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Most of the previous SOC equalization methods for microgrid energy storage target DC microgrids and use centralized control structures, while in recent years many researchers have begun to focus on a decentralized, communication-based implementation of distributed control structures. In this paper, based on the existing research, we use the multi-agent system (MAS) structure and dynamic consistency algorithm (DCA) to realize the estimation of the SOC mean value of microgrid energy storage obtained by each agent in the system. For the problems of the iterative convergence that exists in the distributed control, such as being too slow, too frequent communication, stability, etc., we optimally select the parameters of the consistency iterative summation to improve convergence. In addition, we use the event-driven method to further reduce the unnecessary communication frequency. Finally, a numerical simulation model of the AC microgrid is established by Matlab to verify the effectiveness of the method, which reduces the communication volume by about 50% while maintaining the effect of the control strategy. [ABSTRACT FROM AUTHOR]

In islanded AC microgrids, negative impedance characteristics of AC constant power loads (AC CPLs) easily introduce large signal instability to the system, while energy storage systems sometimes compensate for the dynamic characteristics of AC CPLs, and increase the system stability. Although energy storage control techniques and characteristics have gained a lot of attention, few studies have derived quantitative design guidelines for energy storage systems from the aspect of stability improvement. In order to fill this gap, this paper proposes stability control strategies for bidirectional energy storage converters considering the characteristics of AC CPLs to guarantee large signal stability of islanded AC microgrids. The presented control techniques create quantitative limits for the DC bus voltage loop control parameters of the energy storage DC/DC converter and the integral control loop control parameter of the energy storage DC/AC converter, and also interpret the positive stability influence of energy storage systems and the negative stability influence of AC CPLs. The structure of the paper is as follows. Firstly, DQ coordinate transformation is adopted, and AC microgrid nonlinear models with the energy storage system in charging and discharging states are constructed. Then, large signal models are constructed depending on mixed potential theory. Stability control strategies for bidirectional energy storage converters are obtained, and AC CPLs power, storage system equivalent resistor, and micro power source power are all taken into account. Finally, based on simulation and experimental results, it is obvious that regulating the control parameters of the energy storage converter significantly increases the large signal stability of islanded AC microgrids without extra equipment. The method is very simple and easy to implement. [ABSTRACT FROM AUTHOR]

Modern electrical systems are required to provide increasing standards of power quality, so converters in microgrids need to cooperate to accomplish the requirements efficiently in terms of costs and energy. Currently, power quality compensators (PQCs) are deployed individually, with no capacity to support distant nodes. Motivated by this, this paper proposes a consensus-based scheme, augmented by the conservative power theory (CPT), for controlling clusters of PQCs aiming to improve the imbalance, harmonics and the power factor at multiple nodes of a grid-tied AC microgrid. The CPT calculates the current components that need to be compensated at the point of common coupling (PCC) and local nodes; then, compensations are implemented by using each grid-following converter's remaining volt-ampere capacity, converting them in PQCs and improving the system's efficiency. The proposal yields the non-active power balancing among PQCs compounding a cluster. Constraints of cumulative non-active contribution and maximum disposable power are included in each controller. Also, grid-support components are calculated locally based on shared information from the PCC. Extensive simulations show a seamless compensation (even with time delays) of unbalanced and harmonics current (below 20% each) at selected buses, with control convergences of 0.5–1.5 [s] within clusters and 1.0–3.0 [s] for multi-cluster cooperation. [ABSTRACT FROM AUTHOR]

