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Micro-energy networks integrate distributed energy systems, multiple loads and control devices in parks or communities. Micro-energy networks are capable of realizing multi-energy cogeneration and co-supply functions, which are conducive to enhancing the operational flexibility and economy of multi-energy networks. To effectively cope with the uncertainty of load and new energy output within the micro-energy networks, a polyhedral uncertainty set is established to portray the fluctuation of load and photovoltaic output. An energy hub (EH) coupling matrix describing the topology of micro-energy networks and the coupling relationship between the energy flows is constructed. Then, a two-stage robust optimization economic dispatch model is established for micro-energy networks in both grid-connected and islanded operation modes to minimize the system operation cost. Linear decision rule (LDR) and duality theory are used to develop model-solving strategy. Finally, a case is carried out using micro-energy networks based on improved IEEE 33-node distribution network. The results show that LDR can effectively approximate the relationship between decision variables and uncertain variables through linear affine function within a certain accuracy range, which reduces the difficulty of solving the two-stage robust optimization economic dispatch model.

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This paper proposes a novel method to allocate distributed generation and energy storage as Microgrid-Forming Resources (MFR) to improve the reliability of medium voltage (MV) grids. A graph-based approach is used, which simplifies the circuit analysis and reduces computational time when compared to exhaustive search methods. First, a medium voltage substation-level Nodal Fault-Effect Matrix (NFEM) is derived from the feeder’s topology, which is used for predictive reliability calculations, and calibrated with historical data. Second, a reduced set of scenarios of MFR allocation are generated using a graph community approach and evaluated in terms of reliability improvement and power and energy requirements of the distributed resources. Two case studies are presented based on real grids with different reliability characteristics and the results indicate that the proposed approach is an insightful tool for distribution system planners to evaluate the integration of microgrids to improve system reliability.

The application of microgrids (MG) has a vital role in grid modernization. However, a larger number of installations of electronically coupled distributed energy resources (DER) units in MGs have created opportunities and threats for grid operations. Therefore, efficient and reliable control strategies for MG operation are essential. This article introduces the concept of sector design to show the stabilization of voltage source converter-interfaced DER units in MG applications. The sliding sector design is advanced sliding mode control to obtain a region of attraction such that the state space is divided into attractive and nonattractive regions. In addition, a variable structure-based controlling method has been introduced to drive the trajectories of a plant from the exterior to the interior of the sector and strictly inside throughout the process. In autonomous MG operation, the proposed controller focuses on stabilizing the electronically coupled DER unit's voltage and frequency response. Besides, it confirms the safety of the power conversion system from external disturbances. The system stability is proven using the Lyapunov candidate function. The proposed strategy has been effectively validated through PSCAD/EMTDC. The simulation results have examined the impact of balanced, unbalanced, nonlinear, and dynamic loading patterns in islanded MG. [ABSTRACT FROM AUTHOR]

Among a variety of storage technologies used for energy storage systems, supercapacitors, Pb-Acid Batteries (PABs), and Lithium Batteries (LBs) are widely used for microgrid applications. The supercapacitors with high-power density are suitable for fast power regulations; conversely, the PABs have high-energy density, which makes them suitable for long-term energy management. Since the PABs and supercapacitor can complement each other and overcome mutual defficiencies, their combination as a hybrid energy storage system can be beneficial. However, the LB enjoys both high-energy and high-power densities. Therefore, an LB ESS (LBESS) can similarly function like a Pbacid battery-Supercapacitor Hybrid ESS (PSHESS). However, their topologies, life cycles, and costs are different. This study tends to determine the one which is technically and economically more suitable for applications in islanded microgrids. For this purpose, a frequency control and energy management scheme is proposed. It maintains the balance between demand and supply and also keeps the microgrid frequency within safe operational limits using the least needed sizes for the energy storage systems. Using the simulation results, the sizes and costs of the energy storage systems are determined. For a decade of operation, the PSHESS has incurred almost 49.2% more cost than the LBESS, which makes the LBESS more cost effective. [ABSTRACT FROM AUTHOR]

This paper proposes a Mixed-Integer Linear Programming (MILP) optimization model for the scheduling problem of the interconnected microgrid system. The proposed model is capable of efficiently minimizing the microgrids' total operating costs and improving the entire system's reliability, as it is constrained based on enhancing the interconnected microgrids' reliability. The Expected Energy Not Supplied (EENS) is considered in order to ensure minimizing the interconnected microgrids' power deficiency. Furthermore, the proposed model has the capability to solve the optimization problem considering the islanded operation of the interconnected microgrids, i.e. when disturbances occur on the upstream grid. Numerical simulations on a test system containing three interconnected microgrids are performed to evaluate the effectiveness of the model and the results demonstrate the merits and features of the reliability-constrained optimal scheduling model in minimizing the interconnected microgrids' total operating costs and enhancing the interconnected system reliability. [ABSTRACT FROM AUTHOR]

