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This paper introduces an adaptive voltage and frequency control method for inverter-based distributed generations (DGs) in a multi-microgrid (MMG) structure using distributed cooperative control and adaptive neural networks (ANN). First, model-based controllers are designed using the Lyapunov theory and dynamics of the inverter-based DGs. ANNs are then utilized to approximate these dynamics, resulting in an intelligent controller, which does not require a priori information about DG parameters. Also, the proposed controllers do not require the use of voltage and current proportional-integral controllers normally found in the literature. The effectiveness of the proposed controllers are verified through simulations under different scenarios on an MMG test system. Using Lyapunov analysis, it is proved that the tracking error and the neural network weights are uniformly ultimately bounded, which results in achieving superior dynamic voltage and frequency regulation. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive control scheme for a delta-connected cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) is presented, especially focusing on improving dynamic performance by novel feedforward control method. The method can conspicuously improve the dynamics of zero-sequence circulating current regulation of delta-connected CHB STATCOM especially under grid fault condition as well as load unbalance without excessive dc cell capacitor voltage fluctuation. The validity of the proposed method is demonstrated by mathematical proof. Finally, the full-scaled simulation results and the down-scaled experimental results verify that stable operation is guaranteed for both emulated grid and load unbalance conditions. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid microgrid with series- and parallel-connected microconverters is proposed. A few series-connected low voltage microconverters are used to build a string converter with rated output voltage magnitude. Then, the parallel operation of string converters increases the power rating and redundancy of the microgrid. To realize both grid-tied and islanding microgrid control with proper power sharing, a hierarchical power regulation method is developed with simultaneous point of common coupling (PCC) voltage and microconverter voltage control. First, the well-understood droop control is adopted to obtain PCC voltage reference for power sharing between parallel string converters. Second, a power balancing regulation through microconverter local voltage control is used to redistribute output power demand among series-connected microconverters in a string converter. As the string converter line current measured by all microconverters is the same, it is utilized to develop a novel synchronizer to interconnect the string converter central controller and the microconverter local controllers with only low bandwidth communications. Due to the coordination of string converter central control and microconverter local control, the proposed hybrid microgrid can operate in grid-tied and islanding modes with real and reactive power sharing for each microconverter. [ABSTRACT FROM AUTHOR]

This paper presents model predictive control of dual-mode Z-source inverters (ZSIs) with capability to operate in islanded and grid-connected mode. The transition from islanded to grid-connected mode and vice versa can cause significant deviation in voltage and current due to mismatch in phase, frequency, and amplitude of voltages. The proposed controller offers seamless transition between the two modes of operations. The main predictive controller objectives are direct decoupled power control in grid-connected mode and load voltage regulation in islanded mode. The proposed direct decoupled active and reactive power control in grid-connected mode enables the dual-mode ZSI to behave as a power conditioning unit for ancillary services such as reactive power compensation. The proposed controller features simplicity, seamless transition between modes of operations, fast dynamic response, and small tracking error under steady-state condition of controller objectives. The operation of the proposed system is verified experimentally. [ABSTRACT FROM PUBLISHER]

Motivated by the fact that the location(s) and structural properties of the pinning node(s) affect the algebraic connectivity of a network with respect to the reference value, and thereby its dynamic performance, this paper studies the application of single and multiple pinning of distributed cooperative secondary control of distributed generators (DGs) in an islanded microgrid operation. It is shown that the selection of a pinning set based on the degree of connectivity and distance of leader DG(s) from the rest of the network improves the transient performance for microgrid voltage and frequency regulation. The efficacy of the distributed control strategy based on the proposed algorithms is illustrated via numerical results simulating typical scenarios for a variety of microgrid configurations. [ABSTRACT FROM AUTHOR]

In this paper, we consider a voltage control problem in power distribution grids. The specific goal is that of keeping the voltages within preassigned operating limits by commanding the reactive power output of the microgenerators connected to the grid. We propose three strategies. The first two strategies are purely local, meaning that each microgenerator updates the amount of reactive power to be injected based only on local measurements of the voltages’ magnitude. Instead the third one is distributed, namely, the microgenerators, to perform the updating steps, require some additional information coming from the neighboring agents. The local strategies are simpler to be implemented, but they might fail in solving the voltage control problem. Instead, the distributed one requires the microgenerators to be endowed with communication capabilities, but it is effective in driving the voltages within the admissible intervals and, additionally, it exploits the cooperation among the agents to reach also a power losses minimization objective. Theoretical analysis and extensive numerical simulations are provided to confirm the arguments aforementioned. [ABSTRACT FROM PUBLISHER]