In order to optimize the extraction of solar energy, photovoltaic sources are commonly operated under the control of the so-called maximum power point (MPPT) strategy. However, as the rate of PV installations increases explosively, traditional MPPT algorithms may cause problems such as frequency deviation and power fluctuations, making system frequency stability a challenge due to the inherent intermittent and stochastic nature of PVs. Consequently, in order to reduce the investment and maintenance costs of storage systems, innovative control is expected for PV sources to provide ancillary services for the system, especially for weak systems such as microgrids. In this paper, a novel active power control (APC) strategy, based on characteristic curve fitting, is proposed to flexibly regulate the PV output power. The transient process performance and robustness of the system are improved with the proposed APC strategy. In conjunction, an f–P droop mechanism is designed to provide a frequency regulating (FR) service for the AC microgrid. The comprehensive control strategy unifies the FR function with the traditional MPPT function in a single control structure, allowing the PV source to operate either in the MPPT mode when the system frequency is nominal or in FR mode when the frequency exceeds it. The transition between MPPT and FR is autonomous and fully decentralized, which improves the PV generation efficiency as well as ensuring generation fairness among different parallel PV sources. Importantly, the proposed control strategy does not require any internal bundled energy within the PV generation system to achieve FR capability, but it effectively collaborates with the system-level energy storage system, thus reducing the necessary battery capacity. A detailed dynamic model of a PV generation system is constructed to validate the feasibility and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

The active power‐virtual frequency (P‐ω′) and reactive power‐virtual voltage (Q‐V′) droop control scheme has been suggested for coordinated control of distributed energy resources (DERs) in islanded microgrids. While this method mitigates the issue of active and reactive power coupling in the droop controller, its main limitation is power‐sharing error. Due to the local property of the virtual frequency and voltage, significant steady‐state errors occur for both active and reactive power sharing. In this paper, a decentralized method is proposed to eliminate the mentioned errors without utilization of communication links. In the proposed scheme, P‐ω′ and Q‐V′ droops are realized at the microgrid point of common coupling (PCC). By employing the PCC virtual frequency/voltage for the active/reactive power sharing of all DERs, the sharing errors caused by the local property are eliminated. Small signal stability of the proposed scheme is studied, and Hardware‐in‐the‐Loop (HIL) experimental results are presented to validate the proposed method and highlight the improvements compared with the conventional P‐ω′/Q‐V′ scheme. The results verify the efficacy of the proposed method for providing accurate active and reactive power sharing while regulating the frequency and voltage close to the rated values. [ABSTRACT FROM AUTHOR]

Microgrids (MG) are small-scale electric grids with local voltage control and power management systems to facilitate the high penetration and grid integration of renewable energy resources (RES). The distributed generation units (DGs), including RESs, are connected to (micro) grids through power electronics-based inverters. Therefore, new paradigms are required for voltage and frequency regulation by inverter-interfaced DGs (IIDGs). Notably, employing effective voltage and frequency regulation methods for establishing power-sharing among parallel inverters in MGs is the most critical issue. This paper provides a comprehensive study, comparison, and classification of control methods including communication-based, decentralized, and construction and compensation control techniques. The development of inverter-dominated MGs has caused limitations in employing classical control techniques due to their defective performance in handling non-linear models of IIDGs. To this end, this article reviews and illustrates advanced controllers that can deal with the challenges that are created due to the uncertain and arbitrary impedance characteristics of IIDGs in dynamics/transients. [ABSTRACT FROM AUTHOR]

Linear droop control is a standard approach for the decentralized operation of AC microgrids. While the traditional design of the real and reactive power droop functions does not consider economic aspects, recent approaches present modifications to enable least-cost operation. This paper proposes mixed-integer conic programming (MICP) for computing the real and reactive power droop parameters from a stochastic optimization framework that captures several snapshots of the microgrid’s operation. The model permits distributed generation to switch between grid-forming and grid-feeding modes. Unlike previous works, MICP provides droop parameters that are globally optimal for the design scenarios. Numerical results show that using traditional linear cost-based droop design methods may result in voltage magnitude violations. In contrast, the proposed method gives a solution free of violations and is yet more economical. [ABSTRACT FROM AUTHOR]

