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Purpose: The purpose of the article is to analyze the reduction of PV generation in Germany and Poland and possible ways to solve it. Design/methodology/approach: The analysis was conducted using available secondary data and literature. The study's time scope is from Q1 2013 to Q1 2024. Findings: The answer to the challenges is the continuous modernization of the power grid towards smart grids. A necessary element is increasing the flexibility of the power system through the development of energy storage and new grid services. Research limitations/implications: Limitations apply to the Polish and German markets. Practical implications: Based on the analysis of the Polish and German markets, the authors proposed possible scenarios for the development of PV networks and possible actions aimed at continuous modernization of the power grid towards smart grids by conducting programs to support the development of national network infrastructure. Social implications: Reducing the negative impact on the environment and identifying opportunities to build smart grids. Originality/value: Few studies have examined the effects of integrating RES into power grids. The research gap detailed the analysis of grid deactivation cases in Poland and Germany. The two markets were compared due to their geographical proximity and their significant differences. [ABSTRACT FROM AUTHOR]

Electric power generation is quickly transitioning toward nontraditional inverter-based resources (IBRs). Prevalent devices today are solar PV, wind generators, and battery energy storage systems (BESS) based on electrochemical packs. These IBRs are interconnected throughout the power system via power electronics inverter bridges, which have sophisticated controls. This paper studies the impacts and benefits resulting from the integration of grid forming (GFM) inverters and energy storage on the stability of power systems via replicating real events of loss of generation units that resulted in large load shedding events. First, the authors tuned the power system dynamic model in Power System Simulator for Engineering (PSSE) to replicate the event records and, upon integrating the IBRs, analyzed the system dynamic responses of the BESS. This was conducted for both GFM and grid following (GFL) modes. Additionally, models for Grid Forming Static Synchronous Compensator (GFM STATCOM), were also created and simulated to allow for quantifying the benefits of this technology and a techno-economic analysis compared with GFM BESSs. The results presented in this paper demonstrate the need for industry standardization in the application of GFM inverters to unleash their benefits to the bulk electric grid. The results also demonstrate that the GFM STATCOM is a very capable system that can augment the bulk system inertia, effectively reducing the occurrence of load shedding events. [ABSTRACT FROM AUTHOR]

Grid-forming inverters represent a transformative advancement in power grid management, providing a flexible solution for optimizing energy systems and enhancing grid stability amid the increasing integration of renewable energy and growing grid complexity. This control strategy focuses on regulating voltage and frequency, enabling decentralized energy sources, such as renewables, to synchronize effectively with the island grid, ensuring a stable power supply. This study aims to develop a control algorithm for decentralized grid-forming inverters utilizing renewable energy resources and diesel generators. The proposed system incorporates five distributed generators: a fuel cell, wind turbine, photovoltaic panel, battery energy storage system, and diesel generator. An adaptive fuzzy-based droop controller is employed to meet the study’s objectives, with one component tracking reference voltage and frequency and another addressing the load’s active and reactive power demands. Validation through Matlab/Simulink demonstrates effective voltage and frequency regulation at the point of common coupling, supported by compelling simulation results. The study’s alignment with expected outcomes highlights the system’s reliability and robustness in maintaining grid stability amid dynamic load conditions and renewable energy integration. Furthermore, practical assessments on a test bed validate consistent active and reactive power sharing across all microgrids, reinforcing the proposed control strategy’s effectiveness in optimizing energy management and enhancing power distribution system performance.

This paper presents a grid-forming (GFM) voltage-source inverter (VSI) with direct current regulation for a hybrid wind-solar generator, enabling stable operation at very weak grid conditions and under faults. The GFM-VSI interfaces a hybrid wind-solar generator without an intermediate dc-dc conversion to increase the system efficiency. The wind generator comprises a wind turbine with a permanent magnet synchronous generator (PMSG) interfaced by a voltage-source rectifier (VSR). The PMSG-VSR and a solar photovoltaic (PV) array are connected to the GFM-VSI's dc-side. The VSR is responsible for extracting wind power with a power reserve option. The GFM-VSI is implemented to extract solar power with a power reserve capability and support the grid voltage or reactive power. The stable operation of the proposed system is validated under very weak grid conditions, and it is shown that a similar hybrid wind-solar system with grid-following control is unstable under the same weak grid conditions. A complete small-signal state-space model of the proposed hybrid system is developed and analyzed. Nonlinear time-domain simulations and real-time simulation tests verify the model's accuracy and show the proposed system's effective performance under challenging operating scenarios, such as grid uncertainties and faults.

