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The current pursuit of ambitious decarbonization targets is driving a swift transformation of the power grid, marked by a surge in the production of renewable energy. The expansion on application of renewable energy hinges significantly on Distributed Energy Resources but system operators are grappling with challenges due to the opaque nature of DER operations. This opacity introduces considerable risks to grid stability, as the burgeoning volume of DERs may surpass the existing power network's capacity. In response, the advent of Digital Twins (DT) technology offers a viable remedy by creating virtual counterparts of the physical grid infrastructure that necessitate transmitting minimal data. Digital Twins technology circumvents the hindrances associated with real-time data flows and bolsters the transparency of the system. To foster widespread implementation of DT within the sector, it is imperative to cultivate and validate its application through practical trials. To this end, Power Hardware-in-the-Loop (PHIL) experiments are employed to juxtapose the efficacy of actual power components against that of the DT models. The experiments involve connecting Grid-forming Inverter to a Realtime Digital Simulator (RTDS) for PHIL and DT testing, allowing for an in-depth analysis of the behaviour of photovoltaic inverter. This paper elucidates a platform engineered for immediate simulation tailored to DT and PHIL approaches. This platform is designed to prototype, exhibit, and evaluate grid-forming inverters under different scenarios that are critical for power restoration. With the help of simulation exchange, Perez Model is recommended to add in the DT model to increase the accuracy comparing with the PHIL model. The entire restoration process can therefore be comprehensively represented and analysed. [ABSTRACT FROM AUTHOR]

Grid‐forming inverters play a vital role in connecting renewable energy sources to the grid, and maintaining stable output voltage is essential for system operation. However, traditional dual‐PI voltage–current loop control suffers from slow response and weak disturbance rejection, leading to suboptimal control performance of grid‐connected inverter output voltage. Moreover, employing traditional dual‐PI control in grid‐connected inverters results in low output impedance, making them prone to subsynchronous oscillations and instability under strong grid conditions. To address these challenges, this study introduces a novel dual‐loop control strategy based on linear active disturbance rejection control (LADRC), wherein voltage loop is regulated by LADRC while current loop employs PI control. Quantitative analysis and experimental findings demonstrate that compared with traditional dual‐PI voltage–current loop control, the LADRC‐based dual‐loop control strategy offers higher bandwidth and lower steady‐state error, thereby enhancing the tracking speed and precision of grid‐forming inverter output voltage. The LADRC‐based dual‐loop control strategy reduces output impedance in grid‐forming inverter systems, lowering THD of output voltage and improving harmonic suppression under nonlinear loads. Experimental results show its robustness against strong grid conditions compared with traditional dual‐PI control, ensuring stable output voltage. [ABSTRACT FROM AUTHOR]

Grid-forming inverters (GFMs) have emerged as crucial components in modern power systems, facilitating the integration of renewable energy sources and enhancing grid stability. The significance of GFMs lies in their ability to autonomously establish grid voltage and frequency, enabling grids to form and improve system flexibility. Discussing control methods for grid-forming inverters is paramount due to their crucial role in shaping grid dynamics and ensuring reliable power delivery. This paper explores the fundamental and advanced control methods employed by GFMs, explaining their operational principles and performance characteristics. Basic control methods typically involve droop control, voltage and frequency regulation, and power-balancing techniques to maintain grid stability under varying operating conditions. Advanced control strategies encompass predictive control, model predictive control (MPC), and adaptive control, which influence advanced algorithms and real-time data for enhanced system responsiveness and efficiency. A detailed analysis and performance comparison of different control methods for GFM is presented, highlighting their strengths, limitations, and suitability for diverse grid environments. Through comprehensive studies, this research interprets the ability of various control strategies to mitigate grid disturbances, optimize power flow, and enhance overall system stability. [ABSTRACT FROM AUTHOR]

This paper proposes a control strategy for grid-following inverter control and grid-forming inverter control developed for a Solar Photovoltaic (PV)–battery-integrated microgrid network. A grid-following (GFL) inverter with real and reactive power control in a solar PV-fed system is developed; it uses a Phase Lock Loop (PLL) to track the phase angle of the voltages at the PCC and adopts a vector control strategy to adjust the active and reactive currents that are injected into the power grid. The drawback of a GFL inverter is that it lacks the capability to operate independently when the utility grid is down due to outages or disturbances. The proposed grid-forming (GFM) inverter control with a virtual synchronous machine provides inertia to the grid, generates a stable grid-like voltage and frequency and enables the integration of the grid. The proposed system incorporates a battery energy storage system (BESS) which has inherent energy storage capability and is independent of geographical areas. The GFM control includes voltage and frequency control, enhanced islanding and black start capability and the maintenance of the stability of the grid-integrated system. The proposed model is validated under varying irradiance conditions, load switching, grid outages and temporary faults with fault ride-through (FRT) capability, and fast frequency response and stability are achieved. The proposed model is validated under varying irradiance conditions, load switching, grid outages and line faults incorporating fault ride-through capability in GFM-based control. The proposed controller was simulated in a 100 MW solar PV system and 60 MW BESS using the MATLAB/Simulink 2023 tool, and the experimental setup was validated in a 1 kW grid-connected system. The percentage improvement of the system frequency and voltage with FRT-capable GFM control is 69.3% and 70%, respectively, and the percentage improvement is only 3% for system frequency and 52% for grid voltage in the case of an FRT-capable GFL controller. The simulation and experimental results prove that GFM-based inverter control achieves fast frequency response, and grid stability is also ensured. [ABSTRACT FROM AUTHOR]