In this paper, a fuzzy-secondary-controller-based virtual synchronous generator (VSG) control scheme for interfacing inverters of distributed generation units is proposed for voltage and frequency regulation in microgrids. The proposed control scheme shows significantly improved dynamic performance when compared with the conventional droop control and conventional VSG control through several case studies. Its effectiveness and robustness is further evaluated by sensitivity studies. [ABSTRACT FROM AUTHOR]

Voltage–current (V–I) droop control has been proposed for real and reactive power sharing. Compared with the conventional droop control using real power/frequency and reactive power/voltage, it enhances stability since adding a V–I droop is equivalent to providing a resistor. However, for a system with parallel converters with V–I droops, oscillations are observed when small droop coefficients are adopted. In this paper, a multi-input multioutput (MIMO) model of the system is derived in the frequency domain. This model is based on the $dq$ -reference frame and consists of converter control and the power network model, which is represented by an admittance matrix. Linear system analysis is carried out to identify the root causes of oscillations in both grid-connected mode and autonomous mode. Analysis results are validated by simulation results of detailed model-based systems built in Matlab/Simpowersystems. [ABSTRACT FROM AUTHOR]

This paper presents a novel distributed approach to achieve both bounded voltage and accurate reactive power sharing regulation in ac microgrid. The coupling/trade-off effects between bus voltages and reactive power sharing regulation are first analyzed in detail to provide a guideline for coordinated control design. Furthermore, a containment and consensus-based distributed coordination controller is proposed, by which the bus voltage magnitudes can be bounded within a reasonable range, instead of only controlling average voltage value. Furthermore, the accurate reactive power sharing between distributed generators can be achieved simultaneously. Then, a detailed small-signal model is developed to analyze the stability of the system and the sensitivity of different parameters. Experimental results are presented and compared, where the controller performance, robust performance under communication failure, and plug-and-play operation are successfully verified. [ABSTRACT FROM PUBLISHER]

The distributed secondary control in a microgrid can be used for complementing the function of the primary droop-based control. However, its dynamic performance may be undesirable and furthermore it may introduce new less-damped modes to the system leading to oscillatory responses. Unfortunately, mechanism analysis of the undesirable dynamic performance and the possible oscillations, and more importantly, stabilization of the microgrid with the distributed secondary control have not been reported. To fill this gap, this paper first develops a unified small-signal dynamic model of the microgrid. Based on the developed model, a small-signal stability analysis is utilized to perform the aforementioned mechanism analysis. A distributed optimal controller is thereafter proposed to enhance the system stability and improve the system dynamic performance. The distributed optimal controller can coordinate multiple distributed generation units in the microgrid and exhibits robust performance under a wide range of operating conditions. Finally, theoretical analysis and time-domain simulation results on a benchmark microgrid system are provided to verify the effectiveness of the proposed methods. [ABSTRACT FROM PUBLISHER]

Centralized or decentralized secondary controller is commonly employed to regulate the voltage drop raised by the primary controller. However, in the case of high capacity microgrids (MGs) and long feeders with much voltage drop on the line resistances, the conventional methods may not guarantee the voltage regulation on the load busses within a suitable range. Therefore, in addition to compensate the voltage drop of the primary controller, it is necessary to regulate the voltage of critical loads. In this paper, a new voltage regulation strategy is proposed to regulate the voltage of MG by employing the average voltage of identified critical busses, which are determined by the proposed modal analysis. Numerical steady-state analysis and preliminary simulation results validate effectiveness of the proposed scheme. Furthermore, experimental results are performed to demonstrate the viability of the proposed approach. [ABSTRACT FROM AUTHOR]