Zonal pricing is a well-suited mechanism to incentivize grid-supporting behavior of profit-maximizing producers and consumers operating on a large-scale power system. In zonal electricity markets, local system operators create individual price zones, which provide appropriate price signals depending on local grid conditions such as an excess or shortage of electrical energy in certain regions. In this paper, a real-time zonal pricing controller for AC power networks is presented that ensures frequency and voltage stability as well as Pareto efficiency of the resulting closed-loop equilibria. Based on a dynamic network model that takes line losses and power exchange with adjacent price zones into account, distributed continuous-time control laws are derived which require only neighbor-to-neighbor communication. Application to a real-time congestion management strategy illustrates how spatially and temporally differentiated prices enable grid-supportive operation of the participants without interventions by a superordinate control authority. Effectiveness of different zonal pricing concepts compared to an isolated grid operation is demonstrated through simulations on the IEEE-57 bus system. [ABSTRACT FROM AUTHOR]

Power flow (PF) calculation plays a fundamental role in the steady-state analysis of isolated microgrids (MGs). In this paper, a novel analytical PF algorithm based on holomorphic embedding (HE) is proposed for isolated AC MGs considering hierarchical control (HC). With a deliberate design in embedding technique, the presented algorithm not only inherits the deterministic property of the canonical embedding that enables to derive the upper-branch (operable) solution explicitly, but also is compatible with the HC characteristic of isolated MGs. Furthermore, the embedded PF model features a general structure with a constant recursive matrix, which can accommodate various bus types and network re-configurations (including both radial and meshed networks), as well as non-nonlinearities of voltage-dependent load models. Case studies are carried out on 3 test systems: a 12-bus MG and modified IEEE 33-bus and 123-bus distribution systems. Numerical results reveal the applicability and efficacy of the proposed algorithm. [ABSTRACT FROM AUTHOR]

The world is endeavoring to transition to clean, renewable resource-based power generation systems such as wind and solar due to decreasing fossil fuel availability, a severe increase in power demand, environmental pollution, and global warming caused by unwanted gas emissions from current fossil fuel-based conventional power generation systems. An emerging technology, Microgrid (MG), has the capability to interface power generated from multiple distributed generation energy resources (DGER) such as wind, solar, fuel cells, biomass, geothermal, diesel engine generators, electric vehicles (EVs), and energy storage systems like batteries, compressed air storage, supercapacitors, and flywheels. This complexity in control and protection renders traditional power grids outdated. MGs are designed to operate in two modes: grid-connected mode (GCM) and islanded mode (IM), addressing power demand increases, transmission issues, and electricity storage challenges. MGs offer numerous advantages for power systems, yet their integration into distribution networks presents various challenges in protection systems, including fault current (FC) under different operating conditions, bidirectional fault current flow, unexpected relay tripping, disconnections due to sudden accidents, and cybersecurity issues. This paper provides a comprehensive review of various protection issues associated with MG and its possible solutions as suggested by various researchers. © 2024 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

In this paper, an improved Reinforcement Learning (RL) based approach for adaptive Virtual Synchronous Generator (VSG) control of grid-forming inverters is proposed. High penetration of inverter-based resources causes recurrent frequency stabilization issues. For this matter, a novel model-free Actor–Critic policy optimization technique is proposed to improve frequency transients metrics such as frequency nadir and Rate of Change of Frequency (RoCoF). Stability analysis of the VSG controlled system is conducted through small signal modeling to determine the stability margins of both the virtual inertia parameter J and damping coefficient D p. The control problem is formulated in RL terms using a new technique for state, action and reward construction that improves the learning process of the agents. The proposed approach's efficiency is tested for three RL algorithms, namely, Deep Deterministic Policy Gradient (DDPG), Soft Actor–Critic (SAC), and Twin-Delayed DDPG (TD3) with special attention given to the latter. Various performance metrics such as training time and maximum cumulative episodic reward are used to compare the three algorithms. The resulting agent based VSG adaptive controllers are simulated and then compared for power quality and frequency stabilization capabilities. Finally, a comparison between non-adaptive VSG and TD3 based VSG is presented to conclude on the efficiency improvements achieved by the proposed approach. • Conventional VSG Controllers necessitate recurrent parameter adjustments. • Adaptive VSG parameter adjustment is mandatory during power disturbance events. • Model-free RL-based Adaptive VSG Controllers provide more frequency stabilization. • The TD3 Algorithm is better than DDPG and SAC for Virtual Inertia Optimization. • RL Based Adaptive VSG Methods can be improved by using a novel state formulation. [ABSTRACT FROM AUTHOR]