Hybrid AC–DC microgrid (HMG) is formed by connecting separate AC and DC microgrids with an interlinking converter (IC). In islanded mode of operation of the HMG its AC and DC subgrids are droop controlled to achieve power-sharing. The power flow between the subgrids happens through the IC, which regulates the amount of power being transferred based on the values of the frequency of AC bus voltage and the voltage at the DC bus. This paper presents the utilization of power-electronic converters to form the droop-controlled AC–DC subgrids of an islanded HMG which will be beneficial for further studies of IC and HMG. The detailed design and analysis of the closed-loop control system parameters of both the power converters is presented. The results obtained from simulations carried out in MATLAB/Simulink, and the real-time simulation results obtained by implementation using OPAL-RT OP4500 real-time digital simulator hardware are presented for the verification. [ABSTRACT FROM AUTHOR]

This paper proposes a reliability-based optimal scheduling model of integrated microgrids. The proposed model is capable of delivering optimal scheduling of each individual microgrid and aggregated system. In addition, it is compatible with both grid-connected and islanded operation modes. The developed problem formulation of the proposed model takes into account the reliability indices SAIDI, SAIFI, and CAIDI to evaluate the reliability of the integrated system. Numerical simulations on a modified IEEE-33 bus test system, involving four integrated microgrids, were performed to investigate the advantages and the robustness of the developed model. Furthermore, the impact of merging Battery Energy Storage Systems (BESSs) on the optimization outcome is examined, which demonstrates its significant effectiveness in improving the power systems' reliability. [ABSTRACT FROM AUTHOR]

The integration of distributed generation (DG) into a power distribution network allows the establishment of a microgrid (MG) system when the main grid experiences a malfunction or is undergoing maintenance. In this case, the power-generating capacity of distributed generators may be less than the load demand. This study presents a strategy for the effective utilization of deployed active and reactive power sources under power mismatch conditions in the islanded distribution networks. Initially, the DGs' and capacitors' optimal placement and capacity were identified using the Jaya algorithm (JA) with the aim to reduce power losses in the grid-connected mode. Later, the DG and capacitor combination's optimal power factor was determined to withstand the islanded distribution network's highest possible power demand in the event of a power mismatch. To assess the optimal value of the DG–capacitor pair's operating power factor ( p f s o u r c e ) for the islanded operation, an analytical approach has been proposed that determines the best trade-off between power losses and the under-utilization of accessible generation. The test results on 33-bus and 69-bus IEEE distribution networks demonstrate that holding the islanded network's load power factor ( p f l o a d ) equal to p f s o u r c e during the power imbalance conditions allows the installed distributed sources to effectively operate at full capacity. As expected, the proposed strategy will assist the utility companies in designing efficient energy management or load shedding schemes to effectively cope with the power mismatch conditions. [ABSTRACT FROM AUTHOR]

In this article, a distributed control scheme to compensate for voltage harmonics in islanded microgrids is presented, where each distributed generation (DG) source has a primary control level and a secondary control level. In addition to the voltage and current control loops, the primary control level of DGs includes virtual impedance control loops in the main and harmonic components, which are responsible for dividing the power of the main component and the non-main component (harmonic) between the DGs of the microgrid, respectively. For coordinated operation between the inverters when facing the islanding phenomenon, it is very beneficial to use a droop controller structure. Here, the traditional droop controller is modified in such a way that the power is proportionally divided between the DGs, which causes accurate voltage regulation at the output of the DGs. By presenting a model for the inverter connected to the nonlinear load, a harmonic droop controller is designed. Through the droop controller related to each harmonic, the harmonic voltages are calculated and added to the reference voltage, which improves the quality of the output voltage. Then, the inverter voltage control loop is modified with resistive impedance in the presence of nonlinear loads in such a way that, when combined with the harmonic droop controller, the total harmonic distortion (THD) of the output voltage is significantly reduced. Lastly, the proposed method is implemented on the microgrid through MATLAB software, and the results show the ability of the proposed method to reduce voltage harmonics in the parallel operation of inverters. [ABSTRACT FROM AUTHOR]

