Figure 10B] Figure 10B is a block diagram showing one embodiment of the UniCon Strategy according to an aspect of the present invention.

[0074] [Figure 11A]Figure 11A shows the simulation results of an adaptive inertia scheme for one embodiment of the UniCon scheme, with plots of adaptive gain and constant gain. [Figure 11B] Figure 11B shows the simulation results of an adaptive inertia scheme for one embodiment of the UniCon scheme, and is a plot of variables D and J according to the aspect of the present invention.

[0075] [Figure 12] Figure 12 is a plot of virtual resistance change in response to current according to an embodiment of the present invention.

[0076] [Figure 13A] Figure 13A shows the simulation results of a 4-converter system according to an embodiment of the present invention. [Figure 13B] Figure 13B shows the simulation results of a 4-converter system according to an embodiment of the present invention.

[0077] [Figure 14A] Figure 14A shows the simulation results for a 4-converter system, illustrating the fluctuating inertia during load step / grid connection. [Figure 14B] Figure 14B shows the simulation results of a 4-converter system, illustrating the grid frequency response with and without inertia support from UniCon according to an embodiment of the present invention.

[0078] [Figure 15A] Figure 15A is a flowchart of the algorithm activated to initiate a phase jump algorithm that does not require quadrant jumps according to an embodiment of the present invention.

[0079] [Figure 15B] Figure 15B is a flowchart of a phase jump algorithm called by the algorithm shown in Figure 15A, which does not require quadrant jumps according to an embodiment of the present invention.

[0080] [Figure 15C] Figure 15C shows that the flowchart called by the phase jump algorithm shown in Figure 15B sets the variables to be checked by the phase jump algorithm when the reference voltage phase is maintained at approximately π after passing through the loop of the phase jump algorithm according to an embodiment of the present invention a predetermined number of times.

[0081] [Figure 16A] Figure 16A is a block diagram showing another embodiment of the UniCon strategy according to an aspect of the present invention.

[0082] [Figure 16B] Figures 16B and 16C are block diagrams showing two different implementations of the UniCon strategy shown in Figure 16A, according to an aspect of the present invention. [Figure 16C] Figures 16B and 16C are block diagrams showing two different implementations of the UniCon strategy shown in Figure 16A, according to an aspect of the present invention.

[0083] [Figure 17] Figure 17 is an electrical circuit diagram showing an example grid mode demonstrated in experimental operation as shown in Figures 18A-18C and 19A-19C, and in experimental tests as shown in Figures 20A-20E, according to an aspect of the present invention.

[0084] [Figure 18A] Figure 18A is a plot of power, voltage, and current during experimental operation between isolated operation, subsequent interconnected operation, and subsequent isolated operation of the three converter systems shown in Figure 17 according to an aspect of the present invention.

[0085] [Figure 18B] Figure 18B is a magnified view of the power, voltage, and current of the three converter system during the transition from isolated operation to interconnected operation, as shown in Figure 18A.

[0086] [Figure 18C] Figure 18C is a magnified view of the power, voltage, and current of the three converter systems during the transition from interconnected operation to isolated operation, as shown in Figure 18A.

[0087] [Figure 19A] Figure 19A includes plots of power, voltage, and current during islanded operation, followed by grid-connected operation, and then islanded operation, for experimental operation of the three converter systems shown in Figure 17 according to an aspect of the present invention.

[0088] [Figure 19B] Figure 19B is a magnified view of the power, voltage, and current of the three converter system during the transition from islanded operation to grid-connected operation as shown in Figure 19A.

[0089] [Figure 19C] Figure 19C is a magnified view of the power, voltage, and current of the three converter system during the switch from grid-connected operation to islanded operation, as shown in Figure 19A.

[0090] [Figure 20A] Figure 20A is a plot of experimental data, including voltage and current, for the three converters IBR1, IBR2, and IBR3 shown in Figure 17, during a failure condition, according to an embodiment of the present invention.

[0091] [Figure 20B] Figure 20B is a plot of the power and frequency of each converter IBR1, IBR2, and IBR3 corresponding to the scenarios examined in Figure 20A.

[0092] [Figure 20C]Figure 20C is a plot of the voltage and current of each of the three converters IBR1, IBR2, and IBR3 at the time the failure began, as shown in Figure 20A.

[0093] [Figure 20D] Figure 20D is a plot of the voltage and current of each of the three converters IBR1, IBR2, and IBR3 during fault ride-through, as shown in Figure 20A.

[0094] [Figure 20E] Figure 20E is a plot of the voltage and current of each of the three converters IBR1, IBR2, and IBR3 at the time the fault was resolved, as shown in Figure 20A. [Modes for carrying out the invention]

[0095] As used herein, the term “power electronic converter” is intended to include various types of power electronic converters used to perform a variety of functions (inverse conversion, rectification, etc.) with ratings ranging from a few milliwatts to several thousand kilowatts. Otherwise, the term “power electronic converter” is understood to have the clear and ordinary meaning as understood by those skilled in the art.

[0096] As used herein, the terms “about” or “approximately” for any number or range indicate a preferred tolerance that allows some or all of the components to function for their intended purpose as described herein. More specifically, “about” or “approximately” may refer to a range of values ​​of ±20% of the stated value. For example, “about 90%” may refer to a range of values ​​from 71% to 99%.

[0097] As used herein, terms such as “component,” “module,” “system,” “server,” “processor,” and “memory” are intended to include one or more computer-related units, and examples of computer-related units include, but are not limited to, hardware, firmware, hardware-software combinations, software, or running software. For example, one component may be, but is not limited to, a process, object, executable file, execution context, program, and / or computer running on a processor. For example, an application running on a computing device, and both the computing device and the application, may constitute one component. One or more components can exist within a process and / or execution context, and one component may be localized on one computer and / or distributed across two or more computers. Furthermore, these components can be executable from various computer-readable media containing various data structures. Components can communicate by local and / or remote processes according to signals having one or more data packets, such as data from one component interacting with another component, for example, in a local system, a distributed system, and / or over a network such as the Internet. Computer-readable media may be non-temporary media. Non-temporary computer-readable media include, but are not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable and programmable ROM (EEPROM), flash memory, or other memory technologies, compact disk ROM (CD-ROM), digital multipurpose disk (DVD), or other optical storage devices, magnetic cassettes, magnetic tapes, magnetic disk storage devices, or other magnetic storage devices, or other tangible physical media that can be used to store computer-readable instructions and / or data.

[0098] As used herein, the term “computing system” is intended to include standalone machines or devices, and / or combinations of machines, components, modules, systems, servers, processors, memory, sensors, user interfaces, computing device interfaces, network interfaces, hardware elements, software elements, firmware elements, and other computer-related units. For example, a computing system may include, but is not limited to, one or more general-purpose computers, purpose-specific computers, processors, portable electronic devices, portable electronic medical devices, stationary or semi-stationary electronic medical devices, or other electronic data processing devices.

[0099] As used herein, the term “non-temporary computer-readable medium” includes, but is not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable and programmable ROM (EEPROM), flash memory, or other memory technologies, compact disk ROM (CD-ROM), digital purpose disk (DVD), or other optical storage devices, magnetic cassettes, magnetic tapes, magnetic disk storage devices, or other magnetic storage devices, or other tangible physical media that can be used to store computer-readable information.