• A distributed event/self-triggered H ∞ consensus-based secondary control scheme is proposed to be used in voltage and frequency restoration control of AC microgrids system. • This paper is the pioneer to investigate the impact of event-triggered strategy and a variety of common instability factors (uncertain communication links, external disturbance, and time-varying delay), all in one robust distributed secondary control framework. • This paper designs a self-triggered communication mechanism based on event triggering, which reduces the communication burden and eliminates the computational burden of continuous monitoring of trigger conditions in event-triggered schemes. At present, the distributed cooperative control scheme has been widely used in the secondary control of microgrids (MGs). However, instability factors in communication, as well as the low bandwidth and energy of the communication network, affect the safe and efficient operation of the scheme. In this paper, a distributed self-triggered H ∞ consensus-based secondary control scheme is proposed to be used in voltage and frequency restoration control of AC MGs system. In the research, the external interference, the uncertainty of the communication link, the time-varying delay, and several common instability factors in topology networks are considered. Different from many existing works, this paper designs a self-triggered communication mechanism based on event-triggered, which reduces the communication burden and eliminates the computational burden of continuous monitoring of trigger conditions in event-triggered schemes. By applying the H ∞ theory to the system model, the sufficient conditions for the robust consensus convergence of multi-agents under self-triggered and uncertain time-varying delay communication are obtained using the Lyapunov-Krasovskii theory. Finally, an MGs simulation model was established to evaluate the effectiveness of the proposed control strategy. The results show that the proposed secondary control scheme can restore the voltage and frequency to the reference value, and while greatly reducing the communication burden between agents. [ABSTRACT FROM AUTHOR]

The widespread integration of renewable energy sources to the main electrical grids has led to the increased adoption of AC microgrids. However, the protection of AC microgrids is a challenging task due to inverter interfaces, bidirectional power flow, multiple modes of operation and the requirement for selective phase tripping. This paper presents an innovative artificial neural network (ANN) based approach for fast and accurate identification and localization of symmetrical and asymmetrical faults occurring in the distribution networks of AC microgrids. In the proposed methodology, the three phase and the neutral currents which are sampled at either ends of the distribution lines, undergo discrete wavelet transform to extract the features exhibited during faults in the network. These features are used by two neural networks for classification and localization of the fault. To achieve high accuracy and computational efficiency, the network architectures of the ANNs are optimized, and the extracted features contain the detailed information required for ANNs to clearly distinguish different fault types and locations. A comprehensive evaluation and validation reveal that the proposed scheme accurately and efficiently classifies and localizes faults in AC microgrids. The existing research gap of fault localization in AC microgrids is also addressed through this approach.

Most of the previous SOC equalization methods for microgrid energy storage target DC microgrids and use centralized control structures, while in recent years many researchers have begun to focus on a decentralized, communication-based implementation of distributed control structures. In this paper, based on the existing research, we use the multi-agent system (MAS) structure and dynamic consistency algorithm (DCA) to realize the estimation of the SOC mean value of microgrid energy storage obtained by each agent in the system. For the problems of the iterative convergence that exists in the distributed control, such as being too slow, too frequent communication, stability, etc., we optimally select the parameters of the consistency iterative summation to improve convergence. In addition, we use the event-driven method to further reduce the unnecessary communication frequency. Finally, a numerical simulation model of the AC microgrid is established by Matlab to verify the effectiveness of the method, which reduces the communication volume by about 50% while maintaining the effect of the control strategy.