Changes are being implemented in the electrical power grid to accommodate the increased penetration of renewable energy sources interfaced with grid-connected inverters. The grid-forming (GFM) control paradigm of inverters in active power grids has emerged as a technique through which to tackle the effects of the diminishing dominance of synchronous generators (SGs) and is preferred to the grid-following (GFL) control for providing system control and stability in converter-dominated grids. Therefore, the development of the GFM control is important as the grid advances towards 100% inverter-based grids. In this paper, therefore, we aim to review the changing grid scenario; the behaviour of grid-connected inverter control paradigms and major GFM inverter controls, including their modifications to tackle low inertia, reduced power quality, fault-ride through capability, and reduced stability; and the state-of-the-art GFM models that are pushing the universality of GFM inverter control. [ABSTRACT FROM AUTHOR]

Grid-forming converters are considered a key technology for electrical grids with a high share of renewable energy. However, there is a lack of a general, robust and verifiable definition of what constitutes this behaviour. Therefore, this paper presents three simple scenarios that enable such a definition. They are based on simple black-box simulations that allow manufacturers to protect their development work. It can be shown that grid forming can be clearly distinguished from grid-following controls by their grid behaviour in the proposed scenarios. Extensive results illustrate this and also provide suggestions to deepen the understanding of grid-forming grid behaviour and its impact on system inertia. These findings should inspire the development of a clear definition and ultimately new requirements for converter systems. [ABSTRACT FROM AUTHOR]

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Abstract This paper discusses that the existing grid codes on the fault ride‐through (FRT) operation have certain complications to be implemented in the grid‐forming (GFM) converters. In particular, they do not provide a clear guidelines on the suitable prioritization of the positive‐ and negative‐sequence currents needed for a desired voltage profile during FRT operation. To address this challenge, two FRT controls which are based on (i) voltage balancing priority, and (ii) voltage magnitude priority are investigated for GFM converters. The former gives priority to the elimination of negative‐sequence voltage, while the remaining converter's capacity is used to increase the positive‐sequence voltage. On the contrary, the latter prioritizes the increase of positive‐sequence voltage, while it tries to reduce negative‐sequence voltage by using the remaining converter's capacity. The dynamic performances of these two FRT controls are thoroughly discussed, and the simulation results show that depending on the desired voltage profile during fault, one of them can be implemented.

Ship-to-shore connection is an important technological element that reduces air pollution in ports. Therefore, ports install facilities that allow mooring ships to connect to the port distribution network. By 2025, this will be mandatory for all ports in Europe. This can be a challenging task in most ports due to the different frequency of the network and ship frequency. This problem can be solved by the use of grid-forming static frequency converters. This solution also brings some other advantages: The ship is not threatened by high shore short-circuit currents, and the port distribution network is not affected by the character of the ship load. However, frequency converter software must include a droop control algorithm to ensure that voltage deviations do not exceed the allowed limits during transients. Typical frequency converters used for shore connection are those developed as static frequency converters (SFCs). However, those converters were not developed for large power outputs, which are needed to power large vessels, such as ferries or cruise ships. This paper proposes motor drives that were modified to operate as SFCs. This approach has quite a lot of advantages which are described in this article. This paper describes both a standard shore connection system without a frequency converter and a solution that includes static frequency converters. The paper then focusses on voltage deviation estimations during connection/disconnection of large load (ferry or cruise ship) to static frequency converters. In this work, a high-voltage shore connection (HVSC) simulation model is developed, including a frequency converter, a shoreside transformer, medium-voltage (MV) connection cables, and a power system of the ship, to analyze in detail the behavior of the system in the case of connection or disconnection of the ship load. The model was made in DIgSILENT PowerFactory for the case of a commercial port in southern France. The model gives credible estimations of voltage drops/surges during transient and steady states. [ABSTRACT FROM AUTHOR]

