Unified 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 TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Application No. 63 / 147,630, filed February 9, 2021, the entirety of which is incorporated by reference herein as if set forth in full below.

[0002] (Technical field) The present invention relates generally to power electronics and control thereof, and more particularly to an apparatus and method for centralized control of grid-connected converters. [Background technology]

[0003] The grid is rapidly moving from a system entirely powered by synchronous generators to one with a large penetration of converters fed from distributed energy resources. At low levels of converter penetration, it is reasonable to assume a robust grid and control the converters with a grid-aligned strategy. As converter penetration increases, the same converters need to operate sometimes in grid-connected mode and other times in grid-island mode, which makes control difficult when the converters need to change operating modes. Furthermore, converters behave very differently from generators, especially during transient and fault conditions, often requiring significant changes in control strategies for these two separate modes. Also, it may not be easy to distinguish between the two modes. Real-time control and regulation communication could in principle help to alleviate this problem, but it requires low-latency communication and accurate system knowledge, which is extremely difficult in a geographically distributed and constantly changing system.

[0004] Examples of major issues that can occur with a high penetration of inverter-based resources (IBR) include lack of inertia, converter interaction, reduced system stability, and increased possible mismatches. Converter control often has to deal with resonance, transient overloads leading to current limiting operation, and loss of control over the rest of the system. Major faults and network changes can create system transients that can cause circuit disconnects or oscillatory motions, which can be problematic for protection systems. Finally, the converter to be controlled may not be sized relative to the system to which it is connected. This lack of knowledge is problematic for converter controller design using standard techniques that are often based on knowledge of the system and focused on optimizing converter operation rather than system operation. Summary of the Invention

[0005] A universal control (UniCon) scheme for grid-connected converters is provided herein. One objective of the UniCon scheme is to control converters such that they can migrate from a current synchronous generator dominated grid to an inverter-based resource (IBR) dominated grid.

[0006] In some embodiments, the UniCon scheme automatically allows operation in grid-tied, grid-stabilized, and grid-forming modes. In some embodiments, the converter does not require information about the grid or the connected sources and loads, and can set itself where to operate based on local measurements. In some embodiments, the converter can operate over a wide range of commonly encountered steady-state, transient, and fault conditions. In some embodiments, the converter control strategy can operate with a combination of synchronous generators and grid-connected grid-tied and grid-forming converters, and can operate with robust or fragile grids or microgrids. In some embodiments, the control of the converters is based on the assumption that many (hundreds) converters are connected to the grid, and all converters work together to meet the system objectives. In some embodiments, UniCon provides a universal control strategy for converters on the grid, allowing operation in different modes, including dispatch in grid-tied mode, and automatic load sharing in islanded or microgrid modes. In some embodiments, during transient conditions the converter provides inertial support and improves damping to stabilize and reduce disturbances. In some embodiments, 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. The proposed control strategy has several different elements, all of which need to work together to achieve the desired overall system level operation.

[0007] In one embodiment, the converter controller is configured to control the converter in grid forming mode without relying on a measurement of the frequency of the converter terminal voltage.

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

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

[0010] The converter controller may be configured to synchronize the converter to another grid-forming converter, a microgrid, and / or the grid, having a different frequency and phase but not exceeding the rating of the grid-forming converter.

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

[0012] The converter controller may be configured to control the converter under varying power generation / load conditions.

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

[0014] The converter controller may be configured to synchronize a reference voltage phase of a reference voltage waveform of the converter to a terminal voltage phase of a terminal voltage waveform of the converter in response to a phase jump of the terminal voltage phase.

[0015] The converter controller may be configured to black start a microgrid that includes the converter.

[0016] The converter controller may be configured to manage active and reactive power sharing over a wide range of short circuit and / or X / R (reactance / resistance) ratios. The converter controller may be configured to control the first grid-forming converter to synchronize the first grid-forming converter with the second grid-forming 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-forming converter to synchronize the grid-forming converter to the microgrid and / or the 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 falls out of phase with the converter's reference voltage waveform.

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

[0021] The phase jump module may be configured to synchronize the reference voltage waveform to the terminal voltage waveform without altering 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 an opposite quadrant to avoid the reference voltage waveform and the terminal voltage waveform moving in opposite directions synchronously.

[0024] The phase jump module can be used in response to: 1) the voltage difference between the terminal voltage waveform and the reference voltage waveform being higher than a voltage threshold; or 2) the current flowing through the converter's filter inductor being higher than a current threshold.

[0025] The phase jump module a) measures the phase angle jump by the voltage difference (EV t ) determining a phase angle jump based at least in part on a current through a filter inductor and a terminal voltage waveform of the converter, and b) setting a reference voltage phase such that a product of the angular frequency of the reference voltage waveform and a sampling time is added to the reference voltage phase.

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

[0027] The frequency loop including the adaptive inertial module can be configured to provide an output phase angle (θ(t)). The phase angle jump is jump where:

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 response to any of the following: 1) the voltage difference falls below a voltage threshold and the current through the filter inductor falls below a current threshold; 2) a predetermined number of adjustments to the reference voltage phase have been performed; 3) a predetermined time has elapsed since using the phase jump module; or 4) steps a) to b) have been performed a predetermined number of times.

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

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

[0032] The droop control module can be in 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 an input to the adaptive inertia module.

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

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

[0036] The adaptive inertia module may be configured to synchronize an internal frequency and phase of a reference voltage waveform to a terminal frequency and phase of a terminal voltage waveform without the need to measure the frequency of the terminal voltage waveform.

[0037] The adaptive inertia module may be configured to set an inertia constant based solely 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 be configured to set an inertia constant solely based on any one of the three metrics mentioned above, a combination of any two of the three metrics mentioned above, or all three of the three metrics mentioned above.

[0038] The inertia constant may be based at least in part on a non-linear function of the power difference.

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

[0040] The damping factor of the converter may be based at least in part on the power difference.

[0041] The attenuation factor may be based at least in part on a non-linear function of the power difference.

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

[0043] The virtual impedance module is capable of dynamically varying the effective output impedance of the converter.

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

[0045] The virtual impedance may include a virtual resistor configured to increase resistance with a non-linear function in response to an increase in current.

[0046] The virtual impedance module may further include a constant 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, respectively, may be configured to increase in response to the current being increased using the integrator.

[0048] A frequency loop including the adaptive inertia module may be configured to provide an output frequency that may be maintained constant in a freeze mode when the terminal voltage falls below a predetermined value for a predetermined period of time, and that may exit the freeze mode when the terminal voltage rises 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 converters without relying on a measurement of the frequency of the terminal voltage waveform of the grid-forming converters.

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

[0051] The converter controller may be configured to control the grid-forming converter in an 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 to another grid-forming converter, a microgrid, and / or the grid having a different frequency and phase but not exceeding the rating of the grid-forming converter.

