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Renewable power plant based on the grid-forming converter has a similar performance with the traditional thermal power plant on the function of active support for the frequency and voltage in the power system. An aggregation model is proposed for the frequency and voltage stability analysis of new power system, the overall operation characteristics of the plant are analyzed, and a method for identifying and selecting the parameters of the aggregation model is proposed. The proposed aggregation model can accurately reflect the dynamic process of the interaction between the renewable power plant and the grid, and ensure a quick simulation rate. In comparison with the electromagnetic transient model for grid-forming renewable power plant, the effectiveness of the proposed aggregation model is verified in MATLAB/Simulink. The accuracy and rapidity of the proposed aggregation model is verified in the frequency and voltage stability simulation analysis of power system based on the case study in the IEEE 39 bus system.

When grid-following (GFL) and grid-forming (GFM) converters are interconnected with the grid, they are mutually coupled through the grid impedance. During grid faults, the transient behavior of these converters becomes intricate due to the presence of coupling terms. Instabilities in one converter can propagate to the other, posing challenges to fault ride-through capability. Firstly, a mathematical model was established for a hybrid grid-tied system comprising two converters, investigated the conditions for equilibrium points, and employed phase-plane analysis to elucidate the mutual influence mechanism during faults. Then, a method to adjust the power angle of GFM according to the voltage drop was proposed and used in GFL to improve its phase-locked loop. At the same time, considering the influence of GFL current injection, the fault current of GFM was limited, so as to realized the hybrid fault ride-through control. The proposed approach ensures that GFM converters contribute reactive current support to the grid during faults, while GFL converters adhere to grid guidelines for current injection, thereby preventing overcurrent and phase angle instability in both converters. Finally, simulation results validated the correctness and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Shanghai Jiao Tong University (1006-2467) is the property of Journal of Shanghai Jiao Tong University Editorial Office 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.)

When the Grid-Following (GFL) and the Grid-Forming (GFM) converters are hybrid-connected to the grid, they are coupled through the grid impedance. During grid faults, the transient characteristics of the two converters become more complex due to this coupling. If one of the converters experiences stability issues, it affects the other, making fault ride-through challenging. A mathematical model for the hybrid grid-connected system of the two converters is first established to analyze the existence conditions of the equilibrium point. Using the phase-plane method, the mutual influence mechanism during faults is revealed. Subsequently, a method to adjust the GFM phase angle based on the degree of voltage sag is proposed, which also improves the phase-locked loop (PLL) of the GFL. The influence of GFL current injection is considered to limit the GFM fault current, thereby achieving hybrid fault ride-through control. Finally, the simulation verifies the effectiveness of the proposed control strategy. The results show that the proposed method can adjust the phase angle to support the grid, ensuring that the GFM outputs more reactive current within the maximum allowable current range. Meanwhile, the GFL injects current according to grid guidelines, effectively preventing overcurrent and phase angle instability of the converters.

Introduction: Grid-forming control has received increasing attention for being an effective solution to cope with low-inertia and weak damping systems. Owing to the basic characteristics of transient voltage regulation, inertia support and primary frequency regulation (PFR), virtual synchronous generator (VSG) is the most promising candidate of grid-forming control scheme. The damping characteristic plays a significant role in stabilizing when the system is disturbed. However, the traditional approaches for damping emulation pose a number of problems, such as the introduction of phase-locked loop (PLL) that may lead to stability issues, or the blurring of the functional distinction between damping characteristic and primary frequency regulation. Moreover, the grid strength affects the operational characteristics of the converters. Methods: Based on the background of these issues, firstly, an effective transient damping power strategy is proposed in this paper. In contrast to conventional damping approaches, the proposed scheme provides a positive damping during transient period that suppresses the fluctuation of active power, and has no impact on the steady-state frequency droop characteristic. Ulteriorly, based on small-signal models and classical control theory, an parameters adjustment strategy for both active and reactive power control is proposed to considerably enhance the adaptability of the converter to the variations in grid strength. Results and Discussion: The results obtained from PSCAD/EMTDC and hardware-in-the-loop (HIL) platform verify that proposed control strategy exhibits excellent transient damping effect, the decoupling characteristic between fixed damping coefficient and PFR coefficient, and performs well across a broad spectrum of grid strengths. [ABSTRACT FROM AUTHOR]