In the islanded microgrid, distributed generators are controlled with virtual synchronous generator (VSG) strategy to simulate rotor inertia and droop characteristics of synchronous generators, in order to enhance the voltage and frequency support capabilities. Since the capacity and location distribution of each VSG is random, the VSG output impedance, line impedance and its capacity are mismatched, resulting in inaccurate sharing of reactive power. Based on the study of reactive power sharing schemes without communication and system parameters detection, and aiming at the contradiction between reactive power sharing error and voltage control accuracy of existing schemes, a reactive power sharing strategy based on virtual capacitor is proposed. The strategy simulates characteristics of paralleling capacitor at the VSG output terminal, and compensates the output voltage according to adaptive control of VSG output reactive power, thus to reduce reactive power sharing error, and improve the voltage control accuracy meanwhile. The design of virtual capacitor parameters and a two VSG parallel system stability with proposed strategy are analyzed in this paper. The correctness and effectiveness of the proposed strategy is verified by experiments. [ABSTRACT FROM PUBLISHER]

Series-connected microconverters have been used to integrate distributed energy resources into grid mains through a single-stage power conversion. When this string converter is switched to islanding operation mode, it is preferred to share the local load demand by series microconverter modules in a decentralized manner. Due to the characteristics of series connection, the well-understood P–f droop and Q–E droop control for parallel distributed generation units in a conventional islanding microgrid is not applicable in this case. To solve this problem, the feasibility of using a simple power factor-frequency inverse droop control for series-connected islanding microconverters is discussed in this paper. The proposed method can achieve simultaneous real and reactive power sharing without the assistance of any communications between series microconverters. Furthermore, the accuracy of power sharing is not affected by the variations of microgrid feeder parameters or the disturbance of the point of common coupling (PCC) nonlinear loads. Simulated and experimental results are provided to verify the correctness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper proposes one control paradigm that can operate in both grid-connected and islanded modes, hence does not need any sort of an islanding detection method. The proposed method automatically and seamlessly rides through a fault on the grid side, and controls the microgrid's voltage and frequency during islanded operation. During an islanded operation, it utilizes the combination of distributed generation-energy storage similar to the prime mover of a synchronous generator to control the frequency. A comprehensive active and reactive power control is proposed that minimizes the usage of a local fossil-fuelled auxiliary generator. The method is based on expanding the so-called non-detection zone to all operational scenarios including islanded mode, hence having small “undetectable” voltage and frequency deviation. As soon as the grid is reconnected, the distributed generator is automatically and seamlessly synchronized with the grid. This is achieved through keeping phase-locked loop as a part of the operation in islanded mode without altering its phase angle. The proposed method is validated using PSCAD/ EMTDC simulation. [ABSTRACT FROM PUBLISHER]

P-f and Q-V droop methods are the most common decentralized control methods in islanded microgrid. Although with the P-f droop method, an accurate active power sharing can be achieved among distributed energy resources (DERs), by Q-V droop, the reactive power sharing among DERs often deteriorates due to its highly dependence on the power line impedances and the local load. Variation of frequency and voltage by load changes is another challenge in droop control method. In this paper, a new autonomous control method is proposed to share reactive power among DERs accurately and restore frequency of a microgrid. The proposed method does not require any communication link and so maintains reliability and simplicity of the network. The synchronizing among DERs is obtained by the load change detection, which is accomplished by wavelet transform. The method operation principle is explained and analyzed. Simulation results are presented to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed coordination control method for multiple bidirectional power converters (BPCs) in a hybrid ac/dc microgrid. The researched hybrid ac/dc microgrid is composed of both ac and dc subgrids connected by multiple parallel BPCs, and only the ac subgrid contains storages for the sake of economy. The proposed control method can realize the appropriate power interaction between the two subgrids and make the two subgrids to support each other, so the hybrid microgrid can operate well in both the grid-connected and islanded modes. Concretely, there are three improvements in this proposed control methods. First, a d-q-0 three-axis control strategy instead of the conventional d-q two-axis control strategy is adopted to suppress the circulating current on the ac side generated by the multiple parallel BPCs. And then the square of the dc voltage rather than the dc voltage itself is controlled, which can make the control of the dc voltage linearized and can make the BPCs provide well voltage support for the dc subgrid. Second, the dc droop control is adopted for the dc current sharing. Also, in order to relieve the storages from generating too much reactive power if the ac load reactive power is very large in the ac subgrid, the reactive power droop control is adopted to make the BPCs share some reactive power. Third, taking the capacities and load types of the two subgrids into consideration, a coordinated control strategy based on the dc voltage versus the frequency droop is designed to realize the power interaction between the two subgrids. Furthermore, the stability analysis of the proposed control method is conducted. All the conclusions are verified by the real-time hardware-in-loop tests. [ABSTRACT FROM AUTHOR]