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

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

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

[0056] The converter controller may be configured to synchronize a reference voltage phase of a reference voltage waveform of the grid-forming converter to a terminal voltage phase of a terminal voltage waveform of the grid-forming converter in response to a phase jump of the terminal voltage phase.

[0057] The converter controller may be configured to black start a microgrid that includes the grid-forming converter.

[0058] The above and further aspects of the present invention will be explained in detail with reference to the following description and the accompanying drawings, in which like reference numerals indicate like structural elements and features in the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. The drawings depict one or more implementations of an apparatus according to the present invention by way of example only and not by way of limitation. [Brief description of the drawings]

[0059] [Figure 1] FIG. 1 is a flow diagram illustrating the control structure of a converter according to an embodiment of the present invention.

[0060] [Diagram 2] FIG. 2 is an electrical diagram illustrating an example grid model according to an embodiment of the present invention.

[0061] [Diagram 3] FIG. 3 is a block diagram illustrating one embodiment of a UniCon strategy, according to an aspect of the present invention.

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

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

[0064] [Figure 5A] FIG. 5A is a conceptual diagram showing a simple phase jump to reduce transients when the converter and grid voltages are the same but out of phase.

[0065] [Figure 5B] FIG. 5B is a conceptual diagram illustrating phase jumping followed by quadrant jumping to reduce transients when the converter and grid voltages are the same but out of phase.

[0066] [Figure 6] FIG. 6 is an electrical circuit diagram illustrating a model of two grids each powered by a grid-forming converter according to an embodiment of the present invention.

[0067] [Figure 7A] FIG. 7A is a flow chart of an algorithm operated to initiate a phase jump algorithm according to an embodiment of the present invention.

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

[0069] [Figure 8A] FIG. 8A shows the results of a simulation of connecting two converters, with a plot of the first converter load voltage. [Figure 8B]FIG. 8B shows the results of a simulation of connecting two converters, with a plot of the load voltage of the second converter. [Figure 8C] FIG. 8C shows the results of a simulation of connecting two converters, with a plot of the current in the first converter. [Figure 8D] FIG. 8D shows the results of a simulation of connecting two converters, with a plot of the current in the second converter. [Figure 8E] FIG. 8E shows the results of a simulation of connecting two converters, with a plot of the converter's internal voltages. [Figure 8F] FIG. 8F shows the results of a simulation of the connection of two converters, with a plot of the converter output power. [Figure 8G] FIG. 8G illustrates a simulation result for connecting two converters, plotting frequency for a converter according to an embodiment of the invention.

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

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

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

[0073] [Figure 10B] FIG. 10B is a block diagram illustrating one embodiment of a UniCon strategy, according to an aspect of the present invention.

[0074] [Figure 11A]FIG. 11A shows simulation results of an adaptive inertial scheme for one embodiment of the UniCon scheme, plotting adaptive gain and constant gain. [Figure 11B] FIG. 11B shows simulation results of an adaptive inertial scheme for one embodiment of the UniCon scheme, plotting variables D and J according to an aspect of the present invention.

[0075] [Figure 12] FIG. 12 is a plot of virtual resistance change as a function of current, according to an embodiment of the invention.

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

[0077] [Figure 14A] FIG. 14A shows simulation results for a four-converter system, illustrating the varying inertia during a load step / grid connection. [Figure 14B] FIG. 14B illustrates simulation results for a four-converter system showing grid frequency response with and without inertia assistance from a UniCon in accordance with an embodiment of the present invention.

[0078] [Figure 15A] FIG. 15A is a flow chart of an algorithm that is operated to initiate a phase jumping algorithm that does not require quadrant jumping in accordance with an embodiment of the present invention.

[0079] [Figure 15B] FIG. 15B is a flow chart of a phase jumping algorithm called by the algorithm shown in FIG. 15A that does not require quadrant jumping in accordance with an embodiment of the present invention.

[0080] [Figure 15C] FIG. 15C illustrates that the flow chart called by the phase jump algorithm shown in FIG. 15B sets variables that are checked by the phase jump algorithm if the reference voltage phase remains approximately at π after a predetermined number of iterations through the loop of the phase jump algorithm according to an embodiment of the present invention.

[0081] [Figure 16A] FIG. 16A is a block diagram illustrating another embodiment of a UniCon strategy according to aspects of the present invention.

[0082] [Figure 16B] 16B and 16C are block diagrams illustrating two different implementations of the UniCon strategy shown in FIG. 16A according to an embodiment of the present invention. [Figure 16C] 16B and 16C are block diagrams illustrating two different implementations of the UniCon strategy shown in FIG. 16A according to an embodiment of the present invention.

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

[0084] [Figure 18A] FIG. 18A is a plot of power, voltage, and current of experimental operation of the three-converter system shown in FIG. 17 during isolated operation, followed by interconnected operation, followed by isolated operation, in accordance with an embodiment of the present invention.

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

[0086] [Figure 18C] FIG. 18C is a close-up view of the power, voltage and current of the three-converter system during switching from interconnected to isolated operation as shown in FIG. 18A.

[0087] [Figure 19A] FIG. 19A includes plots of power, voltage, and current during islanded operation, followed by grid-tied operation, followed by islanded operation for experimental operation of the three-converter system shown in FIG. 17 in accordance with an embodiment of the present invention.

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

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

[0090] [Figure 20A] FIG. 20A is a plot of experimental data including voltages and currents of the three converters IBR1, IBR2, IBR3 shown in FIG. 17 during a fault condition in accordance with an embodiment of the present invention.

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

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

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

[0094] [Figure 20E] FIG. 20E is a plot of the voltage and current of each of the three converters IBR1, IBR2, IBR3 at the time the fault is cleared as shown in FIG. 20A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 (inversion, rectification, etc.) rated from a few milliwatts to thousands of kilowatts. Otherwise, the term "power electronic converter" shall be understood to have its clear and ordinary meaning as understood by those skilled in the art.

[0096] As used herein, the term "about" or "approximately" in conjunction with any number or range indicates a suitable tolerance that allows some or all of the components to function for its 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, the terms "component," "module," "system," "server," "processor," "memory," and the like are intended to include one or more computer-related units, examples of which include, but are not limited to, hardware, firmware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to, a process running on a processor, an object, an executable, a context of execution, a program, and / or a computer. As an example, both an application running on a computing device and the computing device may be a component. One or more components may exist within a process and / or context of execution, and a component may be localized on one computer and / or distributed among two or more computers. Furthermore, these components may execute from various computer readable media having various data structures stored thereon. Components may communicate according to signals having one or more data packets, such as data from one component interacting with another component by local and / or remote processes, in a local system, a distributed system, and / or over a network such as the Internet. The computer readable medium may be a non-transitory medium, including, but not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technologies, compact disc ROM (CD-ROM), digital versatile 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 usable to store computer readable instructions and / or data.

