Islanded AC Microgrid Research Articles

A robust control scheme for voltage and reactive power regulation in islanded AC microgrids

In multi-feeder microgrid systems, accurate power sharing and voltage regulation at each load feeder is more challenging than the conventional single-feeder microgrids. This paper presents an enhanced resilient control strategy for multi-feeder micro-network systems by considering the load feeder voltage regulation and power balancing as a quadratic optimization problem. The proposed control strategy is composed of impedance estimator and optimal controller. However, the impedance estimator obtains each load feeder impedance while optimal controller computes the voltage commands for each distributed generator. One of the important aspects of the proposed control scheme is that only the voltage magnitude information is required to transfer at each inverter's controller, with primary aim to achieve the two-fold objectives. This control strategy is also extended to approximate the whole network impedance into solitary load feeder for micro-networks with extensive number of load feeders. This makes the system computationally less burdening. The MATLAB/Simulink and experimental results are obtained under varying load conditions and line parameter uncertainties for radial type multi-feeder microgrid configuration that reflects the effectiveness of the proposed scheme. The verification outcomes prove the proposed method capability of accurate reactive power sharing, regulation of load feeder voltages, and frequency restoration.

Distributed Adaptive Control Scheme for Islanded AC Microgrids With Tolerance to Uncertain Communication Links

This article proposes a fully distributed resilient control scheme for islanded alternating current microgrids to achieve economic operation under uncertain communication links. Based on the well-known hierarchical control structure and consensus algorithm, a distributed resilient economic dispatch algorithm is first designed to determine the optimal generation point, and then, a distributed resilient secondary controller is developed to track the optimal generation point and eliminate the frequency deviation. Different from the robust secondary control schemes, which require the upper bound of uncertainties in dealing with the uncertain communication links and offline design the communication coupling weights through a central controller, the proposed scheme applies adaptive control technology to online design the communication coupling weights based on communication neighbor information, such that the adverse effects of uncertain communication links can be counteracted. Thus, the proposed adaptive scheme is fully distributed in contrast to existing robust schemes. The Lyapunov approach-based theoretical analysis shows that frequency restoration and generation cost minimization can be achieved for the islanded microgrid under some proper assumptions. Finally, the effectiveness of the proposed scheme is verified by several simulation studies in MATLAB/Simulink.

Decentralized Master–Slave Control for Series-Cascaded Islanded AC Microgrid

Series-cascaded microgrids (MGs) are gaining increasing popularity because of their control flexibility and ability to synthesize high voltage levels from low power rated modules. Several control schemes have been reported for autonomous power sharing in series-cascaded MGs that incorporate dispatchable distributed generators (DGs) only. In contrast, this article develops a series-cascaded MG formed by dispatchable and nondispatchable DGs. Achieving power balance as well as voltage and frequency regulation in this MG configuration is a challenging task. In this article, we propose a decentralized master–slave control scheme that regulates voltage and frequency and maintains power balance. The proposed scheme ensures the maximum utilization of nondispatchables, incorporates autonomous power curtailment under light-load conditions, and offers system reliability comparable with the traditional parallel structure. The proposed MG configuration and control scheme are thoroughly validated via simulations and experiments under various operating scenarios.

Optimal Distributed Control of AC Microgrids With Coordinated Voltage Regulation and Reactive Power Sharing

In this paper, we propose an optimal distributed voltage control for grid-forming (GFM) inverters in islanded AC microgrids. An optimization problem is formulated where the distributed generator (DG) output voltage is considered as the control variable with technical constraints on voltage and reactive power output capacity and an objective function that makes a trade-off between voltage regulation and reactive power sharing. A distributed primal-dual gradient based algorithm is developed to solve the formulated optimization problem to address the challenges due to non-separable objective function, unavailable global average voltage, and globally coupled reactive power constraints. The effectiveness of the proposed optimal distributed control is validated through simulations on the 4-DG test microgrid and the modified IEEE 34-bus distribution test system, and the advantages of the proposed control over existing controls are demonstrated.

Cyber-Resilient Sliding-Mode Consensus Secondary Control Scheme for Islanded AC Microgrids

In this article, a cyber-resilient consensus-based distributed control scheme is proposed for islanded ac microgrids. Considering the impacts of false data injection (FDI) attacks on the conventional consensus algorithms, a hybrid solution is presented based on the combination of multiobjective sliding-mode control and communication link quality observer to provide a reliable performance against different types of FDIs. Unlike the commonly developed distributed observer-based cyber-resilient algorithms, this approach aims to form a complete localized solution without the requirements for transmission of extra distributed signals in the secondary layer. Using the proposed method, the system reliability will not be impacted by the cyber intrusions on the distributed observer terms, and the system's dynamic performance is not deteriorated by the counteracting operation resulted from adaptive distributed observers under other system-transient conditions such as load dynamics, which can be even worsened under communication delays. Using cyber-resilient offset compensation terms, performance of the proposed sliding-mode control scheme is enhanced for multiobjective regulation. Integrating the sliding-mode control with communication link observer also ensures effective isolation of the unbounded and extreme cyberattacks. Unlike the former indirect local observers, the proposed scheme operates directly based on monitoring the affected distributed signals rather than only power terms, which enhances its reliability on detection. The effectiveness of the proposed scheme is validated through a real-time model developed in Typhoon HIL-402 real-time simulator as well as the experimental tests.