Currently, research on the small-signal stability of grid-forming converters under two control modes, grid-following and grid-forming, mainly focuses on low-level converters. However, studies on the differences between grid-following modular multilevel converters (MMC) and grid-forming MMC are limited. A comprehensive analysis of the wideband oscillation risk differences in MMC-HVDC systems under these two control modes from the perspective of impedance characteristics is necessary. Therefore, based on the harmonic state-space (HSS) modeling theory, this article establishes wideband sequence impedance models for grid-forming and grid-following MMC. Subsequently, this article identifies key control parameters affecting the impedance characteristics of MMC under two control modes. The wideband oscillation risks of MMC-HVDC systems under both control modes are compared. The study indicates that when the grid strength weakens, grid-following MMC faces instability risks, while grid-forming MMC can maintain stability. In instances where high-frequency resonance peaks exist in the impedance of the grid, both control modes of MMC may face instability risks. Furthermore, by adjusting the obtained key control parameters of MMC, this article effectively suppresses the oscillation risk discussed above. Finally, the analysis results are verified through an electromagnetic transient simulation. [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.)

With the current trend, the power generation in the future power grids is expected to consist of a large number of converter-interfaced generations (CIGs) with a small percentage of synchronous generators (SGs) producing power from hydro, solar thermal, or even nuclear resources. These notable changes pose significant challenges to the secure operation of the power grid. One such issue is new cascading failure mechanisms introduced by CIGs, especially due to the dc-link voltage collapse in a dc-current limited grid-forming converter (GFC) following a generation outage. Therefore, in the online dynamic security assessment (DSA), the system operators would be interested in knowing the critical outages that may trigger this cascading mechanism in the system. To address this issue, this paper presents a contingency screening and ranking approach (which can be a part of a DSA) for generator outages in a modern grid that consists of SGs and GFCs. Our specific focus is to identify the critical outages that may lead to a cascading failure in the system due to a dc-voltage collapse in the GFCs. To that end, first, a novel contingency screening and ranking index for an (N-1) contingency scenario (i.e., outage of a single generator) is derived based on the traditional generator power tracing algorithm. Then, it is shown that the proposed ranking index can be extended to any (N-k) contingency scenarios. Finally, the effectiveness of the proposed approach following generator outages (up to (N-2) contingencies) is demonstrated using detailed time-domain simulations in two different modified test configurations of a 16-machine 68-bus New York-New England test system with GFCs using the MATLAB/Simulink platform.

The broad demonstration of grid-forming converters (GFMs) in microgrid applications has been well documented. Following this, the idea of GFM was assessed for its potential use in large-scale linked networks that include transmission and distribution systems combined with renewable energy sources. As a result, a thorough examination of GFM performance became imperative. This study provides a comprehensive examination of GFMs, encompassing fundamental principles, control mechanisms, implementation strategies, operational aspects, and potential avenues for further research. The capacity of GFMs to withstand system disturbances is of utmost importance for ensuring the overall stability of the system, particularly in situations where inertial response is diminished. Classical control methods are inadequate in addressing transient stability circumstances, necessitating the development of current control approaches to overcome these limitations. The utilization of GFM control is becoming increasingly important in the global initiative to establish a sustainable energy system through the integration of renewable energy. GFM control is particularly significant as it determines the level of controllability over non-dispatchable and variable generation, especially on a larger scale, thus contributing to the practical realization of this initiative. Therefore, an extensive and evaluative examination of many control systems, control theories, and algorithms has been conducted in the last twenty years. This study provides a comprehensive classification of control objectives and applications, emphasizing their importance and effectiveness. The findings of this study are equally applicable to various interdisciplinary fields such as power system operation, power electronics, renewable energy integration, advanced control, and smart grids. [ABSTRACT FROM AUTHOR]

Copyright of Electric Drive is the property of Electric Drive Editorial Office 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 paper presents an adaptation of the microinverter platform from Texas Instruments to incorporate a battery energy storage system (BESS) alongside the development of the BESS system itself. Initially designed for unidirectional power flow between PV panels and an electric grid, the platform required modifications to accommodate bidirectional energy transfer for BESS integration. These modifications encompass software adjustments and hardware enhancements, which are all detailed within the paper. The electrical configuration includes selecting and deploying components such as DCDC power converters, microcontrollers, measured signals, and actuating signals to facilitate battery connection to the platform's DC bus. Furthermore, a supervisory control and data acquisition (SCADA) system is devised for supervisory control and monitoring, with its implementation outlined. Control software tailored for the chosen microcontroller of the DCDC converters is described in terms of structure and functionality. A hardware-in-the-loop (HIL) methodology is employed to validate the proposed modifications and microgrid configuration. Utilizing the real-time simulator OPAL-RT, the paper presents experimental results and their analysis within the considered microgrid environment. [ABSTRACT FROM AUTHOR]