[0098] As used herein, the term "computing system" is intended to include a stand-alone machine or apparatus and / or combinations of machines, components, modules, systems, servers, processors, memory, detectors, user interfaces, computing device interfaces, network interfaces, hardware elements, software elements, firmware elements, and other computer-related units. By way of example, but not limited to, a computing system may include one or more general purpose computers, special purpose 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-transitory computer-readable medium" includes, but is not limited to, random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technologies, compact disc-ROM (CD-ROM), digital versatile 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 incorporated by reference in this patent application should be considered an integral part of this application, except as noted below: to the extent that a term is defined in those incorporated documents in contradiction to a definition expressly or implicitly given in this specification, only the definition in this specification should be considered.

[0101] The components, steps, and materials described below as making up various elements of the disclosed technology are intended to be illustrative and not limiting. Many suitable components, steps, and materials that would perform the same or similar functions as the components, steps, and materials 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] Provided herein is a universal and unified control (UniCon) strategy for power electronic converters ("converters") configured to operate connected to a grid.

[0103] FIG. 1 is a flow diagram illustrating the structure of a converter control structure 100. Utility / owner commands 102 are provided to an upper control layer 104, which may perform, for example, transactional control, forecasting, and / or energy management. The upper control layer 104 may provide data and / or commands, such as, for example, droop settings, grid synchronization, active power dispatch, and / or reactive power dispatch, to a middle control layer 106. The middle control layer 106 may provide, for example, inertial support, dynamic management of active or reactive power, black start microgrid control, transient management, and / or fast settling to a new set point. The middle control layer 106 may provide data and / or commands to a lower control layer 108, which may control, for example, the inner current loop, the inner voltage loop, and / or modulation. The lower control layer 108 may provide switching signals 110 to the converter electronics.

[0104] In some embodiments, a converter controller may be implemented at the mid-level control layer 106. In some embodiments, real-time control of the converters may be based entirely on rules and local measurements that allow the converters to work together and handle the wide range of steady-state, transient, and fault conditions that the converters may experience.

[0105] 2 is an electrical schematic diagram of a model of an example grid 200 to illustrate an example of the technical requirements of the UniCon strategy. A 2 MW distribution feeder 202 is shown with a first microgrid 220 and a second microgrid 240, each of which can operate in grid-connected or grid-island mode (i.e., by opening and closing switch 208). The microgrids 220, 240 are connected to two converter-based distributed energy resources (DERs) 221, 222, 243, 344, respectively. Each of these DERs enables each microgrid 220, 240 to function in grid-connected, grid-island, and microgrid-cluster modes. The DERs 221, 222, 243, 344 can be, for example, based on solar photovoltaics (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. In addition, a partially rated backup generator may be provided. The system is assumed to be geographically distributed (resilient community) and to have low-speed communication. The low-speed communication is interrupted from time to time, but the grid and microgrid must continue to function as suboptimally as possible. A bottom-up black start must be possible for each microgrid 220, 240 and for the connection of the distribution feeder switch 206 when the transmission feed 202 is lost (i.e. when the switch 204 is opened). Finally, although general information is available on the network and sources / loads, accurate information in real time cannot be assumed.

[0107] This is typically a real-world resilient feeder problem and presents many challenges for traditional approaches to controlling converters and microgrids. When grid-connected, the DERs 221, 222, 243, 244 operate in dispatch mode, maximizing energy transfer from PV, optimizing the use of energy storage resources, and providing support to the grid with Volt / VAR support or frequency regulation services. In case of planned disconnection or disconnection caused by grid-side fault, the microgrids 220, 240 continue to operate and balance the load with generation, typically using power-frequency droop characteristics. Operation in microgrid mode can occur with or without a backup gas generator and with or without PV, as two separate microgrids or as an aggregated microgrid. However, many of these tricky cases cause major problems, especially when there is no communication between converters, there is little system knowledge, and control is based on local measurements and a common set of rules. The tricky cases mainly include: Support for grid inertia under conditions of high rate of change of frequency (ROCOF) Connecting the converter to a working grid with minimal transients Connecting an operational microgrid to the grid with minimal transients · To interconnect two microgrids with each other with minimal transients - To quickly settle the system on the PF curve with minimal transients after generation / load changes Dynamic control of P and Q, especially during transients (and Q at steady state) Managing phase jumps after faults or other conditions Black starting a microgrid and forming a bottom-up microgrid cluster Interconnecting microgrids / grids at different frequencies and phases without major transients

[0108] Exemplary embodiments of a controller are presented herein that utilize fast phase jumps in case of large disturbances, dynamic virtual impedance under transients, nonlinearly varying inertial implementations, and partial combinations of these elements to provide a single control strategy that manages steady-state and transient requirements including some or all of the challenges mentioned above. Some embodiments use the grid voltage as the key instantaneous parameter against which the converter reacts.

[0109] The primary goal of UniCon is to be applicable across a wide range of converter topologies and system / network applications, from grid-connected high-voltage source converters (VSCs) to microgrids and microgrid clusters. In some embodiments, a goal of the UniCon strategy is to be vendor and implementation agnostic of the inner voltage / current and protection loops (i.e., the lower control layer 108). In some embodiments, most of the UniCon control is slower than typical converter inner loops and is intended to achieve invariance. In some embodiments, a goal of the UniCon strategy is to create interoperability for all types, ratings, and brands of grid-connected converters. In some embodiments, a goal of the UniCon strategy is to facilitate the further introduction of grid-connected converters.

[0110] 3 is a block diagram illustrating an implementation 300 of an 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 an embodiment of the UniCon strategy 1600 is shown in FIG. 16A. The phase jump module 320 can be configured to achieve fast 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 is configured to generate a phase angle jump θjump , which can measure the phase of the reference voltage θ of the power converter when the terminal voltage waveform of the power converter and the reference voltage waveform of the power converter are suddenly desynchronized. REF The adaptive inertia module 310 can be configured to set an inertia constant of the power converter to synchronize the converter frequency to the grid, the microgrid, and / or the GFC. In some embodiments, the adaptive inertia module can be configured to set the inertia constant of the converter based at least in part on a power difference between the terminal power of the power converter and a reference power of the converter. The virtual impedance module 330 can be configured to limit the current of the power converter in a 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 effective power and frequency or other suitable droop control strategy, as would be understood by one of ordinary skill in the art.