Distributed Finite-Time Event-Triggered Frequency and Voltage Control of AC Microgrids

This paper proposes a finite-time event-triggered secondary frequency and voltage control for islanded AC microgrids (MGs) in a distributed fashion. The proposed control strategy can effectively perform frequency restoration and voltage regulations, while sharing the active and reactive power among the distributed generators (DGs) based on their power ratings. The finite-time control enables a system to reach consensus in a finite period of time enhanced from the asymptotic convergence. The event-triggered communication is utilized to reduce the communication burden among the DG controllers by transmitting data among DGs if an event-triggering condition is satisfied. The performance of the proposed finite-time event-triggered frequency control is verified utilizing a hardware-in-the-loop experimental testbed which simulates an AC MG in Opal-RT.

Temperature Droop-Based Dynamic Reactive Power Sharing Technique to Improve the Lifetime of Power Electronic Converter

Thermal stress because of mean junction temperature (<inline-formula><tex-math notation="LaTeX">$T_{{{jm}}}$</tex-math></inline-formula>) and junction temperature swing (<inline-formula><tex-math notation="LaTeX">$\Delta T_{{j}}$</tex-math></inline-formula>) reduces the lifetime of the power electronic (PE) converters. It is beneficial to distribute the stress evenly among the converters operating in parallel in a system like microgrid to improve their lifetime. The existing techniques usually only distribute stress due to <inline-formula><tex-math notation="LaTeX">$T_{{{jm}}}$</tex-math></inline-formula>. A few techniques are based on the accumulated damage in PE converters do not necessarily mitigate the effect of <inline-formula><tex-math notation="LaTeX">$\Delta T_{{j}}$</tex-math></inline-formula>. Moreover, most of the the existing techniques are not applicable for nondispatchable energy source like PV. The intermittent nature of active power generation profile of the PV inverter further reduces the lifetime of the PE converters operating in an islanded microgrid. To address this issue, a temperature droop-based dynamic reactive power sharing (TDDRPS) technique is proposed for an islanded ac microgrid to distribute the thermal stress due to both <inline-formula><tex-math notation="LaTeX">$T_{{{jm}}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">$\Delta T_{{j}}$</tex-math></inline-formula>. The stability of the proposed TDDRPS technique is mathematically analyzed. Finally, the proposed technique is validated through simulation and experimentation on a scaled down laboratory prototype of microgrid.

Design of droop controller in islanded microgrids using multi‐objective optimisation based on accurate small‐signal model

The inaccuracy of power sharing is a classic problem of droop control when an islanded AC microgrid suffers from high loads and line impedance differences. It degrades system performance and even destroys system stability. This paper originally presents a multi-objective optimisation droop control method to solve such a problem. And three objective functions are presented according to the characteristics of microgrids. First, a complete small-signal model of a microgrid in autonomous operation mode is established. Next, this paper derives constraint conditions for the stable microgrid. Then, this paper originally designs three objective functions for constructing a multi-objective optimisation (MOO) problem to improve control performance. The multiobjective evolutionary algorithm based on decomposition (MOEA/D) is used to obtain the Pareto optimal frontier of the MOO problem. The fuzzy affiliation function method is used to choose the best solution from the generated Pareto optimal frontier. Finally, simulations verified the validity and superiority of the presented method.

Multiobjective Laguerre Functions[formula omitted]Based Discrete[formula omitted]Time Model Predictive Control: A Fast Inner[formula omitted]Loop Controller for Grid[formula omitted]Forming Converters

In islanded ac microgrids, conventional primary control uses inner−loop cascaded linear controller and outer−loop droop to realize local voltage regulation and power sharing. However, it has a poor dynamic performance and fast rate of frequency change following disturbances. This paper addresses these issues by proposing a modified virtual synchronous generator (VSG) control. A Laguerre functions−based discrete−time model predictive control (LF−DMPC) with a multiobjective cost function is incorporated as the inner loop; yielding large prediction horizon, improved dynamic response, and inherent overcurrent protection in the case of faults. The swing−based and the reactive power droop controllers also form the outer loop aimed at inertia emulation and power sharing. The merits of proposed approach are verified by comparisons with conventional droop and VSG controls. Detailed model simulations are conducted on a 2−converter ac microgrid in MATLAB/Simulink to show the efficacy of the proposed controller.