Previous research indicates that post-fault grid-forming (GFM) converters only work in current limiting control (CLC) mode. However, converters integrating the current saturation algorithm (CSA) can operate in both constant voltage control (CVC) and CLC modes. This paper firstly derives the angle-switched model of the grid-connected system governed by CSA and virtual synchronous generator (VSG). Then, combining the fault and three types of post-fault trajectories, critical clearing angle is analytically formulated through the Segmental Equal Area Criterion (SEAC). Finally, a range of values for the saturated current angle is proposed to ensure that the system is stabilized in CVC mode. Notably, the optimal saturated current angle, which is the minimum value within the range, provides the maximum critical clearing angle.

The grid-forming converters (GFMs) have been widely employed to support the power grid. Yet, the phase difference effect (PHE) caused by active power control (APC) on the system's design and stability is still inexplicable. This paper comprehensively investigates the PHE in the GFM under six APCs in terms of the developed impedance models. We observed that GFM exhibits nearly equal dq-frame impedance across six APCs when the PHE is disregarded. However, when accounting for PHE, the impedance of GFM with P-δ droop-based APCs differs significantly from that with P-ω droop- and virtual synchronous generator (VSG)-based APCs. Additionally, the sequence impedance of GFM with P-ω droop- and VSG-based APCs appears to be largely decoupled, whereas P-δ droop-based APCs introduce substantial sequence impedance coupling. Stability and coupling analyses indicate that the P-δ droop-based GFM must work in a weaker grid for stable operation compared to other APC-based GFM. Furthermore, the resulting frequency coupling leads to a higher amplitude of the coupled frequency (2f1-f) than the dominant frequency (f). Lastly, theoretical analysis is verified by experiments, providing new insights into the future research and design guidelines of GFM.

The grid-forming converter that supplies voltage magnitude and frequency is a key element for isolated remote communities, and then wear-out analysis is an essential figure of merit. Therefore, this paper proposes the two-phase three-wire (2$\Phi$3 W) grid forming converter applied in a case study of isolated riverside communities in the Brazilian Amazon region. The proposed xyn-converter displaces the voltage line angle in 180$^{\circ }$, differing from $\alpha \beta$n-converter and abn-converter that displace voltage line angle in 90$^{\circ }$ and 120$^{\circ }$ respectively. First, the authors have also proposed a semiconductor current stress evaluation for 2$\Phi$3 W converters comparing the RMS current. Then, the lifetime evaluation of the three evaluated grid-forming converters is conducted. The results show that the converter operating in the xyn-converter presents the highest reliability among the three converters. Finally, the thermal stress analysis is validated by means of a 2$\Phi$3 W converter prototype with an open semiconductor module. For the evaluated converters, the xyn-converter has the least thermal stress for balanced and unbalanced load conditions and therefore the longest lifespan. The experimental validation shows that the average temperature of the xyn-converter is 19.4% and 23.3% lower than both the abn-converter and the $\alpha \beta$n-converter under balanced load conditions.

Energy storage system based on grid-forming converter (GFMC) is regarded as the key equipment in photovoltaic (PV) system for energy consumption and inertia improvement. However, the design of GFMC aiming at stability improvement of PV & energy storage system (PVESS) is still open to public. Hence, this study takes the PVESS composed of photovoltaic grid-following converter (GFLC) and energy-storage GFMC as the research object, explores the impact of GFMC on the PVESS stability under different operating conditions, and proposes a stability-oriented parameter design method for GFMC. Firstly, the impact of operating conditions on the output impedance of GFLC and GFMC in PVESS is analyzed. Secondly, by treating the GFMC and the grid as an equivalent grid (i.e., an equivalent subsystem) and taking into account the operating status of PVESS, the system stability under different operating conditions is discussed in detail. Results show that the PVESS stability is the worst when the PV output is maximum, and, because the output impedance of GFMC in low-to-mid frequency range is dominated by its filter inductor, the strength of the equivalent grid is seriously affected by the GFMC filter inductor. Hence, as the key factor affecting the PVESS stability, the GFMC filter inductor deserves a careful selection. Subsequently, a comprehensive design for GFMC filter and control parameters is proposed, which can effectively improve the PVESS stability under the weak grid. Hardware-in-the-loop (HIL) experiments are conducted to verify the effectiveness of the proposed method.