[0112] The droop control module 301 can set a reference power for the converter. The reference power (internal to the converter) and the actual power (output of the converter, terminal power, P meas ) is calculated by summing function 302 and fed to non-linear integrators 314, 316 of adaptive inertia module 310. Adaptive inertia module 310 also includes non-linear damper 312. The outputs of non-linear integrators 314, 316 and non-linear damper 312 are in frequency (angular frequency) and are proportional to the nominal angular frequency (ω *) output of the frequency loop 340. The output angular frequency (ω(t)) of the frequency loop 340 is then passed through the integrator 303 to generate the phase angle (θ) of the converter's reference voltage waveform. The phase jump algorithm of the phase jump module 320 directly affects the phase of the converter internal voltage (E) and skips the frequency loop 340, which results in a zero inertial response. The virtual impedance module 330 allows for a decoupling between active and reactive power control. The virtual impedance module 330 also indirectly allows for reactive power control. The virtual impedance module 330 modifies the converter internal voltage reference to emulate series R and series L. The reference voltage phase θ (determined by the phase jump module 320 and the frequency loop 340) REF together with the nominal voltage amplitude (modified by the virtual impedance module 330) form the converter reference voltage waveform. The frequency of the converter reference voltage waveform is allowed to drift and synchronize with the terminal voltage waveform as adjusted by the adaptive inertia module. The end result of this control block is a reference voltage waveform, which can be fed to the converter switching algorithm in the lower control module 308. In some embodiments, the lower control module 308 has similar functionality to the lower control module 108 shown in FIG. 1.

[0113] The synchronization process for microgrids can be very challenging, and traditional strategies usually require time-consuming and precise adjustments. One strategy starts the synchronization process by assuming that the converters have zero initial power and ramp up after grid connection. For example, the virtual impedance is exponentially modified and the current is effectively controlled during synchronization. Another example uses a switching-based technique that allows the converters to synchronize before the current ramps up. However, in many cases, the microgrid is available and the system cannot be shut down for synchronization purposes. Another strategy requires full knowledge of the grid voltage for seamless transition. While synchronization is possible to connect individual units to the system (because the grid voltage is available and can be sensed), in geographically distributed microgrids, many converters do not have access to the grid voltage, and until now, no method has been provided that allows seamless connection of microgrids in a fully distributed manner, independent of the monitoring algorithm. In some embodiments, the phase jump module 320 provides such a feature. In some embodiments, the phase jump module 320 can enhance the performance of single converters, making them “plug-and-play” devices. In some embodiments, the phase jump module 320 can enable the concept of a "fractal grid," meaning that the grid can be divided into small sections during major transients and fast recovery after fault removal.

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

[0115] In some embodiments, the phase jump module 320 may be configured to perform fast voltage phase synchronization and minimize current transients on the grid interconnect.

[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 bring the system in phase with the grid with zero inertia response, a capability that is not possible with existing strategies that are typically based on current limiting algorithms and can lead to current loop saturation and sustained transients.

[0117] 4A and 4B respectively include conceptual diagrams of voltage phase shifts to minimize current transients at the connection to the grid, with FIG. 4A showing a conventional strategy and FIG. 4B showing an embodiment of the UniCon strategy. As shown in FIG. 4A, the conventional approach is based on making the converter voltage amplitude (Vc) the same as the grid voltage (Vg). As shown, the converter is at an initial voltage amplitude 402 and the converter voltage is moved to a new location 406 in the diagram to limit the current from the converter. However, at the new location 406, the reference voltage waveform remains out of phase with the grid / terminal voltage waveform. This approach minimizes current spikes in the interconnection, but the transients persist for a while as the converter tries to make its magnitude and phase the same as the grid. Conversely, the approach utilized by the UniCon strategy shown in FIG. 4B changes the phase of the converter voltage, thereby not only reducing the instantaneous current spikes but also minimizing the transients after phase locking. The converter voltage location 402 has moved to a new location 408 with the same phase angle as the grid voltage location 404, by an angle 410. In a conventional approach (FIG. 4A), the converter does not change direction (always clockwise). In a particular embodiment approach shown here (FIG. 4B), the converter can reverse direction based on the difference between Vc and Vg. In some embodiments, the phase is changed exponentially (non-linearly) to avoid oscillations.

[0118] 5A and 5B each include a conceptual diagram showing a series of voltage phase movements including quadrant jumping. FIG. 5A shows a scenario where the converter voltage and grid voltage are halved in opposite directions by the approach shown in FIG. 4B. In this scenario, Vc moves from a first position 502 to a second position 508 at an angle 510, which is the same magnitude as Vg's position 504. In this scenario, Vc and Vg are not of the same phase, which leads to more transients. To solve this problem, the concept of quadrant jumping is introduced, where the voltage phase jumps to its complement to control the current. FIG. 5B shows Vc moving from the second position 508 to a third position 512, which is the complement in the opposite quadrant, to converge to Vg's position 504.

[0119] In some embodiments, the UniCon strategy is able to 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 quadrant jumping.

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

[0121] Figure 6 is an electrical schematic diagram of a model of two grids both powered by GFCs, i.e., a basic 2GFC system, showing aspects of the UniCon strategy from the perspective of one of the GFCs. The second GFC in this model may also represent a grid. The GFC may include the UniCon strategy 300 shown in Figure 3 at the mid-level control layer 106 as shown in Figure 1.

[0122] Figure 7A is a flow chart of an algorithm 700 that is operated to initiate a phase jump algorithm 720 shown in Figure 7B. This flow chart shows an algorithm that may be executed by one of the GFCs shown in Figure 6 that is not in communication with the other GFCs. For purposes of illustration, the V L1 , V L2 , I L1 , I L2 , V t1 , and V t2 The subscripts "1" and "2" have been omitted.

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

[0124] At block 704, the inductor current (I L ) or inductor voltage (V L ) is the threshold (V L,th ,I L,th ), e.g., 1.5 pu, the algorithm 700 may be activated and proceeds to block 706, otherwise iterates to block 704. In some embodiments, the algorithm 700 may include a delay before proceeding to block 706 for noise removal. For example, the algorithm 700 may include a counter in block 704 that proceeds the algorithm to block 706 once the threshold is reached.

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

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

[0127] At block 710, an inductor voltage polarity variable (V L_pol) to the inductor voltage (V L ) in polarity. Alternatively, blocks 710 and 716 may be omitted.

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

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

[0130] 7B is a flow chart of a phase jump algorithm 720 called by the algorithm 700 shown in FIG. L ) or inductor current (I L In order to reduce the magnitude of the reference voltage phase (θ REF ) can be modified in the optimal trajectory.

[0131] At block 722, a counter is set to zero.

[0132] In block 724, the angle adjustment phase angle jump (θ jump ) is the inductor voltage (V L ), inductor current (I L ), and output voltage (V t ) function. Two exemplary approaches for calculating this trajectory are presented as non-limiting examples.

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

number

number

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

number

number

[0135] At block 726, the angle adjustment (θ jump If the angle adjustment (θ jump ) will be set to a negative value.

[0136] At block 728, the reference voltage phase (θ REF ) with its previous value, the angular frequency × sampling time (Δt), and the angle adjustment (θ jump ) to the sum of

[0137] At block 730, the inductor current (IL ) and inductor voltage (V L ) to their respective thresholds (V L,th ,I L,th ) If both values ​​are less than their respective thresholds, the phase jump algorithm 720 proceeds to block 731 where the counter is incremented. Note that the phase jump algorithm 720 loops to compare the reference voltage phase θ REF , V L and I L For example, the V t If there is a very serious fault where the reference voltage phase θ REF decreases to zero and V L is made close to zero. Then the condition of 730 is met and the phase jump algorithm 720 starts counting.