This paper presents an optimal grid-supporting power-to-gas (GS-P2G) control scheme that coordinates the operation of a grid-forming inverter (GFMI), battery energy storage (BES), and hydrogen energy storage (HES) in a power-to-gas (P2G) system. The design aims to optimally maintain the steady state of the active power flow under critical conditions by smoothing the fluctuating power that flows to the grid from renewable energy sources (RES). P2G contributes to procuring hydrogen supply systems in areas with low-density transmission networks and gas-supply infrastructure. For instance, P2G energy could originate from the RES, which would otherwise be curtailed depending on the network constraints. Furthermore, to prevent waste, HES and existing natural gas networks can potentially store and reuse this energy. The GS-P2G control concept enables GFMI to respond rapidly to the intermittency and variability of wind power, cover the power surplus/deficits by charging/discharging the BES, and prevent the grid voltage and frequency from deviating away from the normal ranges. Therefore, the optimized control scheme presented in this article combines the power management system (PMS) and grid support function with an optimization function for highly RES-penetrated areas. The measured experimental results were evaluated and briefly discussed to demonstrate the performance and feasibility of the proposed control scheme.

This paper discusses that the existing grid codes on the fault ride‐through (FRT) operation have certain complications to be implemented in the grid‐forming (GFM) converters. In particular, they do not provide a clear guidelines on the suitable prioritization of the positive‐ and negative‐sequence currents needed for a desired voltage profile during FRT operation. To address this challenge, two FRT controls which are based on (i) voltage balancing priority, and (ii) voltage magnitude priority are investigated for GFM converters. The former gives priority to the elimination of negative‐sequence voltage, while the remaining converter's capacity is used to increase the positive‐sequence voltage. On the contrary, the latter prioritizes the increase of positive‐sequence voltage, while it tries to reduce negative‐sequence voltage by using the remaining converter's capacity. The dynamic performances of these two FRT controls are thoroughly discussed, and the simulation results show that depending on the desired voltage profile during fault, one of them can be implemented. [ABSTRACT FROM AUTHOR]

A microgrid is a group of interconnected loads and distributed energy resources that can fill the gap between the dependence on a bulk power grid and the transition to renewable energies. The islanded mode presents itself as the most interesting scenario, when local controllers should maintain the power quality standards based on several parameters. A tool specifically focused on the process of parameter tuning of the secondary consensus-based control for inverter-based islanded microgrids was proposed in this paper. One often-quoted drawback in this process is the great number of parameters that must be tuned, even for a very simple microgrid structure. To manage such a large number of parameters, the design of experiments was used in this study. The main motivation for this work was to present an optimized way to define the correct parameters for the secondary consensus control for inverter-based islanded microgrids. The study shows how experimental design methodology can be an efficient tool to tune microgrid parameters, which are typically multi-objective-based experiments. From the results, it is correct to state that the design of experiments is able to reach the optimal setting with a minimal number of experiments, which would be almost impossible to obtain with the trial-and-error method. [ABSTRACT FROM AUTHOR]

With the increasing penetration of power electronic converters in the power system induced by the energy transition, Grid Forming (GFM) technology emerges as crucial for complementing traditional synchronous generators in fulfilling system needs. All over the world, TSOs have started introducing performance-based requirements to define the desired behaviour of GFM units without prescribing specific technical solutions. Based on these specifications, manufacturers design their grid-connected equipment. However, depending on requirements, challenges may arise in optimizing control strategies without hardware modifications, potentially becoming cost-driving factors. Intellectual property protection limits information disclosure, restricting the guidance available to TSOs during cost–benefit assessments. Academic contributions on GFM control and generic models can bridge the gap, providing a fair portrayal of the general behaviour and then facilitates an open discussion on their ability to meet the requirements and contribute to fulfil system needs. This survey paper provides a comprehensive overview of the perspectives offered by these diverse stakeholders. • System needs : review of the main system needs in the context of a high penetration of power electronic converter in the grid • Grid forming : Providing a consistant approach of grid forming including the design of controller, small signal and large disturbance stability • HVDC : analysing the impact of the grid forming control on the HVDC stability [ABSTRACT FROM AUTHOR]

