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This paper presents a distributed complex power sharing approach that solves the problem of active and reactive power sharing in inverter-based islanded microgrids. Previous state-of-the-art strategies based on droop controls with possibly virtual impedance methods and supplemented with hierarchical schemes may provide poor power sharing due to the R/X ratio and mismatched line impedances, even compromising microgrids stability if droop control parameters are not properly set. The novel control approach, that uses a communication network to exchange data among all inverters to fairly inject power, provides a set of appealing properties. First, it achieves accurate active and reactive power sharing for both resistive and inductive power lines with no additional control cost (avoiding unnecessary power injections), and regardless of load changes and connections and disconnections of inverters. Moreover, it ensures an exponential convergence rate, and the tuning of the control parameters can not compromise microgrid stability. The theoretical development relies on a change of variables that linearizes the complex power dynamics, thus easing the control law design task and providing the means for computing the appropriate amplitude and phase for each inverter output voltage. Selected experimental results on a laboratory microgrid certify the applicability and performance of the proposed control scheme. [ABSTRACT FROM AUTHOR]

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

Source-side virtual synchronous machines (VSGs) and load-side VSMs (LVSM) are gradually utilized together in the microgrid to provide virtual inertia and damping. However, instability occurs in the islanded microgrid system due to the interaction dynamics between the VSG and the LVSM, which has been investigated in this article. At first, the dq-frame impedance models of the VSG and the LVSM are established and compared. It is revealed that the d-d channel impedance of LVSM behaves the negative resistor with a V-type magnitude in the low-frequency range, which easily interacts with the d–d channel impedance of the VSG and leads to instability of the system. Thus, the inductor current feedforward control and the additional voltage feedback control are proposed for the VSG to reshape its impedance. It diminishes the impedance magnitude and generates the active impedance of the VSG. In this way, the low-frequency interaction between the VSG and the LVSM can be mitigated. Besides, the proposed control preserves the dynamic performance of the system. Finally, simulations and experiments verify the effectiveness of stability analyses and the proposed suppression method. [ABSTRACT FROM AUTHOR]

This paper applies a new state feedback control to a distributed secondary voltage and frequency control in an islanded microgrid. The problem is focused on the output consensus of the multi-agent systems, which is converted to a first-order dynamic system. The inverter-based distributed generations play as agents in the proposed control strategy. It is assumed that the distributed generators communicate through a communication network modeled by a directed graph (digraph). The distributed output consensus is used to design the secondary controllers. Such innovative controllers synchronize distributed generators' output voltages and frequencies to their reference values by a novel state feedback approach. Compared to the existing consensus protocols, the proposed method provides a different innovative solution to the secondary voltage and frequency control of microgrids, which has a better response in case of communication failures. Finally, extensive and comparative simulations have been presented to verify the validity of the proposed control strategy and the system performance. [ABSTRACT FROM AUTHOR]

Droop control is commonly used for primary control of island microgrids at the expense of voltage deviations. Furthermore, voltage and frequency restoration can be achieved using droop control along with the communication line. However, system complexity increases and reliability decreases due to the communication delay and data loss. Also, overmodulated voltage source converters (VSCs) degrade the ac output in a converter-dominated islanded microgrid. In this article, an improved technique is proposed to control voltage and frequency for an islanded microgrid. The band-pass filter-based droop control (an equivalent secondary control without a communication line) is developed to control distributed inverters to enhance voltage and frequency regulation by reducing response time, overshoot, and steady-state error. Furthermore, a modified proportional integral derivative (MPID) controller is adopted for inner voltage and current loop control to handle the system response fluctuation subject to perturbation and eliminate further voltage quality deterioration. For optimal control action of MPID controller, the controller parameters are tuned using dynamic weighted salp swarm algorithm and validated by both bode plot and step response. Furthermore, an optimal third harmonic injection-sine pulsewidth modulation technique is proposed to reduce the harmonic generation for the entire operation region of the VSC. The proposed control technique is simulated in MATLAB Simulink, and its performance is compared with a state-of-the-art method. Also, the proposed control technique is validated on the OPAL-RT real-time simulator test bench. Investigation revealed that the proposed technique improved the voltage and frequency regulation and effectively minimized overshoots and steady-state errors of the system. [ABSTRACT FROM AUTHOR]

A control strategy of islanded microgrid is proposed in this paper against the harmonic circulation of the inverters and the Point of Common Coupling (PCC) voltage harmonic distortion of the microgrid caused by nonlinear load. The reference voltage of the inverter’s outer voltage loop is given by the decoupling control of the fundamental and harmonic components. Virtual synchronous generator (VSG) is adopted in fundamental domain. In harmonic domain, adaptive harmonic compensation and harmonic virtual impedance are applied to obtain the harmonic components of the reference voltage of the outer voltage loop. The hybrid control strategy combining P control and repetitive control in parallel is adopted in the outer voltage loop. Multi-period repetitive control (MPRC) with fast repetitive control (FRC) is adopted to improve the tracking control performance of the main harmonic components of the given reference voltage. Fractional order delay approximated by Lagrange interpolation polynomial FIR filter is applied in MPRC for different repetitive control periods at the fixed sampling period. The output harmonic current of the inverters can be distributed autonomously according to the capacity of the inverters to suppress the harmonic circulation, and the suppression of the PCC voltage harmonic distortion is realized. Simulation based on Matlab/Simulink and hardware-in-loop(HIL) test are carried out, and the results show that the proposed control strategy is effective.