VSG is a pivotal control strategy for GFM, designed to enhance the damping and inertia of power systems, thereby bolstering their stability. Despite its benefits, VSG can induce a QEO due to dynamic interactions between the GFM and the power system. This QEO, is different from control issues, contrasts with the EO associated with conventional SG, which is rooted in electromechanical factors. To address this phenomenon, the influence mechanism of VSG on QEO of the system is studied. First, a comprehensive control model for the VSG is established and its efficacy in providing damping and inertia is assessed. Subsequently, a small-signal model for the VSG when connected to the grid is developed. Utilizing this model, the system’s characteristic roots and the trajectories of principal state variables, elucidating the impact of VSG control parameters on QEO are analyzed. The validity of the models and the underlying mechanisms are confirmed through extensive simulations. Furthermore, for the key problems that need to be solved urgently in the application of VSG in power system with renewable energy, further thinkings are prospected.

As the integration of renewable energy sources (RESs) and distributed generations (DGs) increases, the need for stable and reliable operation of microgrids (MGs) becomes crucial. However, the inherent low inertia of such systems poses intricate control challenges that necessitate innovative solutions. To tackle these issues, this paper presents the development of robust-adaptive controllers tailored specifically for grid-forming (GFM) converters. The proposed adaptive-robust controllers are designed to accommodate the diverse range of scenarios encountered in low-inertia MGs. The proposed approach applies both the robust control techniques and adaptive control strategies, thereby offering an effective means to ensure stable and seamless converter performance under varying operating conditions. The efficacy of the introduced adaptive-robust controllers for GFM converters is validated within a low-inertia MG, which is characterized by substantial penetration of converter-interfaced resources. The validation also encompasses diverse MG operational scenarios and conditions.

The proportion of the integrated converters using PSC （power synchronization control） in power systems is increasing year by year， but there is a lack of in-depth research on the transient synchronization stability. Therefore， the transient synchronization stability mechanism of integrated grid-forming converters using PSC is analyzed， and measures to improve the transient stability of the converter are proposed. First， the transient stability model of the grid-forming converters with PSC during grid fault is established. Subsequently， based on the phase diagram curve （PDC） method， the destabilization mechanism of the converters and the effect of voltage coupling on the transient synchronous stability are investigated. Moreover， a control strategy to improve the transient synchronous stability by dynamically adjusting the active power command value is proposed based on the effect of voltage coupling. Finally， the electromagnetic transient simulation model of an equivalent system in a city is constructed by using PSCAD/EMTDC， which verifies the correctness of the transient stabilization mechanism of the converter-connected power system and the effectiveness of the stabilization enhancement measures under different fault conditions.

Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office 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.)

Phase-locked loop-based grid-forming converters （PLL-GFCs） can regulate frequency and voltage. However， the transient synchronization stability after their connection to the high-voltage AC power grid is unclear. Therefore， the transient synchronization stability mechanism of the PLL-GFC-based MMC-HVDC （modular multilevel converter-based HVDC） systems is studied. Firstly， the control strategy of the PLL-GFC-based MMC-HVDC systems is introduced， and the power response characteristics are analyzed. Secondly， the grid-connected MMC-HVDC systems' transient synchronization stability model is built. The instability mechanism of the converters after grid faults and the influence of the dynamic changes of the grid current on the transient synchronization stability are studied based on phrase diagram curves. It is concluded that the equilibrium point is crucial to the transient stability after the fault occurs on the inverter-connected power grid. Therefore， an enhancement measure to improve the transient synchronization stability by dynamically adjusting active current command values is proposed. Finally， an electromagnetic transient simulation model is built by PSCAD/EMTDC to verify the correctness of the instability mechanism analysis and the effectiveness of the enhancement measure.