Transient stability is a complex phenomenon presented in multi-machine and multi-converter systems, and it is still considered a key limiting factor for stressed power systems. The increasing integration of non-synchronous generation further emphasises the need to address the challenges of improving the transient stability faced by these power systems. Several studies have focused on developing control strategies for Grid-forming voltage source converter (GFM-VSC) to improve transient stability. These strategies include the use of current limiting algorithms and/or control of active/reactive power injections. This paper investigates the impact of fast voltage boosters (FVBs) and hybrid current limiters (HCLs) on transient stability of power systems with 100% grid-forming VSC-based generators. Short-circuit simulations and critical clearing time analysis are performed to evaluate the effectiveness of HCLs and FVBs in improving transient stability. The simulation results demonstrate the effectiveness of these approaches in avoiding the loss of synchronism. This research contributes to the current studies on transient stability in power systems and provides valuable insights into the potential of HCLs and FVBs as effective approaches to improve system stability. • Grid-forming (GFM) voltage source converters (VSC) and transient stability. • Hybrid current limiters (HCLs) and fast voltage boosters (FVBs) in GFM-VSCs. • FVBs in GFM-VSCs improve transient stability. FVB-L (local) and FVB-WACS (global). • HCLs in GFM-VSCs limit the converter's current and improve transient stability. • FVBs and HCLs are compatible and complementary. [ABSTRACT FROM AUTHOR]

Grid forming inverters are expected to play a key role in future power grids, replacing synchronous generator-based plants. However, the limited current capability of power electronics makes a difference when facing fault induced voltage sags. This work provides a comprehensive review of strategies to handle low voltage ride through events in grid forming inverters. A key contribution of this work is to differentiate between current limiting and transient stability enhancing strategies. Current limiting strategies are classified into voltage and current-based approaches according to the inverter behaviour during the fault. Their performance is evaluated attending to three criteria: (1) transient current limitation capability, related to the self-preservation of the device; and (2) fault current management and (3) transient synchronization stability, key aspects to meet grid code requirements. The modifications that are required to support the grid under asymmetrical faults are also described, focusing on the management and prioritization of positive and negative-sequence fault currents. Transient stability enhancing strategies are classified according to their implementation into (1) power synchronization loop-based and (2) current saturation-based approaches. Their main characteristics are highlighted and compared, whereas their compatibility with previous current limiting strategies is also discussed. Finally, identified open issues and challenges are covered. [Display omitted] • Fault induced voltage sags will lead to overcurrents in grid forming inverters. • Current limiting strategies are classified into voltage and current-based strategies. • Transient current, current contribution and stability will depend on the strategy. • Transient enhancing strategies are used to ensure the stability during faults. • Current limiting strategies can prioritize contribution during asymmetrical faults. [ABSTRACT FROM AUTHOR]

Changes are being implemented in the electrical power grid to accommodate the increased penetration of renewable energy sources interfaced with grid-connected inverters. The grid-forming (GFM) control paradigm of inverters in active power grids has emerged as a technique through which to tackle the effects of the diminishing dominance of synchronous generators (SGs) and is preferred to the grid-following (GFL) control for providing system control and stability in converter-dominated grids. Therefore, the development of the GFM control is important as the grid advances towards 100% inverter-based grids. In this paper, therefore, we aim to review the changing grid scenario; the behaviour of grid-connected inverter control paradigms and major GFM inverter controls, including their modifications to tackle low inertia, reduced power quality, fault-ride through capability, and reduced stability; and the state-of-the-art GFM models that are pushing the universality of GFM inverter control.

Grid-forming converters are considered a key technology for electrical grids with a high share of renewable energy. However, there is a lack of a general, robust and verifiable definition of what constitutes this behaviour. Therefore, this paper presents three simple scenarios that enable such a definition. They are based on simple black-box simulations that allow manufacturers to protect their development work. It can be shown that grid forming can be clearly distinguished from grid-following controls by their grid behaviour in the proposed scenarios. Extensive results illustrate this and also provide suggestions to deepen the understanding of grid-forming grid behaviour and its impact on system inertia. These findings should inspire the development of a clear definition and ultimately new requirements for converter systems.