Power flow (PF) analysis of microgrids (MGs) has been gaining a lot of attention due to the evolution of islanded MGs. To calculate islanded MGs’ PF solution, a globally convergent technique is proposed using Differential Evolution (DE)- a popular optimization algorithm for global non-convex optimization. This paper formulates the PF problem as a constrained optimization problem (COP) considering all the operating conditions of the Droop Controlled Islanded MGs (DCIMGs). To solve the proposed COP, $\epsilon $ DE-NGM, (Epsilon based Differential Evolution with Newton-Gauss-based mutation) is proposed. The proposed algorithm, $\epsilon $ DE-NGM, is a novel variant of DE since it comprises a novel mutation operator, Newton-Gauss-based mutation (NGM). NGM includes all the important features of DE’s mutation strategies as well as reduces the constraint violation by utilizing the information of constraint-space. Numerical experiments validate that the global convergence ability of proposed algorithms in solving COPs than existing state-of-the-art algorithms. Furthermore, the proposed algorithm as a PF tool has better robustness than existing tools on ill- and well-conditioned systems with heavy loads, different limit violations, and inappropriate final solutions (far from the flat start). The performed comparative analysis confirms good agreement of accuracy and efficacy with the existing method for islanded MG’s PF.

In this paper we consider the problem of restoring the voltage for stand-alone inverter-based Microgrids despite the effects of the time-delays arising with the information exchange among the electrical busses. To guarantee that all Distributed Generators (DGs) reach in a finite-time and maintain the voltage set-point, as imposed by a virtual DG acting as a leader, we suggest a novel robust networked-based control protocol that is also able to counteract both the time-varying communication delays and natural fluctuations caused by the primary controllers. The finite-time stability of the whole Microgrid is analytically proven by exploiting Lyapunov-Krasovskii theory and finite-time stability mathematical tools. In so doing, delay-dependent stability conditions are derived as a set of Linear Matrix Inequalities (LMIs), whose solution allows the proper tuning of the control gains such that the control objective is achieved with required transient and steady-state performances. A thorough numerical analysis is carried out on the IEEE 14-bus test system. Simulation results corroborate the analytical derivation and reveal both the effectiveness and the robustness of the suggested controller in ensuring the voltage restoration in finite-time in spite of the effects of time-varying communication delays.

Secondary voltage control of islanded microgrid is one of the main needs of microgrid hierarchical control from one perspective and the realization of stable smart grids from another aspect. This article aims to achieve several goals simultaneously: First, it provides a decentralized structure for controlling and regulating the voltage of DGs, in which the effects of nonlinear terms on the dynamics of the microgrid system are considered without elimination or linearization. The fuzzy method has been used to approximate the uncertainties, the interactions of agents and nonlinear terms, and the adaptive method has been used to obtain the optimal fuzzy approximation gains. Sliding mode rules are also used to obtain a control signal and using Lyapunov's theory, the finite time and robust stability of the closed-loop control system is guaranteed. The finite time approach ensures the achievement of convergence in a finite and definite time, and the robust approach realizes the dynamic stability of the system against uncertainties and disturbances. The results of the microgrid system simulation show well the proposed controller capability in different operating conditions.

Recent advances in inverter-based distributed energy resources (DERs) allow microgrids to operate in grid-connected and islanded modes with ease. However, determining a steady-state load flow solution of a real-world unbalanced and islanded three-phase microgrid remains a challenge. Existing methods are either unsuitable, labor-intensive or computationally demanding for a long-term analysis of islanded microgrids. To address these issues, we propose a practical solution framework that employs externally-updated system frequency, power quantities, and droop characteristics governing the voltage and phase angle of DERs in every iteration as inputs to an off-the-shelf open-source multi-phase, multi-wire, unbalanced power flow software tool to solve an islanded-microgrid power flow. Using our framework, users only need to implement a set of droop equations and update system variables. The power flow solution is accomplished entirely by an off-the-shelf power flow tool, e.g., OpenDSS. Our proposed framework can model a microgrid of any arbitrary configuration and operating condition. We demonstrate and validate the proposed method’s efficacy by comparing its results with those modeled using a time-domain simulation with PSCAD/EMTDC. Finally, an example of a quasi-static time-series study is presented to better illustrate the application of such a tool.

To cope with the volatility and randomness of wind power, photovoltaic (PV) power, and load demands in the islanded microgrid, and also to ensure the safety and economic operation of the islanded microgrid system. A collaborative Distributed model predictive control (Di-MPC) based voltage optimization control strategy is proposed, which considers the strong coupling characteristic of active and reactive power due to the impedance ratio of islanded microgrid, and the requirements of real-time and robustness in the optimization as well. By coordinating the controllable devices in the source-grid-load side of the islanded microgrid, the proposed strategy aims to make full use of the voltage regulation capability of each controllable device. Firstly, by considering the different operating characteristics of the controllable devices, a multi-time scale distributed voltage optimal control model is established. It divides the optimal control process into long-time scale and short-time scale and optimizes for respective objective functions and control variables in different time scales. Secondly, a collaborative Di-MPC-based voltage optimal control strategy is proposed. With the proposed collaboration mechanism, the power output increments of the distributed generators (DGs) are solved in the short-time scale, and the errors in the long-time scale control are also fixed. Finally, the simulation results show that compared with a traditional optimal control method and a centralized model predictive control (CMPC) method, the proposed voltage optimization control strategy can effectively reduce the voltage deviation and fluctuation at each node while ensuring the economic operation of the islanded microgrid system.