This paper presents the analysis, development, and implementation of a new controller for microgrid systems. The developed controller is structured to produce command values for the active and reactive powers ( PC and QC ), and d– q-axis voltage components ( v\text{dC} and v\text{qC}) for each bus in the microgrid system at hand. Values for PC,QC,v\text{dC}, and v\text{qC} are obtained using a power flow formulated in the $d$– $q$-axis frame. The $d$–$q$-axis power flow (DQPF) produces values for PC,QC,v\text{dC} , and v\text{qC} that can meet load demands, respond to variations in power generation within the microgrid, and accommodate the mode of operating the microgrid (interconnected or standalone), while maintaining the voltage and frequency stability. The DQPF-based controller is implemented for performance evaluation using a five-bus microgrid system operated under various conditions, including changes in power generation, step changes in load demands, and changes in mode of operation. Test results demonstrate that the DQPF-based controller can offer simple implementation, accurate and fast responses, and negligible sensitivity to the mode of microgrid operation. [ABSTRACT FROM PUBLISHER]

Today, interests on hybrid ac/dc microgrids, which contain the advantages of both ac and dc microgrids, are growing rapidly. In the hybrid ac/dc microgrid, the parallel-operated ac/dc bidirectional interfacing converters (IFCs) are increasingly used for large capacity renewable energy sources or as the interlinking converters between the ac and dc subsystems. When unbalanced grid faults occur, the active power transferred by the parallel-operated IFCs must be kept constant and oscillation-free to stabilize the dc bus voltage. However, under conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs is affected due to the converters’ current rating limitations. Moreover, unbalanced voltage adverse effects on IFCs (such as output power oscillations, dc-link ripples, and output current enhancement) could be amplified by the number of parallel converters. Therefore, this paper investigates parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposes a novel control strategy to enhance the active power transfer capability with zero active power oscillation. The proposed control strategy employs a new current sharing method which introduces adjustable current reference coefficients for parallel IFCs. In the proposed control strategy, only one IFC, named as redundant IFC, needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power of parallel IFCs. Simulation and experimental results verify the feasibility and effectiveness of the proposed control strategy. [ABSTRACT FROM PUBLISHER]

In this paper, a two-layer network and distributed control method is proposed, where there is a top-layer communication network over a bottom-layer microgrid. The communication network consists of two subgraphs, in which the first is composed of all agents, while the second is only composed of controllable agents. The distributed control laws derived from the first subgraph guarantee the supply–demand balance, while further control laws from the second subgraph reassign the outputs of controllable distributed generators, which ensure active and reactive power are dispatched optimally. However, for reducing the number of edges in the second subgraph, generally a simpler graph instead of a fully connected graph is adopted. In this case, a near-optimal dispatch of active and reactive power can be obtained gradually, only if controllable agents on the second subgraph calculate set points iteratively according to our proposition. Finally, the method is evaluated over seven cases via simulation. The results show that the system performs as desired, even if environmental conditions and load demand fluctuate significantly. In summary, the method can rapidly respond to fluctuations resulting in optimal power sharing. [ABSTRACT FROM PUBLISHER]

This paper deals with a wind–diesel generator hybrid configuration of the microgrid using a voltage source converter (VSC) as a voltage and frequency (VF) controller. The wind power generated by permanent magnet brushless dc generator, and the maximum power is captured by a maximum power point technique using a boost converter with an incremental conductance approach. This power is supplied to the consumer loads and surplus power is stored into battery system (BS). BS is attached at dc link of VSC, which provides load leveling during less or no wind conditions. With such combination of energy resources, a reduced rating, diesel engine-driven squirrel cage induction generator feeds loads, and VSC at point of common coupling (PCC) supports the system when the wind generation is unable to meet out the load demand. Back-propagation feed-forward control algorithm is used for VF control of VSC. This controller provides harmonics elimination, load leveling, and reactive power compensation, and also regulates the voltage at PCC. This microgrid is modeled in MATLAB Sim power tools, and simulation results are produced to verify the appropriate working of both the converters and the overall system. A prototype of the microgrid is also developed to validate the design and model through test results. A comparative study of simulated and experimental work is given in detail. [ABSTRACT FROM PUBLISHER]