The usage of distributed generation sources and the segmentation of power systems into microgrids brings several advantageous features, such as energy independence, lower transmission losses, and improved energy management flexibility. Still, the expected steady abolition of traditional centralized networks and their centralized control schemes introduces several operational difficulties, that can compromise (micro)grid's stability and reliability. Consequently, several approaches to how microgrids should be operated have been proposed, albeit with several salient deficiencies. Precise power exchange, control of frequency and voltage excursions, applicability on different grid topologies, and the possibility to integrate grid codes, which are very important control aspects, are not jointly enabled in a general case. This paper proposes a novel approach, based on a specific goal function and corresponding decentralized control that addresses the mentioned problems and guarantees the microgrid's stability by constraining the frequency and node voltage deviations, simultaneously supporting the desired active power exchange between prosumers. Unlike droop-based and droop-derived control schemes, the proposed approach aims to inherently preserve the frequency and voltage levels within the limits defined by the grid operator or future microgrid codes, thus preventing the generation unit's unwanted disconnection during transients and disturbances. In addition, the control algorithm is independent of network topology, can be applied in both islanded and non-islanded microgrids, and secures system scalability. To confirm the effectiveness of the proposed control strategy, a number of simulation results are presented and thoroughly discussed. • A novel seamlessly scalable self-stabilizing microgrid control framework is proposed. • Guaranteed upper bounds on deviations of node frequencies and voltages are provided. • Precise active power exchange between nodes is enabled. • Priorities of the proposed scheme can be adjusted according to user preferences. • Comprehensive tests in relevant operational scenarios are performed. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the secondary control problems of AC microgrids with physical states (i.e., voltage, frequency and power, etc.) constrained in the process of actual control, namely, under the condition of state constraint. On the basis of the primary control (i.e., droop control), the control signals generated by distributed secondary control algorithm are used to solve the problems of voltage and frequency recovery and power allocation for each distributed generators (DGs). Therefore, the model predictive control (MPC) with the mechanism of rolling optimization is adopted in the second control layer to achieve the above control objectives and solve the physical state constraint problem at the same time. Meanwhile, in order to reduce the communication cost, we designed the self-triggered control based on the prediction mechanism of MPC. In addition, the proposed algorithm of self-triggered MPC does not need sampling and detection at any time, thus avoiding the design of observer and reducing the control complexity. In addition, the Zeno behavior is excluded through detailed analysis. Furthermore, the stability of the algorithm is verified by theoretical derivation of Lyapunov. Finally, the effectiveness of the algorithm is proved by simulation. [ABSTRACT FROM AUTHOR]

Abstract The active power‐virtual frequency (P‐ω′) and reactive power‐virtual voltage (Q‐V′) droop control scheme has been suggested for coordinated control of distributed energy resources (DERs) in islanded microgrids. While this method mitigates the issue of active and reactive power coupling in the droop controller, its main limitation is power‐sharing error. Due to the local property of the virtual frequency and voltage, significant steady‐state errors occur for both active and reactive power sharing. In this paper, a decentralized method is proposed to eliminate the mentioned errors without utilization of communication links. In the proposed scheme, P‐ω′ and Q‐V′ droops are realized at the microgrid point of common coupling (PCC). By employing the PCC virtual frequency/voltage for the active/reactive power sharing of all DERs, the sharing errors caused by the local property are eliminated. Small signal stability of the proposed scheme is studied, and Hardware‐in‐the‐Loop (HIL) experimental results are presented to validate the proposed method and highlight the improvements compared with the conventional P‐ω′/Q‐V′ scheme. The results verify the efficacy of the proposed method for providing accurate active and reactive power sharing while regulating the frequency and voltage close to the rated values.