[0100] Documents referenced by reference in this patent application should be considered integral parts of this application, except in the following cases: to the extent that terms are defined in these referenced documents in a manner that is inconsistent with the definitions expressed or implied herein, only the definitions in this specification should be considered.

[0101] The components, steps, and materials described below as constituting various elements of the disclosed technology are intended for illustrative purposes only, not as limitations. Many preferred components, steps, and materials that perform the same or similar functions as those described herein are intended to be included within the scope of the disclosed technology. Such other components, steps, and materials not described herein may include, but are not limited to, similar components or steps developed after the development of the disclosed technology.

[0102] Here, a universal and centralized control (UniCon) strategy is provided for power electronics converters ("converters") configured to operate in a grid-connected manner.

[0103] Figure 1 is a flowchart showing the structure of the converter control structure 100. Utility / owner commands 102 are provided to a higher control layer 104 capable of performing, for example, transactive control, prediction, and / or energy management. The higher control layer 104 can provide data and / or commands to the intermediate control layer 106, for example, droop setting, grid synchronization, active power supply, and / or reactive power supply. The intermediate control layer 106 can provide, for example, inertia support, dynamic management of active or reactive power, black start microgrid control, transient management, and / or rapid fixing to a new setpoint. The intermediate control layer 106 can provide data and / or commands to the lower control layer 108. The lower control layer 108 can control, for example, internal current loops, internal voltage loops, and / or modulation. The lower control layer 108 can provide switching signals 110 to the converter's electronics.

[0104] In some embodiments, a converter controller may be implemented in the intermediate control layer 106. In some embodiments, real-time control of the converter may be entirely based on rules and local measurements that enable the converters to cooperate and to deal with a wide range of steady-state, transient, and fault conditions that the converters may experience.

[0105] Figure 2 is an electrical circuit diagram of a model grid 200 as an example to illustrate the technical requirements of the UniCon strategy. It shows a 2MW distribution feeder 202 with a first microgrid 220 and a second microgrid 240, which can operate in grid-connected mode or grid-island mode (i.e., by opening and closing switch 208), respectively. Microgrids 220 and 240 are connected to two converter-based distributed energy resources (DERs) 221, 222, 243, and 344, respectively. Each of these DERs enables each microgrid 220 and 240 to function in grid-connected mode, grid-island mode, and microgrid cluster mode. DERs 221, 222, 243, and 344 can be based on, for example, photovoltaic (PV) and storage.

[0106] The main grid 202 itself is powered by a synchronous generator with a short-circuit ratio (SCR) of 20 and an inertia constant (H) of 6 seconds. Additionally, a partially rated backup generator may be provided. This system is geographically distributed (a resilient community) and is assumed to operate with low-speed communications. While low-speed communications will be interrupted occasionally, the grid and microgrids must continue to function as suboptimally as possible. When the transmit feed 202 is lost (i.e., when switch 204 opens), a bottom-up black start must be possible for each microgrid 220, 240, and for the connection of the distribution feeder switch 206. Ultimately, general information is available on the network and source / load, but accurate real-time information cannot be assumed.

[0107] This is typically a real-world resilient feeder problem, presenting many challenges to conventional methods of controlling converters and microgrids. When grid-connected, DER221, 222, 243, and 244 operate in power-feed mode, working to maximize energy transfer from PV, optimize the use of energy storage resources, and provide support to the grid with Volt / VAR support or frequency regulation services. In the event of a planned disconnection or disconnection caused by a grid-side failure, microgrids 220 and 240 continue to operate, typically using power-frequency droop characteristics to balance the load by generating power. Operation in microgrid mode can occur with or without a backup gas generator, and with or without PV, and can occur as two separate microgrids or as a collective microgrid. However, many of these troublesome cases cause significant problems, especially when there is no communication between converters, system knowledge is limited, and control is based on local measurements and a set of common rules. Troublesome cases mainly include: • Support for grid inertia under high rate of change (ROCOF) conditions • When connecting the converter to the operating grid with minimal transients. • When connecting an operating microgrid to a grid with minimal transients. • When interconnecting two microgrids with minimal transients • When the system is quickly anchored to the PF curve with minimal transients after changes in power generation / load. • Dynamic management of P and Q, especially during transient phenomena (Q is also in the steady state). • Management of phase jumps that occur after a failure or under other conditions • When black-starting a microgrid and forming a bottom-up microgrid cluster • Interconnecting microgrids / grids at different frequencies and phases without major transients.

[0108] Here, exemplary embodiments of a controller are presented that realize a single control strategy for managing steady-state and transient requirements, including some or all of the aforementioned challenges, using elements such as high-speed phase jumps in the case of large-scale disturbances, dynamic virtual impedance under transients, nonlinearly fluctuating inertial implementations, and partial coupling of these elements. In some embodiments, grid voltage is used as the instantaneous primary parameter on which the converter reacts.

[0109] The primary objective of UniCon is to be applicable across a wide range of converter topologies and system / network applications, from high-voltage source converters (VSCs) connected to power transmissions to microgrids and microgrid clusters. In some embodiments, one objective of the UniCon strategy is to eliminate vendor constraints and restrictions on the implementation of internal voltage / current and protection loops (i.e., lower control layer 108). In some embodiments, most of the UniCon control is slower than typical converter internal loops and aims to achieve invariance. In some embodiments, one objective of the UniCon strategy is to generate interoperability across all types, ratings, and brands of grid-connected converters. In some embodiments, one objective of the UniCon strategy is to facilitate the further deployment of grid-connected converters.

[0110] Figure 3 is a block diagram showing an implementation 300 of one embodiment of the UniCon strategy, where the UniCon strategy includes a phase jump module 320, an adaptive inertia module 310, and a virtual impedance module 330. A block diagram of one embodiment of the UniCon strategy 1600 is shown in Figure 16A. The phase jump module 320 can be configured to achieve high-speed voltage phase synchronization depending on the interconnection of the converter with the grid, microgrid, and / or grid-forming converter (GFC). In some embodiments, the phase jump module 320 has a phase angle jump θjump It is possible to measure the power converter's reference voltage phase θ, which means that if the synchronization between the power converter's terminal voltage waveform and the power converter's reference voltage waveform is suddenly broken, the power converter's reference voltage phase θ REF The adaptive inertia module 310 can be configured to set the inertia constant of the power converter to synchronize the converter frequency with the grid, microgrid, and / or GFC. In some embodiments, the adaptive inertia module can be configured to set the converter's inertia constant based at least in part on the power difference between the power converter's terminal power and the converter's reference power. The virtual impedance module 330 can be configured to limit the power converter's current in fault ride-through mode. In some embodiments, the virtual impedance module can dynamically vary the effective output impedance of the power converter.

[0111] The UniCon strategy may further include a droop control module 301. In some embodiments, the droop control module 301 may utilize an inverse correlation between active power and frequency or other preferred droop control strategies, as will be understood by those skilled in the art.