When a grid-forming (GFM) inverter is connected to a low- or medium-voltage weak power grid, the line impedance with resistive and inductive characteristics will cause power coupling. Typical GFM decoupling control strategies are designed under nominal line impedance parameters. However, there are deviations between the nominal line impedance and actual parameters, resulting in poor decoupling effects. Aiming at this problem, this paper proposes a power decoupling strategy based on a reduced-order extended state observer (RESO). Firstly, the power dynamic model of the GFM is established based on the dynamic phasor method. Then, the model deviation and power coupling due to line impedance parameter perturbation are estimated as internal disturbances of the system, and the disturbances are compensated on the basis of typical power control strategy and virtual impedance decoupling. Good decoupling performance is obtained under different impedance parameters, improving the control strategy's robustness. Finally, the effectiveness of the proposed method is verified by the results of RT Box hardware-in-the-loop experiments. [ABSTRACT FROM AUTHOR]

With the increasing penetration of distributed energy resources (DER) in microgrids, DER power inverters have become a critical asset for providing power support to these microgrids. Meanwhile, the grid-forming (GFM) inverters, among these DER inverters, have gained significant attention in microgrid applications for their capability to enable the DERs to operate in different microgrid conditions and various operation modes. Moreover, with the implementation of these GFM inverters, smooth operation mode transition, GFM functions as well as black start functions can be obtained to improve the operation of the microgrid systems. In this article, a generalized control method for a single-phase GFM inverter is developed for community microgrid applications, facilitating smooth operation behavior in both operation modes with grid support functions and stable transition for different microgrid conditions. The control design procedure and function analysis of the proposed control method are explained in detail based on the community microgrid system. The effectiveness of the method in this paper is demonstrated on a 10 kW single-phase GFM inverter prototype with comparison to a model predictive method in recent literature. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The current pursuit of ambitious decarbonization targets is driving a swift transformation of the power grid, marked by a surge in the production of renewable energy. The expansion on application of renewable energy hinges significantly on Distributed Energy Resources but system operators are grappling with challenges due to the opaque nature of DER operations. This opacity introduces considerable risks to grid stability, as the burgeoning volume of DERs may surpass the existing power network’s capacity. In response, the advent of Digital Twins (DT) technology offers a viable remedy by creating virtual counterparts of the physical grid infrastructure that necessitate transmitting minimal data. Digital Twins technology circumvents the hindrances associated with real-time data flows and bolsters the transparency of the system. To foster widespread implementation of DT within the sector, it is imperative to cultivate and validate its application through practical trials. To this end, Power Hardware-in-the-Loop (PHIL) experiments are employed to juxtapose the efficacy of actual power components against that of the DT models. The experiments involve connecting Grid-forming Inverter to a Real-time Digital Simulator (RTDS) for PHIL and DT testing, allowing for an in-depth analysis of the behaviour of photovoltaic inverter. This paper elucidates a platform engineered for immediate simulation tailored to DT and PHIL approaches. This platform is designed to prototype, exhibit, and evaluate grid-forming inverters under different scenarios that are critical for power restoration. With the help of simulation exchange, Perez Model is recommended to add in the DT model to increase the accuracy comparing with the PHIL model. The entire restoration process can therefore be comprehensively represented and analysed.

The availability and cost of fossil fuels, natural disasters, aging infrastructure, climate change, and rising electricity consumption have affected today’s power grids. One of the most practical solutions for achieving green and reliable energy is the use of microgrids. The stability of microgrids dominated by electronic converters presents several challenges. Among the problems encountered are the absence of physical inertia, delay in detecting islanding, and loss of stability associated with the transition between operating modes and variations of the load power. To overcome these challenges, this study presents a new robust control strategy based on active disturbance rejection control (ADRC). It is suitable for both islanded and connected operation modes with a single control, without an islanding detection algorithm or Phase-Locked Loop (PLL). The effectiveness of the control strategy is demonstrated through simulations and a comparative analysis with conventional droop control. Flexibility of the transition is also ensured. The proposed control strategy is successfully validated using a TI C2000 DSP TMS320F28335 microcontroller.

Offshore wind is expected to be a major player in the global efforts toward decarbonization, leading to exceptional changes in modern power systems. Understanding the impacts and capabilities of the relatively new and uniquely positioned assets in grids with high integration levels of inverter-based resources, however, is lacking, raising concerns about grid reliability, stability, power quality, and resilience, with the absence of updated grid codes to guide the massive deployment of offshore wind. To help fill the gap, this paper presents an overview of the state-of-the-art technologies of offshore wind power grid integration. First, the paper investigates the most current grid requirements for wind power plant integration, based on a harmonized European Network of Transmission System Operators (ENTSO-E) framework and notable international standards, and it illuminates future directions. The paper discusses the wind turbine and wind power plant control strategies, and new control approaches, such as grid-forming control, are presented in detail. The paper reviews recent research on the ancillary services that offshore wind power plants can potentially provide, which, when harmonized, will not only comply with regulations but also improve the value of the asset. The paper explores topics of wind power plant harmonics, reviewing the latest standards in detail and outlining mitigation methods. The paper also presents stability analysis methods for wind power plants, with discussions centered on validity and computational efficiency. Finally, the paper discusses wind power plant transmission solutions, with a focus on high-voltage direct-current topologies and controls.

Recently, there is rapid integration of power electronic converter (PECs) into the power grid. Most of these PECs are grid-following inverters, where weak grid operation becomes an issue. Research is now shifting focus to grid-forming (GFM) inverters, resembling synchronous generators. The shift towards converter-based generation necessitates accurate PEC models for assessing system dynamics that were previously ignored in conventional power systems. Data-driven modeling (DDM) techniques are becoming valuable tools for capturing the dynamic behavior of advanced control strategies for PECs. This paper proposes using power hardware-in-the-loop experiments to capture dynamic GFM data in the application of DDM techniques. Furthermore, the paper derives an analytical approach to obtaining a mathematical model of GFM inverter dynamics and compares it with the DDM. A square-chirp probing signal was employed to perturb the active and reactive power of the load inside an Opal-RT model. The dynamic response of the GFM inverter, including changes in frequency and voltage, was recorded. This data was then used in a system identification algorithm to derive the GFM DDMs. The effectiveness of DDM is cross-validated with an analytical approach through experimental simulation studies, and the goodness-of-fit for both approaches is compared. Both approaches show more than 85% accuracy in capturing the dynamic response of GFM inverters under different loading conditions.