Microgrid technology is gaining popularity due to its ability to integrate different renewable energy sources and energy storage technologies in grid-connected and islanded modes in an efficient and ecologically friendly way. Because of the absence of inertia, referred to as a weak microgrid, the stable functioning of the system should be maintained in islanded circumstances, necessitating the installation of a robust control system in order to form and maintain its voltage amplitude and frequency constant under all operating conditions (Grid Forming). To that end, this study recommends implementing a robust control method called as Active Disturbance Rejection Control in order to deal with the many uncertainties and disturbances that might occur in such a complex system. This control approach is based on an Extended State Observer, which evaluates all the disturbances that may affect the system in real time and compensates for them in the final control block. An integrated PV-based battery-islanded microgrid is next studied and simulated by using the MATLAB/Simulink Environment in order to illustrate the feasibility of the suggested technique. The obtained results show that the efficacy of the suggested setup to produce and maintain constant voltage and frequency under any operating situation. [ABSTRACT FROM AUTHOR]

The changing energy landscape due to the large scale integration of renewable energy and shutting down of conventional thermal plants has opened up the potential of alternate sources in the blackstart services market. Grid forming wind turbines can do controlled islanded operation independent of an external grid voltage and thus participate in network restoration from the start. However, it is necessary to study the capability of wind turbines to deal with the demanding energization transients in a controlled and stable manner. This work investigates the feasibility of using virtual resistance in the wind turbine converter control to reduce transients during self-transformer inrush and sympathetic interaction from downstream string transformers. This can eliminate the need for pre-insertion resistors during sequential energization. The sensitivity of the AC current and voltage output along with DC link transient to the virtual resistance parameters has also been analyzed using PSCAD simulations. Finally the effectiveness of the proposed method for offshore network energization by a grid forming wind power plant has been tested by comparing to results for a pre-insertion resistor. [ABSTRACT FROM AUTHOR]

In this paper, the hybrid synchronization based grid forming (HS-GFM) control and coordination strategy are proposed for the inverter and boost converter to provide frequency support. As the main contribution, the inertia power and damping power are designed with HS-GFM based coordination strategy between inverter and boost converter. The inertia power can be obtained from DC capacitor and photovoltaic system in deloading mode independently. Besides, the damping power can be realized from the PV inverter in deloading mode. After that, the small signal models of inverter and boost converter considering grid frequency coupling are constructed with simplified single input single output (SISO) model to guide the parameter selection. Further, the grid synchronization stability, synchronization bandwidth and grid strength influence on the frequency response of the PV inverter can be analyzed with SISO model. Compared with conventional DC voltage based GFM, HS-GFM can improve stability especially in strong grid with slight reduction of synchronization bandwidth while it has the risk of instability in weak grid if parameters are not proper. Compared with phase locked loop (PLL) based control, the proposed HS-GFM control achieves a fast response under frequency excursion with better stability in weak grid. Finally, realtime and offline simulations are carried out to verify the proposed strategy and analysis. • Inertia and damping power are designed with hybrid synchronization based GFM control. • The DC side thevenin equivalence of boost converter is obtained for SISO analysis. • SISO model of PV inverter is built for stability analysis and parameter selection. • Comparisons with conventional GFM and phase locked loop based control are presented. [ABSTRACT FROM AUTHOR]

Vehicle-to-grid (V2G) technology offers an innovative solution to provide grid services using electric vehicle (EV) batteries. This work proposes a novel grid forming (GFM) controller for V2G applications which can ensure the voltage source behaviour and provide support to the grid, regardless of the state of charge (SoC) and the charging requirements of the battery. This is achieved by integrating a SoC controller that modifies the classical GFM algorithm. The controller only relies on the SoC and power measurements, achieving a decentralized without the need of communication. With the proposed approach, the V2G charger will always behave as a voltage source that provides inertial response and voltage support to the grid. The primary frequency regulation, on the other hand, will depend on the battery status and charging needs. Frequency regulation could be enabled, disabled or it could be limited to some perturbations (under/over frequencies). Moreover, the SoC controller provides freedom to tune the frequency support of the device, limiting it to inertial response or extending its contribution along the time. The tuning of the system parameters is addressed in detail to ensure a damped response under all the operation scenarios provided by the SoC controller. Its performance and stability is evaluated using small signal transfer functions. Finally, it is validated both in simulation and experimentally. • Bidirectional electric chargers can provide support to the grid. • Chargers supporting the grid need to consider the user charging requirements. • A modified grid forming strategy could meet grid support and charging requirements. • The charger will provide support based on local measurements and state of charge. • Up to four different grid forming operation modes are identified and validated. [ABSTRACT FROM AUTHOR]

