Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control Finally, the future research trends on islanded microgrids are also discussed in this paper

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Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

The increasing integration of the distributed renewable energy sources highlights the requirement to design various control strategies for microgrids (MGs) and microgrid clusters (MGCs). The multiagent system (MAS)-based distributed coordinated control strategies show the benefits to balance the power and energy, stabilize voltage and frequency, achieve economic and coordinated operation among the MGs and MGCs. However, the complex and diverse combinations of distributed generations (DGs) in MAS increase the complexity of system control and operation. In order to design the optimized configuration and control strategy using MAS, the topology models and mathematic models such as the graph topology model, noncooperative game model, the genetic algorithm, and particle swarm optimization algorithm are summarized. The merits and drawbacks of these control methods are compared. Moreover, since the consensus is a vital problem in the complex dynamical systems, the distributed MAS-based consensus protocols are systematically reviewed. On the other hand, the communication delay issue, which is inevitable no matter in the low- or high-bandwidth communication networks, is crucial to maintain the stability of the MGs and MGCs with fixed and random delays. Various control strategies to compensate the effect of communication delays have been reviewed, such as the neural network-based predictive control, the weighted average predictive control, the gain scheduling scheme, and synchronization schemes based on the multitimer model for the case of fixed communication delay, and the generalized predictive control, networked predictive control, model predictive control, Smith predictor, $H_{\infty}$ -based control, sliding mode control for the random communication delay scenarios. Furthermore, various control methods have been summarized to describe switching topologies in MAS with different objectives, such as the plug-in or plug-out of DGs in an MG, and the plug-in or plug-out of MGs in an MGC, and multiagent-based energy coordination and the economic dispatch of the MGC. Finally, the future research directions of the multiagent-based distributed coordinated control and optimization in MGs and MGCs are also presented.

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MAS-Based Distributed Coordinated Control and Optimization in Microgrid and Microgrid Clusters: A Comprehensive Overview

Communication infrastructure (CI) in microgrids (MGs) allows for the application of different control architectures for the secondary control (SC) layer. The use of new SC architectures involving CI is motivated by the need to increase MG resilience and handle the intermittent nature of distributed generation units. The structure of SC is classified into three main categories, including centralized SC (CSC) with a CI, distributed SC (DISC) generally with a low-data-rate CI, and decentralized SC (DESC) with communication-free infrastructure. To meet the MGs’ operational constraints and optimize performance, control and communication must be utilized simultaneously in different control layers. In this survey, we review and classify all types of SC policies from CI-based methods to communication-free policies, including CSC, averaging-based DISC, consensus-based DISC methods, containment pinning consensus, event-triggered DISC, washout-filter-based DESC, and state-estimation-based DESC. Each structure is scrutinized from the viewpoint of the relevant literature. Challenges such as clock drifts, cyber-security threats, and the advantage of event-triggered approaches are presented. Fully decentralized approaches based on state-estimation and observation methods are also addressed. Although these approaches eliminate the need of any CI for the voltage and frequency restoration, during black start process or other functionalities related to the tertiary layer, a CI is required. Power hardware-in-the-loop experimental tests are carried out to compare the merits and applicability of different SC structures.

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On the Secondary Control Architectures of AC Microgrids: An Overview

This paper presents a distributed, robust, finite-time secondary control for both voltage and frequency restoration of an islanded microgrid with droop-controlled inverter-based distributed generators (DGs). The distributed cooperative secondary control is fully distributed (i.e., uses only the information of neighboring DGs that can communicate with one another through a sparse communication network). In contrast to existing distributed methods that require a detailed model of the system (such as line impedances, loads, other DG units parameters, and even the microgrid configuration, which are practically unknown), the proposed protocols are synthesized by considering the unmodeled dynamics, unknown disturbances, and uncertainties in their models. The other novel idea in this paper is that the consensus-based distributed controllers restore the islanded microgrid's voltage magnitudes and frequency to their reference values for all DGs within finite time, irrespective of parametric uncertainties, unmodeled dynamics, and disturbances, while providing accurate real-power sharing. Moreover, the proposed method considers the coupling between the frequency and voltage of the islanded microgrid. Unlike conventional distributed controllers, the proposed approach quickly reaches consensus and exhibits a more accurate robust performance. Finally, we verify the proposed control strategy's performance using the MATLAB/SimPowerSystems toolbox.

Distributed Robust Finite-Time Secondary Voltage and Frequency Control of Islanded Microgrids

The development of microgrids is an advantageous option for integrating rapidly growing renewable energies. However, the stochastic nature of renewable energies and variable power demand have created many challenges like unstable voltage/frequency and complicated power management and interaction with the utility grid. Recently, predictive control with its fast transient response and flexibility to accommodate different constraints has presented huge potentials in microgrid applications. This paper provides a comprehensive review of model predictive control (MPC) in individual and interconnected microgrids, including both converter-level and grid-level control strategies applied to three layers of the hierarchical control architecture. This survey shows that MPC is at the beginning of the application in microgrids and that it emerges as a competitive alternative to conventional methods in voltage regulation, frequency control, power flow management and economic operation optimization. Also, some of the most important trends in MPC development have been highlighted and discussed as future perspectives.