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A virtual synchronous generator (VSG) strategy can introduce the rotational inertia and damping characteristics of the synchronous generator to the static inverter, e.g., PV, wind generation, and ESS, which are used to enhance the system frequency support characteristics of the micro-grid. However, under various operations of the VSG, the inertia and damping support capabilities are different, and the conventional VSG strategy with a fixed control coefficient sacrifices a certain degree of dynamic regulation performance with less robustness. This research proposes an improved VSG strategy with adaptive inertia and damping coefficients to increase the flexibility of VSG. To this end, a mathematical model is first established to analyze the impact of various parameters on the characteristics of VSG, and then the root trajectory is used to explore the impacts of various rotational inertia and damping coefficients on the stability ofthe VSG system. Second, an improved control strategy with adaptive inertia and damping coefficients is proposed based on the characteristics of the second- order system and the system frequency deviations. Finally, a simulation system with the VSG is constructed based on MATLAB/Simulink. The effectiveness of the proposed control strategy is verified by comparing the simulation results to the conventional control strategy with the fixed control coefficients under over- frequency and under-frequency disturbances. [ABSTRACT FROM AUTHOR]

Grid-forming inverters, which are represented by droop control and virtual synchronous generator control, have been widely studied and applied because of their excellent grid-supporting ability and smooth off-grid switching. When a grid-forming inverter is connected to a microgrid or utility grid, the control loops of active power and reactive power will be coupled because of the voltage phase difference, which will affect the power control performance. This paper first derives the small-signal linearized model of the system, based on which a frequency feedforward control and an amplitude feedforward control are proposed to decouple the active power and reactive power control loops, respectively. The proposed decoupling strategy directly modifies the reference values through feedforward with an easily implementable principle that is applicable to various control coordinate systems, control coordinate systems, and control structures. By comparing system models with and without the proposed decoupling strategy, its effectiveness can be theoretically proven. Time-domain simulations and hardware experiments are presented to further validate its effectiveness. [ABSTRACT FROM AUTHOR]

Virtual inertia (VI) control schemes implemented in inverter-based resources (IBRs) present a promising solution for mitigating the absence of inertial response in power systems after the decommissioning of synchronous generators (SGs). The interaction of multiple grid-following (GFL) and grid-forming (GFM) inverters with the power grid demands a thorough understanding, as different control mechanisms can lead to undesired interference that could severely impact the grid’s stability. Power hardware-in-the-loop (PHIL) simulation is a verification technique that effectively bridges the gap between pure simulation and field demonstration. However, the topic of PHIL testing frameworks for multiple inverters equipped with VI control schemes has not been extensively addressed. To address this gap, this study presents a PHIL test framework that enables the verification of the performance and possible interference of multiple hardware inverters. The proposed framework enables the testing of inverters with different rated capacities and control mechanisms under equivalent capacity and control parameter conditions. This framework allows for the testing of various combinations of two inverters, from different manufacturers, to assess their performance and possible interference. The inverter prototypes from different manufacturers will be combined and tested with the proposed PHIL test framework to examine performance and potential interference for all combinations of the two inverters.

A virtual synchronous generator (VSG) strategy can introduce the rotational inertia and damping characteristics of the synchronous generator to the static inverter, e.g., PV, wind generation, and ESS, which are used to enhance the system frequency support characteristics of the micro-grid. However, under various operations of the VSG, the inertia and damping support capabilities are different, and the conventional VSG strategy with a fixed control coefficient sacrifices a certain degree of dynamic regulation performance with less robustness. This research proposes an improved VSG strategy with adaptive inertia and damping coefficients to increase the flexibility of VSG. To this end, a mathematical model is first established to analyze the impact of various parameters on the characteristics of VSG, and then the root trajectory is used to explore the impacts of various rotational inertia and damping coefficients on the stability of the VSG system. Second, an improved control strategy with adaptive inertia and damping coefficients is proposed based on the characteristics of the second-order system and the system frequency deviations. Finally, a simulation system with the VSG is constructed based on MATLAB/Simulink. The effectiveness of the proposed control strategy is verified by comparing the simulation results to the conventional control strategy with the fixed control coefficients under over-frequency and under-frequency disturbances.

Grid-forming inverters are the essential components in the effort to integrate renewable energy resources into stand-alone power systems and microgrids. Performance of these inverters directly depends on their control parameters embodied in the controller. Even the most conscientiously designed controller will exhibit suboptimal performance upon implementation due to the presence of parasitic elements in the existing hardware. Hence, the controller has to be tuned and optimized. In the present article, the process of implementation, laboratory verification, and tuning of a matching-controlled grid-forming inverter is presented. In order to assess the efficiency of the grid-forming controller, its operation has been tested and analyzed in blackstart, steady state, and transient operation. For this purpose, a systematic sensitivity analysis has been conducted and the control parameters have been tuned in laboratory tests. The laboratory results verify proper operation of a 7 kW grid-forming inverter in all three test scenarios. After applying the proposed method on the tested grid-forming inverter in steady state operation, total harmonic distortion (THD) of the output voltage is less than 0.5% for its practical loading range (maximum THD is less than 1% in no-load condition). The system is able to blackstart and supply the loads. Finally, the studied grid-forming inverter is stable in the presence of severe step load changes and disturbances, i.e., voltage overshoot is managed well and compensated for with a low settling time using this approach. [ABSTRACT FROM AUTHOR]