[0138] At block 731, a counter is incremented.

[0139] The counter is compared to a counter threshold (counter_set) in block 732. If the counter is greater than the counter threshold, the phase jump algorithm 720 proceeds to block 734 where the phase jump algorithm 720 ends. If the counter is less than the counter threshold, the phase jump algorithm 720 proceeds to block 724 where it repeats its loop.

[0140] Returning to the main algorithm 700 of Figure 7A, once the phase jump algorithm 720 is completed at block 734, the main algorithm 700 proceeds to block 704.

[0141] At block 704, the inductor current (I L ) or inductor voltage (V L ) at their respective thresholds (V L,th ,I L,th), the main algorithm 700 proceeds to block 706. Note that the condition at block 704 is initially NO as the phase jump algorithm 720 is finished. However, the loop continues to operate. If the changes to the reference voltage phase by the phase jump algorithm 720 do not synchronize the reference voltage waveform with the terminal voltage waveform, the current and / or voltage (I L and / or V L ) will rise again soon and the condition at block 704 will become YES. The Ph_j_ctr value is calculated as the loop is executed multiple times to determine the current and / or voltage (I L and / or V L ) occurs.

[0142] At block 706, the control counter variable (Phase_j_ctrl) is incremented by one so that it becomes two for the second pass through the loop.

[0143] At block 708 , the control counter variable (Phase_j_ctrl) is not equal to 1 during the second pass through the loop, so the algorithm 700 proceeds to block 714 .

[0144] At block 714 , if the control counter variable (Phase_j_ctrl) is equal to 2, then the method proceeds to block 716 , otherwise the method 700 proceeds to block 740 .

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

[0146] In block 718, the quadrant, reference phase angle of the converter internal voltage phasor (θ) is determined. If the reference voltage phase (θ) is in the first or fourth quadrant, the reference voltage phase (θ) will jump to its complement in the second or third quadrant. The method then proceeds to a phase jump algorithm 720 as shown in FIG. 7B.

[0147] At block 740, a fault ride-through algorithm is initiated that uses the virtual impedance module 330. Thus, the fault ride-through algorithm is initiated once more than two loops of the main algorithm 700 are completed. In an embodiment that includes optional blocks 710, 716, the fault ride-through algorithm initiates a fault ride-through algorithm that uses the virtual impedance module 330 to ride through the V between the first and second loops of the main algorithm 700 during the second loop. L The polarity of the signal can be changed.

[0148] This procedure leads to a smooth reconnection of two or more microgrids. Nevertheless, if the other one is a robust grid or the fault causes a steep 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, go to 720). This time, the phase jumps to the second / third quadrant where the grid voltage vector is (block 718). Again, if the current rises for the third time (Ph_j_ctr==3), it means that it is not a synchronous transient but a fault situation. Therefore, the fault ride-through algorithm (block 740) will be activated.

[0149] Once the current and voltage are under control, the main algorithm 700 ends and the frequency loop control 340 (FIG. 3) adjusts the reference voltage phase (θ REF ) control is performed again. This time, both sides (e.g., E in Fig. 6)inv1 and E inv2 ) have the same angle but different frequency values. In order to minimize the transients due to the different frequency values ​​and make them converge smoothly, an adaptive inertial scheme in the adaptive inertial module 310 is utilized.

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

[0151] Each converter supplies its own local load and at t=1.485s the switch is closed. The rating of the first converter (120V, 30A) is twice as large as the rating of the second converter (120A, 15a). As shown in Figures 8A-8E, when connected, one converter is positive at its peak while the other converter is negative at its peak, and therefore the voltage at the inductor is maximum, i.e. 2pu. In this case, converter 2 has smaller dimensions and therefore reaches its threshold sooner, thus minimizing the jump transient. Figures 8A and 8B demonstrate that the voltage at the load goes through a high frequency transient for 1 / 8 of a cycle and then remains stable. 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 starts to adjust the frequency, and the power sharing follows the droop curve, as shown in Figures 8F and 8G. This result fully demonstrates that it is possible to interconnect two GFCs while limiting the voltage and current transients.

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

[0153] Current state-of-the-art grid forming technologies are mostly designed to mimic the behavior of synchronous generators (SGs) and are called virtual synchronous generators (VSGs). However, VSGs inherit the same drawbacks from SGs, such as oscillatory response during transients. Choosing a large value for the virtual moment of inertia provides better inertial support and effectively limits the ROCOF, but at the expense of oscillations and slow response. On the other hand, choosing a low value for J provides a faster transient response but with larger overshoots and violations of the ROCOF tolerance.

[0154] To avoid this problem, "variable inertia" has also been proposed to effectively control the frequency nadir and damp the oscillations. In one example, a bang-bang control scheme is used to control Δω and dω m Change the inertia constants based on the value of / dt. The problem is that these values ​​are not easy to measure. m Measuring / dt requires suppression of noise amplified by the derivative, while measuring Δω requires knowledge of the grid-side frequency, which is difficult especially during transients. The algorithm also ensures that the system trajectory passes through the desired setpoint and then takes action to improve the non-optimal response.

[0155] Recently, this principle has been improved by varying D and J simultaneously, or by introducing a continuously varying J. The latter is m Although this improves the control scheme by eliminating the need for / dt measurements, its drawback is that the grid frequency is assumed to be constant and 60Hz. While this is likely for grid-connected operation, for isolated microgrids where the frequency is changed very frequently, this is not an accurate assumption. Several experiments have demonstrated that when the grid frequency is changed other than the nominal value, the response becomes very 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. No measurement of the grid frequency is required and the system is able to damp the response and move optimally to a new operating point regardless of the grid frequency.

[0156] A large inertia constant (J) creates extra oscillations, and a small J leads to a large frequency nadir. To avoid these problems, the UniCon strategy can speed up the system when the frequency is far away from the new set point (which can be any number), and gradually slow down the response as the system approaches the new set point. This can be achieved by choosing a nonlinear curve for J, whose value changes smoothly based on the error. Additionally, the damping coefficient (D) can be nonlinear. A polynomial function is a good candidate with this property.

[0157] 9A is an example of a nonlinear inertia constant (J) suitable 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 may be used to reduce the time required to change two variables, namely P rated and ΔP.

