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Active and reactive powers should be properly shared among sources to avoid instability problems and to improve reliability in inverter-based micro grids. This can be done using a number of control techniques including hierarchical, centralized and decentralized control. Decentralized control techniques offer extra reliability to the system in addition to the plug and play functionality of distributed generators . This control method is conventionally done using a droop controller. However, the droop control method has certain limitations, particularly in terms of the reactive power sharing. The improper reactive power sharing can lead to serious problems when reactive power surges occur, such as severe circulating reactive powers, instability issues and even a complete shutdown of inverters. The aim of this work is to present a new method for a proper sharing of reactive power by utilizing a low-bandwidth communication through power lines. Using these communication signals, the virtual impedances of the inverters are automatically adjusted to achieve accurate reactive power sharing. The method is described and analysed in detail, including stability analysis related to communication loss and delay effects. The same communication through power lines can be used to share additional information within the micro grid to achieve other control tasks such as secondary control. The proposed method is verified by simulations and using a hardware prototype. Results show that the proposed algorithm is able to accurately achieve reactive power sharing while maintaining the expected power quality at load buses.

This paper proposes a tube model predictive control method considering uncertainties of renewable energy generation in the low-voltage distribution network. For relatively low dependence on communication networks, a decentralized voltage control framework is urgently needed to be developed for voltage regulation in the low-voltage distribution network. Opposed to the traditional voltage control techniques, a novel decentralized tube model predictive voltage control (TMPC) strategy is proposed. First, this strategy entails the formulation of a discrete model encompassing distributed photovoltaic units and energy storage systems intertwined with the distribution grid model based on voltage sensitivity. Then, considering the worst case of voltage deviations caused by uncertain and stochastic power fluctuation, we specifically discuss a robust positively invariant to determine the control section of controllers. Further, the strategy employs tube model predictive controllers encompassing nominal and auxiliary controllers in distribution areas. The nominal controller generates a central path for different voltage operating points of each distribution area, and the auxiliary controller makes all trajectories lie in a bounded neighborhood called a tube. The effectiveness of the proposed control strategy is investigated on a test system containing renewable energy resources.

Active and reactive powers should be properly shared among sources to avoid instability problems and to improve reliability in inverter-based micro grids. This can be done using a number of control techniques including hierarchical, centralized and decentralized control. Decentralized control techniques offer extra reliability to the system in addition to the plug and play functionality of distributed generators. This control method is conventionally done using a droop controller. However, the droop control method has certain limitations, particularly in terms of the reactive power sharing. The improper reactive power sharing can lead to serious problems when reactive power surges occur, such as severe circulating reactive powers, instability issues and even a complete shutdown of inverters. The aim of this work is to present a new method for a proper sharing of reactive power by utilizing a low-bandwidth communication through power lines. Using these communication signals, the virtual impedances of the inverters are automatically adjusted to achieve accurate reactive power sharing. The method is described and analysed in detail, including stability analysis related to communication loss and delay effects. The same communication through power lines can be used to share additional information within the micro grid to achieve other control tasks such as secondary control. The proposed method is verified by simulations and using a hardware prototype. Results show that the proposed algorithm is able to accurately achieve reactive power sharing while maintaining the expected power quality at load buses. • The limitations of conventional droop control in reactive power sharing are discussed in the paper along with the undesired outcomes resulted from improper reactive power sharing control such as circulating reactive power, and damage or shutdown of inverters. • A new approach for sending and receiving signals between inverters optimized for microgrids is developed along with its hardware realization. The method is tested through simulation and on a microgrid lab setup and the results show the effectiveness of the method through enhancing reactive power sharing. • Compared with conventional methods used for signal sharing between inverters, the developed method has the advantage of being less invasive as it utilizes the power network as a communication medium without introducing a communication network. [ABSTRACT FROM AUTHOR]