Gas-fired reciprocating engine plants (GREPs) are widely used in power supply systems of industrial facilities, which allows for ensuring the operation of electrical loads in case of accidents in the power system. Operating experience attests to the fact that during islanded operations, GREPs are shut down by process protections or protective relays in the event of severe disturbances. This leads to complete load shedding, which is accompanied by losses and damage to industrial facilities. Severe disturbances include the following ones: large load surges on GREPs due to one of them being switched off, the group starting of electric motors, and load shedding (more than 50%) during short circuits or disconnection of process lines. Energy storage systems (ESS) have the ability to compensate for instantaneous power imbalances to prevent GREPs from switching off. The authors of this study have developed methods for intelligent control of the ESS that allow one to solve two problems: prevention of GREPs shutdowns under short-term frequency and voltage deviations as well as preservation of the calendar and cycling lifetime of battery storage (BS) of the GREP. The first method does not require performing the calculation of adjustments of control actions for active and reactive power on the ESS online but rather determines them by the value of frequency deviations and the voltage sag configuration, which greatly simplifies the system of automatic control of the ESS. The second method, which consists in dividing the steady-state power/frequency characteristic into sections with different droops that are chosen depending on the current load of the ESS and the battery state of charge, and offsetting it according to a specified pattern, allows for preventing the premature loss of power capacity of the ESS BS. [ABSTRACT FROM AUTHOR]

Islanded operation is a key tool to improve the reliability and operational resilience of distribution grids. To enable islanded operation with minimal hardware changes, it is convenient to operate already installed PV units in droop-controlled grid-forming mode. This paper analyzes the main adaptive strategies that allow to implement the P / ω droop principle in double-stage units interfacing intermittent energy sources. The advantages and unwanted behaviors of each strategy are illustrated through a thorough simulation-based evaluation, which leads to a set of guidelines for the design of novel improved adaptive P / ω droop strategies. • Analysis of "frequency signaling" and "proportional power sharing" capabilities. • Illustration of why such capabilities can be lost with intermittent energy sources. • Simulation-based comparison of existing adaptive P / ω droop strategies. • Guidelines to facilitate the design of novel adaptive P / ω droop strategies. [ABSTRACT FROM AUTHOR]

The integration of distributed generation (DG) into a power distribution network allows the establishment of a microgrid (MG) system when the main grid experiences a malfunction or is undergoing maintenance. In this case, the power-generating capacity of distributed generators may be less than the load demand. This study presents a strategy for the effective utilization of deployed active and reactive power sources under power mismatch conditions in the islanded distribution networks. Initially, the DGs’ and capacitors’ optimal placement and capacity were identified using the Jaya algorithm (JA) with the aim to reduce power losses in the grid-connected mode. Later, the DG and capacitor combination’s optimal power factor was determined to withstand the islanded distribution network’s highest possible power demand in the event of a power mismatch. To assess the optimal value of the DG–capacitor pair’s operating power factor (pfsource) for the islanded operation, an analytical approach has been proposed that determines the best trade-off between power losses and the under-utilization of accessible generation. The test results on 33-bus and 69-bus IEEE distribution networks demonstrate that holding the islanded network’s load power factor (pfload) equal to pfsource during the power imbalance conditions allows the installed distributed sources to effectively operate at full capacity. As expected, the proposed strategy will assist the utility companies in designing efficient energy management or load shedding schemes to effectively cope with the power mismatch conditions.

The power system is moving away from the centralized generation paradigm. One of the current trends is the microgrid concept, where loads, small generators and renewable energy resources (RERs) that are in close proximity are controlled as one entity. Microgrids also allow for an increase in power availability as they can continue to supply electric power to loads even in the absence of a connection to the main grid. During the transition to islanded operation, microgrids may be subject to frequency disturbances caused by the power imbalance between load and generation. When microgrids contain high shares of renewable energy, the challenge is significantly higher due to the control strategies that aim to maximize power production, which are typically applied to RERs and render them insensitive to grid changes. Therefore, new control strategies need to be developed to enable the participation of RERs in the support of the frequency response. This work proposes a predictive control strategy that is based on a generalized predictive controller (GPC) being applied to the grid side converter of a doubly fed induction generator (DFIG) wind turbine to enable frequency support capabilities. The control objective was to track a time varying power reference signal that was generated according to the deviation from the nominal frequency, thereby enabling the energy storage device to inject power into the microgrid without a communication system. The GPC is a controller belonging to the family of model predictive controllers (MPCs), the main principles of which are the use of a system model to predict future states and the choice of an optimal input to ensure that the reference values are followed. To validate the proposed control strategy, a microgrid was simulated in MATLAB Simscape Electrical. The frequency response using the proposed GPC strategy was compared to another MPC-based strategy, known as finite control set, and a scenario in which the DFIG was not equipped with frequency support capabilities. The results show that the proposed strategy was able to improve the frequency response of the microgrid, reduce frequency oscillations and increase the value of the frequency nadir. [ABSTRACT FROM AUTHOR]