[0112] The droop control module 301 allows setting the reference power for the converter. Reference power (internal to the converter) and actual power (converter output, terminal power, P meas The error between ( ) and ( ) is calculated by the addition function 302 and supplied to the nonlinear integrators 314 and 316 of the adaptive inertia module 310. The adaptive inertia module 310 also has a nonlinear attenuator 312. The outputs of the nonlinear integrators 314 and 316 and the nonlinear attenuator 312 are frequencies (angular frequencies), and the nominal angular frequency (ω) of the converter. *) are summed. Thereafter, the angular frequency (ω(t)) output by the frequency loop 340 passes through the integrator 303 to generate the phase angle (θ) of the reference voltage waveform of the converter. The phase jump algorithm of the phase jump module 320 directly affects the phase of the internal voltage (E) of the converter, skips the frequency loop 340, and as a result, results in a zero inertia response. The virtual impedance module 330 enables separation between active power control and reactive power control. The virtual impedance module 330 also enables reactive power control indirectly. The virtual impedance module 330 changes the internal voltage reference of the converter to emulate a series R and a series L. The reference voltage phase θ (determined by the phase jump module 320 and the frequency loop 340) REF forms the reference voltage waveform of the converter together with the nominal voltage amplitude (changed by the virtual impedance module 330). The frequency of the converter reference voltage waveform can drift and synchronize with the terminal voltage waveform to be adjusted by the adaptive inertia module. The final result of this control block is the reference voltage waveform, which can be supplied to the converter switching algorithm in the lower control module 308. In some embodiments, the lower control module 308 has a function similar to the lower control module 108 shown in FIG. 1.

[0113] The synchronization process for microgrids can be extremely challenging, and conventional strategies typically require time-consuming and precise adjustments. One strategy initiates the synchronization process by assuming that the converter's initial power is zero and that it ramps up after grid connection. For example, a virtual impedance is changed exponentially, and the current is effectively controlled during synchronization. Another example uses a switching-based technique that allows the converter to synchronize before the current ramps up. However, in many cases, the microgrid is operational, and the system cannot be shut down for synchronization purposes. Another strategy requires complete knowledge of the grid voltage for a seamless transition. While it is possible to synchronize individual units to connect them to the system (since the grid voltage is available and can be detected), in geographically distributed microgrids, many converters do not have access to the grid voltage, and to date, there has been no monitoring algorithm-independent method to enable seamless connection of microgrids in a fully distributed state. In some embodiments, the phase jump module 320 provides such a feature. In some embodiments, the phase jump module 320 enhances the performance of individual converters, making them "plug-and-play" devices. In some embodiments, the phase jump module 320 can enable the concept of a “fractal grid.” A “fractal grid” means that the grid can be divided into smaller sections during the major transient and rapid recovery after fault removal.

[0114] In one embodiment of the UniCon strategy, the synchronization process can be implemented in two parts: initial phase synchronization to limit voltage / current transients, and subsequent frequency synchronization. Phase synchronization can be facilitated by a phase jump module 320. Frequency synchronization can be facilitated by an adaptive inertia module 310.

[0115] In some embodiments, the phase jump module 320 can be configured to perform high-speed voltage phase synchronization and achieve minimum current transients at grid interconnects.

[0116] In some embodiments, the phase jump module 320 includes a phase jump algorithm configured to directly control the voltage angle of the converter's internal voltage waveform to effectively limit the current and move the system to the same phase as the grid with a zero-inertia response. Such functionality is not possible with existing strategies, which typically rely on current-limiting algorithms that can lead to current loop saturation or sustained transients.

[0117] Figures 4A and 4B contain conceptual diagrams of voltage phase shifts to minimize current transients at the grid connection, respectively. Figure 4A shows a conventional strategy, and Figure 4B shows one aspect of the UniCon strategy. As shown in Figure 4A, the conventional method is based on making the converter voltage amplitude (Vc) the same as the grid voltage (Vg). As illustrated, the converter is at an initial voltage amplitude 402, and the converter voltage moves to a new position 406 in the figure to limit the current from the converter. However, at the new position 406, the reference voltage waveform remains out of phase with the grid / terminal voltage waveform. This method minimizes current spikes during interconnection, but the transient persists for a while because the converter attempts to match its magnitude and phase with the grid. Conversely, the method used by the UniCon strategy shown in Figure 4B changes the phase of the converter voltage, thereby reducing not only instantaneous current spikes but also transients after phase synchronization. The converter voltage position 402 moved by an angle 410 to a new position 408 having the same phase angle as the grid voltage position 404. In the conventional method (Figure 4A), the converter does not change direction (always clockwise). In the method of the specific embodiment shown here (Figure 4B), the converter can reverse direction based on the difference between Vc and Vg. In some embodiments, the phase is changed exponentially (nonlinearly) to avoid oscillations.

[0118] Figures 5A and 5B each contain conceptual diagrams illustrating a series of voltage phase shifts, including quadrant jumps. Figure 5A shows a scenario in which the converter voltage and grid voltage are halved in opposite directions using the method shown in Figure 4B. In this scenario, Vc moves from the first position 502 at an angle of 510 to the second position 508, which is the same magnitude as Vg at position 504. In this scenario, since Vc and Vg are not in phase, transients are increased. To solve this problem, the concept of quadrant jumps is introduced, in which the voltage phase jumps to its supplementary value to control the current. Figure 5B shows Vc moving from the second position 508 to the third position 512, which is the supplementary value in the opposite quadrant, and converging to Vg at position 504.

[0119] In some embodiments, the UniCon strategy can detect the need for a phase jump and pick the correct direction for the phase change, the magnitude of the phase change at each time step, and the need for a quadrant jump.

[0120] Figures 4B, 5A, and 5B show only two possible scenarios. The UniCon strategy can be configured to synchronize the reference voltage phase with the terminal voltage phase, according to the strategies shown in Figures 4B, 5A, and 5B.

[0121] Figure 6 is an electrical circuit diagram of a basic 2GFC system, a model of two grids powered by GFCs, illustrating an embodiment of the UniCon strategy from the perspective of one of the GFCs. The second GFC in this model may also represent a single grid. The GFC may include the UniCon strategy 300 shown in Figure 3 at the intermediate control layer 106, as shown in Figure 1.

[0122] Figure 7A is a flowchart of algorithm 700 which is activated to initiate the phase jump algorithm 720 shown in Figure 7B. This flowchart shows an algorithm that can be executed by one of the GFCs shown in Figure 6 that does not communicate with other GFCs. For illustrative purposes, V in Figures 7A and 7B L1 , V L2 , I L1 , I L2 , V t1 , and V t2 The subscripts "1" and "2" have been omitted.

[0123] In block 702, the algorithm control counter variable (Phase_j_ctrl) can be set to zero.

[0124] In block 704, the inductor current (I L ) or inductor voltage (V L ) is the threshold (V L,th ,I L,th If the threshold exceeds, for example, 1.5 pu, then algorithm 700 can be activated, and algorithm 700 proceeds to block 706; otherwise, block 704 is repeated. In some embodiments, algorithm 700 may include a delay before proceeding to block 706 for noise reduction. For example, algorithm 700 may include a counter in block 704 that, once a threshold is reached, advances the algorithm to block 706.

[0125] In block 706, the algorithm control counter variable (Phase_j_ctrl) is incremented by 1.

[0126] In block 708, if the value of the algorithm control counter variable (Phase_j_ctrl) is 1, algorithm 700 proceeds to block 710; otherwise, the algorithm proceeds to block 714.