Power-flow-based energy management of hierarchically controlled islanded AC microgrids

This paper presents a novel approach of employing hierarchical control to optimize the operation of islanded AC microgrids. The proposed method is an offline, centralized, power-flow-based energy management scheme which includes primary and secondary control dynamics in a modified power-flow formulation. The inner power-flow level maintains bus voltages and system frequency within the desired range and ensures power balance in the network. The outer optimization level ensures that the various components remain within their operational constraints, while optimizing a system-level objective. Two case studies are explored using a modified 14-bus medium voltage (MV) CIGRE benchmark microgrid to validate the proposed energy management algorithm. The first case includes minimization of conventional generator operating cost and renewable energy curtailment, both with linear and non-linear loads, and the second case includes minimization of conventional generator operating cost with load shedding. The results obtained from the case studies show the efficiency of the proposed energy management algorithm, and evidence its reliability for the optimal operation of islanded AC microgrids with multiple renewable energy sources.

Stability Boundary Analysis of Islanded Droop-Based Microgrids Using an Autonomous Shooting Method

This paper presents a stability analysis for droop-based islanded AC microgrids via an autonomous shooting method based on bifurcation theory. Shooting methods have been used for the periodic steady-state analysis of electrical systems with harmonic or unbalanced components with a fixed fundamental frequency; however, these methods cannot be directly used for the analysis of microgrids because, due to the their nature, the microgrids frequency has small variations depending on their operative point. In this way, a new system transformation is introduced in this work to change the droop-controlled microgrid mathematical model from an non-autonomous system into an autonomous system. By removing the explicit time dependency, the steady-state solution can be obtained with a shooting methods and the stability of the system calculated. Three case studies are presented, where unbalances and nonlinearities are included, for stability analysis based on bifurcation analysis; the bifurcations indicate qualitative changes in the dynamics of the system, thus delimiting the operating zones of nonlinear systems, which is important for practical designs. The model transformation is validated through time-domain simulation comparisons, and it is demonstrated through the bifurcation analysis that the instability of the microgrid is caused by supercritical Neimark–Sacker bifurcations, and the dynamical system phase portraits are presented.

Reactive power sharing and voltage restoration in islanded AC microgrids

Microgrids (MG) are a new and innovative concept in modern distribution networks. Several challenges are associated with the operation and control of MG networks. Active and reactive power sharing among energy resources interfaced through power electronic conversion stages is a major challenge. Although active power sharing can be achieved under varying scenarios, sharing of reactive power between distributed generation units is difficult to achieve. This paper presents a novel and innovative control scheme to ensure sharing of reactive power between Distributed generation units within an autonomous, islanded AC microgrid. A framework composed of novel multiagent moving average estimators is proposed to make the participating nodes share active and reactive power among themselves. Detailed case studies are carried out in MATLAB and Simulink environment to verify the efficacy of the proposed control scheme. Furthermore, a lab scale MG set up is implemented to verify the operation of the control scheme in an MG environment.

A novel distributed secondary voltage control method for AC microgrids based on voltage observer

Since the islanded AC Microgrid is affected by the impedance of line, it is difficult to coordinate the voltage regulation and reactive load distribution. A distributed secondary voltage controller based on observer is proposed for isolated AC Microgrids, and it does not need more voltage communications. This method can guarantee the estimated average voltage equals to average of actual output voltages that can converge to the nominal values, and realize accurate proportional load sharing in a microgrid. Then, the convergence of the voltage observer is proved. Finally, the proposed control method is verified by time-domain simulation.

A Distributed Current Sharing Strategy for Islanded AC Microgrids Based on Low-Bandwidth Communication

Bus voltage regulation and current sharing are significant to ensure the normal operation of islanded microgrids. In this paper, a low-bandwidth communication (LBC)-based distributed current sharing strategy for islanded ac resistive microgrids is proposed, especially when the wire impedances of parallel-connected inverters are mismatched. In a general case, all the balanced, unbalanced and harmonic components of load current should be shared accurately. Therefore, in the local controller, those components of inverter output currents are extracted and regulated to follow the component average values generated based on LBC. To guarantee the bus voltage regulation, the influence of current sharing loops on voltage references is eliminated by subtracting the average voltage reference compensation from the current sharing controller outputs. A simplified inverter small-signal model is established, based on which the system stability is analyzed. Besides, the steady-state bus voltage regulation performance is discussed. The simulation in MatLab/Simulink is used to demonstrate the current sharing performance and bus voltage regulation ability. A hardware-in-loop platform of an ac microgrid prototype is built, where the power stage is in a real-time simulator and the controllers are implemented with DSPs. The results validate the effectiveness of the proposed strategy.