This paper describes the concept of virtual oscillator control (VOC) for parallel-connected single-phase inverters (SPIs). The principal idea is to introduce a series of weakly coupled oscillators (Deadzone oscillator-based VOC and Van der Pol oscillator-based VOC) that can be used for the regulation of single-phase power inverters in an islanded microgrid (MG). Its dynamic equations are used to provide the frequency and amplitude references of the inverters. In these traditional methods, there is always the presence of a 3−order harmonic in the output voltage, which causes a significant amount of 3−order harmonic current in the system. The non-linear dynamical equations of the oscillator are analyzed and its non-linear current source (NCS) is made simpler in order to develop new VOC for SPIs that can effectively get rid of the 3−order harmonic component in the oscillator's output voltage. Finally, an extensive comparison of Deadzone-based VOC (Dz-VOC), Van der pol-based VOC (VdP-VOC), and new VOC-based controllers with linear and nonlinear loads is presented. The new VOC-based controller minimizes the 3-order harmonic component in the output voltage and achieves a quicker response compared to the conventional VdP-VOC-based controller. Simulation results of the Dz-VOC, VdP-VOC, and proposed new VOC-based control methods with different loads (Resistive, Linear RLC, non-linear) were compared and analyzed in detail. The total harmonic distortion (THD) of the current in Dz-VOC, VdP-VOC, and new VOC are 1.98%, 1.11%, and 0.21%, respectively. The 3rd harmonic is dominant in both Dz and VdP VOCs, while in the new VOC the 3rd harmonic is very less and below 0.1%. The new VOC is also showing good current sharing and fast voltage synchronization (within 0.2 s). Hardware experimentation is also carried out to analyze the efficacy of the proposed new VOC controlled SPI in standalone MG. The results clearly depict that the new VOC control strategy is quite efficient in handling the output voltage harmonics and situations of different loadings in the standalone MG. [ABSTRACT FROM AUTHOR]

Frequency stability is one of the major challenges in renewable energy source-based microgrids. This issue can be resolved by employing virtual inertia control that helps in improving damping properties and stability. In applying this control method, a phase-locked loop (PLL) is used to acquire the assessment of the frequency data of the microgrid. Nonetheless, the application of PLL in microgrids produces larger frequency oscillation due to its system dynamics. To solve such issues, we propose a robust Interval Type-2 Fuzzy Logic PID Controller (IT2FPID) for the virtual inertia control to limit the unwanted frequency measurement impacts while simultaneously improving the microgrid system stability. Further, a novel improved parasitism predation algorithm (IPPA) algorithm is proposed to tune the controller parameters. The performance of our proposed solutions is studied by comparing the simulation results with a few standard techniques considering the change in system parameters and different paces of renewable energy source penetration. [ABSTRACT FROM AUTHOR]

The balanced, unbalanced and harmonic current sharing inaccuracy caused by the mismatched wire impedances of distributed generation (DG) units influences the normal operation of islanded microgrids. To deal with this issue, an iterative virtual impedance regulation (IVIR) strategy is proposed in this paper, both the resistive and reactive parts in the output impedances of DG units are regulated. The whole process is decomposed into several successive iterations. In each iteration, the fixed virtual impedance and adaptive virtual impedance regulation are combined to generate a virtual resistance increment and uniform virtual reactance increment according to the uniform output current magnitude and phase angle respectively. Based on these increments, the virtual impedance at fundamental-negative and harmonic frequencies also are modified. With the proposed strategy, all the balanced, unbalanced and harmonic current sharing is accurate with a relatively small virtual impedance. Only the local current information is required for the IVIR process, and the communication is merely used to exchange the VIR mode status flags at the beginning and the end of the IVIR process, the dependency on communication is minimized. The feasibility of the proposed strategy is verified with the simulation and experimental results. [ABSTRACT FROM AUTHOR]

A battery energy storage system (BESS) can play a critical role in regulating system frequency and voltage in an islanded microgrid. A $\mu$ -synthesis-based robust control has been proposed for dc link voltage regulation of BESS for achieving frequency regulation and voltage quality enhancement of islanded microgrid. Variation in the operating condition of ac microgrid affects the operating condition of the BESS’ converter. This controller synthesis accounts for such uncertain variations as parametric uncertainty. The stability and performance of the proposed controller can be guaranteed for bounded parametric variations. The bounds on parameters are selected based on practical limitations of BESS. In this article, the proposed controller's performance is tested on an islanded CIGRE TF C6:04:02 benchmark low voltage ac microgrid system. The importance of dc link voltage regulation is analyzed based on performance comparison with a benchmark controller. The controller performance is also validated using a real-time Typhoon HIL emulator. [ABSTRACT FROM AUTHOR]

