Abstract-Flexibility and reliability of AC microgrids are enhanced by reducing the dependence on communications in multi-layer hierarchical control. In this sense, this paper presents a secondary control with no communications for islanded microgrids. It is based on a control scheme that switches between two configurations according to a time-dependent protocol. With this proposal, an excellent performance in terms of transient response and frequency restoration in steady-state is achieved. The paper also presents design considerations for the control parameters. In addition, the theoretical predictions are validated by experimental results from a laboratory microgrid.Index Terms-Distributed generation, hierarchical control, islanded microgrids, secondary control, switched control, time-dependent protocol. I. INTRODUCTIONICROGRIDS have recently emerged as a potential solution for electrical supply due to their advantages as environmental impacts reduction, reliability, and flexible operability [1], [2]. Basically, a microgrid is a cluster of distributed generators (DGs), loads, and energy storage systems which can operate in two possible modes: gridconnected or islanded [3].In islanded mode, microgrids are disconnected from the main grid intentionally or by an electrical fault and the DGs provide active and reactive power to the local loads [4]. To control frequency, voltage amplitude or power sharing, a multi-layer hierarchical scheme can be implemented, normallyManuscript received Month xx, 2xxx; revised Month xx, xxxx; accepted Month x, xxxx. This work has been supported by ELAC2014/ESE0034 from the European Union and its linked Spanish national project PCIN-2015-001. We also appreciate the support from the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund (FEDER) under projects ENE2015-64087-C2-1-R and TEC2013-46938-R.Juan M. Rey is with Escuela de Ingenierías Eléctrica, Electrónica y de Telecomunicaciones (E3T), Universidad Industrial de Santander (UIS), 680002 Bucaramanga, Colombia and with the Electronic Engineering Department, Technical University of Catalonia, Av. Victor Balaguer s/n, 08800 Vilanova i Ia Geltrú, Spain (e-mail: juanmrey@uis.edu.co).Pau Marti and Manel Velasco are with the Automatic Control Department, Technical University of Catalonia, Pau Gargallo 5, 08028 Barcelona, Spain.Jaume Miret and Miguel Castilla are with the Electronic Engineering Department, Technical University of Catalonia, Av. Victor Balaguer s/n, 08800 Vilanova i la Geltrú, Spain. divided in three control layers: primary, secondary and tertiary layer [5].Primary layer rapidly controls power sharing and stabilizes frequency and output voltage amplitude in each DG. However, steady-state errors or deviations are introduced, which are corrected by the secondary layer. Tertiary layer manages and coordinates the operation of the power flows at optimal points considering economic concerns (i.e., energy prices and electricity markets) and grid requirements (i.e., reactive power flows contr...

This paper presents a Continuous Control Set Model Predictive Control with a receding horizon for a three-phase voltage source inverter with an LCL filter. In this proposal, a reduced model of the converter with an embedded integrator and a Kalman filter are used to obtain the inverter-side currents without oscillation. Then, a cost function makes use of these currents to generate the optimum duty cycles for the space vector modulator. With the proposed method, active damping performances, a zero state error and a reduction of the computational burden are achieved. Compared to the Finite Control Set Model Predictive Control, the proposed method operates with a fixed switching frequency. Simulations and experimental results show that this proposal works correctly even in the case of grid harmonics and voltage sags.

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Hierarchical schemes are widely used for the design of inverter-based AC microgrids control strategies. To ensure a reliable operation, hierarchical control must consider together all the functionalities that allow the regulation of key variables and guarantee a safe transition between operation modes. Conventionally, in the literature are proposed three-layer schemes which present relevant drawbacks: they include limited functionalities and they use droop method for the primary layer which, despite its decentralized nature, suffers from issues that have motivated the development of alternative strategies. Considering this, the contribution of this paper is twofold. First, a droop-free hierarchical control strategy that satisfy a proper operation of AC microgrids is proposed. Control objectives such as power-sharing, frequency regulation, optimal power dispatch, and voltage regulation are considered. Second, a closed-loop small-signal model, which facilitates the control parameters design and fills a gap in the literature is presented. Differences between the proposal and previous controls are discussed. Selected tests are carried out in a laboratory microgrid under different conditions, including normal operation and the response to failures in the central controller and to communication impairments. The experimental results show a good performance of the proposal even in adverse conditions.

In islanded microgrids, voltage source inverters working in parallel are expected to provide regulation of the local frequency while granting active power sharing. This paper presents a local control approach at each inverter based on an event-driven operation of a parameter-varying filter. It ensures perfect active power sharing and controllable accuracy for frequency restoration without requiring the exchange of control data between inverters over the communication network. The paper includes stability analysis and design guidelines for the control parameters using a modeling approach that considers the interaction between inverters. Selected experimental results on a three-inverter laboratory microgrid corroborate the effectiveness of the proposed control scheme, and outlines its advantages with respect to previous similar schemes and the performance cost that implies not using communications.

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