This paper proposes a tube model predictive control method considering uncertainties of renewable energy generation in the low-voltage distribution network. For relatively low dependence on communication networks, a decentralized voltage control framework is urgently needed to be developed for voltage regulation in the low-voltage distribution network. Opposed to the traditional voltage control techniques, a novel decentralized tube model predictive voltage control (TMPC) strategy is proposed. First, this strategy entails the formulation of a discrete model encompassing distributed photovoltaic units and energy storage systems intertwined with the distribution grid model based on voltage sensitivity. Then, considering the worst case of voltage deviations caused by uncertain and stochastic power fluctuation, we specifically discuss a robust positively invariant to determine the control section of controllers. Further, the strategy employs tube model predictive controllers encompassing nominal and auxiliary controllers in distribution areas. The nominal controller generates a central path for different voltage operating points of each distribution area, and the auxiliary controller makes all trajectories lie in a bounded neighborhood called a tube. The effectiveness of the proposed control strategy is investigated on a test system containing renewable energy resources. • A novel decentralized control structure is proposed to serve voltage control for distribution networks, which can cooperate for accurate voltage control with random and fluctuant renewable energy. • In the nominal controller, operation points of DGs are generated considering the generation and load scheduling. In the auxiliary controller, detailed control objectives and constraints are proposed for suppressing disturbances of DGs. • To balance the conservation and performance, a worst-case-based robust positively invariant is developed to quantify the range of tubes to cope with the randomness. [ABSTRACT FROM AUTHOR]

Highlights • MPC based decentralized control scheme is proposed for hybrid source converters and parallel-connected VSI for an islanded microgrids. • To maintain dc bus voltage, and balancing the SoC among DGs, the MPVC method is proposed for multiple PV/Battery hybrid source converters. • Enhancing real and reactive power sharing among several DG units under mismatched feeder impedance, predictive hierarchical control is proposed for VSIs. • The proposed control strategy enhances voltage quality, load sharing at the PCC under fluctuating power generation, and various load conditions. Abstract This article presents a decentralized power management scheme based on Model Predictive Control (MPC) approach for islanded ac microgrid. This control structure can be categorized into dc-dc converter control and Voltage Source Inverter (VSI) control. The Model Predictive Voltage Control (MPVC) method is deployed for multiple PV/Battery hybrid source converters and parallel connected inverters in droop controlled microgrids. In the proposed strategy, the fluctuating PV output can be smoothened by controlling the PV converter. In addition, bidirectional battery converter maintains dc-bus voltage and State of Charge (SoC) balancing of batteries. Droop control is further incorporated with MPVC to control the power converters for ensuring stable ac output voltage and proper load sharing. [ABSTRACT FROM AUTHOR]

For distribution network operators to make effective decisions about real-time applications, they should have complete knowledge of all system variables. However, measuring all variables is infeasible due to the large number of system buses and the consequently high cost of measurement devices. Network operators are thus in serious need of methods that can estimate system voltages and currents with only a few measuring devices. This paper presents a novel voltage, current, and power loss estimation technique for distribution networks characterized by a high level of distributed generation (DG) penetration. The proposed method is based on online measurements from smart meters (SMs) placed at a few selected locations in addition to the measurements from DGs production meters; the estimation is derived without any pseudo measurements. The ingenuity of the proposed technique is that the SM locations are dependent on the network topology only, which means that their locations remain unchanged regardless of penetration levels and/or DG injection points. The proposed technique also includes consideration of variations in X/R ratios and laterals. The developed algorithm was implemented and tested on three radial distribution feeders to show the capability of the proposed technique for estimation for balanced as well as unbalanced distribution networks. The results of a comparison with the actual load flow demonstrate the accuracy and effectiveness of the new technique. [ABSTRACT FROM AUTHOR]

This paper proposes a new control strategy of single-phase Voltage Controlled Voltage Source Inverter (VCVSI) based on Two Revolving Field Theory (TRFT) for integrating the single-phase static as well as dynamic ac loads to the DC Microgrid (DCMG). In the developed control scheme, the output ac voltage/current of the single-phase VCVSI has been decomposed into two counter rotating vectors, referred as Forward-Positive (FP) and Backward-Negative (BN) vectors, by using TRFT principle. The α-β components of the FP and BN rotating vectors have been obtained using the orthogonal transformation. The new control scheme, with dual controllers; one for the FP vectors and another for the BN vectors of the output ac voltage/current of the VCVSI, has been implemented into two rotating d-q synchronous reference frames, using a feed-forward and two feed-back control loops. The objective of the proposed controllers is to establish the VCVSI rated output ac voltage with low total harmonic, faster transient response after disturbances, and to ensure needed power flow from the microgrid to the single-phase loads. Simulations have been carried out to show the effectiveness of the developed control scheme along with the proposed DCMG in the islanded mode, under different operating conditions. [ABSTRACT FROM AUTHOR]