[0127] In block 710, the inductor voltage polarity variable (V L_pol) is the inductor voltage (V L Set it to be equal to the polarity of ). Alternatively, blocks 710 and 716 may be omitted.

[0128] In block 712, the quadrant of the converter's internal voltage phasor (θ), i.e., the reference phase angle, is determined. If the internal voltage phasor (θ) is in the second or third quadrant, the phasor (θ) will jump to its supplementary value in the first or fourth quadrant.

[0129] In block 720, the phase jump algorithm 720 is initiated.

[0130] Figure 7B is a flowchart of the phase jump algorithm 720, which is invoked by algorithm 700 shown in Figure 7A. The phase jump algorithm 720 uses an inductor voltage (V L ) or inductor current (I L To reduce the magnitude of the reference voltage phase (θ), REF It is possible to make the trajectory change to the optimal one.

[0131] In block 722, set the counter to zero.

[0132] In block 724, the phase angle jump (θ) is used for angle adjustment. jump ) is the inductor voltage (V L ), inductor current (I L ), and output voltage (V t It is calculated based on the function of ). Two exemplary methods for calculating this trajectory are presented as unrestricted examples.

[0133] Method 1: The purpose of this method is |(EV t Minimize |I| max The limitation is to this. In this method, the phase is derived using a polynomial function (such as tanh(x)) where the independent variable is the inductor voltage. Consider the basic equations of the converter:

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[0134] Method 2: The objective of this method is to minimize |I| and |(EV) t The limit is )|<Δ. In this method, state feedback control is applied to limit the current, as will be explained below. Here again, we consider the basic equation of the converter in equation (1). u(t) is

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[0135] In block 726, angle adjustment (θ jump If ) is in the first or fourth quadrant, angle adjustment (θ jump ) will be set to a negative value.

[0136] In block 728, the reference voltage phase (θ REF ) is calculated using the previous value, angular frequency × sampling time (Δt), and angle adjustment (θ). jump Set the total to ).

[0137] In block 730, the inductor current (IL ) and inductor voltage (V L ) and their respective thresholds (V L,th ,I L,th ) is compared. If both values ​​are below their respective thresholds, the phase jump algorithm 720 proceeds to block 731 and the counter is incremented. Note that the loop of the phase jump algorithm 720 is the reference voltage phase θ REF V L and I L It is configured to move in the direction of decreasing. For example, V t If an extremely serious fault exists where = 0, then the reference voltage phase θ REF It decreases to zero, V L Set this value to a value close to zero. Then, the condition for 730 is met, and the phase jump algorithm 720 begins the aggregation.

[0138] In block 731, the counter is incremented.

[0139] In block 732, the counter is compared to the counter threshold (counter_set). If the counter is greater than the counter threshold, the phase jump algorithm 720 proceeds to block 734 and terminates. If the counter is less than the counter threshold, the phase jump algorithm 720 proceeds to block 724 and repeats the loop.

[0140] Returning to the main algorithm 700 in Figure 7A, once the phase jump algorithm 720 has finished in block 734, the main algorithm 700 proceeds to block 704.

[0141] In block 704, the inductor current (I L ) or inductor voltage (V L ) any of those thresholds (V L,th ,I L,thIf it is greater than (I), the main algorithm 700 proceeds to block 706. Note that the condition in block 704 is initially NO when the phase jump algorithm 720 finishes. However, the loop continues to operate. If the change in the phase of the reference voltage due to the phase jump algorithm 720 does not synchronize the reference voltage waveform with the terminal voltage waveform, the current and / or voltage (I L and / or V L ) rises again immediately, and the condition in block 704 becomes YES. The Ph_j_ctr value is determined when the loop is executed multiple times, and the current and / or voltage (I L and / or V L It is predetermined to increase the probability that an increase will occur.

[0142] In block 706, the control counter variable (Phase_j_ctrl) is incremented by 1, so that it becomes 2 the second time the loop is executed.

[0143] In block 708, the control counter variable (Phase_j_ctrl) is not 1 when the loop is executed for the second time, so algorithm 700 proceeds to block 714.

[0144] In block 714, since the control counter variable (Phase_j_ctrl) is 2, the method proceeds to block 716; otherwise, method 700 proceeds to block 740.

[0145] In block 716, the inductor voltage polarity variable (V L _pol) and inductor voltage (V L Compare the polarity of ( ). If they are the same, method 700 proceeds to block 718; otherwise, method proceeds to block 740. Alternatively, blocks 710 and 716 may be omitted.

[0146] In block 718, the quadrant and reference phase angle of the converter's internal voltage phasor (θ) are determined. If the reference voltage phase (θ) is in the first or fourth quadrant, the reference voltage phase (θ) will jump to its supplementary value in the second or third quadrant. Then, the process proceeds to the phase jump algorithm 720, as shown in Figure 7B.

[0147] In block 740, a fault ride-through algorithm using the virtual impedance module 330 is started. Thus, the fault ride-through algorithm is started when two or more loops of the main algorithm 700 have completed. In embodiments including optional blocks 710, 716, the fault ride-through algorithm is started during the second loop between the first and second loops of the main algorithm 700. L It can be initiated when the polarity changes.

[0148] This procedure leads to the smooth reconnection of two or more microgrids. Nevertheless, if the other is a robust grid or if a fault causes a sharp rise in current, further steps are required to suppress the transient. In the first case (Ph_j_ctr==1 in block 708), under certain conditions, i.e., when the grid voltage is in the second or third quadrant (block 712), the voltage / current may exceed the limit, and the phase jump algorithm 720 will be activated for the second time (Ph_j_ctr==2 in block 714, proceeding to 720). This time, the phase jumps to the second / third quadrant where the grid voltage vector is located (block 718). Again, when the current rises for the third time (Ph_j_ctr==3), it means that it is a fault condition, not a synchronous transient. Therefore, the fault ride-through algorithm (block 740) will be activated.

[0149] Once the current and voltage are controlled, the main algorithm 700 terminates, and the frequency loop control 340 (Figure 3) starts, setting the reference voltage phase (θ REF Control of the ) is performed again. At this time, both sides (for example, E in Figure 6)inv1 and E inv2 Although the angles are the same, the frequency values ​​are different. To minimize transient phenomena caused by the different frequency values ​​and to smoothly converge them, the adaptive inertia scheme of the adaptive inertia module 310 is used.

[0150] Figures 8A to 8G show the simulation results for a two-converter connection. To demonstrate the functionality of algorithm 700 of the phase jump module 320, the two-converter system shown in Figure 6 is simulated in MATLAB Simulink. Figure 8A is a plot of the load voltage of the first converter. Figure 8B is a plot of the load voltage of the second converter. Figure 8C is a plot of the current of the first converter. Figure 8D is a plot of the current of the second converter. Figure 8E is a plot of the internal voltage of the converters. Figure 8F is a plot of the output power of the converters. Figure 8G is a plot of the frequency of the converters.