In this paper, an enhanced hierarchical control structure with multiple current loop damping schemes for voltage unbalance and harmonics compensation (UHC) in ac islanded microgrid is proposed to address unequal power sharing problems. The distributed generation (DG) is properly controlled to autonomously compensate voltage unbalance and harmonics while sharing the compensation effort for the real power, reactive power, and unbalance and harmonic powers. The proposed control system of the microgrid mainly consists of the positive sequence real and reactive power droop controllers, voltage and current controllers, the selective virtual impedance loop, the unbalance and harmonics compensators, the secondary control for voltage amplitude and frequency restoration, and the auxiliary control to achieve a high-voltage quality at the point of common coupling. By using the proposed unbalance and harmonics compensation, the auxiliary control, and the virtual positive/negative-sequence impedance loops at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencies, an accurate power sharing is achieved. Moreover, the low bandwidth communication (LBC) technique is adopted to send the compensation command of the secondary control and auxiliary control from the microgrid control center to the local controllers of DG unit. Finally, the hardware-in-the-loop results using dSPACE 1006 platform are presented to demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM PUBLISHER]

The coupling between the active and the reactive power of the droop-controlled inverter is critical in the low-voltage microgrid. The conventional virtual power-based control strategy can decouple the active and the reactive power of the inverter, whereas the active power of the load is probably not equally shared among the parallel inverters in the microgrid. To tackle this issue, this paper proposes an improved virtual power-based control method for the droop-controlled parallel inverters with a unified rotation angle in the power transformation. Meanwhile, the optimal value of the unified rotation angle is rigorously derived through investigating the relative stability of the system by innovatively adopting Routh Stability Criterion. Moreover, the proposed method is enhanced by introducing low-pass filters in the coupling path for further mitigating the coupling between the active and the reactive power of the inverter. Additionally, the stability analysis and parameter design for the proposed method are comprehensively presented. Finally, the effectiveness of the proposed method is validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, an inverter using resistive-capacitive output impedance (RC-type inverter) is proposed not only to provide fast reactive power sharing to support microgrid voltage, and but also to reduce circulating currents and damp high-frequency resonances among inverters. Introducing the RC virtual impedance loop, the inverter provides fast transient response. Based on the RC-type inverter modeling, the comparative frequency-domain analysis of equivalent output impedances is discussed, and the impact of the virtual complex impedance over the circulating currents and high-frequency resonances among parallel inverters is quantitatively analyzed. The control parameters are systematically selected, and effect of virtual complex impedance on the inverter output voltage is depicted. The RC-type inverter can reduce circulating currents and damp resonances due to different equivalent output impedances of inverter, and line impedances. Simulation and experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Four-leg dc–ac power converters are widely used for the power grids to manage grid voltage unbalance caused by the interconnection of single-phase or three-phase unbalanced loads. These converters can further be connected in parallel to increase the overall power rating. The control of these converters poses a particular challenge if they are placed far apart with no links between them (e.g., in islanded microgrids). This challenge is studied in this paper with each four-leg converter designed to have improved common current sharing and selective voltage-quality enhancement. The common current sharing, including zero sequence component, is necessary since loads are spread over the microgrid and they are hence the common responsibility of all converters. The voltage-quality enhancement consideration should however be more selective since different loads have different sensitivity levels towards voltage disturbances. Converters connected to the more sensitive load buses should therefore be selectively triggered for compensation when voltage unbalances at their protected buses exceed the predefined thresholds. The proposed scheme is therefore different from conventional centralized schemes protecting only a common bus. Simulation and experimental results obtained have verified the effectiveness of the proposed scheme when applied to a four-wire islanded microgrid. [ABSTRACT FROM AUTHOR]

Unlike the typical microgrid with a common ac bus, networked microgrid always suffers more serious reactive power sharing issues due to its complex inner structure. In such case, the system reactive power sharing error cannot be easily evaluated and eliminated. So, this paper proposes a wireless control strategy that employs optimized virtual impedance controllers and local load measurements for the reactive power sharing in networked microgrid. First, from the modeling of microgrid network, an estimation method for network reactive power sharing error is derived. Through the estimation method-based network feature analyses, corresponding design for virtual impedance controller is presented. Then, by introducing genetic algorithm, virtual impedance controller parameters of each distributed generation unit are optimized, which aims to minimize the microgrid global reactive power sharing error. The parameter optimization process is performed offline in microgrid configuration stage. By using these optimized virtual impedance controllers, the reactive power sharing performance of a networked microgird can be greatly improved. Finally, the accuracy of the estimation method is validated by MATLAB simulation results, and the feasibility of the proposed virtual impedance optimization method is verified through real power experiments. [ABSTRACT FROM PUBLISHER]