Use of distributed generators in distribution networks is proved highly reliable and economical. In utility practice at the present time, accidental islanding is an undesirable mode of operation as it can harm the personnel and other connected systems. Therefore, islanding must be detected and the islanded distributed generators must be disconnected from the rest of the system. This paper proposes a new islanding detection method which provides a reliable detection in two levels. At first level wavelet energy entropy is used as an indicator to suspect islanding and activate second level method. Second level method is an active frequency drift method with positive feedback and pulsating chopping fraction that vary the output voltage frequency of distributed generators to realize the anti islanding protections. The performance of the proposed method is simulated in MATLAB environment. The proposed algorithm provides fast islanding detection with least disturbance in inverter’s grid connected operation. [ABSTRACT FROM AUTHOR]

This paper presents an efficient and fast converging optimization algorithm based on a modification of the traditional firefly method for optimal sizing and siting of voltage controlled distributed generators in balanced/unbalanced distribution systems. The proposed algorithm modifies the traditional firefly method to be able to deal with the practically constrained optimization problems by proposing formulas for tuning the algorithm parameters and updating equations. The proposed algorithm is implemented in MATLAB environment and tested on the 69-bus feeder system, the IEEE 37 nodes feeder and the IEEE 123-nodes feeder in order to minimize the system power loss via robustly detection of the optimal location and size of the distributed generators without violating the system constraints. Validation of the proposed method is done by comparing the obtained results with published results obtained from other competing methods. [ABSTRACT FROM AUTHOR]

• A VSG-based hierarchical control scheme is proposed for DGs in active distribution network. • Multiple VSG-based distributed generators can enhance the inertia of system through regulating the equivalent output power. • In order to minimize the voltage deviation and power loss, we adopt model predictive control to cooperate DGs. With the increasing penetration of renewable energy, converters need to help enhance the stability and rigidity of the power system. This paper proposes an inertia-enhanced method with the hierarchical control scheme for active distribution network to support inertia for the utility grid. The paper first analyzes the mechanism and influence of inertia in power systems and quantifies the inertia which an active distribution network can supply. Then, two-layer control structure is designed for favorable control performance. At the primary control layer, a virtual synchronous generator control strategy is adopted for instant inertia support under frequency fluctuation. The objective of primary control is to track the control references rapidly and accurately. At the secondary control layer, the objective is to coordinate the active and reactive power output of distributed generators to keep voltage deviations and power loss of internal distribution network operating at a comparatively appropriate level. The secondary control strategy is based on model predictive control method, which can consider the physical and control constraints of distributed generators and find the optimal reference signals for all the primary controllers to track. The inertia of power system is apparently enhanced with the hierarchical inertia-enhanced scheme. Finally, simulation based on Matlab/Simulink is carried out, and the feasibility and validity of proposed control strategy are verified. [ABSTRACT FROM AUTHOR]

Microgrids comprising multiple distributed resources are being increasingly considered for integration into electricity networks. Considering the multiplicity of the distributed resources in a single location and distributed nature of such single entities, it is impractical to represent them as detailed models in power system simulations. Hence, it will be attractive for electricity utilities to employ simplified models of microgrids in dynamic studies. This paper proposes a method for representing grid-tied microgrids as simplified generator or load units in such studies. For this purpose, the paper takes an approach in investigating the applicability of modal analysis as a tool for model equivalencing of grid-tied microgrids with inverter and non-inverter interfaced distributed generators. Validity of the reduced order dynamic equivalents is tested under different operating conditions: power export and import modes, with different load types and fault conditions. Scalability of the model equivalencing approach is demonstrated using a case study developed based on the IEEE 13-Bus test feeder. Furthermore, the model equivalencing is applied on microgrids in a multi-microgrid environment to validate the methodology. The dynamic model equivalent of the microgrid can be represented as a current source at the point of common coupling while retaining all imperative dynamic characteristics. The proposed methodology is also applicable to distribution network clusters comprising distributed generators. [ABSTRACT FROM AUTHOR]