This paper presents a combined control scheme for the grid-connected energy storage system (ESS). There are two control modes: the power control mode for the charging or discharging condition and the energy control mode for the stand-by condition. In the power control, a derivative element is added in the inertial power-frequency droop as a simple power system stabilizer to enhance the system damping. The energy control is developed from the power control by considering the energy storage dynamics. During system disturbances, both control modes are able to provide autonomous grid support. The small-signal model and the large-signal transient model of the ESS with the proposed control scheme are derived. As a result, the output power and/or the stored energy of ESS is in proportional to the frequency deviation and will be restored to the pre-disturbance condition. By selecting appropriate control parameters, the stored energy is controlled within a safe range constrained by the frequency limits without manual interventions. The primary frequency regulation with both power-frequency and energy-frequency proportional relationships is realized by the energy control, which coordinates with the grid secondary frequency regulation. The control performance and the derived energy model are validated by detailed time-domain simulation studies. [ABSTRACT FROM AUTHOR]

A microgrid is a group of interconnected loads and distributed energy resources that can fill the gap between the dependence on a bulk power grid and the transition to renewable energies. The islanded mode presents itself as the most interesting scenario, when local controllers should maintain the power quality standards based on several parameters. A tool specifically focused on the process of parameter tuning of the secondary consensus-based control for inverter-based islanded microgrids was proposed in this paper. One often-quoted drawback in this process is the great number of parameters that must be tuned, even for a very simple microgrid structure. To manage such a large number of parameters, the design of experiments was used in this study. The main motivation for this work was to present an optimized way to define the correct parameters for the secondary consensus control for inverter-based islanded microgrids. The study shows how experimental design methodology can be an efficient tool to tune microgrid parameters, which are typically multi-objective-based experiments. From the results, it is correct to state that the design of experiments is able to reach the optimal setting with a minimal number of experiments, which would be almost impossible to obtain with the trial-and-error method.

An enhanced grid forming (GFM) control strategy is proposed for modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission. Compared with the traditional GFM control with only AC side control, the proposed method can achieve the decoupled AC and DC side control. The AC side control includes the hybrid synchronization unit and the cascaded voltage-current loop. The AC side control realizes the inertia emulation of the synchronous machine (SM) in a wide range of short circuit ratio. The DC side control realizes the DC power or DC voltage control by controlling the inserted submodule number with the inner DC current loop. The DC side control imitates the primary frequency control of the governor for SM. Based on the decoupled AC and DC side control, the inertia and primary frequency control are obtained from the AC and DC side control, respectively, instead of only the AC side from the traditional GFM control. It provides complete mapping from the AC side and governor side of the SM to the AC side and DC side of the MMC. The system model is built to guide the parameter design, and the simulations are provided to validate the proposed methods. • Hybrid synchronization with AC voltage current loop emulates the inertia of SM. • The inserted submodule number is modified to regulate the DC power and/or DC voltage. • System modelling is provided for parameter selection. [ABSTRACT FROM AUTHOR]

Grid forming control for modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system is favorable to support the AC systems. This paper presents the hybrid AC-DC impedance model for grid forming based MMC-HVDC with wind farm integration. Comparisons of the hybrid AC-DC impedance with the traditional AC/DC impedance are presented, and the hybrid AC-DC impedance does not contain right half-plane poles caused by DC/AC coupling. Then, the network based impedance analysis is carried out to analyze the stability of grid-forming based MMC-HVDC with wind farm integration. It is found that grid forming based MMC-HVDC may lack damping with two modes of oscillations. Large inertia, DC/AC virtual impedance and wind farm integration can dampen the first mode oscillation, while only AC virtual impedance is effective for the second mode oscillation. The stability comparison of grid forming and grid following based wind farm integration is also presented. Moreover, MMC-HVDC with grid forming control cannot improve the stability of grid following based wind farm integration in weak grid where grid forming based wind farm performs better. Therefore, both wind farm and MMC-HVDC with grid forming control are better in weak grid. Simulation results in PSCAD/EMTDC validate the corresponding analysis. • The hybrid AC-DC impedance based modeling methods for GFM based MMC are presented. • Network based stability analyses are presented with the hybrid AC-DC impedance. • The stability comparison of GFM and GFL based WF integration is presented. [ABSTRACT FROM AUTHOR]