In this article, a smart inverter model that executes ancillary services with automated decisions is presented, such as power sharing and voltage and frequency stabilization, compensation of unbalance voltage, mitigation of harmonic content, and the balance of generation and demand. The droop control was utilized for power-sharing between the distributed generations; the mitigation of negative-sequence voltage is used for unbalanced voltage compensation, and finally, a feed-forward current is adopted for harmonic compensation. Therefore, the smart inverter was controlled through a supervisory system, identifying which ancillary function to activate at certain times. Results show the successful operation of the system and can be used as validation of the proposed control strategies, including the automated decisions in the microgrid. [ABSTRACT FROM AUTHOR]

Increasing power demand, aging distribution systems and concerns towards greenhouse gas emissions have resulted in the increased occurrence of distributed generation (DG) within distribution networks. The conventional protection methods designed for passive radial distribution networks may become redundant with large bidirectional power flow and dynamic network topology. Additionally, the anti-islanding protection currently employed limits the benefits of wide-scale DG installation and autonomous operation. The microgrid concept can solve these problems, but several challenges must be overcome before practical implementation. Besides bi-directional power flow, the vast variance between the fault current in grid-connected and autonomous mode and the arbitrary output impedance of the inverter-interfaced DG units in fault conditions and current limiting mode pose a challenge to the protection schemes that use traditional overcurrent protection devices. Many researchers have proposed various techniques, but a robust protection scheme capable of protecting microgrids against different faults for both modes of operation under dynamic network topologies and being financially viable is still to be developed. Hence, the main objective of this paper is to critically review various AC microgrid protection methods proposed in the literature, focusing on analysing the recent protection approaches using modern intelligent techniques. Open research problems and future research trends in AC microgrid protection are also presented in this research. • Thorough examination of the challenges associated with microgrid implementation. • Critical review of recently proposed AC microgrid protection strategies. • Comparison of protection methodologies and functional attributes. • Summary of feature extraction techniques and features extracted in AI-based methods. • Discussion on open research problems and future trends in AC microgrid protection. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of optimally operating a set of grid-forming devices in an AC microgrid when a detailed network model is not available. The main aim of the approach is to maximize the power sharing of the controllable grid-forming devices and to maintain the frequency and the nodal voltages of the microgrid as close as possible to their corresponding references. The proposed control architecture is conformed by a local control layer in each grid-forming device that intends to emulate the performance of a synchronous machine and a centralized secondary controller composed of two complementary tools that coordinates the setpoints of the grid-forming devices: an online feedback optimization algorithm and an automatic generation control. The proposed method has been validated through simulations and hardware-in-the-loop tests, evidencing its good performance and robustness under different conditions. • A new control scheme to optimally operate a set of grid-forming Converter Interfaced Generators (CIGs) is proposed • A PLL-free primary control layer guarantees active power sharing and frequency support. • The secondary controller ensures reactive power sharing and optimal nodal voltages. • The secondary controller operates without a microgrid model, ensuring operational constraints. • The controller manages islanded low-voltage microgrids with 100% of renewable based CIGs. [ABSTRACT FROM AUTHOR]

Hierarchical control of AC microgrids, despite providing stable voltage, frequency, and optimal power distribution, is susceptible to false data injection (FDI) attacks on its secondary control. Electric power systems in countries such as the United States and Venezuela have suffered cyberattacks. This paper proposes a redundant secondary control method to resist FDI attacks. This method uses distributed generators (DGs) online as redundant units on standby, generating dynamic corrections to the attacked DGs, thus eliminating the attack impact. It effectively addresses the numerical tampering and addition brought by FDI to the frequency and voltage setting values of the secondary control, ensuring the microgrid can still output stable frequency and voltage. The principle of this control method is direct and practical; it does not require complex controllers and can respond quickly to FDI attacks. Then, focusing on a microgrid system containing six DGs, the setting values of the microgrid system are tampered with by multiple values in different time periods, the simulation results confirm that the proposed method can effectively resist FDI attacks under various scenarios. Finally, field programmable gate array (FPGA)-in-the-loop experiments further verify the effectiveness of the proposed method.