Grid-forming (GFM) converters are a promising solution to enable large scale integration of renewable energy sources into the power system. However, due to the intrinsic voltage-source behaviour of GFM converters, current limitation during large grid disturbances is challenging. This article presents a novel limitation strategy that preserves the GFM properties of the converter and at the same time effectively limits the converter current to the desired value. Through the limitation of the converter's internal voltage, stable operation even during faults and in case of large frequency disturbances in the grid is achieved. Experimental results show the effectiveness of the proposed current limitation strategy in case of various grid disturbances.

The grid‐forming converter (GFC) is used to regulate the voltage and frequency of a local grid. However, due to the limited current capacity, the GFC must actively modify its control strategy when a serious external fault occurs, thus to avoid the possible damage caused by overcurrent. This paper proposes a novel voltage reference generation method for GFC based on the external equivalent impedance magnitude (EEIM), which has the following merits: (i) the faulty phase currents and the non‐faulty phase voltage can be controlled within their operational limits. (ii) The voltage reference unbalance factor (VRUF) of the GFC can be adjusted flexibly if necessary. Then, in order to achieve the optimal support effect for the faulty phase voltages, a two‐stage GFC fault control strategy is proposed in this paper. In the first stage, the fault type is judged by computing and comparing the EEIM of each phase. In the second stage, the voltage reference mode is determined according to the fault type, and the VRUF is adjusted to its optimum value through a search algorithm. Simulation results based on Matlab/Simulink verify the effectiveness of the proposed GFC fault control strategy in terms of current limiting and faulty phase voltage support. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

One of the main capabilities of grid‐forming converters is the ability to operate in both islanded and grid‐connected modes. The grid‐forming converters work autonomously and behave as an AC voltage source when they are isolated from the bulk power system. In a grid‐connected mode, it is necessary for them to offer a set of grid‐forming capabilities for enhancing system inertia and strength. A grid‐forming converter may experience significant fluctuations in voltage, current, and frequency when the transition occurs. This paper investigates an improved adaptive model predictive control (AMPC) method for grid‐forming converters to achieve a smooth transition from islanded to grid‐connected mode and balance the contradiction between frequency stability and response speed. The AMPC method uses the angular frequency and phase angle as the performance indicators and introduces the angular frequency variations to adaptively adjust its weight coefficient in the objective function. The implementations and effects of the proposed AMPC method on the grid‐forming converter are theoretically evaluated. Compared to the conventional control, the proposed control scheme can not only suppress voltage, current, and frequency fluctuations, but also achieve a smooth transition. Additionally, the frequency control strategy in the islanded mode is improved for better independent operation. Finally, experiment tests on a controller‐level hardware‐in‐the‐loop simulation platform are carried out to validate the proposed control strategy for achieving a smooth transition. [ABSTRACT FROM AUTHOR]

This article proposes a fault ride through (FRT) technique for a high-order virtual synchronous generator (VSG) that adjusts its virtual armature resistance. When a fault is detected by a dedicated algorithm, the proposed control adjusts the resistance parameter accordingly. The main contribution of this article is to adjust the virtual resistance directly in the machine model to limit the current during faults, unlike other techniques proposed in the literature that add another control loop to produce the virtual impedance effects. To validate the effectiveness of the proposed control, a hardware-in-the-loop real-time simulation platform was adopted using a Typhoon HIL 402 device and a Texas Instruments F28379D digital controller. The results demonstrate that the control effectively limits the converter's current while still contributing to raising the system's critical clearing time (CCT) and improving transient stability. The proposed FRT strategy is validated in a three-phase fault scenario in which a 500 kVA–480 V converter's peak fault current is reduced from 5 kA to 1.4 kA, depending on the resistance value adjusted. The transient stability is also analyzed in 30 different scenarios and the VSG support on the CCT is reduced by 23 ms on average. However, when compared to the baseline scenario without the VSG, the system still sees an increase in CCT with the current limiting control applied. Additionally, the control allows the VSG to smoothly transition to island mode in a scenario where the fault is cleared and the grid is disconnected by a protection system. [ABSTRACT FROM AUTHOR]