Virtual synchronous generators (VSGs) are one of the most relevant solutions to integrate renewable energy in weak grids and microgrids. They indeed provide inverters characteristics of rotating machines (inertia for instance) that are useful for stabilizing the system, notably in the context of the high variability of the production. Thanks to the virtual characteristics of the VSG, the virtual parameters of the emulated synchronous machine can be optimally adapted online as a function of the electric environment of the inverter. We call that inverter's control a polymorphic VSG. The online adaptation of the critical control parameters of the VSG helps reduce the risk of deterioration of the inverter's constituents that might be induced by harsh events (frequent in weak grids) but, more importantly, improves the robustness of the system. In this paper, four implementations of a polymorphic VSG controller are compared on a simple microgrid study case to a complete VSG model. For the test, polymorphic VSGs have to minimize frequency and voltage oscillations while withstanding short circuits, which is typically a requirement for units in this context. One of the controls is based on recurrent optimization over a prediction time horizon, and two sub-optimal ones target practical implementation in industrial inverters with limited computational power. Results show a clear reduction in incidents in the microgrid thanks to the controllers. The error reduction with the complete polymorphic VSG is up to 100% for the voltage, 32% for the currents, and 79% for the duty ratio. Those values are decreased by 30 to 50% with the sub-optimal controllers but for a reduction in the computational burden of more than 97%. Recommendations are proposed for the development of an auto-adaptive polymorphic VSG from a high technology-readiness-level perspective, i.e., targeting a compromise between error reduction and computational burden. [ABSTRACT FROM AUTHOR]

The grid-forming inverter (GFI) plays an indispensable role in the microgrid (MG). Thus, it is critical for a microgrid to control the GFI flexibly and accurately, because of its voltage-forming and frequency regulation capabilities. The conventional proportional integral (PI) control strategy is usually used to control the GFI. However, the conventional PI control strategy has a high output voltage total harmonic distortion (THD) and deteriorative performance under external disturbances, which results in several problems in system flexibility, reliability and stability. This paper proposed a novel adaptive sliding mode control (ASMC) based cascaded control of the GFI system, which involves an ASMC-based inner current loop and outer voltage loop, and shows some superior performances of low THD, fast dynamic tracking performance and good robustness. Both the MATLAB/SIMULINK-based simulation and StarSim Modelling Tech-based semi-physical experimental results were presented to validate a perspective: the control strategy proposed by this paper has a better performance than the conventional PI control strategy under transient and steady-state condition. [ABSTRACT FROM AUTHOR]

Grid-forming inverters (GFMIs) are promising technologies that can replace some of the capabilities traditionally provided by synchronous generators (SGs), such as inertial response. While there have been many simulation-based studies on GFMIs, performance testing has not been adequately discussed. Power hardware-in-the-loop (PHIL) simulation is an attractive option for testing GFMIs. The interaction between GFMI and power systems can be observed under a variety of conditions, including low-inertia and contingency conditions. Since PHIL testing was primarily used for grid-following inverters (GFLIs), the PHIL configuration needs to be adjusted to test GFMIs, which respond faster to changes in grid voltage than GFLIs. This paper proposes the PHIL setup for testing GFMIs. It utilizes the PHIL interface developed for testing GFLIs and adjusts it for testing GFMIs. The stability and accuracy of the PHIL testing are evaluated in terms of frequency stability in low-inertia power systems by comparing test and simulation results.

Grid-forming inverters (GFMs) have emerged as crucial components in modern power systems, facilitating the integration of renewable energy sources and enhancing grid stability. The significance of GFMs lies in their ability to autonomously establish grid voltage and frequency, enabling grids to form and improve system flexibility. Discussing control methods for grid-forming inverters is paramount due to their crucial role in shaping grid dynamics and ensuring reliable power delivery. This paper explores the fundamental and advanced control methods employed by GFMs, explaining their operational principles and performance characteristics. Basic control methods typically involve droop control, voltage and frequency regulation, and power-balancing techniques to maintain grid stability under varying operating conditions. Advanced control strategies encompass predictive control, model predictive control (MPC), and adaptive control, which influence advanced algorithms and real-time data for enhanced system responsiveness and efficiency. A detailed analysis and performance comparison of different control methods for GFM is presented, highlighting their strengths, limitations, and suitability for diverse grid environments. Through comprehensive studies, this research interprets the ability of various control strategies to mitigate grid disturbances, optimize power flow, and enhance overall system stability.

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Grid-forming inverters are anticipated to be integrated more into future smart microgrids commencing the function of traditional power generators. The grid-forming inverter can generate a reference frequency and voltage itself without assistance from the main grid. This paper comprehensively investigates grid-forming inverter modelling and control methodology. A decentralized method employing an active power versus frequency P − f droop and a reactive power versus voltage Q − V droop is exploited to drive the operation of the grid-forming inverter. This decentralized method ensures balancing the supply and demand beside the power-sharing task between two or more inverters. The performance of the grid-forming inverter is examined by monitoring the frequency and RMS voltage of the inverter bus for three different periods of a varying PQ load. In addition, the performance of the resultant droop is compared with the assumed droop to validate the effectiveness of the proposed method. Finally, two grid-forming inverters equipped with the same droop characteristics are connected to a single load to observe the power-sharing concept among them. All simulations are implemented and executed using Matlab/Simulink version R2014b. [ABSTRACT FROM AUTHOR]

The hybrid power- and voltage-based synchronization control method has shown potential for enhancing the stability of grid-forming (GFM) inverters. However, its effectiveness may be compromised if other control loops are not properly designed. To address the control-loop interactions, this paper presents a design-oriented analysis method for multiloop-controlled GFM inverters. The method begins by identifying the dominant oscillation modes through modal analysis. The sensitivities of damping ratios to control parameters are then determined for the dominant modes, which allows for characterization of control-loop interactions. A co-design method of GFM control is next developed based on the sensitivity analysis. Lastly, simulations and experimental results are presented to confirm the effectiveness of the method.