In this article, a distributed control scheme to compensate for voltage harmonics in islanded microgrids is presented, where each distributed generation (DG) source has a primary control level and a secondary control level. In addition to the voltage and current control loops, the primary control level of DGs includes virtual impedance control loops in the main and harmonic components, which are responsible for dividing the power of the main component and the non-main component (harmonic) between the DGs of the microgrid, respectively. For coordinated operation between the inverters when facing the islanding phenomenon, it is very beneficial to use a droop controller structure. Here, the traditional droop controller is modified in such a way that the power is proportionally divided between the DGs, which causes accurate voltage regulation at the output of the DGs. By presenting a model for the inverter connected to the nonlinear load, a harmonic droop controller is designed. Through the droop controller related to each harmonic, the harmonic voltages are calculated and added to the reference voltage, which improves the quality of the output voltage. Then, the inverter voltage control loop is modified with resistive impedance in the presence of nonlinear loads in such a way that, when combined with the harmonic droop controller, the total harmonic distortion (THD) of the output voltage is significantly reduced. Lastly, the proposed method is implemented on the microgrid through MATLAB software, and the results show the ability of the proposed method to reduce voltage harmonics in the parallel operation of inverters.

Solar photovoltaic (SPV) system performance degrades due to the presence of nonlinear/unbalanced loads at the point of common coupling (PCC). These loads also cause the current in the ac network's neutral conductor, which causes additional losses and the voltage difference between the neutral conductor and the ground. This work addresses these problems and presents the control algorithm for the SPV system to improve its performance and the power quality (PQ) at the PCC, to reduce the current flow in the ac network's neutral conductor, and to maintain the balanced operation in the ac network at unbalanced loads. The battery energy storage is incorporated with the SPV system to improve the supply's reliability at the load end by operating the system in the islanded mode (ILM) when the ac network supply is interrupted. A new application of the variable zero attracting (VZA) least mean square (LMS) adaptive control algorithm is presented for the PQ improvement in both grid-connected mode (GCM) and ILM operations. The VZA component reduces the steady-state oscillation in the LMS algorithm without compromising its dynamic response. The seamless operation is used to avoid any abnormality in voltages and currents of the system operating mode changes from the ILM to the GCM after the grid recovery. The performance of the system is tested on the developed Simulation model and further verified by the experimental study performed on a reduced rating prototype. [ABSTRACT FROM AUTHOR]

An energy management control strategy is proposed for an islanded AC microgrid with the hybrid energy storage system, including the battery and the supercapacitor (SC). According to the state of charge of the battery, the photovoltaic system can work in either maximum power point tracking mode or load power tracking mode to prevent the battery from over charging. Similarly, the load shedding control is adopted to prevent the battery from over discharging. A virtual impedance control strategy is proposed to achieve effective power sharing in hybrid energy storage systems, where the battery provides steady state power and the SC only supports transient power fluctuations. The terminal voltage of the SC can be restored to the initial value automatically by introducing a high-pass filter in the voltage control loop. The AC bus voltage remains constant using the voltage secondary controller to compensate the voltage drop caused by the virtual impedance control strategy. The simulation results under typical working conditions verify the correctness and effectiveness of the proposed control strategy.

Gas-fired reciprocating engine plants (GREPs) are widely used in power supply systems of industrial facilities, which allows for ensuring the operation of electrical loads in case of accidents in the power system. Operating experience attests to the fact that during islanded operations, GREPs are shut down by process protections or protective relays in the event of severe disturbances. This leads to complete load shedding, which is accompanied by losses and damage to industrial facilities. Severe disturbances include the following ones: large load surges on GREPs due to one of them being switched off, the group starting of electric motors, and load shedding (more than 50%) during short circuits or disconnection of process lines. Energy storage systems (ESS) have the ability to compensate for instantaneous power imbalances to prevent GREPs from switching off. The authors of this study have developed methods for intelligent control of the ESS that allow one to solve two problems: prevention of GREPs shutdowns under short-term frequency and voltage deviations as well as preservation of the calendar and cycling lifetime of battery storage (BS) of the GREP. The first method does not require performing the calculation of adjustments of control actions for active and reactive power on the ESS online but rather determines them by the value of frequency deviations and the voltage sag configuration, which greatly simplifies the system of automatic control of the ESS. The second method, which consists in dividing the steady-state power/frequency characteristic into sections with different droops that are chosen depending on the current load of the ESS and the battery state of charge, and offsetting it according to a specified pattern, allows for preventing the premature loss of power capacity of the ESS BS.