Summary: Due to lack of support from the main grid, the problem of frequency over‐limitation is easily caused during heavy load disturbance occurs in the islanded microgrid (MG). For this reason, an improved curve‐shaped droop control strategy, which is composed by virtual synchronous generator (VSG) and an improved droop control, is designed to ensure the frequency stable of the MG system. In the contributed strategy, when the frequency of MG is changed within the normal range, the VSG control will be adopted to maintain the stability of the output power and frequency, this method can ensure that the frequency can be adjusted within the stable range rapidly; when the frequency deviation is serious, the proposed strategy will select the improved droop control strategy to ensure the output power equals to the load demand, and to ensure that the controlled frequency cannot lower than the minimum value. In addition, in order to suppress the voltage deviation and the system oscillation caused by the switching of the control methods, a voltage synchronization control strategy is also proposed, which can realize the smooth switching of the two control strategies and guarantees the voltage stable of the MG system. The effectiveness of the proposed methods is verified by simulation experiments via matlab/Simulink software. [ABSTRACT FROM AUTHOR]

Global Positioning System (GPS) based control has recently been reported as a replacement of droop control to achieve fixed frequency operation in islanded microgrids. This article presents the perspective that the power sharing performance of a general microgrid synchronized by GPS is essentially determined by equivalent output impedance. A novel adaptive virtual impedance control approach, which implements different values of virtual resistance for d- and q-axis, is proposed accordingly. The virtual resistance concept is implemented comprising a basic local implementation for output impedance shaping, and a sparse resistance tuning network for the compensation of mismatched feeder impedance, which demands no knowledge of actual output impedance. The resistance tuning network utilizes the consensus protocol and only requires neighboring interactions among DG units. A complete tuning of output resistance for a given load condition results in accurate active and reactive power sharing even after communication is interrupted and will still outperform conventional droop control methods if load changes during the interruption. Small-signal analysis based on delay differential equations model of the overall microgrid is performed to investigate the adverse impact of communication delays on system stability. The efficacy of the proposed approach is validated by both simulation and experimentation. [ABSTRACT FROM AUTHOR]

An increased integration of distributed generators into microgrids presents a technical challenge to maintain voltage and frequency stability, especially in islanded or autonomous operation. For stability assessment of the microgrid, a proper initialization is particularly needed to get the right steady-state operating point when the widely used induction motor (IM) loads are mainly considered. Since a conventional power-flow approach is not straightforward and suitable for initializing the IM loads in the isolated microgrid network due to an unavoidable discrepancy between initial-bus scheduled and actual values of IM's reactive powers. To eliminate this mismatch, this paper presents a precise power-flow initialization using a unified Newton–Raphson approach. The correct steady-state initializations are demonstrated using 6bus and 13bus microgrid systems. IEEE standard 33, 38, and 69bus distribution networks are employed to explore computational performances of the proposed algorithm for the case where a large number of IM loads are incorporated via distribution feeder loads. [ABSTRACT FROM AUTHOR]

In remote areas, extending a power line to the primary electricity grid can be very expensive and power losses are high, making connections to the grid almost impossible. A well-designed microgrid that integrates renewable energy resources can help remote areas reduce investment costs and power losses while providing a reliable power source. Therefore, investigating the design of an independent and economically practical microgrid system for these areas is necessary and plays an important role. This paper introduces a design procedure to design an isolated microgrid using HOMER software (HOMERPro 3.14.5) for remote areas. In Vietnam, due to the obstruction of the mountainous terrain or the isolated island location, many remote areas or islands need electrification. A simple case study of a hybrid system with a 60 kW peak load demand on Con Dao island in Vietnam is used to illustrate the proposed design method. Specifically, a hybrid system that includes a PV system, batteries, and a diesel generator is designed. To provide the full information of the designed hybrid system designed, each solution is analyzed and evaluated in detail according to the sensitivity parameters. [ABSTRACT FROM AUTHOR]

Secondary voltage control of islanded microgrid is one of the main needs of microgrid hierarchical control from one perspective and the realization of stable smart grids from another aspect. This article aims to achieve several goals simultaneously: First, it provides a decentralized structure for controlling and regulating the voltage of DGs, in which the effects of nonlinear terms on the dynamics of the microgrid system are considered without elimination or linearization. The fuzzy method has been used to approximate the uncertainties, the interactions of agents and nonlinear terms, and the adaptive method has been used to obtain the optimal fuzzy approximation gains. Sliding mode rules are also used to obtain a control signal and using Lyapunov's theory, the finite time and robust stability of the closed-loop control system is guaranteed. The finite time approach ensures the achievement of convergence in a finite and definite time, and the robust approach realizes the dynamic stability of the system against uncertainties and disturbances. The results of the microgrid system simulation show well the proposed controller capability in different operating conditions. [ABSTRACT FROM AUTHOR]

An islanded microgrid (IMG) having renewable energy sources (RES), dispatchable sources and battery energy storage (BES) is an effective approach to supply the electricity in remote areas. The reliable operation of IMG is quite complex and challenging task, due to uncertain behaviour of RES, loads and energy bid. In this paper, an IMG is simulated to maximise the revenue considering a dynamic optimisation objective subjected to various constraints of system components for two different cases, i.e. considering the stationary and the dynamic energy bid. Artificial bee colony (ABC) and its variants based on global best (GABC), and global and personal best with asynchronous scaling factors (GPSABC) are implemented and the obtained results are also compared with the existing optimisation techniques. Results of GPSABC improve the revenues and offers better accuracy and stability parameters. Further, the effect on revenue results with different ramp rates of diesel generators is described through GPSABC.