Developing advanced control functions for inverter-based resources (IBRs) to replace the inertial response of retiring synchronous generators (SGs) is progressing rapidly. Although various control mechanisms have been proposed for grid-following (GFL) inverters and grid-forming (GFM) inverters, the comprehensive comparison of their performance in contributing to grid stabilization based on hardware testings has not been studied well. This paper performs power hardware-in-the-loop (PHIL) testing for multiple inverter prototypes from different manufacturers and compares their performance concerning the difference in inverter types, control methods, and IBR penetration conditions in the power system. It also tests the possible interference between islanding detection methods (IDMs) and frequency control capabilities of GFL and GFM inverters.

In recent years, integrating renewable energy sources (RESs) has achieved significant attention due to the growing demand for sustainable energy solutions. Inverter-interfaced Islanded Microgrids (IGs) have appeared as an advantageous approach to integrating RESs into the power grid. Grid-forming inverters (GFIs) are a critical component of IGs, and their synchronization is essential for stable and reliable operation. The literature has widely proposed soft transition, pre-synchronization, and re-synchronization to synchronize IGs to the main grid. However, methods for synchronizing GFIs in the islanded microgrid are restricted. Parallel operation of GFIs is required to guarantee the high-power demand of IG and improve voltage-frequency stability. For parallel operation, GFIs must be synchronized with each other. In the conventional synchronization control systems that are highly nonlinear, the linear proportional-integral (PI) controllers are commonly used in synchronization loops without considering the nonlinearity resulting from the initial condition dependency and cross-coupling. Thus, conventional synchronization methods can be exposed to concerns of stable operation, narrow operation area, and performance degradation. This study proposes a new linearized synchronization control system for GFIs in IGs. In this way, it is possible to analytically design robust linear controllers and ensure a stable operation, high performance, and wide (full) operation area. In addition, a new soft-commissioning method is proposed to deactivate synchronization loops and soft-start the synced GFI. The proposed system has been tested in real-time and CHIL hardware setups for two 550 kW GFIs operating in parallel, and the results in the perfect agreement are presented in this study.

Grid-forming (GFM) inverters are promising technologies in future power systems. Although the voltage-source characteristic of the GFM inverter has been validated to enhance the stabilities in low-inertia power systems, modifying protective function mechanisms is needed from grid-following (GFL) inverters with the current-source characteristic. Therefore, the protective functions that coexist with grid stabilization capabilities in GFM inverters and their verification methods should be studied for practical deployment. This paper applies the existing Japanese conformance testing on prototypes of GFL and GFM inverters to reveal issues in the existing conformance testing framework, which was not designed to consider the voltage-source characteristics of GFM inverters. The test results identify the three main issues of GFM inverters in the existing test framework. The discussion based on the results will provide helpful knowledge to design future provisions on the functional requirements of GFM inverters and their verification methods.

Grid-forming inverters, which are represented by droop control and virtual synchronous generator control, have been widely studied and applied because of their excellent grid-supporting ability and smooth off-grid switching. When a grid-forming inverter is connected to a microgrid or utility grid, the control loops of active power and reactive power will be coupled because of the voltage phase difference, which will affect the power control performance. This paper first derives the small-signal linearized model of the system, based on which a frequency feedforward control and an amplitude feedforward control are proposed to decouple the active power and reactive power control loops, respectively. The proposed decoupling strategy directly modifies the reference values through feedforward with an easily implementable principle that is applicable to various control coordinate systems, control coordinate systems, and control structures. By comparing system models with and without the proposed decoupling strategy, its effectiveness can be theoretically proven. Time-domain simulations and hardware experiments are presented to further validate its effectiveness.

Grid-forming inverters are the essential components in the effort to integrate renewable energy resources into stand-alone power systems and microgrids. Performance of these inverters directly depends on their control parameters embodied in the controller. Even the most conscientiously designed controller will exhibit suboptimal performance upon implementation due to the presence of parasitic elements in the existing hardware. Hence, the controller has to be tuned and optimized. In the present article, the process of implementation, laboratory verification, and tuning of a matching-controlled grid-forming inverter is presented. In order to assess the efficiency of the grid-forming controller, its operation has been tested and analyzed in blackstart, steady state, and transient operation. For this purpose, a systematic sensitivity analysis has been conducted and the control parameters have been tuned in laboratory tests. The laboratory results verify proper operation of a 7 kW grid-forming inverter in all three test scenarios. After applying the proposed method on the tested grid-forming inverter in steady state operation, total harmonic distortion (THD) of the output voltage is less than 0.5% for its practical loading range (maximum THD is less than 1% in no-load condition). The system is able to blackstart and supply the loads. Finally, the studied grid-forming inverter is stable in the presence of severe step load changes and disturbances, i.e., voltage overshoot is managed well and compensated for with a low settling time using this approach.