Due to the increasing penetration of distributed renewable energy generation systems, their participation in power system restoration is gaining a growing interest. For the black start of a system without synchronous generation, Wind Energy Conversion Systems (WECSs) must be able to control voltage and frequency. However, WECSs have been traditionally controlled as current sources. This paper proposes a decentralized control system for the black start of WECSs operating in parallel. In comparison with known methods for operation of distributed energy resources without synchronous generation, this paper specifically addresses the requirements of WECSs. Total wind power is regulated to match the demand during the black start process. This power is shared between WECSs according to the wind power available in each of wind turbine using a droop control characteristic between the voltage and active power. Using the so-called reactive power synchronization method, the WECSs can operate in synchronism even if wind power is significantly different in each wind turbine. Simulation results for a case study where two WECSs are connected in parallel are presented to validate the proposal. [ABSTRACT FROM AUTHOR]

This paper proposes a novel nested restoration decision system (NRDS), which aims to minimize unused capacity of distributed generation (DG) for service restoration due to contingency during islanding stage. The proposed algorithm is based on the concept of nested microgrid formation, where the network control schemes are framed on a layered structure. The power-sharing strategy between neighboring microgrids in the network is from outer to inner microgrid loads, where the outermost microgrid exchanges power with utility grid only. The first stage refers to pre-fault scenario, which finds a solution for networked microgrid distributed generation as the initial setting. Furthermore, the second stage calculates additional redistribution requirements based on respective microgrid's deficiency using a solution index matrix. The proposed control has been evaluated on a networked microgrid system, while performing islanding. [ABSTRACT FROM AUTHOR]

This paper deals with static voltage stability assessments in the context of microgrids. It is well known that distributed systems have been changing their operation mode from passive to active due to a high level of distributed resource penetration. Hence, operating of the system in islanded mode could be considered on a routine or emergency basis, and the voltage stability analysis become vital in any planning or operation study in the distribution level of electrical power systems. In this way, this paper develops a novel formulation for voltage stability analysis in islanded systems, showing the differences with respect to traditional methodologies, mainly when they neglect the absence of a swing bus and the frequency of the system as a variable. Further, the present work compares the load margins of a distribution system when it works in both modes, in grid-connected mode and islanded mode, considering the operation strategies of the most usual Power Electronic Converters. The results are carried out in the IEEE-37 Node Test Feeder. [ABSTRACT FROM AUTHOR]

This study presents a differential protection scheme based on ensemble empirical mode decomposition (EEMD) for AC microgrid. The fault current level is significantly lower during the islanded operation of a microgrid, which leads to the malfunction of the traditional over-current protection scheme. The proposed differential protection scheme uses the spectral differential energy of the first intrinsic mode function extracted from the decomposition of the current signal using EEMD for effective fault detection in the AC microgrid. The proposed EEMD-based differential protection scheme is validated on 10 bus and modified IEEE 34-bus AC microgrid test systems during various shunt faults. Moreover, the performance of the proposed EEMD-based differential protection scheme is evaluated under high fault impedance scenarios. The simulation results reveal that the proposed differential protection scheme can effectively detect the faulty line in an AC microgrid during seamless islanded and grid-tied operations. [ABSTRACT FROM AUTHOR]

The increasing penetration of distributed generations enables an innovative operation paradigm that allows islanded operation to enhance the resilience of the distribution grid. In this study, a cyber‐physical oriented islanding strategy is proposed by coordinating centralised and distributed control to achieve seamless islanding transition and operational flexibility in emergency conditions. A cyber‐physical control structure is developed to mitigate various disturbances (e.g. emergencies or fluctuations) according to different operation conditions. Specifically, the distributed fault isolation and seamless islanding transition are coordinated to mitigate the outage caused by unplanned islanding, while a secondary control is proposed to support primary control by reducing the power fluctuations during islanded operation. With a rapid response speed, the local cyber‐physical devices are coordinated to accomplish islanding separation by selecting a feasible islanded area even under an unplanned islanding situation. A field test was conducted on a practical distribution network in China, and the results demonstrated the effectiveness and feasibility of the proposed islanding strategy. [ABSTRACT FROM AUTHOR]

This work proposes a control algorithm to be used in the synchronization process for the reconnection of an autonomous microgrid with the main grid. The control loop acts in the phase mismatch between the voltages and utilizes an adjustable parameter to regulate the transient dynamic, allowing for a smooth synchronization process. Simulations were made in order to evaluate its performance; then the algorithm was implemented in the TMS320F28377S digital signal processor and tested in a reduced-scale autonomous microgrid prototype using DRV8305EVM static converters. The results confirm that the proposed algorithm allows for smooth synchronization and reconnection processes, with the possibility of adjusting the smoothness numerically, providing another scientific criterion for the synchronization analysis, validating the proposal. [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.)