[0158] FIG. 9B is an example of a nonlinear damping coefficient (D). A virtual synchronous generator can be modeled by an RC circuit, where 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 components of the circuit. In an RLC circuit, to have a deterministic damping response with frequency, the virtual R is:

number

number

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

[0160] Figure 10B is a block diagram of an implementation 1000 of one embodiment of the UniCon strategy, omitting the phase jump module 320 and the virtual impedance module 330. The implementation 1000 includes components and modules 1001, 1002, 1003, 1004, 1008, 1010, 1012, 1014, 1016 configured similarly to the corresponding components and modules 301, 302, 303, 304, 308, 310, 312, 314, 316 shown in Figure 3. A droop control module 1001 is employed to move the system to a new steady-state operating point after an initial fast 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 = 1 s, a step change in the grid frequency occurs, jumping from 60 Hz to 60.3 Hz. Then, at t = 2 s, the grid frequency returns to the nominal value. An example of this scenario is when two or more grid-forming converters are isolated by a fault and need to be synchronized and reconnected after fault clearance. In such a scenario, a frequency drift between the grid-forming converters occurs and a frequency step change can be effectively observed on their terminals.

[0162] Figure 11A shows the results of small gain 1103, large gain 1102, and adaptive inertia 1101. As can be seen, the variable inertia scheme exhibits a damped response, where the movement is fast when the error is large, but slows down as the set point is approached.

[0163] Figure 11B shows how the inertia and damping gains vary based on the response. With nonlinearly varying inertia, the step response characteristics of the system are better than the constant gain case because the constant gain does not make the compromises that designers must make when using constant gain.

[0164] Another important element of the converter's control is its ability to ride through a fault and support the grid by providing enough current. To achieve this, a varying virtual resistor 334 is used. This method helps the converter avoid going into saturation mode and operate within its linear range. If a fast transient is present in the system, the UniCon 300 first tries to manage it by using the phase jump module 320. After several attempts without success, the algorithm 700 detects this as a fault condition and activates the fault ride-through algorithm 740, which increases the virtual resistor 334 to limit the current. Once the voltage returns to its normal value, the virtual resistor 334 is reduced again to its nominal value. The virtual inductor 332 in steady state separates P and Q for better power sharing during the initial transient. The virtual inductor can be varied to allow the system to inject a stable sinusoidal current during the transient.

[0165] A virtual resistor consists of two terms: R inst and R flt It consists of R inst R changes rapidly to limit the instantaneous current and therefore protect the semiconductor switch. flt varies with the virtual inductor based on an integrator, allowing the converter to inject a stable current during a fault.

[0166] FIG. 12 illustrates the first term R of a virtual resistor that varies with current in one embodiment. inst This is the plot.

[0167] The second term R flt varies at the same rate as the virtual inductor to allow 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 vary according to Equations 5-7 shown below.

number

number

number

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

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

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

[0171] At t=1.2s, the two microgrids 220, 240 are connected together to the switch 208 to form a larger microgrid. Note that none of the converters 221, 222, 243, 244 have a priori information about the possible interconnection of the microgrids 220, 240.

[0172] At t=2.5s a load is added stepwise to the system.

[0173] At t=4.5s a fault occurs and the system goes into a fault ride-through state until t=4.8s when the fault is cleared.

[0174] The system then starts decreasing the virtual resistor to restore the voltage and synchronize the converter again with adaptive inertia.

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

[0176] Simulation results show that the system is able to manage all transients and move smoothly between different modes of operation.

[0177] FIG. 14A shows how the inertia constant J and damping coefficient D vary to damp the frequency response at t=2.5 s and t=7 s during 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 and a UniCon controlled converter is added to support it with an inertial response. It is observed that not only can the second converter improve the frequency nadir by varying its inertia constant, but it can also damp the response to eliminate oscillations induced on the grid. In summary, the simulation results demonstrate the effectiveness of UniCon in a variety of scenarios.

[0179] FIG. 15A is a flow chart of an algorithm 1500 that is operated to initiate a phase jump algorithm 1520 shown in FIG. 15B, which utilizes a reference voltage phase status check algorithm 1550. Flow chart 1500 illustrates an algorithm that can be executed by one of the GFCs shown in FIG. 6 without communicating with the other GFCs. For purposes of illustration, the V L1 , V L2 , I L1 , I L2 , V t1 , and V t2 The subscripts "1" and "2" have been omitted.

[0180] 15A-15C show an algorithm for implementing phase jumping that does not require quadrant phase jumping and can be used as an alternative to algorithm 700 shown in FIGS. 7A and 7B.

[0181] In Figure 15A, blocks 1502, 1504, 1506, 1508, 1540 may be configured similarly to corresponding blocks 702, 704, 706, 708, 740 of the main algorithm 700 shown in Figure 7A. The main algorithm 1500 shown in Figure 15A does not have the set and check voltage polarity blocks 710, 716 and the quadrant jumping blocks 712, 718 of the main algorithm 700 shown in Figure 7A. It is noted that the set and check polarity blocks 710, 716 are optional features of the main algorithm 700 shown in Figure 7.

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

[0183] At block 1536, dθ REF Block 1536 is executed following the increment counter block 1531.

[0184] At block 1550, θ REFCalls the status check algorithm 1550. The status check algorithm 1550 checks if block 1530 is YES (i.e., V L and I L is called for each loop of the phase jump algorithm 1520 if θ is less than its respective threshold. REF But, θ REF If some calls to the status check algorithm 1550 bring it close to π, then θ REF The status check algorithm 1550 sets the status check "Stuck_status" to one.

[0185] FIG. 15C shows the REF FIG. 15 shows a flow diagram of a status check algorithm 1550.

[0186] At block 1552, the absolute value |θ REF If |-π| is less than the threshold ε, increment the counter “stuck_ctr”.

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

[0188] At block 1556, θ REF The status check algorithm 1550 is completed and the phase jump algorithm 1520 shown in FIG.

[0189] In block 1538, the dθ obtained in block 1536 is REF If the most recent measurement of / dt is less than zero, then θ jump is π-θ jump θ REF This causes the value to jump to the next quadrant.

[0190] In block 1539, if “Stuck_status” is 1 (set by block 1554 of FIG. 15C), then θ REF is set to zero. Therefore, regardless of whether a quadrant jump occurs in block 1538, θ REF But, θ REF After the number of calls to the status check algorithm 1550, “stuck_set”,, the value of θ remains close to π. REF is set to zero in block 1539.

[0191] Figure 16A is a block diagram of another embodiment of the UniCon strategy 1600. Figures 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 a variety of different implementations shown in Figures 16B and 16C. This includes a control technique without a current controller as shown in Figure 16B, and a control technique with an additional control loop such as a current controller as shown in Figure 16C.

[0192] FIG. 17 is an electrical schematic diagram of a model of an example grid tested with experiments as shown in FIGS. 18A-18C and 19A-19C and as shown in FIGS. 20A-20E. The example grid includes three converters IBR1, IBR2, IBR3. The example grid includes a first microgrid (microgrid 1) including first and second converters IBR1, IBR2, and a second microgrid (microgrid 2) including a third inverter IBR3. The converters IBR1, IBR2, IBR3 do not communicate with each other. FIGS. 18A-18C show seamless interconnection and disconnection of two microgrids without communication between the two microgrids. FIGS. 19A-19C show seamless grid connection and grid disconnection of a microgrid including three converters. FIGS. 20A-20E show smooth and stable operation of the converters during a fault.