In this paper, a robust local controller has been designed to balance the power for distributed energy resources (DERs) in an islanded microgrid. Three different DER types are considered in this study; photovoltaic systems, battery energy storage systems, and synchronous generators. Since DER dynamics are nonlinear and uncertain, which may destabilize the power system or decrease the performance, distributed robust nonlinear controllers are designed for the DERs. They are based on the Lyapunov stabilization theory and super-twisting integral sliding mode control which guarantees system stability and optimality simultaneously. The reference signals for each DER are generated by a supervisory controller as a power management system. The controllers proposed in this work are robust, have fast response times, and most importantly, the control signals satisfy physical system constraints. The designed controller stability and effectiveness are also verified using numerical simulations.

A new modified extended state observer (MESO)-based robustness voltage sliding mode control (SMC) strategy is proposed for an AC islanded microgrid under system uncertainties including system parameter and load variation. First, the disturbance effect on the system is regarded as a lumped uncertainty, and a state space model of the uncertain islanded microgrid system is established. Then, a modified extended state observer is designed to estimate external disturbances and internal perturbation. Finally, considering the lumped uncertainty, a sliding mode controller with a multi-power reaching law is proposed to enable the output voltage of the system to track its reference voltage rapidly and accurately, and to enhance the robustness of the system. The simulation results confirm that the proposed robustness voltage control strategy can perform satisfactory voltage control and demonstrate a strong capability to reject parameter and load variation.

With the increase of distributed generation penetration, the economic operation of microgrid has been fully developed. For islanded microgrid, due to the lack of large grid support and the limited capacity of the distributed generation (DG), the energy supply of microgrid may not meet the need of users in real time. Therefore, it is of great significance to solve the problem of insufficient power supply of islanded microgrid and improve the economy of its operation. Firstly, this paper analyses the frequency characteristics of DG, and establishes power models oriented to the frequency regulation characteristics of microgrid and introduces the mathematical model of DG. Then, an islanded microgrid day-ahead optimization scheduling model considering frequency regulation characteristics of units and demand response (DR) is proposed. The frequency characteristics of controllable units and DR are considered in the model, and different strategies are selected to analyze their impact on microgrid economy. The example shows that the optimization scheduling model of microgrid established in this paper can reduce the power supply pressure during peak load periods, alleviate the problem of insufficient power supply, and effectively improve the overall benefit of microgrid, which verifies the rationality of the model.

In recent years, with the increase of distributed generation (DG) penetration rate, the economic operation of microgrid (MG) has been fully developed, but the energy consumption system based on fossil energy has increased the contradiction of the economy, energy and environment. The process of accelerating the development and utilization of renewable energy are a new focus of social concern. This paper first analyzes the basic characteristics and related policies of the distributed power and demand response (DR) in MG, and introduces the evaluation index of the islanded microgrid. Then, established a multi-objective optimization scheduling model considering the operation performance of islanded microgrid. In model, considered the generation cost of microsources, environmental management cost and power supply reliability of MG, and selected different scene analysis and verification. Used the improved particle swarm optimization-exterior point method (IPSO-EPM) to solve the problem. When the constraint condition is included in the objective function, the problem of falling into the local extremum during planning is avoided as much as possible. Finally, an islanded microgrid is taken as an example to verify the rationality of the proposed model and the effectiveness of the algorithm.

Model predictive control (MPC) facilitates online optimal resource scheduling in electrical networks, thermal systems, water networks, process industry to name a few. In electrical systems, the capability of MPC can be used not only to minimise operating costs but also to improve renewable energy utilisation and energy storage system degradation. This work assesses the application of MPC for energy management in an islanded microgrid with PV generation and hybrid storage system composed of battery, supercapacitor and regenerative fuel cell. The objective is to improve the utilisation of renewable generation, the operational efficiency of the microgrid and the reduction in rate of degradation of storage systems. The improvements in energy scheduling, achieved with MPC, are highlighted through comparison with a heuristic based method, like Fuzzy inference. Simulated behaviour of an islanded microgrid with the MPC and fuzzy based energy management schemes will be studied for the same. Apart from this, the study also carries out an analysis of the computational demand resulting from the use of MPC in the energy management stage. It is concluded that, compared to heuristic methods, MPC ensures improved performance in an islanded microgrid.

Supply and demand balance is required to assure the normal operation of microgrids. When unintentional islanding occurs in a microgrid, a load shedding (LS) control scheme is necessary to achieve the power balance and frequency stability of an islanded microgrid due to its limited power generation. However, the LS control of a microgrid faces two key challenges, i.e., how to determine the appropriate LS amount and LS objects in an islanded microgrid. To solve these problems, this paper proposes a coordinated LS control scheme based on Double-Q learning for an islanded microgrid. In the proposed coordinated LS control scheme, aiming at the LS amount problem in the process of unintentional islanding, this paper considers the relationship between the active power and frequency deviation of each distributed energy resource (DER) and uses the frequency deviation measured locally to calculate the LS amount and guide LS actions. Aiming at the problem of LS object selection in the process of unintentional islanding, this paper describes the LS control problem as a Markov decision process (MDP). The load priority is considered in a reward value function to ensure the uninterrupted power supply of critical loads in a microgrid. The two value functions of Double-Q learning are used to make the two Q value tables measure each other, which can reduce the overestimation of Q values and improve the accuracy of decision-making. The performance of the proposed coordinated LS control scheme is verified by a microgrid model based on a modified IEEE 13-bus system. The test results verify the feasibility and effectiveness of the proposed coordinated LS control scheme.

