Grid-tied inverters are widely used for interfacing renewable energy sources or storage devices to low-voltage electrical power distribution systems. Lately, a number of different control techniques have been proposed to address the emerging requirements of the smart power system scenario, in terms of both functionalities and performance. This paper reviews the techniques proposed for the implementation of current-controlled or voltage-controlled inverters in microgrids. By referring to a voltage source inverter with LCL output filter, the different control architectures are classified as single-, double-, and triple- loop. Then, the functionalities that are needed or recommended in the grid-connected, islanded, and autonomous operating modes of the grid-tied inverter are identified and their implementation in the different control structures is discussed. To validate the analysis and to better illustrate the merits and limitations of the most effective solutions, six control strategies are finally implemented and experimentally compared on a single-phase, grid-connected inverter setup.

Low-voltage microgrids can be seen as the basic tiles of the smart grid patchwork owing to their capability to efficiently manage the distributed energy resources (DERs) in aggregate form. They can support the grid in terms of demand response, power quality, ride through capability, and at the same time, they can ensure electrical continuity to the loads, even in case of grid failure. This paper describes a simple and effective approach to manage microgrids by synergistic control of the power electronic interfaces acting therein, i.e., the utility interface (UI), installed at the point of common coupling with the utility and the energy gateways (EGs), interfacing the DERs with the distribution grid. The proposed master-slave control uses the UI as control master for the EGs. In grid-connected operation, the UI performs as a grid-supporting unit and dispatches active and reactive power references to the EGs so as to improve energy efficiency and power quality; in islanded operation, the UI performs as a grid-forming voltage source and ensures the power balance by exploiting every power source and energy storage unit available in the microgrid. This paper discusses the theoretical background, architecture, and algorithms of the proposed master-slave control and demonstrates the resulting microgrid performance by means of simulation and experimental results.

This paper proposes an effective technique to control the power flow among different phases of a three-phase four-wire distribution power system by means of single-phase converters arbitrarily connected among the phases. The aim is to enhance the power quality at the point-of-common-coupling of a microgrid, improve voltage profile through the lines, and reduce the overall distribution losses. The technique is based on a master/slave organization where the distributed single-phase converters act as slave units driven by a centralized master controller. Active, reactive, and unbalance power terms are processed by the master controller and shared proportionally among distributed energy resources to achieve the compensation target at the point-of-common-coupling. The proposed control technique is evaluated in simulation considering the model of a real urban power distribution grid under non-sinusoidal and asymmetrical voltage conditions. The main results, concerning both steady-state and transient conditions, are finally reported and discussed.

This paper presents a simple and robust control technique for distributed energy resources (DERs) in microgrids. The technique utilizes the full potential of DERs during grid-connected and islanded operating modes. In grid-connected mode, the control pursues quasi-optimum operation of the microgrid so as to reduce the distribution losses and voltage deviations while fully exploiting renewable energy sources. In islanded mode, it effectively manages any available energy source, including storage devices, to ensure a safe and smooth autonomous operation of the microgrid. In addition, prompt adaptation to variations of the generated and absorbed power is ensured in each operating condition. The proposed control can be implemented by an Information and Communication Technology architecture, which is inherently flexible and scalable, allows plug-and-play integration of distributed energy sources, and does not involve time-critical communications.

This paper proposes a local power-based droop controller for distributed energy resource converters in dc microgrids that are connected to upstream grids by grid-interface converters. During normal operation, the grid-interface converter imposes the microgrid bus voltage and the proposed controller allows power flow regulation at distributed energy resource converters output. On the other hand, during abnormal operation of the grid-interface converter (e.g., due to faults in the upstream grid), the proposed controller allows bus voltage regulation by droop control. Notably, the controller can autonomously convert from power flow control to droop control, without any need of bus voltage variations detection schemes or communication with other microgrid components, which enables seamless transitions between these two modes of operation. Considering distributed energy resource converters employing the power-based droop control, the operation modes of a single converter and of the whole microgrid are defined and investigated herein. The controller design is also introduced. Furthermore, the power sharing performance of this control approach is analyzed and compared with that of classical droop control. The experimental results from a laboratory-scale dc microgrid prototype are reported to show the final performances of the proposed power-based droop control.

Droop-controlled distributed energy resource converters in dc microgrids usually show low output impedances. When coupled with ac systems, second-order harmonics typically appear on the dc-bus voltage, causing significant harmonic currents at the converters resource side. In this paper, we show how to reduce such undesired currents by means of notch filters and resonant regulators included in the converters control loops. The main characteristics of these techniques in terms of harmonic attenuation and stability are systematically investigated. In particular, it is shown that the voltage control-loop bandwidth is limited to be below twice the line frequency to avoid instability. Then, a modified notch filter and a modified resonant regulator are proposed, allowing to remove the constraint on the voltage loop bandwidth. The resulting methods (i.e., the notch filter, the resonant regulator, and their corresponding modified versions) are evaluated in terms of output impedance and stability. Experimental results from a dc microgrid prototype composed of three dc-dc converters and one dc-ac converter, all with a rated power of 5 kW, are reported.