The development trend of high shares of renewables and power electronics has increased the demand for new energy converters in the power system, but there is a lack of systematic research on the stability of different types of converters when transmitting power, which is worth exploring in depth. In this study, the power transfer capabilities of grid-forming and grid-following converters are investigated separately through an equivalent circuit diagram and phasor diagram when connected to the grid, and a quantitative relationship between converters' power transmission limit and short circuit ratio under static stability conditions is obtained, leading to the conclusion that, in terms of power transmission, grid-forming converters are more suitable for weak grids with high damping and low inertia, whereas grid-following converters are more suitable for strong grids with high inertia. The conclusions are further verified by constructing the converter grid-connected models for different grid strengths through the PLECS simulation platform and the real-time simulation RTBOX1 and F28379D launchpad platform. [ABSTRACT FROM AUTHOR]

Compared with current source converters, voltage source converters (grid-forming converters) have better frequency support capabilities, voltage support capabilities, and regulation performance, thus they have broad application prospects. However, the grid-forming (GFM) converter has insufficient current control ability, and it easily causes problems such as overcurrent issues when a fault occurs. Thus, this insufficiency is one of the most important challenges the GFM converter is faced with. Aiming to solve the problems mentioned above, this paper proposes a control method of a GFM converter achieved with a low-pass filter structure and an additional current loop. The additional current loop controls the dq-axis current components by acting on the outer loop to generate appropriate phase and voltage amplitude reference. The low-pass filter structure is used to solve the system frequency stability problem caused by the inclusion of the additional current loop. On the premise of ensuring that the system frequency meets the grid-connection requirements, the proposed strategy rapidly limits the output current within the allowable range and guarantees expected voltage source characteristics of the converter during the fault period. Finally, the effectiveness and superiority of the proposed control strategy are verified by MATLAB/Simulink simulations. [ABSTRACT FROM AUTHOR]

The virtual synchronous generator (VSG) control is a promising solution for the grid-forming converters to enhance the inertia of the converter-based power grid. The virtual inertia and output impedance of the VSG are the important behaviors to be focused. However, the virtual inertia of grid-forming converters may cause active power oscillation due to the interaction with the power inertia of grid through the low-frequency band grid impedance. Besides, the resonance can be also triggered by the interaction between middle-frequency impedance of power grid and output impedance of converter. In order to test the stability of grid-forming converter, accurate emulation of grid behaviors from more aspects, including inertia and impedance under wider frequency ranges, is becoming critical. This article proposes a grid emulation method to mimic the inertia and impedance characteristics of power grid, targeting for the stability test of grid-forming converter under multifrequency bands. A high-switching-frequency converter and a low-switching-frequency converter are adopted in the proposed emulator structure to achieve both high capability in respect to control bandwidth and power level. Moreover, the virtual impedance control is integrated with the VSG control to emulate the line impedance and inertia within relatively wider frequency ranges. The realization of virtual impedance is achieved without derivation terms nor low pass filter. Finally, the performance of the proposed emulator is verified by various simulations and experimental measurements. [ABSTRACT FROM AUTHOR]

Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office 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 increase proportion of power converters causes stability problems in the radial grid. To address these issues, this paper firstly proposes a unified modeling procedure and applies it to two types of converters. Secondly, the impedance models are used to analyze the stability of a radial grid with converters under four cases. The influence of the short circuit ratio (SCR) and line impedance on system stability are investigated. Then, a comprehensive sensitivity analysis is performed in order to investigate the effects of parameters variations on the closed-loop poles of the system under different conditions. Finally, the accuracy of the model and theoretical analysis are verified by simulations and Hardware-in-the-Loop (HiL) tests.

Power systems around the globe are undergoing a transformation characterized by a massive deployment of converter-interfaced generation (CIG) to effectively combat climate change. However, achieving a seamless transition from current power systems dominated by synchronous generators (SGs) to future ones with high levels of CIG requires overcoming several technical challenges. From a frequency stability perspective, reduced system inertia increases the frequency nadir after a loss of generation thereby endangering frequency stability. In this context, this paper proposes a novel methodology for determining control requirements to impose on CIG as their penetration in the network increases. Results of a case study based on the Chilean grid projected for the year 2046 show that, if only grid-following converters without frequency control capability are deployed, a maximum CIG penetration level of 75% can be achieved without threatening frequency stability. The Chilean system can reach a 99% CIG penetration, provided that the remaining CIGs are deployed in grid-following with frequency support capability. Finally, we show that if the last SG is replaced with a grid-forming converter, the system can still sustain frequency stability and exhibits a good dynamic performance. These results demonstrate that, at least from a frequency stability viewpoint, achieving a 100% based CIG system is technically possible. The proposed methodology can be used by energy regulators to define the control requirements necessary to impose on CIG for achieving renewable energy targets in a secure way. Although the obtained results are particular for the Chilean system, the proposed methodology can be applied to any power system.