• A decision-making strategy of the fault restoration method considering uncertainties of photovoltage generators and loads is presented in two subsequent steps. • The interval sets number for PV outputs is achieved using the methods of the deep belief network and the particle swarm optimization. • To facilitate the solution of the robust model, the dual-transformation and the optimal equidistant piecewise-linear algorithms are introduced to relax the original objective function into a linear form. The intermittent and stochastic power injections of the penetration of photovoltaic (PV) generators make the processing of fault restoration in the distribution network need to consider more uncertainties. A new scheme of robust fault restoration methods, aiming to maximize the recovery of outage power, is proposed for unbalanced distribution networks utilizing affine numbers to describe the uncertainty of PV outputs. Besides, the safe operation of the network is taken into account as its constraint condition. This restoration method is presented in two subsequent steps. The first step is the restoration of the island power outage, with the objective to reasonably isolate the power outage area containing the black-start PV to achieve island power supply recovery. Then the second step is to maximize the use of the remaining capacity of the main network and restore the power outage to ensure the power supply of the entire network. To solve the mathematical restoration model accurately, the piecewise linear approximation method based on the best isometric idea is introduced to relax the original objective function into a linear solvable form. An efficient decomposition algorithm, named column-and-constraints generation (C&CG), is used to solve the model, which can further improve the computational efficiency of the algorithm. Numerical tests on different test systems demonstrate that the proposed robust restoration methodology yields obvious advantages in resisting system uncertainties by contrast with the existing deterministic restoration methods. [ABSTRACT FROM AUTHOR]

A variety of distributed generators (DGs) are integrated in distribution system which is usually operated under three-phase unbalanced conditions. The zero and negative sequence impedances of DGs may vary within a large range. In this paper, the convergence problem caused by the zero and negative sequence impedances of DGs in forward/backward sweep three-phase power flow is found through numerical experiments. The reason of this phenomenon is explained and an impedance compensation method is proposed to solve this problem. [ABSTRACT FROM AUTHOR]

This paper presents a novel quasi-oppositional teaching learning based optimization (QOTLBO) methodology in order to find the optimal location of distributed generator to simultaneously optimize power loss, voltage stability index and voltage deviation of radial distribution network. The basic disadvantage of the original teaching learning based optimization (TLBO) algorithm is that it gives a near optimal solution rather than an optimal one in a limited iteration cycles. In this paper, opposition based learning (OBL) and quasi OBL concepts are introduced in original TLBO algorithm for improving the convergence speed and simulation results of TLBO. In order to show the effectiveness and superiority, the proposed algorithms are tested on 33-bus, 69-bus and 118-bus radial distribution networks. The simulation results of the proposed methods are compared with those obtained by other artificial intelligence techniques like GA/PSO, GA, PSO and loss sensitivity factor simulated annealing (LSFSA). The results show that the QOTLBO surpasses the other techniques in terms of solution quality. [ABSTRACT FROM AUTHOR]

Highlights: [•] Problem of load growth and its effect on distribution system performance parameter is addressed. [•] Distributed generators are placed at multiple nodes optimally. [•] Techno-economic benefits are studied. [•] Improvement in distribution system performance parameter is reported. [ABSTRACT FROM AUTHOR]

Abstract: Distributed generators (DGs) integrated into distribution networks could affect the host network in number of ways. In this paper an assessment of the impact of the DG continuously operated, which output behavior is non-stochastic, on the reliability of supply in the industrial networks, is performed. A set of new indices representing impact of DG on the interruption frequency, interruption duration and expected energy not supplied to every single customer, is introduced. In addition, a new composite index compiling the impacts of DG on the overall reliability of supply, including interruption frequency, interruption duration and expected energy not supplied, is proposed. Results obtained from several case studies using real-life industrial network are presented and discussed. [Copyright &y& Elsevier]

• A centralized control strategy for an AC meshed MG is presented. • The strategy facilitates the management of power systems including several MGs. • The MG can be operated as a Virtual Synchronous Machine (VSM). • An extended load flow model is used to identify the PQ production of each DG source. This paper proposes a centralized control strategy for distributed generation (DG) sources connected to an AC meshed microgrid (MG). Through the proposed strategy, power production of distributed generators is managed such that the injected active and reactive powers at the point of common coupling are equal to predefined set-points. Thus, the MG is able to provide the main ancillary services including frequency and voltage control. An extended load flow model is used to identify active and reactive power production of each DG unit such that the total power injected to the utility grid satisfies the required setpoints. Hence, from the point of view of the transmission system operator (TSO), the whole MG can be seen as a single entity that behaves as a «PQ» generator. This control is intended to facilitate the management of power systems including a large number of microgrids and DG sources in terms of stability analysis, optimal reactive power dispatch, optimal power flow, etc. Furthermore, since the centralized control allows to set «PQ» power production to any predefined value, it was proposed to operate the whole MG using the concept of Virtual Synchronous Machine (VSM). It has been shown that, through the proposed control, the MG can reproduce the behavior of conventional synchronous generators during normal conditions and during faults. [ABSTRACT FROM AUTHOR]