With the high integration of power electronic technologies in microgrids, the reliability assessment considering power electronic devices has become a hot topic. However, so far no research has considered the impact of the operation failure probability of power electronic equipment on the overall reliability of the microgrid. This paper aims to construct a holistic operation failure rate model of power electronic systems based on the overall reliability assessment of islanded microgrid with high penetration of renewable energy sources (RESs). In addition, to improve the reliability of islanded microgrid, the conventional battery energy storage system (BESS) is replaced by the hybrid energy storage system (HESS). Based on the proposed model, the operation failure models for the power electronic modules in microgrid are built and tested, and then the sensitivity analysis is performed for exploring the influence of various factors on the reliability of the microgrid.

Load flow analysis is a significant tool for proper planning, operation, and dynamic analysis of a power system that provides the steady-state values of voltage magnitudes and angles at the fundamental frequency. However, due to the absence of a slack bus in an islanded microgrid, modified load flow algorithms should be adopted considering the system frequency as one of the solution variables. This paper proposes the application of nature-inspired hybrid optimization algorithms for solving the load flow problem of islanded microgrids. Several nature-inspired algorithms such as genetic algorithm (GA), differential evolution (DE), flower pollination algorithm (FPA), and grasshopper optimization algorithm (GOA) are separately merged with imperialistic competitive algorithm (ICA) to form four hybrid algorithms named as ICGA, ICDE, ICFPA, and ICGOA. Performances of these algorithms are tested on the 6-bus test system and the modified IEEE 37-bus test system. A comparison among the proposed algorithms is carried out in terms of statistical analysis conducted using SPSS statistics software. From the statistical analysis, it is identified that on an average, ICDE takes less number of iterations and consequently needs less execution time compared with other algorithms in solving the load flow problem of islanded microgrids.

Dump load (DL) utilization at low demand hours in highly penetrated islanded microgrid is of great importance to offer voltage and frequency regulation. Additionally, load flow (LF) convergence is vital to optimize the working states of the DL allocation problem. Hence, more analysis is necessary to highlight the significance of DL in power regulation while observing the influence of LF on solution accuracy. This article proposes two LF techniques derived from backward/forward sweep (BFS), viz., general BFS (GBFS) and improved special BFS (SBFS-II). The latter is based on global voltage shared between generating units, while the former has a more general approach by considering generating bus’s local voltage. The optimal sizing and sitting of DL with optimum droop sets are determined using the mixed-integer distributed ant colony optimization (MIDACO) with the two new LF methods. The optimization problem was formulated to minimize voltage and frequency deviations as well as power losses. The problem was validated on IEEE 69- and 118-bus systems and compared with established metaheuristics. Results show that DL allocation using MIDACO with SBFS-II and GBFS has improved the solution speed and accuracy, respectively. Furthermore, the enhanced voltage and frequency results highlight DL as an efficient power management solution.

This paper focuses on designing a robust control law to manage the demand response of islanded microgrids composed of shifting and adjusting loads. On one side, Hybrid Dynamical System theory is suited here, because the microgrid model is composed of continuous-time dynamics (the energy stored in the battery and the adjustable loads), and discrete-time dynamics (the shifting loads). On the other side, Multi Agent System theory is used to control the adjusting loads in order to guarantee a consensus between them. Hence, non-uniform convergence of the State of Charge of the battery to a given reference is ensured. Robustness with respect to plug and play of any load and parameter variations is also ensured. Experimental results from a laboratory-scale microgrid validate the approach. [ABSTRACT FROM AUTHOR]

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Islanded, renewable energy microgrids for use at remote operating facilities reduce logistical burdens associated with fossil fuel based electrical power sources and provide greater operational flexibility; however, energy generation can be intrinsically intermittent on renewable microgrids. This intermittent electrical generation can be mitigated with electrical energy storage. Liquid air energy storage (LAES) is one promising technology proposed to meet this energy storage issue due to its high energy density and lack of geographical constraints. Small-scale microgrids may not have enough excess capacity to store pressurized liquid air (LA), and instead may rely on unpressurized LA storage and their associated unpressurized power recovery systems. Using commercial off-the-shelf components, this article conducts a performance-based tradespace study for several dual opening, unpressurized Dewar designs for use with Stirling- or Peltier-based power recovery cycles. The dual opening Dewar design is found to be ineffective for the short-term LA storage times necessary for small-scale microgrid use, primarily due to excessive conductive thermal losses; however, the design may be useful as a LA receiver and immediate use energy storage medium for a connected Stirling generator. A proposed alternative solution using a self-pressurized Dewar for LA storage and transport for microgrid applications is presented for future work.