Virtual synchronous generators (VSGs) are one of the most relevant solutions to integrate renewable energy in weak grids and microgrids. They indeed provide inverters characteristics of rotating machines (inertia for instance) that are useful for stabilizing the system, notably in the context of the high variability of the production. Thanks to the virtual characteristics of the VSG, the virtual parameters of the emulated synchronous machine can be optimally adapted online as a function of the electric environment of the inverter. We call that inverter’s control a polymorphic VSG. The online adaptation of the critical control parameters of the VSG helps reduce the risk of deterioration of the inverter’s constituents that might be induced by harsh events (frequent in weak grids) but, more importantly, improves the robustness of the system. In this paper, four implementations of a polymorphic VSG controller are compared on a simple microgrid study case to a complete VSG model. For the test, polymorphic VSGs have to minimize frequency and voltage oscillations while withstanding short circuits, which is typically a requirement for units in this context. One of the controls is based on recurrent optimization over a prediction time horizon, and two sub-optimal ones target practical implementation in industrial inverters with limited computational power. Results show a clear reduction in incidents in the microgrid thanks to the controllers. The error reduction with the complete polymorphic VSG is up to 100% for the voltage, 32% for the currents, and 79% for the duty ratio. Those values are decreased by 30 to 50% with the sub-optimal controllers but for a reduction in the computational burden of more than 97%. Recommendations are proposed for the development of an auto-adaptive polymorphic VSG from a high technology-readiness-level perspective, i.e., targeting a compromise between error reduction and computational burden.

The objective of this paper is to analyze and identify the range of voltage balancing capability of grid-forming inverters serving three-phase unbalanced loads. These inverters are designed to compensate for voltage imbalance by controlling the negative-sequence components of voltage and current. However, the magnitude of the negative-sequence current an inverter can supply is limited by its relatively low rated current. Moreover, it becomes more challenging to estimate the amount of current needed for an unbalanced load when the inverter is interfaced using a delta-wye grounded interconnection transformer. Therefore, we investigate the range of negative-sequence current the inverter can supply and derive formulas to determine the minimum inverter’s capacity required to compensate for voltage imbalance while supplying unbalanced loads connected through a delta-wye grounded transformer. The proposed formulas can be used to estimate the capacity of an inverter in lieu of detailed analyses and electromagnetic transient simulations. The proposed equation is implemented in small and large scale microgrid systems and validated using a detailed model developed in PSCAD/EMTDC.

This article compares two strategies for seamless (re)connection of grid-forming inverters to a microgrid powered by droop-controlled inverters. While an incoming inverter must be synced to the microgrid, seamless syncing and power-sharing are technical challenges for grid-forming inverters. In the first strategy, called the output-sync method, an incoming inverter is synced to the microgrid, and then the circuit breaker is closed for power-sharing. In the second strategy, called the controller-sync method, the inverter initially contributes at zero power following the microgrid frequency, and then the controller is transferred to the mode of power-sharing. Remarkably, the circuit breaker can be kept closed during the syncing process using the controller-sync method as the inverter starts from zero power contribution. In this novel strategy, two controller sets run in parallel, i.e., two parallel control paths are in place for obtaining the magnitude and the phase angle of the PWM reference signal. These parallel paths are synced for seamless transitions when only one of them is engaged to generate the PWM reference signal. The efficacy of these control strategies has been tested in a hardware setup of a microgrid fed by two 5kVA 208V droop-controlled inverters, and the results are presented in this article. This article is also accompanied by a video clip demonstrating the performance of the output-sync and controller-sync methods in the laboratory.

Driven by the demands of carbon footprint reduction, the conventional fossil-based generators are gradually being replaced by grid-forming inverter. One crucial aspect of grid-forming inverter is synchronization stability, which is not only related to the inverter power-frequency (P-f) dynamic but also influenced by power grid characteristics as well as grid impedances. From the perspective of a local inverter, it is difficult to know the detailed information. As a result, severe oscillations and even the loss of synchronization may occur and pose a great threat to the power grid. To overcome this problem, a black-box approach is presented in this article. Initially, a frequency-domain identification approach is proposed to acquire the power grid characteristic by the inverter itself. Based on the measurement results, the P-f dynamic of grid-forming inverter is explicitly reshaped to ensure the synchronization stability. The proposed methodology does not require any prior power system knowledge or the trivial modeling process. Finally, the effectiveness of the proposed method is verified by hardware experimental results. [ABSTRACT FROM AUTHOR]

Currently, there is a large-scale penetration of renewable energy sources (RES) in power systems around the world, most of which are based on the power converter technology. The result of such penetration is the emergence of frequency and voltage regulation problems in power systems with RESs. An effective way to solve the mentioned problems is the use of energy storage systems (ESS), among which hydrogen ESSs are promising. However, the quality of such solution mainly depends on the approach to the power converters control. One of the most effective is a grid-forming power converter control strategy. Within this strategy, many scientific groups have proposed the concept of a virtual synchronous generator (VSG). However, the existing VSG models use a series structure with a stiff direction of signals, which completely mimics conventional synchronous generators. In this regard, the fundamental problems inherent in such a structure arise, e.g., a coupling between active and reactive power control. This paper describes a freely configurable VSG model (FC-VSG), in which the structural control blocks can be transferred from one level to another, and the levels themselves can be installed not only in series, but also in parallel. The case studies carried out made it possible to identify the configuration of the FC-VSG, which excludes the fundamental problems inherent in the conventional VSG structures. At the same time, the most efficient and reliable frequency and voltage regulation is provided, which has been proved by qualitative and quantitative comparison, including with the conventional VSG structure. [ABSTRACT FROM AUTHOR]

The presence of unbalanced loads in power systems creates voltage imbalances that perturb the operation of sensitive loads, such as induction motors, power electronics converters, and adjustable speed drives. This article presents a novel control scheme that achieves accurate voltage balance at any node of an islanded ac microgrid with seamless integration with the existing hierarchical control of the microgrid. A master–slave architecture is adopted for the unbalance compensation due to its low network traffic requirements and its modular conception. The unbalance compensation follows a collaborative principle where current injections are computed as a function of the current rating of each inverter, thus avoiding the overcurrent stress of the unbalance compensation by a single inverter. This article presents the stability analysis and the design guidelines for the control parameters. The performance of the control proposal has been successfully tested in a laboratory microgrid. Selected experimental results are presented. [ABSTRACT FROM AUTHOR]