• A low-bandwidth communication (LBC)-based distributed strategy to distribute load current accurately among parallel-connected inverters with mismatched wire impedances in an islanded microgrid under different load conditions. • (The balanced, unbalanced and harmonic components of the output current are extracted with the cascaded delayed signal cancelation (CDSC)-based algorithms and transformed into different synchronous reference frames, then they are exchanged among all the inverters. • For each inverter, each output current component is regulated with a proportional-integral (PI) controller to follow the generated component reference. From the PI controller outputs, the average voltage reference compensation is subtracted, to eliminate the influence of current sharing loops on voltage regulation. • A simplified inverter small-signal model is built, based on which the system stability is analyzed and the regulators in average current sharing loops are designed. • The load current can be distributed accurately among all DGs according to their rated power, and the bus voltage can be regulated in its desired range. Bus voltage regulation and current sharing are significant to ensure the normal operation of islanded microgrids. In this paper, a low-bandwidth communication (LBC)-based distributed current sharing strategy for islanded ac resistive microgrids is proposed, especially when the wire impedances of parallel-connected inverters are mismatched. In a general case, all the balanced, unbalanced and harmonic components of load current should be shared accurately. Therefore, in the local controller, those components of inverter output currents are extracted and regulated to follow the component average values generated based on LBC. To guarantee the bus voltage regulation, the influence of current sharing loops on voltage references is eliminated by subtracting the average voltage reference compensation from the current sharing controller outputs. A simplified inverter small-signal model is established, based on which the system stability is analyzed. Besides, the steady-state bus voltage regulation performance is discussed. The simulation in MatLab/Simulink is used to demonstrate the current sharing performance and bus voltage regulation ability. A hardware-in-loop platform of an ac microgrid prototype is built, where the power stage is in a real-time simulator and the controllers are implemented with DSPs. The results validate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This article details and solves a small-signal stability-constrained optimal power flow (SSSC-OPF) for inverter-based ac microgrids. To ensure a sufficient stability margin during optimal generation, a small-signal stability constraint is embedded into the conventional OPF formulation. This condition is enforced using a Lyapunov stability equation. A reduced-order model of the microgrid is adopted to alleviate the computational burden involved in solving the resulting SSSC-OPF. Even then, the resulting stability conditions are highly nonlinear and cannot be handled using the existing methods. To tackle the nonconvexity in the SSSC-OPF due to the presence of the nonlinear stability constraint, two distinct convex relaxation approaches, namely semidefinite programming and parabolic relaxations, are developed. A heuristic penalty function is added to the objective function of the relaxed SSSC-OPF, which is solved sequentially to obtain a feasible point. While off-the-shelf tools fail to produce any feasible point within hours, the proposed approach enables us to solve the SSSC-OPF in near real time. The efficacy of the proposed SSSC-OPF is evaluated by performing numerical studies on multiple benchmarks as well as real-time studies on a microgrid system built in a controller/hardware-in-the-loop setup. [ABSTRACT FROM AUTHOR]