The power system is migrating to 100% penetration of the converter-interfaced generation (CIG). The grid-forming converter-based CIG has been proposed to be a solution to replace the synchronous generation (SG) in order to form the grid and achieve the 100% penetration target. However, besides the generation, whether the protection system can coordinate this transition is still an open question. Although the grid-forming converter operates as the same manner as the SG, unlike SG, it physically has constraints on its current output which in transient leads to different operation to the SG. This paper aims to investigate whether these different transients have effects on the protection system, specifically, investigate if the protection system remains reliable when the system migrates to 100% penetration. The investigation is based on the hardware in-the-loop experiment, considering different converter control, different type of the fault, different length of the line and different fault location. The result shows that the grid-forming converter has a reliable coordination with the protection system in response to the fault.

As a flexible and reliable way for distributed energy consumption and integration, the converters-dominated microgrid has attracted more and more attention recently. Owing to the low inertia and high nonlinearity of power converters, the islanded microgrid under large-signal disturbances is easily suffer to transient stability problems. To support the stable operation of microgrid, grid-forming converters are extensively investigated and applied. In this paper, an improved equal area criterion (IEAC) is proposed for the transient stability analysis of a simple microgrid equipped with both grid-following and grid-forming converters. Firstly, a simplified second-order model of the microgrid is established, which contains a nonlinear damping term relying on the power angle. Then, the limitation of conventional equal area criterion (EAC) for transient stability analysis is revealed, which completely ignores the unfavorable influence of negative damping and may lead to erroneous judgement. To fill this gap, an improved equal area criterion is proposed to derive the transient stability conditions for islanded microgrid, which could improve the estimation accuracy of stability boundaries. Moreover, impacts of system parameters and controller parameters on transient stability are studied, which provides useful guidelines for the parameter optimization of grid-forming and grid-following converters. Eventually, simulation and hardware-in-loop experiments are conducted to verify the effectiveness and superiority of the proposed method. [ABSTRACT FROM AUTHOR]

Offshore wind power plants (WPPs) play a pivotal role in achieving a CO2-neutral electricity sector. However, their grid connection presents significant challenges and costs. High-voltage alternating current (HVAC) offers a cost-effective solution, but it may necessitate the installation of a static synchronous compensator (STATCOM) at the onshore bus to ensure stability and compliance with connection requirements. Integrating energy storage into the STATCOM (ES-STATCOM) enables the provision of ancillary services, such as inertial response and frequency support, which require active power. This paper utilizes the generalized Nyquist criterion to demonstrate that operating the ES-STATCOM with grid-forming control enhances the stability margin of the grid-connected WPP when compared to operating it with grid-following control. Furthermore, it illustrates through network frequency perturbation (NFP) plots that the overall WPP, comprising grid-following wind turbines and a grid-forming ES-STATCOM, imparts grid-forming behavior at the grid-connection point. • Small-signal model of HVAC-offshore wind farms is derived. • Mathematical models are used to evaluate small-signal stability of wind farms. • Grid-forming control for ES-STATCOM enhances stability of HVAC-offshore wind farms. • NFP plots are utilized to evaluate grid-forming capability of offshore wind farms. • Grid-forming ES-STATCOM can offer grid-forming capability to overall wind farm. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the transient stability of a state-of-the-art grid-forming complex-droop control (i.e., dispatchable virtual oscillator control, dVOC) under current saturation. We quantify the saturation level of a converter by introducing the concept of degree of saturation (DoS), and we propose a provably stable current-limiting control with saturation-informed feedback, which feeds the degree of saturation back to the inner voltage-control loop and the outer grid-forming loop. As a result, although the output current is saturated, the voltage phase angle can still be generated from an internal virtual voltage-source node that is governed by an equivalent complex-droop control. We prove that the proposed control achieves transient stability during current saturation under grid faults. We also provide parametric stability conditions for multi-converter systems under grid-connected and islanded scenarios. The stability performance of the current-limiting control is validated with various case studies. • Current saturation can destabilize grid-forming converters since it leads to current-dependent variable impedance. • Saturation information is fed back to the controller to ensure transient stability under current saturation. • Transient stability conditions are established for current-saturated grid-forming converters. [ABSTRACT FROM AUTHOR]