Reliable power system operation with 100% inverter-based resources (IBRs) is an unsolved and challenging problem. One of the most challenging factors is ensuring power system stability after N-1 contingencies. This paper presents a promising solution using an operator support system (OSS) to enable stable operation of power system with up to 100% IBR generation. The OSS consists of two components. First is dynamic security assessment to evaluate the system resiliency, and identify critical N-1 contingencies that could endanger the system. The second component, as the key technology behind the OSS, is dynamic security optimization (DSO). The DSO optimizes the control parameters of generators and inverters to improve the stability of the system towards the identified N-1 contingencies. The key to system with 100% IBRs, as emphasized in many recent studies, is to establish the grid frequency reference using grid-forming (GFM) inverters. We show through high-fidelity Electro-Magnetic-Transient (EMT) simulations of the future generation models of Hawai‘i Island system with 100% IBR capacity that a system with 100% IBRs can be operated stably with the help of GFM inverters, and appropriate controller parameters can be found by DSO for the inverters. The DSO is verified via 28 critical N-1 contingencies of Hawai‘i Island system identified by Hawaiian Electric. The simulation results verify the effectiveness of DSO, and show significant stability improvement from DSO. [ABSTRACT FROM AUTHOR]

In the past decade, inverter-integrated energy sources have experienced rapid growth, which leads to operating challenges associated with reduced system inertia and intermittent power generation, which can cause instability and performance issues of the power system. Improved control schemes for inverters are necessary to ensure the stability and resilience of the power system. Grid-forming inverters dampen frequency fluctuations in the power system, while grid-following inverters can aggravate frequency problems with increased penetration. This paper aims at reviewing the role of grid-forming inverters in the power system, including their topology, control strategies, challenges, sizing, and location. In order to facilitate continued research in this field, a comprehensive literature review and classification of the studies are conducted, followed by research gaps and suggestions for future studies. [ABSTRACT FROM AUTHOR]

The high-penetrated renewables enable flexible and versatile dispatchable resources in power systems. As the digitalization process improves with the help of information and communication technology (ICT), a real-time online control framework is essential to regulate such a low-inertia power system dominated by renewables. To enable control practices, a control-oriented model is developed linking load increments to frequency dynamics. Based on such a control-oriented model, a model predictive control (MPC)–based online feedback algorithm is well designed by the virtue of rapid data transmission enabled by digitalization. The MPC optimization is a convex quadratic programming, where an objective function that balances frequency deviations and load increments is formulated and operational constraints are integrated into a matrix-type inequality. In the end, the proposed MPC-based load-frequency control framework is illustrated by several case studies, where validations and analyses of constraints inside the model are also included.

More and more renewable energy sources are integrated into power grids, leading to a power electronic-based low-inertia power system. The grid-forming (GFM) inverter is an effective method for improving the inertia of the system. However, with the increased GFM inverters in the system, how the multiple control parameters affect the frequency response is still not clear. In this study, first, the power-phase model of the power grid is established; then, a small-signal distributed frequency model of the GFM inverter-based power system is established associating with the power-phase model of the power grid and the power-frequency model of the GFM inverter. Based on the proposed model, the influence of the multiple parameters to the frequency response is analyzed. It is concluded that both the inertia and damping coefficient affect the settling time, overshoot, and oscillation of the frequency. Finally, the simulation results verify the proposed model and the conclusion.

Grid-forming (GFM) inverters are promising solutions for future power systems with high penetration of renewables. The successful design of GFM inverters is crucial for achieving current limiting, enabling them to withstand disturbances such as voltage sags and phase jumps in grid voltages. However, when the parameters are not properly tuned, a cascaded voltage and current controller can compromise grid stabilization performance due to issues like controller windup, post-fault recovery failure, and harmonic instability. To address this challenge, this paper proposes a dual control scheme consisting of GFM and current control that work in parallel. The dual control scheme incorporates emulation signal generators integrated with the dual controllers. This integration allows for adjusting their internal states during standby mode to track to references, enabling a seamless transition between the running and standby modes. Numerical simulations and experiments demonstrate that the proposed controller exhibits promising performance under both normal and faulted conditions. • The proposed controller eliminates the issues caused by the cascaded controller. • The proposed controller avoids transient instability during the current limitation. • The proposed emulation signal generator enables smooth switching between GFM and current control. [ABSTRACT FROM AUTHOR]

In remote arctic communities, where access to a bulk electrical grid interconnection is not available, the implementation of islanded microgrids has been the most viable way to produce and distribute electricity services to their inhabitants. Historically, these islanded grids have relied primarily on diesel generators or hydropower resources to supply the baseload. However, this practice can result in increased expense due to the high costs associated with fuel transportation and the significant amounts of on-site storage necessary when fuel transportation is unavailable during winter months. In order to mitigate this problem, arctic microgrids have started to transition to a hybrid-source operational mode by incorporating renewable energy sources that are inherently variable in nature, such as wind or solar. Due to their highly stochastic behavior, these hybrid-source islanded microgrids can pose potential issues related to power quality due to introduction of rapid net load fluctuations and inability of diesel generators to respond rapidly. In addition, non-firm stochastic sources can require significant idling diesel generator resources to serve as spinning reserves, which is inefficient and wasteful. This work studies the problems that may arise in the transient dynamics of a real-world hybrid diesel microgrid when subjected to a loss of wind generation. Moreover, this work proposes a transition from a diesel spinning reserve to a battery energy-storage system (BESS) operating reserve scheme. The study of the proposed transition is important in establishing the fundamental implication of transient dynamics and the potential benefits of integrating a BESS as a spinning reserve in terms of stability, frequency nadir, and transient voltage deviation. The methods to investigate and validate the transient dynamics relied on both electromagnetic simulation models of GFMIs and a commercially available GFMI in an experimental power hardware-in-the-loop setup. The simulation results showed that the proposed operating reserve scheme improves the power quality of the system in terms of voltage deviation and frequency nadir when the microgrid is subjected to a loss of wind generation scenario. Depending on the simulation cases, adding a GFMI reduced the frequency nadir between 65.3% and 86.7%. Moreover, the reductions in the voltage deviations improved between 3.6% and 23.0%. From these results it can be concluded that the integration of a GFMI can reduce the frequency nadir in a hybrid diesel microgrid, and in turn, reduce diesel consumption, which improves system reliability while reducing fuel expenses. Furthermore, the novelty of this work relies on the fact that the offline simulation results were validated using a power hardware-in-the-loop platform that incorporated a 100 kVA commercially available GFMI as the device under test. [ABSTRACT FROM AUTHOR]