The share of electrical energy from renewable sources has increased considerably in recent years in an attempt to reduce greenhouse gas emissions. To mitigate the uncertainties of these sources and to balance energy production with consumption, an energy storage system (ESS) based on water electrolysis to produce hydrogen is studied. It can be applied to AC microgrids, where several renewable energy sources and several loads may be connected, which is the focus of the study. When excess electricity production is converted into hydrogen via water electrolysis, low DC voltages and high currents are applied, which needs specific power converters. The use of a three-phase, buck-type current source converter, in a single conversion stage, allows for an adjustable DC voltage to be obtained at the terminals of the electrolyzer from a three-phase AC microgrid. The voltage control is preferred to the current control in order to improve the durability of the system. The classical control of the buck-type rectifier is generally done using two loops that correspond only to the control of its output variables. The lack of control of the input variables may generate oscillations of the grid current. Our contribution in this article is to propose a new control for the buck-type rectifier that controls both the input and output variables of the converter to avoid these grid current oscillations, without the use of active damping methods. The suggested control method is based on an approach using the flatness properties of differential systems: it ensures the large-signal stability of the converter. The proposed control shows better results than the classical control, especially in oscillation mitigation and dynamic performances with respect to the rejection of disturbances caused by a load step. [ABSTRACT FROM AUTHOR]

• An adaptive virtual impedance regulation strategy is proposed to eliminate the wire impedance mismatch. • Both the resistive and inductive parts of inverter impedance are regulated distributedly, which ensures accurate current distribution. • The current is shared both in steady state and transient accurately. In islanded mocrogrids, the mismatch among the equivalent impedances (sum of output impedance and wire impedance) degrades the current sharing performance. It is an essential mission to eliminate this mismatch to achieve accurate current sharing in different conditions. About this issue, several aspects should be considered. Firstly, for a general microgrid, the inverter equivalent impedances are unknown, both the resistance and reactance should be considered. Secondly, all the balanced, unbalanced and harmonic components of load current should be shared according to the power ratings of distributed generators. Besides, not only the current sharing in steady states but also during transients should be achieved. Herein, a low-bandwidth communication-based distributed current sharing strategy for parallel-connected inverters is proposed in this paper. The virtual resistance and reactance are adaptively regulated according to the magnitude error and phase angle error of the inverter uniform output current respectively. Then they can be used to reshape the inverter equivalent impedance at fundamental positive-sequence frequency, fundamental negative-sequence frequency and harmonic frequencies. With the proposed method, all the three aspects mentioned above can be guaranteed without knowing inverter wire impedances in advance. The simulation and experiments on an islanded microgrid prototype are developed to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Microgrids (MG) are small-scale electric grids with local voltage control and power management systems to facilitate the high penetration and grid integration of renewable energy resources (RES). The distributed generation units (DGs), including RESs, are connected to (micro) grids through power electronics-based inverters. Therefore, new paradigms are required for voltage and frequency regulation by inverter-interfaced DGs (IIDGs). Notably, employing effective voltage and frequency regulation methods for establishing power-sharing among parallel inverters in MGs is the most critical issue. This paper provides a comprehensive study, comparison, and classification of control methods including communication-based, decentralized, and construction and compensation control techniques. The development of inverter-dominated MGs has caused limitations in employing classical control techniques due to their defective performance in handling non-linear models of IIDGs. To this end, this article reviews and illustrates advanced controllers that can deal with the challenges that are created due to the uncertain and arbitrary impedance characteristics of IIDGs in dynamics/transients.

In this paper, we investigate the secondary control problems of AC microgrids with physical states (i.e., voltage, frequency and power, etc.) constrained in the process of actual control, namely, under the condition of state constraint. On the basis of the primary control (i.e., droop control), the control signals generated by distributed secondary control algorithm are used to solve the problems of voltage and frequency recovery and power allocation for each distributed generators (DGs). Therefore, the model predictive control (MPC) with the mechanism of rolling optimization is adopted in the second control layer to achieve the above control objectives and solve the physical state constraint problem at the same time. Meanwhile, in order to reduce the communication cost, we designed the self-triggered control based on the prediction mechanism of MPC. In addition, the proposed algorithm of self-triggered MPC does not need sampling and detection at any time, thus avoiding the design of observer and reducing the control complexity. In addition, the Zeno behavior is excluded through detailed analysis. Furthermore, the stability of the algorithm is verified by theoretical derivation of Lyapunov. Finally, the effectiveness of the algorithm is proved by simulation.