An islanded microgrid consisting of grid-forming converters, being a high-order nonlinear system, exhibits rich nonlinear dynamical phenomena. The use of appropriate reduced-order models offers useful physical insights into the behavior of the system without the need for excessive computational resources. In this article, we derive a number of reduced-order models capable of describing the slow-scale dynamics of an islanded microgrid comprising a number of grid-forming converters. It is shown that slow-scale Hopf and homoclinic bifurcation behaviors arise from the stability of the voltage loops of grid-forming converters and are unrelated to the transmission network dynamics. Therefore, omitting the network dynamics does not affect the accuracy of reduced-order models in representing the slow-scale dynamics of the system. This is especially beneficial for modeling the microgrid with a complex transmission network. Furthermore, on this basis, all inner loops can be omitted when studying saddle-node bifurcation, leading to the development of power-flow-based reduced-order models. Finally, the stability of an islanded microgrid with a complex transmission network is evaluated. [ABSTRACT FROM AUTHOR]

This article proposes a grid-forming based decoupling approach, which effectively mitigates the dynamic interactions between active and reactive power, especially suitable for system stabilization in ultrastrong grids with short-circuit ratio over 30. The physical coupling behaviors are first revealed by analytical studies, where two potential resonances, respectively, located in 40–50 Hz and 0–10 Hz are discovered with mathematical proofs. Based on the results, the decoupling controller is then designed with transfer function approximation for practical usage. The control method not only suppresses the couplings but also cancels two pairs of conjugate poles through decoupling paths. Thanks to the cancellation, the stability margin is remarkably increased in stiff grids, and therefore, a significant bandwidth boost on the power synchronization control can be realized if desired. The experimental results are finally performed to verify the proposed decoupling method. [ABSTRACT FROM AUTHOR]

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This letter develops an equivalent circuit model of a grid-forming (GFM) converter with a circular current limiter and analyzes its transient stability. It is revealed that the inner control loop can be simplified as a voltage source behind an equivalent resistor. Based on the developed model, theoretical analysis and experimental tests demonstrate that the transient stability of a $P-f$ droop-controlled GFM converter with the circular current limiter can be assured whenever there exist stable equilibrium points. [ABSTRACT FROM AUTHOR]

The Stability of grid-forming converters (GFCs) interfacing renewable resources in a power system with multiple synchronous generators (SGs) is studied in the context of primary frequency response. The GFCs are divided into two classes based on the control methods – class-A: droop control, dispatchable virtual oscillator control (dVOC), and virtual synchronous machine (VSM); and class-B: matching control. First, averaged phasor models of these GFC classes are developed, which can be seamlessly integrated with positive sequence fundamental frequency planning models. Next, simplified averaged models are derived to study the stability of the dc-link voltage of the GFCs under dc-side current limitation in a generic multimachine system during primary frequency response. To that end, sufficiency conditions for stability of both classes and that of instability for class-A GFCs are established. Finally, the proposed stability conditions are verified using detailed switched models of two small systems and phasor models of a 4-machine and a 16-machine IEEE benchmark systems. [ABSTRACT FROM AUTHOR]

This paper deals with the development of a control technique for the control of voltage source converters (VSCs) as an interface between renewable energy sources (RESs) and power grids. The lack of inertia in converter-based generators and their unconventional dynamic behavior increase the negative impacts on the power grid, unlike the operation of the conventional synchronous generators (SGs). In this sense, the proposed control technique emulates the behavior of SGs in the control loop of the interfaced converter acting as a grid-forming power generator. Thus, supportive functionalities for enhancing electric grid stability (e.g., frequency oscillation damping) will be provided by interfaced converter as a virtual synchronous generator (VSG). The main targets of the proposed control technique include virtual inertia emulation, frequency oscillation damping, primary and secondary frequency regulation, and voltage regulation. The main contribution of this work over the existing methods is that the optimal values for swing equation parameters are employed to ensure well-designed damping feature of the VSG-based converter which adapts grid-connected and islanded operating modes, enabling a smooth resynchronizing to the grid as its plug-and-play capability requirement. The control structure is evaluated through simulation in MATLAB/Simulink.