Future Active Distribution Grids (ADGs) will incorporate a plethora of Distributed Generators (DGs) and other Distributed Energy Resources (DERs), allowing them to provide ancillary services in grid-connected mode and, if necessary, operate in an islanded mode to increase reliability and resilience. In this paper, we investigate the ability of an ADG to provide Frequency Control (FC) in grid-connected mode and ensure reliable islanded operation for a pre-specified time period. First, we formulate the operation of the grid participating in European-type FC markets as a centralized multi-period optimal power flow problem with a rolling horizon of 24 hours. Then, we include constraints to the grid-connected operational problem to guarantee the ability to switch to islanded operation at every time instant. Finally, we explore the technical and economic feasibility of offering these services on a balanced low-voltage distribution network. The results show that the proposed scheme is able to offer and respond to different FC products, while ensuring that there is adequate energy capacity at every time step to satisfy critical load in the islanded mode. [ABSTRACT FROM AUTHOR]

Abstract With the insertion of distributed generation in distribution networks, security analysis becomes crucial, in islanded operation or when the system is connected to the grid. Dynamic stability analyses are necessary in order to achieve minimum levels of security and reliability. Such analyses are obtained through the solution of the algebraic-differential set of equations that model the distribution grid and their generators, which requires numerical solution methods. Therefore, this paper presents a computational tool for transient stability and large disturbance frequency stability analysis, considering distribution systems with the insertion of biomass-fed distributed generation, which may, or may not, have speed regulation systems. The presented tool may be employed to assess assorted system indexes, such as the severity of disturbances, the quality of a given operating condition, in terms of voltage and frequency operating limits, and the response of both the system and the generators in face of the action of speed regulators.

This article proposes a new control paradigm for the seamless operation of a permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS) in both islanded and grid-connected modes. Conventionally, grid-connected WECS are controlled to extract the maximum power from the wind, known as maximum power point tracking (MPPT). However, in islanded operation, the objective is to control the power generated from the WECS to match the load demand, which here is called load following power generation (LFPG). This article proposes a power management system (PMS) which integrates the MPPT and LFPG capabilities of the WECS into a single-control paradigm. This enables the WECS to seamlessly achieve MPPT in grid operation, LFPG in islanded mode and low-voltage ride-through (LVRT) during fault. The proposed scheme is validated using MATLAB/SIMULINK simulations.

In this paper, a new and generalized model for the optimal operation of microgrids is presented. The proposed mathematical model considers both the grid-connected (GC) and islanded (IS) operational modes. First, a mixed integer non-linear programming (MINLP) formulation is introduced, modeling the microgrid as an unbalanced ac three-phase electrical distribution system, comprising distributed generator (DG) units, battery systems and wind turbines. In GC mode, the frequency and the voltage magnitude references are imposed by the main grid at the point of common couple, while in IS mode, it is assumed that the DG units operate with droop control. Additionally, a set of convexification procedures are introduced in order to approximate the original MINLP model into a new convex formulation that can be solved using commercial solvers. The proposed model has been tested in a 25-bus microgrid for different scenarios, including one where a degradation of the voltage magnitude reference is observed. Results show that the proposed model is able to properly define the operational mode of the microgrid, based on the technical constraints of the system. [ABSTRACT FROM AUTHOR]

This study proposes a decentralised robust fuzzy control strategy for islanded operation of an AC microgrid with voltage source inverters. The objective is to design a robust controller for regulating the load voltage and sharing power among distributed generators (DGs) in the presence of uncertainties in the system and non‐linear loads. The AC microgrid consists of parallel DGs connected to a main AC bus. A Takagi‐Sugeno fuzzy approach is developed in this article to achieve stability and desired performance in dealing with non‐linearities in the islanded microgrid and H∞ criterion is used to obtain a robust control strategy in presence of uncertainties raised by unmodelled and high frequency dynamics. Therefore, a non‐convex condition in H∞ optimisation problem is converted to a convex linear matrix inequality (LMI) condition and is solved by MATLAB LMI toolbox. In order to develop a decentralized system, P–f and Q–V droop controllers are used to specify set‐points for local inverter controllers in each DG. The effectiveness of the proposed control strategy in presence of constant power load and DG accidental outage is validated by the simulation of a microgrid test system in MATLAB SimPowerSystems toolbox. Comparison with cascaded proportional integral controller shows advantages of robust T‐S controller. [ABSTRACT FROM AUTHOR]