[0151] Each converter supplies power to its own local load, and the switch is closed at t=1.485s. The rating of the first converter (120V, 30A) is twice as high as the rating of the second converter (120A, 15A). As shown in Figures 8A to 8E, when connected, one converter has a positive peak, while the other converter has a negative peak, and therefore the voltage across the inductor is at its maximum, i.e., 2pu. In this case, since converter 2 has smaller dimensions, it reaches its threshold earlier and therefore jumps, minimizing the transient. Figures 8A and 8B demonstrate that the voltage across the load passes through a high-frequency transient for 1 / 8 of the cycle and is then maintained in a stable state. Figures 8C and 8D demonstrate that the currents of both converters are within their peak ratings. Once the phase jump is completed by the phase jump module 320, the frequency loop 340 begins frequency regulation, and power sharing follows the droop curve, as shown in Figures 8F and 8G. This result fully demonstrates that the two GFCs can be interconnected while limiting voltage and current transients.

[0152] In some embodiments, the synchronization process may have two parts: initial phase synchronization to limit voltage / current transients, regulated by a phase jump module 320; and subsequent frequency synchronization, regulated by a frequency loop 340 including adaptive inertia modules 314, 316 and a nonlinear damping module 312. The first part of the regulation by the phase jump module 320 is as previously disclosed. The second part, by the frequency loop 340 including the adaptive inertia module 314 and the nonlinear damping module 312, is disclosed below.

[0153] The technologies used to form grids at the current level of technology are usually designed to mimic the operation of synchronous generators (SGs) and are called virtual synchronous generators (VSGs). However, VSGs inherit the same drawbacks as SGs, such as oscillating responses during transients. Choosing a large value for the virtual moment of inertia provides better inertial support and effectively limits the ROCOF, but conversely, it results in oscillating and slow response. On the other hand, choosing a low value for J results in a faster transient response, but increases overshoot and violates the tolerance range of the ROCOF.

[0154] To circumvent this problem, a "variable inertia" has been proposed that effectively controls the lowest frequency point to dampen vibrations. In one example, a bang-bang control scheme is used to control Δω and dω m The constant of inertia is changed based on the value of / dt. The problem is that measuring these values ​​is not easy. dω m Measuring / dt requires suppressing noise amplified by the derivative, while measuring Δω requires knowledge of the grid-side frequency, which is particularly difficult during transients. The algorithm also ensures the system's trajectory passes through a desired setpoint and then performs actions to improve suboptimal responses.

[0155] Recently, this principle has been improved by simultaneously changing D and J, or by introducing a continuously changing J. The latter is dω m While this improves the control scheme by eliminating the need for / dt measurement, its drawback is that it assumes a constant grid frequency of 60 Hz. This is likely to occur for grid-connected operation, but it is not an accurate assumption for isolated microgrids where the frequency changes very frequently. Several experiments have demonstrated that when the grid frequency is changed from the nominal value, the response becomes extremely slow. One objective of some embodiments of the UniCon strategy is to further improve the operation of the VSG by introducing a nonlinear adaptive inertia scheme that dynamically varies the gain based on the system response. Grid frequency measurement is not required, and the system can attenuate the response to optimally move to a new operating point regardless of the grid frequency.

[0156] A large inertia constant (J) creates extraneous oscillations, while a small J leads to a large low frequency point. To avoid these problems, the UniCon strategy allows the system to be fast when the frequency is far from a new setpoint (which can be any number), and gradually decrease in response speed as the system approaches the new setpoint. This can be achieved by selecting a nonlinear curve for J, where the value changes smoothly based on the error. Furthermore, the damping coefficient (D) can be nonlinear. A polynomial function is a good candidate with this property.

[0157] Figure 9A shows an example of a suitable nonlinear inertia constant (J) for the variable inertia module 314 of the UniCon Strategy 300. In some embodiments, the inertia constant (J) includes a polynomial function. Additionally or alternatively, adaptive inertia uses two variables, namely P, to prevent the converter from being overloaded. rated And it can be a function of ΔP.

[0158] Figure 9B shows an example of a nonlinear damping coefficient (D). A model of a virtual synchronous generator can be constructed using an RC circuit. Here, the values ​​of C and R are proportional to J and D. In this case, the swing equation corresponds to the resonance that occurs between the virtual C and the inductive component of the circuit. In an RLC circuit, because it has a deterministic damping response for frequency, the virtual R is...

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[0159] Figure 10A is an electrical circuit diagram of an example grid model used to illustrate the operation of implementation mode 1000 of one embodiment of the UniCon strategy shown in Figure 10B.

[0160] Figure 10B is a block diagram of implementation 1000 of one embodiment of the UniCon strategy, with the phase jump module 320 and virtual impedance module 330 omitted. Implementation 1000 includes components and modules 1001, 1002, 1003, 1004, 1008, 1010, 1012, 1014, and 1016, configured similarly to the corresponding components and modules 301, 302, 303, 304, 308, 310, 312, 314, and 316 shown in Figure 3. A droop control module 1001 is employed to move the system to a new steady-state operating point after the initial high-speed transient.

[0161] To verify the effectiveness of the variable inertia scheme, the system shown in Figure 10A and the control scheme shown in Figure 10B are simulated in MATLAB Simulink. The scenario is as follows: At t=1s, a step change occurs in the grid frequency, with the frequency jumping from 60Hz to 60.3Hz. Then, at t=2s, the grid frequency returns to its nominal value. One embodiment of this scenario is when two or more grid forming converters are isolated due to failure and need to be synchronized and reconnected after fault removal. In such a scenario, frequency drift occurs between the grid forming converters, and the frequency step change can be effectively observed at their terminals.

[0162] Figure 11A shows the results for small gain 1103, large gain 1102, and adaptive inertia 1101. As can be seen, the variable inertia scheme exhibits a damping response. Here, this motion is fast when the error is large, but slows down as it approaches the setpoint.

[0163] Figure 11B shows how the inertia and damping gains change based on the response. Due to the nonlinearly fluctuating inertia, the system's step response characteristics are better than those of a constant gain because a constant gain does not reconcile with the compromises that designers must make when using a constant gain.

[0164] Another crucial element of converter control is the ability to overcome faults and support the grid by supplying sufficient current. To achieve this, a fluctuating virtual resistor 334 is used. This method helps the converter avoid entering saturation mode and operate within its linear range. If a fast transient is present in the system, UniCon300 attempts to manage it first by using the phase jump module 320. If several attempts fail, algorithm 700 detects this as a fault condition and activates fault ride-through algorithm 740, which increases the virtual resistor 334 to limit the current. Once the voltage returns to normal, the virtual resistor 334 decreases again to its nominal value. In a steady state, the virtual inductor 332 separates P and Q for better power sharing during the initial transient. The virtual inductor can change during the transient to allow the system to inject a stable sinusoidal current.

[0165] The virtual resistor consists of two terms, namely R inst and R flt It consists of R. inst It changes rapidly to limit the instantaneous current, thereby protecting the semiconductor switch. flt This changes along with the virtual inductor based on the integrator, allowing the converter to inject a stable current during a fault.

[0166] Figure 12 shows the first term R of a virtual resistor that changes with current in one embodiment. inst This is the plot.

[0167] Second term R flt This changes at the same rate as the virtual inductor, allowing the converter to inject a stable current during a fault. When the root mean square (RMS) current exceeds a threshold, the virtual resistor and inductor change according to equations 5 to 7 shown below.