In order to improve robust operating performance and enhance bus voltage stability, a learning observer-based fault-tolerant control strategy is proposed for the distributed generation in islanded microgrid with sensor faults and uncertain disturbances. Firstly, the output feedback control theory and the linear matrix inequality method are used to design closed-loop controller for the voltage source inverter of distributed generation; secondly, a fault-tolerant model and control structure of the distributed generation in an islanded microgrid with sensor faults is analyzed. By employing the fault output signal conversion filter and proportional derivative type learning observer, the online estimation and real-time compensation of the sensor fault signal are realized. Thirdly, the system synthesis of output feedback control and fault-tolerant control is completed. Finally, the multi-scenario sensor fault scheme simulation experiment verifies that the proposed control strategy has strong sensor fault tolerance and adaptability.

Sliding mode control can restrain the perturbations generated from the intermittence of the renewable energy generation and the randomness of local loads when microgrids are operating in islanded mode. However, the microgrid consists of several subsystems and the interactions among them will cause the chattering problems under the overall sliding mode control. In this paper, the chattering restraint issues for voltage control of the islanded microgrid with a ring topology structure are investigated based on the decentralized adaptive sliding mode control strategies. Firstly, we construct a tracking error system with interconnections considering the power transmission among subsystems and nominal values of system states. Secondly, we design linear matrix inequalities (LMIs) according to the H∞ attenuation performance of the system external disturbances. Then, the tracking error performance and the control precision are guaranteed via the asymptotic stabilities of integral sliding mode surfaces. Adaptive laws are utilized to address the chattering problems of the sliding mode control. Finally, simulation results verify the effectiveness of the proposed decentralized control methods.

Recently, several numerical algorithms have been proposed to solve the power flow (PF) problems of islanded microgrids (MGs). However, these algorithms approximate the steady-state model of the distributed generation units (DGs) as linear equations. In some cases, this linear approximation leads to inaccurate PF solutions due to dead-zone and generation limits of the given DGs. To fill this gap, we propose a new non-linear approximation of the steady-state model of the DGs considering dead-zone and generation limits of the DG. Besides, an inversion-free PF algorithm based on the improved Newton-Traub composition (INTC) is proposed for solving PF problems of grid-connected and islanded MGs more robustly and accurately, where the inverse of the Jacobian matrix is updated by using the Broyden’s method. To include the power losses of the neutral conductor and grounding resistance, four-conductor based PF formulation is proposed. The numerical results on several test systems suggest that the proposed algorithm can provide an accurate steady-state solution as compared to other state-of-the-art algorithms. [ABSTRACT FROM AUTHOR]

With microgrids gaining more popularity in effectively meeting the increasing power demand, the operational challenges keep on increasing hand-in-hand. Especially when the microgrid work in islanded mode, it has to supply its area loads and maintain the system parameters as per the standards. This requires systematic coordination among the distributed generators so that the real and reactive power sharing is proportional to their ratings, simultaneously maintaining the system voltage and frequency at the rated values. While a droop controller shares the real power accurately, it is ineffective for precise reactive power sharing. The errors in reactive power sharing are compensated by adding virtual impedance in series with the distributed generator. The key objective of the work is to estimate the virtual impedance value to nullify the reactive power sharing errors without the need for communication links. A fuzzy logic controller is proposed to estimate the value for virtual impedance based on the real and reactive power demand at that particular instant. The virtual impedance dynamically changes depending on the load demand in order to compensate for the feeder impedance mismatch and hence called adaptive virtual impedance. Thus with the proposed fuzzy logic controller, accurate reactive power sharing is made possible without the requirement of communication links and the knowledge of feeder impedance. A secondary controller is also included to restore the frequency and the voltage to the rated values. A microgrid with equally and unequally rated DGs has been simulated in Matlab Simulink with the proposed fuzzy controller and successfully validated. [ABSTRACT FROM AUTHOR]

A solar photovoltaic (SPV), battery energy storage (BES), and a wind-driven SEIG-based islanded microgrid (MG) system is developed and utilized to provide continuous power to remote areas and electrical vehicle (EV) charging station (CS). The CS is primarily designed to use the extra power during reduced load to charge the EV battery. To synchronize and control the proposed MG system and to extract fundamental component, a Savitzky–Golay filter (SGF) (Hasan et al. in IEEE Trans Instrum Meas 71:1–6, 2022. https://doi.org/10.1109/TIM.2022.3196946)-based phase-locked loop (PLL) control approach is employed. The SGF control with VSC provides harmonics suppression, stabilization of frequency, quick response with lesser oscillations, and enhancement of the overall power quality (PQ) of the MG system under numerous operating scenarios such as uncertain conditions of load, wind, and solar power. Further, an MG suffers from problems of deviation in generation and continuous dynamic load demand; therefore, for power leveling between the load and power generation sources, a BES is connected across the DC link of the VSC through a bidirectional DC–DC converter (BDC). Moreover, to improve CS operational efficiency, the CS also performs vehicle-to-home (V2H) and vehicle-to-vehicle (V2V) power transfer. The observed results show the dynamic behavior of the proposed islanded system, which is confirmed through an experimental prototype. The CS follows the IEEE-1547 standard in all working modes, and the total harmonic distortion (THD) of voltage & current is below 5%. [ABSTRACT FROM AUTHOR]

In this paper, a nonlinear, bounded, distributed secondary control (DSC) method is proposed to coordinate all the distributed generators (DGs) in islanded AC microgrids (MGs). This proposed consensus-based DSC strategy can not only guarantee the restoration control of frequency and voltage but also realize an accurate active power sharing control. Through introducing a nonlinear dynamic from beta cumulative distribution function (CDF), the convergence speed of DSC is accelerated, the asymptotical convergence of DSC is ensured, and the transient overshoot of DSC is diminished compared with traditional DSC. Moreover, by ensuring the Lipchitz continuity characteristic of the control algorithm, the common chattering phenomenon in non-Lipchitz DSC scheme is eliminated. The stability and performance of the proposed DSC are also analyzed in this paper. An islanded AC microgrid test system with four inverter-based DGs is built in MATLAB/SIMULINK to further validate the effeteness of the proposed DSC strategy.