We have proposed a concept model of emergency islanded operation by using distributed generators (DGs) such as photovoltaics, electric vehicles, or batteries, which can provide power to customers via undamaged distribution network in order to develop the resilient power system against large disasters, and it is named Islanded Distribution Network (IDN). Since there is no guarantee that the three‐phase generation system is interconnected to the IDN, the single‐phase DGs that are installed in the distribution network may be treated as main generators in the IDN operation. If the IDN has only single‐phase DGs, it is difficult to regulate the three‐phase voltage within the allowable range and to compensate the unbalance voltage. The aim of this study is to develop the method to supply three‐phase balanced voltage by single‐phase generators in the IDN. First, the operating condition of the generators is proposed for the supply of three‐phase balanced voltage in the IDN model by algebra calculation. The control method for three single‐phase generators has been developed by using the conditions obtained from the derivation of the generators conditions. [ABSTRACT FROM AUTHOR]

Local secondary control has been successfully used to regulate the frequency of inverter-based islanded microgrids without using communications. In this scenario, noticeable steady-state deviations have been observed in active power sharing caused by the inherent clock drift of the digital processors that implement the local control of inverters. This paper presents a control scheme that performs frequency regulation and improves the active power sharing under high-load conditions, thus alleviating the impact of clock drifts in this situation. The study introduces a theoretical analysis that quantifies the steady-state deviations in active power sharing. It also includes a design procedure for the control parameters based on static and dynamic specifications. Experimental tests validate the expected features of the proposed control. The experimental setup is based on a laboratory microgrid equipped with three independent digital signal processors with different clock drifts. [ABSTRACT FROM AUTHOR]

This paper proposes a unified control based utility-interactive uninterruptible power converter (UIUPC). The proposed control enables the UIUPC to inherently transfer from a $ p-q$ control strategy in grid-connected mode to a voltage–frequency control strategy in autonomous mode and vice-versa. This happens seamlessly with the same control topology. The need to switch between two separate control architectures is thus eliminated. Critical islanding detection and synchronizing mechanisms are also not needed in the proposed control method. The unified controller proposed in this paper is derived from the concept of controlling the perturbations in the magnitude and speed of the point of common coupling (PCC) space vector. The PCC space vector is compared with a reference space vector applied in the direction of the PCC space vector on an instantaneous basis to extract the perturbations. The reference system of the proposed control is linked to the dynamics of the PCC space vector. This offers the ability of parallel operation of similar unified control UIUPC. Thus, in addition to the above said advantages, the proposed control also puts forth an alternative to paralleling approaches like conventional droop and master–slave configurations. The effectiveness of the proposed control is validated by simulation and experimentation. [ABSTRACT FROM AUTHOR]

The deployment of hybrid ac/dc microgrids is a promising alternative for using local distributed energy resources to serve both ac and dc loads. Hybrid ac/dc microgrids, like other types of microgrids, are subjected to dynamic instability when operated in island mode. This paper analyzes the small-signal stability of islanded hybrid ac/dc microgrids and proposes a droop-based control mechanism to coordinate the operation of ac and dc sections. A general approach is presented which uses a linearized state-space model of microgrids for establishing adequate stability margins in hybrid ac/dc microgrid operations. The proposed case studies are conducted to investigate the small-signal behavior of a hybrid ac/dc microgrid based on time-domain simulations and frequency-domain analyses. [ABSTRACT FROM AUTHOR]