[0193] 18A is a plot of power, voltage and current during isolated operation, followed by interconnected operation, followed by isolated operation of the three converter system. The grid connect switch remains open, but the microgrid interconnect breaker is opened after being closed in FIG. 17.

[0194] Figure 18B is an expanded view of the powers, voltages and currents during switching of the three converter system from isolated operation as shown in Figure 18A to interconnected operation. 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 out of phase with the voltage waveform V2 of the second microgrid. The currents I1, I2, I3 of the three converters IBR1, IBR2, IBR3 are shown. The powers P1, P2, P3 of the three converters IBR1, IBR2, IBR3 are shown.

[0195] Figure 18C is a close-up view of the powers, voltages and currents of the three converter system during switching from interconnected operation as shown in Figure 18A to isolated operation. 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, I3 of the three converters IBR1, IBR2, IBR3 are shown. The powers P1, P2, P3 of the three converters IBR1, IBR2, IBR3 are shown.

[0196] Figure 19A includes plots of power, voltage, and current during islanded operation, followed by grid-tied operation, followed by islanded operation of the three converter system. The grid-tied switch remains closed and then opens, but the microgrid interconnect breaker remains closed in Figure 17.

[0197] FIG. 19B is an expanded view of the power, voltage, and current during switching from islanded operation to grid-connected operation of the three converter system as shown in FIG. 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 to connect the two microgrids to the grid in grid-connected operation. In islanded operation, the microgrid voltage waveform uGV is out of phase with the grid voltage waveform GV. Over about one cycle, the voltage waveform of the microgrid uVG phase jumps in phase with the grid voltage waveform GV. The currents I1, I2, I3 of the three converters IBR1, IBR2, IBR3 are shown. The powers P1, P2, P3 of the three converters IBR1, IBR2, IBR3 are shown.

[0198] Figure 19C is an expanded view of the power, voltage and current of the three converter system during switching 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 to isolate the microgrid from the grid. The currents I1, I2, I3 of the three converters IBR1, IBR2, IBR3 are shown. The powers P1, P2, P3 of the three converters IBR1, IBR2, IBR3 are shown. The grid and microgrid voltage waveforms uGV, GV are shown.

[0199] 20A is a plot of experimental data including voltages and currents of three converters IBR1, IBR2, 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 a connection between the first and second microgrids. When the fault begins, the system rides through the fault until it is cleared. The adapted virtual impedance together with the adaptive inertia scheme allows smooth and stable operation of the inverter during the fault.

[0200] FIG. 20B is a plot of power and frequency for each converter IBR1, IBR2, IBR3 corresponding to the scenario tested in FIG. 20A.

[0201] FIG. 20C is a plot of the voltage and current of each of the three converters IBR1, IBR2, IBR3 when the fault begins as shown in FIG. 20A.

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

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

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

[0205] Also disclosed herein is the ability to control grid forming converters with control schemes that include p-ω droop control, nonlinear integrator (inertia) and nonlinear damping, fast phase change blocks, and virtual impedance blocks.

[0206] The feasibility of the proposed scheme was demonstrated by simulation and experimental results.

[0207] In some embodiments, the UniCon strategy can be implemented in hardware (e.g., a chipset) that is preferably integrable into a wide range of converter configurations. In some embodiments, the UniCon strategy can be implemented in software or firmware that is preferably applicable to a wide range of converter configurations. Some embodiments include a converter that implements the UniCon strategy.

[0208] It is understood that the embodiments and claims disclosed herein are not limited in their application to the details of construction and arrangement of the components described herein and illustrated in the drawings. Rather, the specification and drawings provide examples of possible embodiments. The embodiments and claims disclosed herein are further capable of other embodiments and can be implemented and carried out in various ways. It is also understood that the phraseology and terminology used herein are for the purpose of description and are not to be construed as limiting the scope of the claims. The embodiments disclosed herein are not limited to the specifics of any UniCon strategy disclosed herein. In particular, an embodiment can include individual modules, components thereof, or methods associated therewith. As can be understood by one skilled in the art, compatible features of the disclosed embodiments can be combined. As can be understood by one skilled in the art, features of the disclosed embodiments can be combined with features not disclosed herein.

[0209] It will thus be apparent to those skilled in the art that the principles on which this application and claims are based may be readily used as a basis for the designing of other structures, methods and systems for carrying out some of the purposes of the embodiments and claims described herein, and it is important that the claims be regarded as including such equivalent structures.

[0210] Moreover, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public, particularly those skilled in the art who may not be familiar with patent and legal language and texts, to quickly determine, upon reading, the nature and principles of the technical disclosure of the present application. The Abstract is not intended to define the scope of the claims of the present application, nor is it intended to limit the scope of the claims in any way. Instead, it is intended that the disclosed technology be defined by the appended claims.

Claims

1. A converter controller configured to control the converter in a mode selected from the group consisting of a grid-aligned mode, a grid-stabilized mode, a grid-forming mode, and combinations thereof, based solely on local measurements and without relying on information about the grid.

2. The converter controller of claim 1 , further configured to control the converter in the grid-forming mode without relying on a measurement of a frequency of a converter terminal voltage.

3. The converter controller includes: or further configured to control the converter in an island mode after being disconnected from a network including at least one of another converter, a microgrid, or a grid in the grid-forming mode; or and further configured to synchronize the converter in the grid-forming mode to at least one of another grid converter, a microgrid, or a grid in the grid-forming mode. The converter controller of claim 1 , wherein the converter controller is one or more of:

4. 2. The converter controller of claim 1, wherein the converter controller is further configured to synchronize the converter in the grid-forming mode to at least one of another converter in the grid-forming mode, a microgrid, or a grid having a different frequency and phase but not exceeding a rating of the converter in the grid-forming mode.

5. The converter controller of claim 1 , wherein the converter controller is further configured to control the converter in the grid forming mode under conditions of high frequency rate of change.

6. The converter controller of claim 1 , wherein the converter controller is further configured to control the converter in the grid-forming mode under varying generation / load conditions.

7. The converter controller includes: or A reference voltage phase of a reference voltage waveform of the converter in the grid formation mode is synchronized with a terminal voltage phase of a terminal voltage waveform of the converter in the grid formation mode in accordance with a phase jump of the terminal voltage phase.

2. The converter controller of claim 1, wherein the at least one of:

8. The converter controller of claim 1 , wherein the converter controller is further configured to black start a microgrid including the converter in the grid-forming mode.

9. The converter controller of claim 1 , further configured to manage active and reactive power sharing over a wide range of short circuit ratios and / or X / R (reactance / resistance) ratios.