As the penetration rate of intermittent renewable energy continues to increase in a microgrid, the operational model of interconnected microgrid changes faster and more frequently. To maintain the stability and security of the microgrid, short-term reliability assessment is necessary and must be adapted to the time-varying characteristics of the islanded microgrid. In view of this, this paper proposes a new short-term reliability assessment method based on the time-varying probability ordered tree screening algorithm. First, a screening algorithm based on the time-varying probability ordered tree is proposed, and the number of system states is reduced by screening the system state with high probability at each time. Then, according to the different impact ranges, two event types of load point power supply interruption is defined, and the formulas are given for calculating the time-varying probability. On this basis, the loss of load probability (LOLP) index and the expected demand not supplied (EDNS) index are defined to assess the short-term reliability of the islanded microgrid. Finally, the proposed method is modeled by MATLAB, and a case study is performed on the European low-voltage microgrid system to verify its correctness and effectiveness.

This paper proposes a novel decentralized control strategy for multiparallel grid-forming distributed generations (DGs) in an islanded microgrid. Different from most existing droop-based hierarchical control methods, the proposed scheme imposes a fixed system frequency that is independent of the load conditions. Additionally, a proper point of common coupling (PCC) voltage amplitude maintaining and proportional power sharing can be simultaneously achieved when dealing with variations in the load and DGs plug in/off. The transient performance of the system, as well as the power supply reliability, can be improved, as no extra restoration control layer or communications between the inverters are needed. Furthermore, the power-sharing ratio among the DG units can be changed online without affecting the voltage regulation performance, which enhances the power management flexibility. The PCC voltage regulation and the power-sharing performance of the proposed control strategy are ensured via the Lyapunov method. Finally, the effectiveness and practicability of the proposed approach are verified through real-time simulations and hardware experimental tests.

Maintaining frequency stability of low inertia microgrids with high penetration of renewable energy sources (RESs) is a critical challenge. Solving this challenge, the inertia of microgrids would be enhanced by virtual inertia control-based energy storage systems. However, in such systems, the virtual inertia constant is fixed and selection of its value will significantly affect frequency stability of microgrids under different penetration levels of RESs. Higher frequency oscillations may occur due to the fixed virtual inertia constant or unsuitable selection of its value. To overcome such a problem and provide adaptive inertia control, this paper proposes a self-adaptive virtual inertia control system using fuzzy logic for ensuring stable frequency stabilization, which is required for successful microgrid operation in the presence of high RESs penetration. In this concept, the virtual inertia constant is automatically adjusted based on input signals of real power injection of RESs and system frequency deviations, avoiding unsuitable selection and delivering rapid inertia response. To verify the efficiency of the proposed control method, the contrastive simulation results are compared with the conventional method for serious load disturbances and various rates of RESs penetration. The proposed control method shows remarkable performance in transient response improvement and fast damping of oscillations, preserving robustness of operation.

The consensus control method based on a multi-agent system has been widely applied in the distributed control and optimization of microgrids. However, the following drawbacks are still common in current research: (1) ignoring the influence of consensus control commands on the synchronization stability of the physical grid under primary control; (2) only focusing on improving one property ofcontrol performance, lacking comprehensive considerations of multiple properties. With the aim of solving these problems, in this paper we propose a weight-adaptive robust control strategy for implementing distributed frequency regulation of islanded microgrids. Firstly, the frequency synchronization stability of the physical layer is analyzed by means of a coupled oscillator theory and the design objectives of the controllable parameters for the information layer are formed. Subsequently, the relationship between the weight coefficients and the two important control performances of convergence speed and delay robustness is strictly analyzed. Based on this, an adaptive coefficient that can be autonomously adjusted according to the frequency deviation is designed to achieve a trade-off between convergence speed and delay robustness. Finally, three simulation studies are presented to verify the effectiveness of the proposed control strategy.

Today’s clean technologies related to microgrids are approaching towards the smart nanogrid system. It fulfils the demand of the electricity throughout the world by proper using of renewable energy sources and energy storage systems. Still, the microgrid (MG) power plant control has enriched to a level that it will require complicated and smooth control in the grid interaction including distributed islanding operation. The load dynamics and uncertainties are the common issues which hampers the frequency, voltage and power profile of the MG that is responsible to damage the load and power system. The design of robust fuzzy logic controller (FLC) has been proposed in this research article to regulate the performances of three-phase islanded MG. The performance of the proposed FLC has been examined under different loading condition whose robustness has been evaluated under faulty condition. The investigated performances of the MG ensure high tracking and robust performance of the proposed FLC. [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.)