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[0168] Returning to Figure 2, to better verify the performance of the UniCon strategy in a real-world scenario, the system, including the four converters 221, 222, 243, and 244 shown, was created in MATLAB Simulink. The UniCon system 300 shown in Figure 3 is run as the main controller. The simulation uses the parameters shown in Figure 2.

[0169] Figures 13A and 13B show the simulation results. Figure 13A shows the power, current, output voltage, and frequency of the four converters 221, 222, 243, and 244. Figure 13B shows the internal voltages of converters 221, 222, 243, and 244.

[0170] At t=0s, all converters begin to ramp up and supply power to the local load. The first and second converters 221 and 222 form the first microgrid 220, and the second microgrid 240 includes the third and fourth converters 243 and 244.

[0171] At t=1.2s, both microgrids 220 and 240 are connected to switch 208, forming a larger microgrid. Note that none of the converters 221, 222, 243, and 244 have prior information regarding the possible interconnections of microgrids 220 and 240.

[0172] At t=2.5s, the load is applied to the system in stages.

[0173] At t=4.5s, a fault occurs, and the system enters a fault ride-through state until the fault is resolved at t=4.8s.

[0174] Next, the system begins to decrease the virtual resistance, restoring the voltage and synchronizing the converter again by adaptive inertia.

[0175] Finally, the microgrid is connected to the grid at t=7s.

[0176] The simulation results show that the system can manage all transient phenomena and smoothly transition between different modes of operation.

[0177] Figure 14A shows how the inertia constant J and the damping coefficient D change to attenuate the frequency response at t=2.5s and t=7s between two case scenarios (load change and grid connection).

[0178] Figure 14B shows the grid frequency response in another scenario, where a large converter is simulated as the grid, with a UniCon-controlled converter added and supported by its inertial response. It is observed that the second converter can not only improve the lowest frequency point by varying its inertial constant, but also attenuate the response to eliminate oscillations in the grid. In summary, the simulation results demonstrate the effectiveness of UniCon in various scenarios.

[0179] Figure 15A is a flowchart of algorithm 1500 which is activated to initiate the phase jump algorithm 1520 shown in Figure 15B, and the phase jump algorithm 1520 utilizes the reference voltage phase status check algorithm 1550. Flowchart 1500 shows an algorithm that can be executed by one of the GFCs shown in Figure 6 without communicating with the other GFCs. For illustrative purposes, V in Figures 15A to 15C L1 , V L2 , I L1 , I L2 , V t1 , and V t2 The subscripts "1" and "2" have been omitted.

[0180] Figures 15A to 15C show algorithms that achieve a phase jump without requiring a quadrant phase jump and can be used as an alternative to algorithm 700 shown in Figures 7A and 7B.

[0181] In Figure 15A, blocks 1502, 1504, 1506, 1508, and 1540 can be configured in the same way as the corresponding blocks 702, 704, 706, 708, and 740 of the main algorithm 700 shown in Figure 7A. The main algorithm 1500 is shown in Figure 15A. The main algorithm 1500 shown in Figure 15A does not have the voltage polarity setting and verification blocks 710, 716 and the quadrant jump blocks 712, 718 of the main algorithm 700 shown in Figure 7A. Note that the polarity setting and verification blocks 710 and 716 are optional features of the main algorithm 700 shown in Figure 7.

[0182] Figure 15B shows a phase jump algorithm 1520 called by the main algorithm 1500 shown in Figure 15A. In Figure 15B, blocks 1522, 1524, 1528, 1530, 1531, 1532, and 1534 can be configured similarly to the corresponding blocks 722, 724, 730, 731, 732, and 734 shown in Figure 7B. The phase jump algorithm 1520 shown in Figure 15B does not have a block corresponding to the phase jump block 726 of the phase jump algorithm 720 shown in Figure 7B. The phase jump algorithm 1520 shown in Figure 15B includes blocks 1536, 1550, 1538, and 1539 that do not have a corresponding block in the phase jump algorithm 720 of Figure 7B.

[0183] In block 1536, dθ REF / dt is measured and stored. Block 1536 is executed following the increment counter block 1531.

[0184] In block 1550, θ REFCall the status check algorithm 1550. The status check algorithm 1550 is called every time through the loop of the phase jump algorithm 1520 when the block 1530 is YES (that is, when V L and I L are each less than their respective threshold values). When θ REF comes close to π through several calls to the status check algorithm 1550, θ REF the status check algorithm 1550 sets the status check "Stuck_status" to 1. When θ[[ID=]] REF the status check algorithm 1550 sets the status check "Stuck_status" to 1.

[0185] Figure 15C is a diagram showing the flowchart of the status check algorithm 1550. REF Figure 15C is a diagram showing the flowchart of the status check algorithm 1550.

[0186] In block 1552, if the absolute value |θ REF - π| is less than the threshold value ε, increment the counter "stuck_ctr".

[0187] In block 1554, if the counter "stuck_ctr" is greater than a predetermined number "stuck_set", set the status check "Stuck_status" variable to 1.

[0188] In block 1556, when the status check algorithm 1550 for θ REF ends, the phase jump algorithm 1520 shown in Figure 15B proceeds to block 1532. If the condition of block 1532 is YES, the loop ends and the phase jump algorithm 1520 proceeds to block 1538.

[0189] In block 1538, if the latest measurement of dθ REF / dt obtained in block 1536 is less than zero, θ jump is set to π - θ jump , and θ REF will jump to the next quadrant.

[0190] In block 1539, when "Stuck_status" is 1 (set by block 1554 in FIG. 15C), θ REF is set to zero. Thus, regardless of whether a quadrant jump occurred in block 1538, θ REF is, θ REF maintained at a value close to π after the number "stuck_set" of calls to the status check algorithm 1550, then θ REF is set to zero in block 1539.

[0191] FIG. 16A is a block diagram of another embodiment of the UniCon strategy 1600. FIGS. 16B and 16C are block diagrams of two different implementations of the UniCon strategy 1600. The UniCon strategy 1600 can be implemented in the middle layer and can be added to the various different implementations shown in FIGS. 16B and 16C. This includes control techniques without a current controller as shown in FIG. 16B and control techniques with an additional control loop such as the current controller shown in FIG. 16C.

[0192] Figure 17 is an electrical circuit diagram of an example grid model, which was tested by experiments as shown in Figures 18A-18C and 19A-19C, as well as by experiments as shown in Figures 20A-20E. The example grid includes three converters IBR1, IBR2, and IBR3. The example grid includes a first microgrid (microgrid 1) including first and second converters IBR1 and IBR2, and a second microgrid (microgrid 2) including a third inverter IBR3. Converters IBR1, IBR2, and IBR3 do not communicate with each other. Figures 18A-18C show that two microgrids seamlessly interconnect and disconnect without communication between them. Figures 19A-19C show seamless grid connection and grid disconnection of a microgrid containing three converters. Figures 20A-20E show smooth and stable operation of the converters during a failure.

[0193] Figure 18A plots the power, voltage, and current of the three converter system during isolated operation, followed by interconnected operation, and then isolated operation again. The grid connection switch remains open, while the microgrid interconnection breaker is opened after being closed in Figure 17.