Decentralized Secondary Control for Frequency Restoration and Power Allocation in Islanded Ac Microgrids

Decentralized Secondary Control for Frequency Restoration and Power Allocation in Islanded Ac Microgrids

Distributed secondary control of islanded AC microgrids considering topology switching and plug-and-play operation

This paper proposes a distributed cooperative secondary voltage and frequency control scheme of a DG based islanded AC microgrid subjected to change in the topology of the microgrid communication network. The proposed control algorithm has been extended to take care of microgrid voltage and frequency profile in case of loss of communication of any DG in the network which facilitates plug-and-play operation of DGs. The centralized control schemes of an islanded microgrid pose multiple challenges in the case of maintaining microgrid voltage and frequency profile during fixed communication topology. The distributed controller gain in the proposed algorithm has been calculated using the linear quadratic regulator (LQR) principle. The adaptivity of the controller has also been introduced to consider the coupling gain dynamics among the DGs. An exhaustive Matlab simulation validates the effectiveness of the proposed distributed adaptive control design of the microgrid.

Fault-Tolerant, Distributed Control for Emerging, VSC-Based, Islanded Microgrids—An Approach Based on Simultaneous Passive Fault Detection

This paper proposes a fault-tolerant control and a robust, distributed, simultaneous, passive fault detection and isolation (or FDI) for islanded ac microgrids (MGs), including heterogeneous distributed generations and battery energy storage systems (BESSs). Disturbances and both actuator and sensor faults are considered in this paper to design a robust distributed control. The nature of the considered faults is general to deal with any sources, e.g., cyber threats (in the form of data integrity attacks) and sensor issues. In this regard, a mixed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_\infty / H_{-}$ </tex-math></inline-formula> formulation is introduced in order to detect and eliminate the undesirable effects of the aforementioned defected components simultaneously, thereby achieving robust performance. Besides, sufficient conditions based on extended linear matrix inequality (also known as LMI) are outlined. Voltage and frequency regulation, active power-sharing, and the state of charge balance of BESSs under faulty conditions are investigated. Different scenarios are tested on a detailed model of a test MG system. The effectiveness of the proposed scheme on mitigating the impacts of faults and disturbances on the distributed energy resources’ (DERs’) performance is evaluated through comprehensive and comparative simulations based on MATLAB/Simulink. Moreover, experimental studies are conducted in order to validate the results in the corresponding practical scenarios.

Control Based on Linear Matrix Inequalities for Power Converters of an Islanded AC Microgrid

Control Based on Linear Matrix Inequalities for Power Converters of an Islanded AC Microgrid

Voltage/Frequency Regulation With Optimal Load Dispatch in Microgrids Using SMC Based Distributed Cooperative Control

Majority of the contemporary hierarchical control strategies for microgrids are either centralized or distributed, relying on leader-follower (or master-slave) consensus for secondary frequency and/or voltage regulation. Thus, in either case, these strategies are susceptible to single-point-failure (SPF). This potential research gap motivated the authors to propose a distributed three-layered hierarchical control strategy applicable to droop-controlled islanded AC microgrids. The proposed strategy can simultaneously ensure (i) frequency and voltage regulation of DGs within the prescribed frequency and voltage deviation limits as per IEC 60034-1 standard (i.e., &#x00B1;2% and &#x00B1;5%, respectively) without relying on leader-follower consensus at the secondary level, (ii) distributed economic dispatch of active power with minimum error, and (iii) distributed reactive power dispatch with plausible error. The proposed technique is fully distributed and shares the computation and communication burden among the neighboring nodes using a sparse communication network, thus, it is insusceptible to SPF. The feasibility of the proposed technique is guaranteed through various time-domain based numerical simulations executed in Matlab/Simulink under different loading conditions and microgrid expansion.

Varying Negative Sequence Virtual Impedance Adaptively for Enhanced Unbalanced Power Sharing Among DGs in Islanded AC Microgrids

Inverter based distributed generators (DGs) are increasingly being employed for interfacing renewable energy sources into the grid. Droop control is a very popular technique for ensuring parallel operation of these sources based on local measurements alone, in which the inverters are made to emulate the behaviour of synchronous generators of the power system. However, several drawbacks of the droop control technique are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing among the distributed generators (DGs) due to the feeder/line impedances. The performance further deteriorates in the presence of unbalanced loads. This work presents an adaptive technique for adjusting the negative sequence (NS) virtual impedance of the DG, based on its output current, without the need of communication or extra sensors, to achieve enhanced unbalanced power sharing. The proposed control is shown to have the capability of seamless integration with the existing conventional and inverse droop techniques. The methodology is validated through simulations for different DG ratings, controls and on a modified 13 bus system. Small signal eigenvalue analysis about the operating point is used to determine the stable range of control parameters and the strategy is validated on an experimental setup in the laboratory.