Microgrids (MGs) are the emergent solution to overcome the current electricity demand. The MGs provide the facility to operate in both isolated and grid-connected modes. For both operating modes, Distributed Generation (DG) inverters are operating under grid forming or grid following control modes. During mode switching, the MG experiences enormous fluctuations, which occur due to the unidirectional islanding event. This paper presents a control strategy by using the modified power control scheme, current controller, and DC linked voltage controller scheme to ensure the operational mode transfer smoothly from the grid-connected to the islanded mode and vice versa. The proposed control scheme is applied to a three-phase distributed energy resource-based MG system with fixed loads. The simulation results validate the effectiveness of the control technique while tested at the point of common coupling and also at the time of mode transfer. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMC) can be used in several applications, especially (but not only) in high-voltage direct current (HVDC) and STATCOM. In order to develop experimental scaled-down test benches for lab validation, several projects have developed MMCs with a limited number of cells, but they need to use pulse width modulation (PWM) techniques to achieve acceptable power quality (because nearest level modulation (NLM), common in HVDC applications with hundreds of levels, cannot achieve sufficient power quality unless the number of cells is high enough). The present paper proposes a new concept which is based on designing arms with a single cell. This allows to have the simplest possible converter that maintains the structure of an MMC. While all the inner controllers of large-scale HVDC MMCs are included, the only remarkable difference is that PWM is used and NLM cannot be implemented. As this is also a limitation for other low voltage MMC, the proposed concept is suggested for scaled-down low voltage applications. The paper includes the design and construction of the converter, the definition and implementation of the converter controllers, and the converter testing, with detailed dynamic simulations and an experimental setup.

In more-electronics power systems, grid-forming power converters, which operate as ac voltage sources, regulate the grid frequency and voltages in replacement of synchronous generators. Notably, grid impedances greatly influence the small signal and voltage stability of grid-forming converters. As such, prior knowledge of grid impedances can be very helpful for controller design. However, grid impedance estimation schemes are normally designed for current-controlled grid-following converters. Moreover, they are either very complicated or only yield grid inductances in a generally intrusive way. To fill this research gap, an impedance estimation method well suited to grid-forming converters is proposed. The method consists of four operating modes, which work well in voltage and power control cases. In the voltage control case, the voltage amplitude perturbation or phase angle information is exploited. Subsequently, the grid inductance and resistance are derived from power measurement. Alternatively, the active or reactive power information serves to estimate the grid impedance in the power control case. The proposed method features an easy implementation without any harmonic distortion, safety concern, or dependence on control parameters. Moreover, the method operates nonintrusively in most scenarios. Furthermore, a novel Kalman filtering scheme is proposed to provide added incentives. Finally, simulation and experimental results validate the effectiveness and simplicity of the proposed method. [ABSTRACT FROM AUTHOR]

With the increasing integration of power electronics interfaced distributed generators, transient stability assessment of grid-connected inverters subjected to large grid disturbances is of vital importance for the secure and resilient operation of the power grid. Dispatchable virtual oscillator control (dVOC) is an emerging approach to implement nonlinear control of grid-forming inverters. Through coordinate transformation, a simple first-order nonlinear power angle dynamic equation is uncovered from the complex oscillator dynamics. Furthermore, this article proposes a concise and straightforward graphical approach to assess transient stability of dVOC using vector field on the circle. To provide a more in-depth analysis, a complete large-signal model is derived and the impact of dVOC voltage amplitude dynamics is analyzed. For comparison, transient stability of the currently prevalent droop control is also assessed using phase portraits. Salient transient stability features of dVOC and droop control during grid faults are summarized and compared. The theoretical analysis is validated by controller hardware-in-the-loop testbed using industry-grade hardware. [ABSTRACT FROM AUTHOR]

In this paper, the selective laser melting (SLM) forming method was combined with the requirement of boiling heat transfer for porous structure to study the forming process and boiling heat transfer characteristics of porous structure of G-surface lattice formed by SLM. 316L stainless steel powder was used to rasterize the G-surface structure. Besides, porosity and surface morphology of G-surface lattice were analyzed. Deionized water was used for boiling heat transfer experiment to study the effect of technological parameters on boiling heat transfer enhancement. On this basis, the bubble escape modes in different stages of boiling heat transfer of G-surface lattice were considered, and the delay of the critical heat flux caused by G-surface lattice was explained. This study provides a useful reference for the design of lattice heat cooling structure. [ABSTRACT FROM AUTHOR]