This work proposes positive- and negative-sequence equivalent circuits of grid-forming inverters for steady-state analysis. The proposed models are especially attractive for performing long-duration voltage regulation analysis and short-circuit studies involving grid-forming inverters. Our proposed equivalent circuit models are based on the inverter’s voltage and current control loops in the $\alpha \beta $ and dq frames. For this reason, they operate according to prescribed control functions and specified impedances (i.e., filter impedance, current limiter block, virtual admittance block, and PI/PR controller block). The equivalent circuit model accuracy is validated by comparing system steady-state voltage and current responses obtained by detailed time-domain models in PSCAD/EMTDC to those by the equivalent circuit models implemented in steady-state load flow program (e.g., OpenDSS). Two distinct control structures implemented in the $\alpha \beta $ and dq frames are used for the validation. Single line-to-ground and line-to-line-to-ground faults are simulated in a small islanded microgrid as well as the IEEE 34-node test feeder. Fault impedances varying from 0 to 5 ohms are simulated. We show that the equivalent models precisely replicate the steady-state response of the detailed time-domain models.

Unlike a synchronous generator that could withstand a large overcurrent, an inverter-based distributed generation (DG) has low thermal inertia, and the inverter is likely damaged by overcurrents during grid faults. In this article, a new strategy, namely positive- and negative-sequence limiting with stability enhanced P-f droop control (PNSL-SEPFC), is proposed to limit the output currents and active power of droop-controlled inverters in islanded microgrids. This strategy is easy to implement in the inverter controller and does not require any fault detection. Inverter stability is analyzed mathematically, which gives guidelines to design the parameters of the PNSL-SEPFC strategy. PSCAD/EMTDC simulation based on a four-DG microgrid shows that the proposed PNSL-SEPFC can limit inverter output currents and powers with better performance under both symmetrical and asymmetrical faults. Furthermore, hardware experiments demonstrate that the proposed PNSL-SEPFC can ensure the inverters riding through grid faults safely and stably. (A video of experimental waveforms is attached.) [ABSTRACT FROM AUTHOR]

The increasing penetration of behind-the-meter distributed energy resources opens up opportunities to improve distribution grids’ resilience and reliability. In this context, the paper studies islanded operation of radial low-voltage (LV) distribution feeders with a high penetration of prosumers equipped with grid-forming inverters. In contrast to the existing studies that focused on droop control as the main source of instability, we extend the analysis to the microgrid topology, in particular the number of grid-forming inverters. The eigenvalue analysis shows that with the increasing number of grid-forming inverters, the system becomes practically unstable with its critical eigenvalue approaching zero as progressively more inverters are added. Furthermore, we extend the analysis by explicitly modelling the DC side dynamics, including the DC/DC converter with its controller and inherent dynamic response of the DC energy source. Eigenvalue analysis shows that the DC voltage controller impacts the system stability by introducing new dominant eigenvalues. The results suggest that the DC energy source should have a fast response to ensure a stable operation. The small-signal stability results, benchmarked against time-domain simulations, indicate that careful parameter tuning is required to ensure stable islanded operation of LV feeders. [ABSTRACT FROM AUTHOR]

Summary: In this paper, an improved fractional order notch filter (IFONF)‐based grid‐forming inverter control scheme is employed to mitigate the harmonics in photovoltaic system applications. Moreover, a fuzzy gain tuning proportional‐integral‐derivative (FTPID) controller is developed for the dc‐link voltage to maintain its reference value. The proposed FTPID‐IFONF control scheme is executed to evaluate the active fundamental weight components (FWC) of the load currents and distorted grid voltages consequently generates the PWM pulses to the grid‐forming inverter for power quality enrichment in a grid integration with photovoltaic applications. The integer order based filters have some constraints due to differentiator term and fixed integrator. For this reason, an improved design of an integer order notch filter is suggested in the proposed FTPID‐IFONF controller scheme. The behavior of the proposed FTPID‐IFONF control scheme is found good in‐terms of elimination of dc offset, grid‐synchronization, separation of fundamental components, adaptive behavior and harmonic elimination at the consumer terminals. The proposed controller is validated under several load and grid dynamic conditions such as steady‐state, grid voltage sag‐swell, distorted, load dynamic and removed conditions on MATLAB/Simulink platform. A prototype PV integrated with grid setup is constructed to validate the proposed system in the real‐time. The FTPID‐IFONF control scheme is well justified under several grid and load dynamic conditions and the harmonic distortions of the grid characteristics are found well based on the limits of IEEE‐519 standards. A comparative study is also analyzed to show the effectiveness of the controller with recently published works. [ABSTRACT FROM AUTHOR]