Model predictive control of microgrids – An overview

One of the well-known methods to share active and reactive power in microgrids (MGs) is droop control. A disadvantage of this method is that in steady state the frequency of the MG deviates from the nominal value and has to be restored using a secondary control system (SCS). The signal obtained at the output of the SCS is transmitted using a communication channel to the generation sources in the MG, correcting the frequency. However, communication channels are prone to time delays, which should be considered in the design of the SCS; otherwise, the operation of the MG could be compromised. In this paper, two new SCSs control schemes are discussed to deal with this issue: 1) a model predictive controller (MPC); and 2) a Smith predictor-based controller. The performance of both control methodologies are compared with that obtained using a conventional proportional integral-based SCS using simulation work. Stability analysis based on small signal models and participation factors is also realized. It is concluded that in terms of robustness, the MPC has better performance.

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Secondary Control Strategies for Frequency Restoration in Islanded Microgrids With Consideration of Communication Delays

This paper studies strategies to minimize the electromechanical interaction (EMI) within aircraft power systems. With the growth of electrical power on-board aircraft, the interaction between the electrical systems and the engine core will become significant. The behaviour of electrical loads (on/off, transient etc.) will have significant impacts on the engine shaft, such as producing transient vibrations, creating stability problems and reducing the efficiency etc. To avoid these problems, an advanced electrical power management system (PMS) is required. This paper introduces novel loading methods for PMS applications to minimize the interactions between electrical and mechanical systems. The strategies, referred as Single Level Multi-edge Switching Loads (SLME), Multilevel Loading (MLL), and Multi-load Single Level Multi-edge Switching Loads (MSLME) are developed based on the Posicast method. An insight look of the developed technique has been studied using the zero-pole root locus. It is demonstrated that the excited poles in the system are cancelled by the addition of zeros, and thus supressed the EMI vibrations.

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Minimization of electro-mechanical interaction with posicast strategies for more-electric aircraft applications

Rotational systems such as aircraft engine drivetrains are subject to vibrations that can damage shafts. Torsional vibrations in drivetrains can be excited by the connection of loads to the generator due to electromechanical interaction. This problem is particularly relevant in new aircraft because the drivetrain is flexible and the electrical power system (EPS) load is high. To extend the lifespan of the aircraft engine, the electromechanical interaction must be considered. Since real-time constants of the electrical and mechanical systems have very different magnitudes, the simulation time can be high. Furthermore, highly detailed models of the electrical system have unnecessary complexity for the study of electromechanical interactions. For these reasons, modelling using reduced order systems is fundamental. Past studies of electromechanical interaction in aircraft engines developed models that allow the analysis of the torsional vibration, but these are difficult to implement. In this study, a reduced order electromechanical interaction system for aircraft applications is proposed and validated using experimental results. The proposed system uses a reduced drivetrain, simplified EPS, and sensorless measurement of the vibrations. The excitation of torsional vibrations obtained is compared with past studies to prove that the reduced order system is valid for studying the electromechanical interactions.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-epa.2019.0122

Modelling of reduced electromechanical interaction system for aircraft applications

Active Magnetic Bearing (AMB) technology is becoming attractive for several reasons such as friction-free suspension and high-speed operation, high reliability, and vibrations exemption. These desirable features come at the cost of an increased complexity of the system, which includes position sensors, a power electronic converter and a control system dedicated to the AMBs. This paper focus on the control system design of an AMB featuring a Wheatstone bridge winding configuration. To achieve a high-bandwidth current control able to generate the desired forces, a Finite Control Set Model Predictive Control (FCS-MPC) has been proposed in this paper. The proposed FCS-MPC includes an estimation of the AMB coils inductance, which can be used to improve the control system performance and, at the same time, provide a rotor position estimation without the need of additional sensing coils and possibly without the use of conventional position sensors. The AMB is modelled considering finite element simulation results in order to evaluate the relation between coil inductance variation and rotor position. A standard PI position control is also included in the system. Finally, the control system is validated through simulation.

Predictive Control For An Active Magnetic Bearing System With Sensorless Position Control

Aircraft drivetrains connect the engine to the electrical power system. In most cases, the drivetrains are relatively flexible and have vibration modes with values below 100 Hz to reduce weight and size. Therefore, electrical loads’ connection and disconnection may excite torsional vibrations in the machine's shaft, reducing the drivetrains’ lifespan. This interaction is known as electromechanical interaction. This issue can be mitigated using an input-shaping strategy, which reduces the excitation of torsional vibrations by connecting the electrical loads following a pattern, dependent on the drivetrain's natural frequencies. However, since this method is based on the knowledge of the vibration modes attributes, it can be susceptible to parameter's uncertainty. In this article, a pulsating input shaping method's robustness is assessed, analyzing simulation and experimental results. The effect of the inductances is analyzed, and a strategy to reduce its effect is proposed. Furthermore, the effect of uncertainty in the mechanical parameters is evaluated, and theoretical analysis is carried out to establish safe operating limits. The theoretical analysis is experimentally validated.

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Constanza Ahumada

Papers

Secondary Control Strategies for Frequency Restoration in Islanded Microgrids With Consideration of Communication Delays

Minimization of electro-mechanical interaction with posicast strategies for more-electric aircraft applications

Modelling of reduced electromechanical interaction system for aircraft applications

Predictive Control For An Active Magnetic Bearing System With Sensorless Position Control

Evaluation of Input-Shaping Control Robustness for the Reduction of Torsional Vibrations