This paper presents a framework for the coordination of distributed energy resources (DERs) and demand response (DR) for voltage and frequency support of islanded microgrids. The proposed method basically relies on extracting information from real and reactive power sensitivities at different buses for minimizing the voltage and frequency deviations of the islanded microgrid. To this aim, a new power flow procedure is adopted in which the frequency deviation appears as an additional state variable. This helps to calculate the required setpoints for the DERs as well as the amount of the demanded power curtailed through the controllable loads to meet the overall goal. The loads are classified based on their a-priori known controllability degree. To minimize the manipulated load, the most effective buses are selected based on their associated sensitivity values. In the grid-connected mode, the total operation cost is minimized, while the microgrid bus voltages are maintained within the pre-specified acceptable range. In both modes, the whole process is formulated as a multiobjective problem solved by the particle swarm optimization (PSO). The control procedure involves a series of commands for which the incident command system (ICS) is used as a secure communication structure. The performance of the proposed control framework is evaluated for the case of a typical MV microgrid in both grid-connected and islanded modes. [ABSTRACT FROM PUBLISHER]

In this paper, we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive tradeoff between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances, or loads. The distributed architecture allows for flexibility and redundancy, eliminating the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation. [ABSTRACT FROM PUBLISHER]

In this paper, restorations for both voltage and frequency in the droop-controlled inverter-based islanded microgrid (MG) are addressed. A distributed finite-time control approach is used in the voltage restoration which enables the voltages at all the distributed generations (DGs) to converge to the reference value in finite time, and thus, the voltage and frequency control design can be separated. Then, a consensus-based distributed frequency control is proposed for frequency restoration, subject to certain control input constraints. Our control strategies are implemented on the local DGs, and thus, no central controller is required in contrast to existing control schemes proposed so far. By allowing these controllers to communicate with their neighboring controllers, the proposed control strategy can restore both voltage and frequency to their respective reference values while having accurate real power sharing, under a sufficient local stability condition established. An islanded MG test system consisting of four DGs is built in MATLAB to illustrate our design approach, and the results validate our proposed control strategy. [ABSTRACT FROM PUBLISHER]

Weather-dependent photovoltaic (PV) system power variation may become frequent and rapid, which could be regarded as perturbation for the microgrid (MG). This perturbation would lead to voltage and frequency fluctuations and dramatically degrade the MG performances. Conventional secondary control strategy could restore the voltage and frequency after a certain time delay, but it cannot satisfy the speed demands when frequent and rapid perturbations appear. To overcome the limitation, this paper proposes a novel real-time voltage and frequency compensation strategy to suppress the fluctuations. The proposed strategy realizes voltage regulation by an adaptive virtual impedance loop and realizes frequency regulation by a virtual frequency-impedance loop. The novel real-time strategy is simple to be implemented and needless of communication; therefore, it is much more suitable for the PV-based MG. Finally, a PV-based MG is established as laboratory prototype, and experimental results validate the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper proposes a smooth transition control strategy for voltage-source inverters between stand-alone (SA) and grid-connected (GC) modes of operation. In both GC and SA modes of operation, the inverters use the droop control method to regulate the real power flow without the need of any external communication between them. One of the distributed energy sources working in the microgrid system is designated as a dispatch unit (DU) to facilitate interconnection with utility grid and achieve smooth transition between the modes of operations. During transition modes, this selected unit takes extra responsibility to ensure the continuous power delivery to the load. For smooth transition from GC to SA mode of operation, the DU compensates for the grid current immediately after the grid disconnection, and then, other units pick up this responsibility gradually through their individual droop controllers. The DU also adjusts its output power to synchronize the inverters with the grid during the transition from SA to GC mode. The simulation and experimental results, as well as the stability analysis, are provided to verify the effectiveness of the proposed control technique. [ABSTRACT FROM AUTHOR]