Highlights • The fault current level of the feeders drastically decreases during islanded operation. • The protection will not properly operate if the overcurrent settings are not changed. • It is proposed a method for identification of the grid operating conditions. • The method is based on the use of a local scheme installed close to the protection device. Abstract Currently, it is considered that distributed synchronous generators may operate in intentional-islanding operation mode, in which a portion of the power grid, which consists of load and distributed generation, is isolated from the remainder of the utility system. However, the fault current level of the feeders drastically decreases when the utility grid is disconnected and the protection will not properly operate if the overcurrent settings are not changed. In this paper it is proposed a method for identification of the grid operating conditions, utility connected or islanded. The method is based on the use of a local scheme installed close to the protection device. The scheme uses a thyristor which is fired during a short waveform interval of voltage. The thyristor voltage and current are monitored and used to calculate the grid equivalent impedance. According to the grid equivalent impedance it is possible to determine whether the utility is connected or not. Simulations considering a distribution system and a distributed synchronous generator are used to validate the proposed method. The results show that the method can be used to update the protection settings when the operating condition of the system is changed from utility-connected to islanded operation mode or vice-versa. As a result, the method allows implementing an adaptive protection without the use of communication system or can be used as a backup to increase the resilience in case the communication is lost. [ABSTRACT FROM AUTHOR]

This paper introduces a novel approach to enhance the stability of islanded grids with multiple virtual synchronous generators (VSGs) in-operation. The proposed method mainly relies on the concept of “center of inertia” (COI) response of the system. We discuss that an islanded grid with ‘ n ’ connected VSGs, can experience up to ‘ n-1 ’ oscillation frequencies due to swing power between VSGs, while the COI-frequency only indicates the major disturbances. The main idea of this method is to control the input power of VSGs to suppress the power fluctuations and reduce oscillations of VSGs relative to COI-frequency. For each VSG, a controller which includes three proportional, integral and derivative terms is used to match the VSG speed with COI-frequency in transient conditions. The state-space analysis shows that these parameters provide a high degree of freedom for adjusting the grid stability-indices to desired values. In addition, the method is able to effectively eliminate the adverse effect of the high impedance of VSGs to achieve desired performances. [ABSTRACT FROM AUTHOR]

This paper proposes a current-controlled voltage-mode control scheme for dispatchable electronically coupled distributed energy resource (DER) units based on sliding-mode control (SMC) strategy. The proposed control strategy provides fast and stable control on the terminal voltage and frequency of DER unit and ensures protection of the power-electronic interface to external faults. In addition it maintains the quality of the output voltage of the host DER unit in spite of unbalanced and/or distorted load currents. Performance of the proposed SMC strategy is demonstrated through time-domain simulation of a single islanded DER unit, starting from black-start and subjected to various operating scenarios, and it is compared to the performance of a proportional-integral (PI) based control strategy. Also a current-mode control strategy is proposed for DER units based on SMC. The performance of both voltage- and current-mode control strategies are demonstrated in a sample master-slave organized three-unit microgrid. [ABSTRACT FROM AUTHOR]

With the rapid development of the micro-grid associated with new and clean energies, the smooth switching between grid-connected and islanded operation modes of the micro-grid is a key issue that needs to be addressed urgently. In traditional solutions, V/f (Voltage/frequency) control is adopted for the master micro sources when the micro-grid works in islanded mode, while PQ (real and reactive power) control is adopted when in grid-connected mode. However, when the two controllers switch when mode transfer occurs, transient oscillations usually occur and thereafter the dynamic response will be degraded. This paper considers an archetypical micro-grid with Wind-PV-ES (Wind, Photovoltaic and Energy Storage) hybrid system, which forms the basis of our case study. The underlying reason for such transient oscillation is analyzed in this paper. Thereafter a modified control strategy for seamless mode transfer is designed and implemented. An improved PQ control method is designed by which the output of the PQ controller always synchronously tracks the output of the V/f controller for micro-grid switches from islanded mode to grid-connected; furthermore, a dq rotating coordinate synchronization based V/f control method is proposed for transition from grid-connected mode to islanded mode. Finally, experiments and analysis are undertaken on some basic and important operating cases; the results in our case study indicate that the modified control strategy is effective in dominating the micro-grid during mode transfer and thus yielding significantly better performances.

The increasing penetration of microgrids in distribution networks, as a viable option for end-use customers to increase load-point reliability and power quality, will result in formation of many interconnected microgrids in a not so far future. This paper considers a case in which multiple microgrids are geographically close and electrically connected, and studies anticipated interactions among these microgrids and also between the microgrids and the utility grid, during grid-connected and islanded operation modes. A model for the optimal scheduling of integrated microgrids is further proposed. The model is first developed with the objective of minimizing the aggregated operation cost and is accordingly decomposed into individual optimal scheduling problems using the Lagrangian relaxation method to take prevailing privacy issues into account. The microgrids capability in operating in the islanded mode for multiple hours is scrutinized by a T–τ islanding criterion. Numerical simulations exhibit the merits and the effectiveness of the proposed model via simulations on a system of integrated microgrids. [ABSTRACT FROM AUTHOR]