[0194] Figure 18B is a magnified view of the power, voltage, and current of the three-converter system during the transition from isolated operation to interconnected operation as shown in Figure 18A. During isolated operation, the two microgrids are isolated from each other and connected to interconnected operation. In isolated operation, the voltage waveform V1 of the first microgrid is not in phase with the voltage waveform V2 of the second microgrid. The currents I1, I2, and I3 of the three converters IBR1, IBR2, and IBR3 are illustrated. The powers P1, P2, and P3 of the three converters IBR1, IBR2, and IBR3 are illustrated.

[0195] Figure 18C is an enlarged view of the power, voltage, and current of the three-converter system during the switching from interconnected operation to isolated operation as shown in Figure 18A. During interconnected operation, the two microgrids are connected to each other, and during isolated operation, they are switched to be isolated from each other. The currents I1, I2, and I3 of the three converters IBR1, IBR2, and IBR3 are shown. The powers P1, P2, and P3 of the three converters IBR1, IBR2, and IBR3 are shown.

[0196] Figure 19A includes plots of power, voltage, and current for the three converter system during islanded operation, followed by grid-connected operation, and then the subsequent islanded operation. The grid-connected switching maintains a closed state and is subsequently opened, while the microgrid interconnect breaker remains closed in Figure 17.

[0197] Figure 19B is a magnified view of the power, voltage, and current of the three-converter system during the transition from islanded operation to grid-connected operation, as shown in Figure 19A. During islanded operation, the two microgrids are connected to each other and the grid connection switch is open. The grid connection switch is closed, connecting the two microgrids to the grid in grid-connected operation. In islanded operation, the microgrid voltage waveform uGV is not in phase with the grid voltage waveform GV. For approximately one cycle, the voltage waveform of the microgrid phase jump uVG is synchronized with the grid voltage waveform GV. The currents I1, I2, and I3 of the three converters IBR1, IBR2, and IBR3 are shown. The powers P1, P2, and P3 of the three converters IBR1, IBR2, and IBR3 are shown.

[0198] Figure 19C is an enlarged view of the power, voltage, and current of the three converter system during the transition from grid-connected operation to islanded operation, as shown in Figure 19A. During grid-connected operation, the two microgrids are connected to the grid, and then in islanded operation, the grid-connected switches are opened, isolating the microgrids from the grid. The currents I1, I2, and I3 of the three converters IBR1, IBR2, and IBR3 are shown. The powers P1, P2, and P3 of the three converters IBR1, IBR2, and IBR3 are shown. The voltage waveforms uGV and GV of the grid and microgrids are shown.

[0199] Figure 20A is a plot of experimental data, including voltages and currents, for the three converters IBR1, IBR2, and IBR3 isolated from the grid during a fault condition when a fault occurs between the first converter IBR1 and the second converter IBR2 while the third converter IBR3 is connected to the first converter IBR1 by the connection between the first and second microgrids. Once this fault begins, the system overcomes the fault and resolves it. The adapted virtual impedance, along with the adaptive inertia scheme, enables smooth and stable operation of the inverter during the fault.

[0200] Figure 20B is a plot of the power and frequency of each converter IBR1, IBR2, and IBR3 corresponding to the scenarios examined in Figure 20A.

[0201] Figure 20C is a plot of the voltage and current of each of the three converters IBR1, IBR2, and IBR3 at the time the failure began, as shown in Figure 20A.

[0202] Figure 20D is a plot of the voltage and current of each of the three converters IBR1, IBR2, and IBR3 during fault ride-through, as shown in Figure 20A.

[0203] Figure 20E is a plot of the voltage and current of each of the three converters IBR1, IBR2, and IBR3 when the fault was resolved, as shown in Figure 20A.

[0204] Here, an adaptive inertia scheme for the GFC is disclosed. This scheme may enable the converter to seamlessly connect to a grid or another converter and handle load disturbances with minimal voltage / current transients and a damped frequency response. Here, adaptive inertia corresponds to the GFC's ability to adapt its operation from a zero-inertia system to a highly inertia system as needed and based only on local information. To achieve this adaptive inertia operation, the UniCon strategy includes two strategies: a first strategy which includes a fast phase jump strategy (Figures 7A and 7B) that limits voltage / current transients by achieving voltage phase synchronization with the grid / converter immediately after interconnection; and a second strategy which includes a nonlinear controller (Figure 9) based on nonlinear (integrator) inertia and a nonlinear damping scheme that achieves a damped frequency response after interconnection to the grid / converter or changes in load.

[0205] The ability to control the grid-forming converter by a control scheme is also disclosed. This ability includes p-ω droop control, a nonlinear integrator (inertia) and nonlinear damping, a fast phase change block, and a virtual impedance block.

[0206] The feasibility of the proposed scheme was demonstrated through simulations and experimental results.

[0207] In some embodiments, the UniCon strategy can be implemented in hardware (e.g., a chipset) that can be integrated into a broad converter configuration. In some embodiments, the UniCon strategy can be implemented in software or firmware that can be applied to a broad converter configuration. Some embodiments include a converter that implements the UniCon strategy.

[0208] It will be understood that the embodiments and claims disclosed herein are not limited in their intended use to the structural and arrangement details of the components described herein and shown in the drawings. Rather, this specification and the drawings provide examples of conceivable embodiments. The embodiments and claims disclosed herein are further possible in other forms and can be implemented and performed in various ways. It will also be understood that the expressions and terms used herein are for illustrative purposes only and should not be considered to limit the claims. The embodiments disclosed herein are not limited to any particular feature of any UniCon Strategy disclosed herein. Specifically, an embodiment may include individual modules, their components, or methods associated therewith. As will be understood by those skilled in the art, compatible features of the disclosed embodiments are combinable. As will be understood by those skilled in the art, features of the embodiments disclosed herein are combinable with features not disclosed herein.

[0209] Therefore, it will be apparent to those skilled in the art that the principles on which the present application and claims are based can be readily used as the basis for the design of other structures, methods, and systems to accomplish the embodiments and some of the objectives of the claims described herein. It is therefore important that the claims be considered to include such equivalent structures.

[0210] Furthermore, the purpose of the abstract is to enable the United States Patent and Trademark Office and the public (in particular, those skilled in the art who are unfamiliar with patent and legal terminology and language) to quickly determine the essence and principles of the technical disclosure of this application upon reading it. The abstract is not intended to define the claims of this application, nor to limit the scope of the claims in any way. Rather, it is intended that the disclosed technology is defined by the attached claims.

Claims

1. A converter controller configured to control a converter connected to a grid in a mode based solely on local measurements, without relying on information about the grid, A phase jump module configured to set the reference voltage phase of the converter in synchronization with the terminal voltage waveform of the converter with zero inertia, An adaptive inertia module configured to set the inertia constant of the converter based at least in part on the power difference between the terminal power of the converter and the reference power of the converter, A converter controller including a virtual impedance module that dynamically varies the effective output impedance of the converter.

2. The converter controller according to claim 1, further configured to control the converter in grid formation mode without relying on measured values ​​of the frequency of the converter terminal voltage.

3. The aforementioned converter controller The converter is further configured to control another converter, microgrid, or island mode after being disconnected from the network including the grid in the grid formation mode, or The converter is further configured to synchronize with another converter, microgrid, or one or more of the grids in the grid formation mode. The converter controller according to claim 2, wherein one or more of the above are performed.