Different microsources have different frequency regulation functions and capabilities. The droop control can allocate power among the microsources according to the operation demand during system dynamics; however, the steady-state frequency often deviates from the rated value because of the droop characteristics. To ensure the precise condition of power and the stability of frequency even in a low-voltage network, this paper puts forward an improved droop control algorithm based on coordinate rotational transformation. With the ability to accurately regulate the unbalance power, this method realizes self-discipline parallel operation of microsources. Furthermore, an adaptive inverse control strategy applied to modified power conditioning is developed. With an online adjustment of modified P-f droop coefficient for the frequency of microgrid to track the rated frequency, the strategy guarantees maintaining the frequency of microgrid at the rated value and meeting the important customers' frequency requirements. The simulation results from a multibus microgrid show the validity and feasibility of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In this paper, a reactive power sharing strategy that employs communication and the virtual impedance concept is proposed to enhance the accuracy of reactive power sharing in an islanded microgrid. Communication is utilized to facilitate the tuning of adaptive virtual impedances in order to compensate for the mismatch in voltage drops across feeders. Once the virtual impedances are tuned for a given load operating point, the strategy will result in accurate reactive power sharing even if communication is disrupted. If the load changes while communication is unavailable, the sharing accuracy is reduced, but the proposed strategy will still outperform the conventional droop control method. In addition, the reactive power sharing accuracy based on the proposed strategy is immune to the time delay in the communication channel. The sensitivity of the tuned controller parameters to changes in the system operating point is also explored. The control strategy is straightforward to implement and does not require knowledge of the feeder impedances. The feasibility and effectiveness of the proposed strategy are validated using simulation and experimental results from a 2-kVA microgrid. [ABSTRACT FROM AUTHOR]

This paper presents two cost-prioritized droop sche- mes for distributed generators (DGs) in a rural or islanded microgrid. Dispatch prioritization of the schemes allows autonomous identification of the appropriate DGs for generation, in accordance to the overall load conditions of the microgrid. The result is a lower total generation cost for the microgrid when compared to other droop schemes. An experimental system has been implemented and tested with results showing a higher saving achieved by the proposed schemes. [ABSTRACT FROM AUTHOR]

In multibus islanded microgrids, the power quality requirements for different areas and buses can be different. This paper proposes a hierarchical control to realize optimal unbalance compensation for satisfying the power quality requirements in different areas. Primary and secondary controllers are applied to realize unbalance compensation for critical bus, and at the same time, to make distributed generators (DGs) equally share the compensation efforts. Tertiary control, which inherently is an optimization method, is implemented to adjust the compensating effort of each DG considering the voltage unbalance limits in local buses and DG terminals. This method realizes multipower-quality-level control in a multibus islanded system by optimally utilizing DGs as distributed compensators and saves the investment for additional compensation equipment. Hardware-in-the-loop results demonstrate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

This paper presents the fundamental concepts of a generalized central power management system and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units. DER units are divided into voltage-controlled and power-controlled DER units. The frequency of each DER unit is determined by its independent internal oscillator and all oscillators are synchronized by a common time-reference signal based on a global positioning system (GPS). The power management system (PMS) specifies the power and voltage set points for the local controllers of each DER unit. A linear, time-invariant, multivariable, robust, decentralized control system is designed to track the setpoints. Each control agent guarantees fast tracking, zero steady state error, and robust performance despite uncertainties of the microgrid parameters, topology, and the operating point. Existence conditions, control design procedures, eigenanalysis and robust stability analysis of the closed-loop system, and performance of the control strategy based on digital time-domain simulation studies in PSCAD/EMTDC platform, are reported. Performance of the control system is also verified based on hardware-in-the-loop (HIL) studies in the RTDS environment. [ABSTRACT FROM AUTHOR]