10. A voltage shaping controller configured to use measurements of voltage and current available at the interconnection terminals to determine an instantaneous AC voltage reference to be synthesized by a power conversion stage; Seamlessly transition between system states without the need for system knowledge or external communication Automatically forming one or more distributed, ad-hoc, bottom-up microgrids as needed; Autonomously providing variable inertia as needed to assist in one or more of grid synchronization, fault ride-through, and / or power sharing; Coordinate sinusoidal fault current injection and transient current limiting by providing a variable virtual impedance one or more adaptive controller loops configured to and a phase jump algorithm unit configured to reduce a synchronization response time and a transient current limit.

11. A phase jump module configured to set a reference voltage phase of the converter synchronously with a terminal voltage waveform (Vt) of the converter with zero inertia; an adaptive inertia module configured to set an inertia constant of the converter based at least in part on a power difference between a terminal power of the converter and a reference power of the converter; The universal converter controller of claim 10 , further comprising: a virtual impedance module configured to dynamically vary an effective output impedance of the converter.

12. The universal converter controller of claim 11 , wherein the converter controller is further configured to control the converter in a grid-forming mode to synchronize the converter in the grid-forming mode to a microgrid and / or a grid.

13. The universal converter controller of claim 11 , wherein the phase jump module is further configured to set the reference voltage phase to a phase of the terminal voltage waveform with zero inertia response.

14. The phase jump module includes: a voltage difference between the terminal voltage waveform and the reference voltage waveform is greater than a voltage threshold; or The universal converter controller of claim 11 , wherein the universal converter controller is used in response to a current through a filter inductor of the converter being greater than a current threshold.

15. The universal converter controller of claim 11 , wherein the phase jump module is further configured to synchronize the reference voltage waveform to the terminal voltage waveform without altering a voltage amplitude of the reference voltage waveform.

16. The universal converter controller of claim 11 , wherein the phase jump module is further configured to non-linearly adjust the reference voltage phase.

17. 12. The universal converter controller of claim 11, wherein the phase jump module is further configured to jump the reference voltage phase to an opposite quadrant to avoid the reference voltage waveform and the terminal voltage waveform moving in opposite directions to each other in sync.

18. The phase jump module includes: The phase angle jump is defined as the voltage difference (E−V ｔ ), and / or a current through a filter inductor of the converter, and / or a terminal voltage of the terminal voltage waveform; 12. The universal converter controller of claim 11, further configured to set the reference voltage phase such that the phase angle jump and a product of an angular frequency of the reference voltage waveform and a sampling time are added to the reference voltage phase.

19. The phase angle jump is a function of the voltage difference (f), i.e., f(E−V ｔ )) and 20. The universal converter controller of claim 18, wherein the function (f) is proportional to a constant, a linear function, or a non-linear function.

20. The function (f) is the hyperbolic tangent (tanh(E−V ｔ 20. The universal converter controller of claim 19, comprising:

21. a frequency loop including the adaptive inertial module configured to provide an output phase angle (θ(t)); The phase angle jump is [0010] 20. The universal converter controller of claim 18, wherein:

22. The universal converter controller of claim 18 , wherein the phase jump module is further configured to repeat the determining and setting steps until the phase jump module is removed.

23. 20. The universal converter controller of claim 18, wherein the derivative of the terminal voltage angle is measured and used to determine whether a jump to the opposite quadrant is required.

24. 20. The universal converter controller of claim 18, wherein the terminal voltage angle is constantly monitored and set to zero when the terminal voltage angle approaches π for a predefined set time.

25. The phase jump module includes: the voltage difference falls below a voltage threshold and / or the current through the inductive impedance falls below a current threshold; The adjustment of the reference voltage phase is performed a predetermined number of times; A predetermined time has elapsed since the phase jump module was used; or 20. The universal converter controller of claim 18, wherein the universal converter controller is removed in response to at least one of the determining and setting steps having occurred a predetermined number of times.

26. The universal converter controller of claim 11 , further comprising a droop control module configured to set a reference power of the converter.

27. 27. The universal converter controller of claim 26, wherein the droop control module is configured to set a reference power of the converter based at least in part on an inverse correlation between an active power and a frequency of the converter.

28. 27. The universal converter controller of claim 26, wherein the droop control module is in a feedback loop with the adaptive inertia module.

29. 27. The universal converter controller of claim 26, wherein the difference between the output of the droop control module and terminal power is provided as an input to the adaptive inertia module.

30. 27. The universal converter controller of claim 26, wherein the sum of the output of the adaptive inertia module and a reference angular frequency of the reference voltage waveform is provided as an input to an integrator to generate an angle.

31. 12. The universal converter controller of claim 11, wherein the adaptive inertia module is further configured to synchronize an internal frequency and phase of the reference voltage waveform to a terminal frequency and phase of the terminal voltage waveform without the need to measure the frequency of the terminal voltage waveform.

32. The adaptive inertia module determines the inertia constant as: the measured power of said converter; the difference between the reference power and the measured power, or 12. The universal converter controller of claim 11, further configured to set based on at least one of: a terminal voltage amplitude of the terminal voltage waveform.

33. The universal converter controller of claim 11 , wherein the inertia constant is based at least in part on a non-linear function of the power difference.

34. The universal converter controller of claim 11 , wherein the inertia constant is based at least in part on a polynomial function of the power difference.

35. The universal converter controller of claim 11 , wherein a damping factor of the converter is based at least in part on the power difference.

36. The universal converter controller of claim 11, wherein the damping coefficient of the converter is based at least in part on a non-linear function of the power difference.

37. The universal converter controller of claim 11 , wherein a damping coefficient of the converter is proportional to the inverse square root of the inertia constant.

38. The universal converter controller of claim 11 , wherein the virtual impedance module includes a virtual impedance configured to increase impedance in response to an increase in current.

39. 39. The universal converter controller of claim 38, wherein the virtual impedance includes a virtual resistor component configured to vary the resistance with a non-linear function to limit the instantaneous current through a filter inductor of the converter below a predetermined threshold.

40. 40. The universal converter controller of claim 39, wherein the virtual impedance module further comprises a constant virtual inductor.

41. the virtual impedance includes a virtual resistor component and a virtual inductor component; 39. The universal converter controller of claim 38, wherein the inductance of the virtual inductor component and the resistance of the virtual resistor component are each configured to increase in response to a current being greater than a predetermined threshold using a proportional-integral-derivative controller.

42. The virtual impedance module includes a virtual resistor and a constant virtual inductor; 39. The universal converter controller of claim 38, wherein the inductance of the constant virtual inductor and the resistance of the virtual resistor are each configured to decrease in response to a current falling below a predetermined threshold using an integrator.

43. a frequency loop including the adaptive inertial module configured to provide an output frequency; 12. The universal converter controller of claim 11, wherein the output frequency is maintained constant in a freeze mode when the terminal voltage falls below a predetermined value for a predetermined period of time.

44. 44. The universal converter controller of claim 43, wherein the output frequency goes out of the freeze mode when the terminal voltage is greater than a predetermined value for a predetermined period of time.

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