4. The converter controller according to claim 2, wherein the converter controller is further configured to synchronize the converter with one or more other converters, microgrids, or grids in the grid formation mode, having different frequencies and phases but not exceeding the ratings of the converter in the grid formation mode.

5. The converter controller according to claim 1, wherein the converter controller is further configured to control the converter in a state where power generation / load is changing.

6. The converter controller according to claim 1, further configured to dynamically manage active and reactive power during transient phenomena.

7. The converter controller according to claim 1, further configured to synchronize the reference voltage phase of the reference voltage waveform of the converter with the terminal voltage phase of the terminal voltage waveform of the converter in accordance with the phase jump of the terminal voltage phase.

8. The converter controller according to claim 1, wherein the converter controller is further configured to black start the microgrid including the converter.

9. The converter controller according to claim 1, further configured to manage active and reactive power sharing in the fluctuating short-circuit ratio and / or X / R (reactance / resistance) ratio of the grid's synchronous generators.

10. The converter controller according to claim 1, further configured to control the converter in grid formation mode to synchronize the converter with a microgrid and / or grid.

11. The converter controller according to claim 1, wherein the phase jump module is further configured to set the reference voltage phase to the phase of the terminal voltage waveform by zero inertia response.

12. The aforementioned phase jump module is The voltage difference between the terminal voltage waveform and the reference voltage waveform of the converter is higher than the voltage threshold, or The converter controller according to claim 1, used in accordance with the fact that the current flowing through the filter inductor of the converter is greater than a current threshold.

13. The converter controller according to claim 1, wherein the phase jump module is further configured to synchronize the reference voltage waveform of the converter with the terminal voltage waveform without changing the voltage amplitude of the reference voltage waveform of the converter.

14. The converter controller according to claim 1, wherein the phase jump module is further configured to nonlinearly adjust the reference voltage phase.

15. The converter controller according to claim 1, wherein the phase jump module is further configured to jump the reference voltage phase to the opposite quadrant in order to prevent the reference voltage waveform and the terminal voltage waveform of the converter from moving synchronously in opposite directions.

16. The aforementioned phase jump module is The phase angle jump is determined at least partially based on the voltage difference between the reference voltage waveform of the converter and the terminal voltage waveform, and / or the current flowing through the filter inductor of the converter, and / or the terminal voltage of the terminal voltage waveform, The converter controller according to claim 1, further configured to set the reference voltage phase such that the phase angular jump and the product of the angular frequency of the reference voltage waveform and the sampling time are added to the reference voltage phase.

17. The phase angle jump is a function of the voltage difference between the reference voltage waveform and the terminal voltage waveform (f), (i.e., f(E-V)) ｔ It is proportional to )) and The converter controller according to claim 16, wherein the function (f) is proportional to a constant, a linear function, or a nonlinear function.

18. The above function (f) is the hyperbolic tangent (tanh(E-V) ｔ The converter controller according to claim 17, including )).

19. The frequency loop including the adaptive inertia module is configured to provide an output phase angle (θ(t)), The aforementioned phase angle jump is, [Math 1] The converter controller according to claim 16, which is equal to (where i(t) is the filter inductor current, Vt is the terminal voltage waveform, and E is the reference voltage waveform).

20. The converter controller according to claim 16, wherein the phase jump module is further configured to repeat the step of determining the phase angle jump by setting the reference voltage phase until the phase jump module is removed.

21. The converter controller according to claim 16, wherein the derivative of the terminal voltage angle is measured, and the derivative is used to determine whether a jump to the opposite quadrant is necessary.

22. The converter controller according to claim 16, which constantly observes the terminal voltage angle and sets the terminal voltage angle to zero when the terminal voltage angle approaches π in a predetermined set time.

23. The aforementioned phase jump module is The voltage difference falls below the voltage threshold, and / or the current flowing through the inductive impedance falls below the current threshold. The adjustment of the reference voltage phase is performed a predetermined number of times. A predetermined amount of time has elapsed since the use of the phase jump module, or The converter controller according to claim 16, which is removed in accordance with at least one of the following: the step of determining the phase angle jump by setting the reference voltage phase is performed a predetermined number of times.

24. The converter controller according to claim 1, further comprising a droop control module configured to set the reference power of the converter.

25. The converter controller according to claim 24, wherein the droop control module is further configured to set the reference power of the converter on at least partly the inverse correlation between the active power and frequency of the converter.

26. The converter controller according to claim 24, wherein the droop control module is located in a feedback loop with the adaptive inertia module.

27. The converter controller according to claim 24, wherein the difference between the output of the droop control module and the terminal power is provided as an input to the adaptive inertia module.

28. The converter controller according to claim 24, wherein the sum of the output of the adaptive inertia module and the reference angular frequency of the reference voltage waveform is provided as input to an integrator to generate an angle.

29. The converter controller according to claim 1, wherein the adaptive inertia module is further configured to synchronize the internal frequency and phase of the reference voltage waveform with the terminal frequency and phase of the terminal voltage waveform without the need to measure the frequency of the terminal voltage waveform.

30. The adaptive inertia module uses the inertia constant as follows: The measured power of the converter, The difference between the aforementioned reference power and the measured power, or The converter controller according to claim 1, further configured to be set based solely on the terminal voltage amplitude of the terminal voltage waveform.

31. The converter controller according to claim 1, wherein the constant of inertia is at least partially based on a nonlinear function of the power difference.

32. The converter controller according to claim 1, wherein the inertia constant is at least partially based on a polynomial function of the power difference.

33. The converter controller according to claim 1, wherein the attenuation coefficient of the converter is at least partially based on the power difference.

34. The converter controller according to claim 1, wherein the attenuation coefficient of the converter is at least partially based on the nonlinear function of the power difference.

35. The converter controller according to claim 1, wherein the damping coefficient of the converter is proportional to the inverse square root of the constant of inertia.

36. The converter controller according to claim 1, wherein the virtual impedance module includes an adaptive virtual impedance configured to increase impedance in response to an increase in current.

37. The converter controller according to claim 36, wherein the virtual impedance module includes a virtual resistor configured to vary its resistance using a nonlinear function to limit the instantaneous current flowing through the filter inductor of the converter to below a predetermined threshold.

38. The converter controller according to claim 37, wherein the virtual impedance module further includes a virtual inductor.

39. The virtual impedance module includes a virtual resistor and a virtual inductor, The converter controller according to claim 36, wherein the inductance of the virtual inductor and the resistance of the virtual resistor are configured to increase in response to the current exceeding a predetermined threshold using an integrator.

40. The virtual impedance module includes a virtual resistor and a virtual inductor, The converter controller according to claim 36, wherein the inductance of the virtual inductor and the resistance of the virtual resistor are configured to decrease as the current becomes smaller than a predetermined threshold using an integrator.

41. The frequency loop, including the adaptive inertia module, is configured to provide an output frequency. The converter controller according to claim 1, wherein the output frequency is maintained constant in freeze mode when the terminal voltage falls below a predetermined value for a predetermined period of time.

42. The converter controller according to claim 41, wherein the output frequency exits the freeze mode when the terminal voltage becomes higher than a predetermined value for a predetermined period of time.

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