Distributed secondary control (DSC) is a new approach for microgrids (MGs) by which frequency, voltage, and power can be regulated by using only local unit controllers. Such a solution is necessary for anticipated scenarios that have an increased number of distributed generators (DGs) within the MG. Due to the constrained traffic pattern required by the secondary control, it is viable to implement a dedicated local area communication functionality among the local controllers. This paper presents a new wireless-based robust communication algorithm for the DSC of MGs. The algorithm tightly couples the communication and the control functionality, such that the transmission errors are absorbed through an averaging operation performed in each local controller, resulting in a very high reliability. Furthermore, transmissions from each DG are periodic and prescheduled broadcasts, and in this way, contention over the shared wireless medium is avoided. Real-time simulation and experimental results are presented in order to evaluate the feasibility and robustness endowed by the proposed algorithm. The results indicate that the proposed algorithm is very robust with respect to communication impairments, such as packet delays and random packet losses. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel load-flow analysis (LFA) algorithm for droop-based islanded microgrids (DBIM). The standard LFA is not applicable for DBIM due to the absence of a node with fixed reference voltage. As the voltage in the islanded microgrid depends on the droop relations, these relations are included as part of the load-flow equations. The proposed LFA is used with particle swarm optimization to select the droop parameters that optimize the reactive power sharing. Voltage compensation terms are also suggested to improve voltage regulation. By using the proposed LFA, a modeling procedure is suggested to check the stability and stabilize the microgrid. The effectiveness of the proposed methods is verified through different simulation studies. [ABSTRACT FROM AUTHOR]

Paralleled operation of voltage-source inverters (VSIs) is currently achieved by using voltage/frequency droop control techniques which requires the knowledge of the system parameters. Otherwise, centralized control techniques with robust communication among VSIs controllers are also used. This paper presents a new control strategy which allows the load sharing between the power sources of an ac microgrid without centralized controller or any communication among the VSIs; only local measurements of voltage and output current are used. The dispatchable sources (e.g., fuel cells) of the microgrid are operated using voltage control with a direct droop scheme, and the nondispatchables or intermittent ones (e.g., wind turbine generators) are operated using power control with a complementary inverse droop scheme (D-Droop + I-Droop). The number of operating sources can be changed online without any modification needed on the VSI controllers. The proposed VSI controllers are based on the variable frequency adaptive linear neuron with frequency-locked loop for the VSIs system synchronization, voltage/power and signal estimation. Experimental results using field-programmable gate array devices for the implementation of each VSI control in the microgrid test bench demonstrated the validity of the proposition. [ABSTRACT FROM PUBLISHER]

The parallel operation of inverters in microgrids is mainly based on the droop method. The conventional voltage droop method consists of adjusting the output voltage frequency and amplitude to achieve autonomous power sharing without control wire interconnections. Nevertheless, the conventional voltage droop method shows several drawbacks, such as complicated inner multiloop feedback control, and most importantly, frequency and voltage deviations. This paper proposes a new control strategy in microgrid applications by drooping the virtual flux instead of the inverter output voltage. First, the relationship between the inverter virtual flux and the active and reactive powers is mathematically obtained. This is used to develop a new flux droop method. In addition, a small-signal model is developed in order to design the main control parameters and study the system dynamics and stability. Furthermore, a direct flux control algorithm is employed to regulate the virtual flux according to the droop controller, which avoids the use of proportional-integral controllers and pulse-width modulation modulators. Both the simulation and experimental results show that the proposed flux droop strategy can achieve active and reactive power sharing with much lower frequency deviation than the conventional voltage droop method, thus highlighting the potential use in microgrid applications. [ABSTRACT FROM AUTHOR]

As colleges prepare or revise their power engineering courses to include discussions on the smart grid, the microgrid, distributed energy sources and renewable energy resources, the need for new material suitable for inclusion in a typical 4-year undergraduate engineering course has never been more obvious. Toward this effort, an old machines lab was remodeled and converted into a small power network that can be operated in both grid-connected and grid-islanded microgrid modes. The intent of this paper is to devise laboratory experiments to supplement a course in conventional power systems analysis at the undergraduate level. During the remodeling process, two of six synchronous machines in the lab were converted to represent distributed generating units within a laboratory-based microgrid. The use of electromechanical distributed generators (EDG) with a low start-up time allows the system to operate as a conventional power network with provisions for frequency and voltage variations. Students can experiment with voltage control and power sharing between the EDGs in islanded mode. A Laboratory Grid Central Controller (LGCC) was developed in LabVIEW® to coordinate the operation of the EDGs, the utility grid and the loads. Its functions are similar to those of a central dispatch office, albeit on a much smaller scale. A one credit hour lab course held on this laboratory was offered for the first time during the Fall semester of 2012 at Missouri S&T. [ABSTRACT FROM PUBLISHER]

Presents closure on the discussion to the above named paper. [ABSTRACT FROM AUTHOR]