In this article, we investigate grid-forming and grid-following control strategies starting from a nonlinear state-space modeling viewpoint. An electronic synchronous machine is an inverter whose integral of the dc-bus measurement generates the angle of the instantaneous modulation vector. We show how this minimal augmentation constitutes an exact physical realization without requiring inner-current loops. The dc-link capacitance becomes the equivalent rotational inertia of the converter. Additional features, such as a phase locked loop, a voltage controller, and a power tracking mechanism are then designed via two energy-shaping techniques. One energy function is used to implement a grid-following control scheme, via the inherent synchronizing torque, while the other is used to implement a grid-forming control scheme. An alternative interpretation of active-power droop is suggested by this method. The results are first derived systematically, and then evaluated experimentally on a front-to-front setup. [ABSTRACT FROM AUTHOR]

With an increasing capacity of inverter-based generation and with a 100% renewable energy power system on the horizon, grid forming converters have the potential to become the prevalent control mode in the grid. Thus, the correct performance of these devices is going to be crucial for system stability and security of supply. Most research related to the grid-forming control is focused on normal operating conditions, although significant effort has been devoted to current limitation strategies to ensure Low Voltage Ride Through (LVRT) capability. However, most contributions usually consider only balanced faults. This paper, proposes a new current limiting method based on the well-known threshold virtual impedance (TVI) that keeps the voltage source behaviour associated to the grid forming (GFM) capability, even when the current limit is reached, while reducing the voltage unbalance according to user-defined settings.

Solid state transformer (SST) integrated hybrid dc–ac microgrid (HMG) has wide operating capabilities in presence of dc–ac loads, renewable energy resources, and energy storage. This paper proposes control strategies for localized control of zonal hybrid microgrid, enabled by the SST and centralized energy storage devices (ESD) integrated using dual active bridge (DAB) converter. The control of DAB dc–dc converter is modified for zero circulating power flow (ZCPF) operation in both SST and ESD. Power management and coordination control used in this paper ensures the power supply reliability of the zonal HMG integrated through the SST. Microgrid system, along with its proposed controllers, provides a stabilized low-voltage dc and ac network ports, in both grid feeding and forming modes. The performance of the configured HMG system is evaluated under various power management scenarios using the MATLAB software platform. Further, the SST architecture, with its proposed controller, is tested on FPGA-based hardware platform. [ABSTRACT FROM AUTHOR]

Power sharing control among grid forming dc sources employing a conventional voltage droop approach meets inaccurate load sharing and unacceptable voltage regulation performance. Thereby, communication-based secondary and supervisory controllers have been presented to overcome the aforementioned issues. Furthermore, with the aim of eliminating communication system, frequency-based droop approaches have been introduced for low-voltage dc grids where the frequency of the superimposed ac signal onto the dc voltage is proportional to the output power. However, in reality, dc grid structures can be applied to medium and high voltage applications with different X/R ratios. This paper generalizes the frequency-based droop scheme for dc microgrids with different low, medium, and high X/R ratios. Obtained simulation and experiment results demonstrate the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

There is an increasing need to apply rigorous model-order-reduction techniques in the analysis of large-scale networks of inverter-based distributed generation resources due to the limitations of existing simulation tools. Various coherency-based aggregation techniques have long been used to construct reduced-order dynamic models of large-scale synchronous machine (SM) networks. Such techniques have the advantage of preserving the nonlinear nature of the dynamic model throughout the order-reduction process, enabling the efficient and accurate analysis of large-scale network dynamics during large disturbances such as fault events. This paper proposes the application of a rigorous coherency-based aggregation technique to the analysis of large-scale networks of grid-forming droop-controlled inverters. A rapid and powerful generalized eigenvalue perturbation technique for coherency identification, previously only applied to SM networks, is adapted to grid-forming droop-controlled inverter networks. The resulting reduced-order models are physically insightful and are capable of accurately reproducing the system response in the aftermath of large disturbances. For some networks, a rigorously-derived condition of coherency can be difficult to achieve, given the expected range of L–C–L filter impedances. To remedy this limitation, the potential for high-bandwidth inverter control to enforce the conditions that allow for coherency of droop-controlled inverters has been investigated and confirmed using a controller hardware-in-the loop testbed. Using this approach, the use of simple nonlinear aggregate inverter models to accurately model large sections of the inverter network can be more rigorously justified. [ABSTRACT FROM AUTHOR]