There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

DC and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.

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Hierarchical control of droop-controlled DC and AC microgrids — a general approach towards standardization

Energy management means to optimize one of the most complex and important technical creations that we know: the energy system. While there is plenty of experience in optimizing energy generation and distribution, it is the demand side that receives increasing attention by research and industry. Demand Side Management (DSM) is a portfolio of measures to improve the energy system at the side of consumption. It ranges from improving energy efficiency by using better materials, over smart energy tariffs with incentives for certain consumption patterns, up to sophisticated real-time control of distributed energy resources. This paper gives an overview and a taxonomy for DSM, analyzes the various types of DSM, and gives an outlook on the latest demonstration projects in this domain.

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Demand Side Management: Demand Response, Intelligent Energy Systems, and Smart Loads

The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.

Control of Power Converters in AC Microgrids

The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

This paper proposes the utilization of a bidirectional DC-DC converter with sliding mode control in a wind distributed system employing the switched reluctance generator (SRG). The SRG wind system is connected to a three-phase AC utility grid by means of a voltage source converter. The presented proposal employes the DC-DC converter to regulate the optimum excitation voltage at the SRG side, while the VSC is responsible to maintain the DC bus voltage in a fixed value to allow the DC-AC conversion with the required voltage level and frequency. To face the problem of wide voltage variations, sliding mode control is applied to ensure the desired PWM duty cycle to the bidirectional DC-DC converter. SRG control is implemented through direct power control by current hysteresis, for low speeds, and by voltage single pulse control, for high speeds of operation. A careful examination about the behavior of the system considering the whole SRG operating speed range is carried out. Simulation results show that the proposal system is applicable and the applied sliding mode controller has rapid an robust behavior, as desired.

Wind Distributed System Based on Switched Reluctance Generator Using a Bidirectional DC-DC Converter with Sliding Mode Control

A VICINITY platform-based load scheduling method by considering the smart parking and household smart appliances for renewable energy resources integrated residential microgrids is proposed in this paper. The proposed method can shift the local loads and publish the vacant number of parking slot with a real-time electric vehicle charging price to VICINITY platform according to the renewable energy generation and the usage of the parking spaces to reduce energy cost and increase resident's revenue. The experimental platform-based real-life cross domain lab testing verifies the effectiveness of the proposed control approach.

VICINITY Platform-based Load Scheduling Method by Considering Smart Parking and Smart Appliance

The applicability of a two-degree of freedom (DOF) internal model-based voltage control (IMVC) scheme for DC Microgrids (DCMGs) with unknown external disturbances is explored in this paper. One of the most challenging aspects of voltage control in DC/DC converters is maintaining voltage reference tracking under unknown external disturbances and measurement noise while the load changes simultaneously. This paper develops a voltage control framework to address these challenges by a plug-and-play internal model voltage controller for voltage-source converters (VSCs) at the primary level. The effectiveness of the proposed control scheme is evaluated by a MATLAB/Simulink testbed, considering unknown external disturbances under various rapid voltage reference changes and load profile changes in multiple case study scenarios.

Internal Model-based Voltage Control for DC Microgrids Under Unknown External Disturbances

This paper discusses the control design for an islanded microgrid in order to ensure acceptable performance in terms of voltage quality and load sharing by focusing on transient conditions. To this aim, state feedback decoupling approach has been applied. Experimental tests have been performed to demonstrate the effectiveness of the proposed approach.

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Control design of VSIs to enhance transient performance in microgrids

In this paper, a control strategy for the parallel operation of three-phase inverters forming an online uninterruptible power system (UPS) is presented. The UPS system consists of a cluster of paralleled inverters with LC filters directly connected to an AC critical bus and an AC/DC forming a DC bus. The proposed control scheme is performed on two layers: (i) a local layer that contains a “reactive power vs phase” in order to synchronize the phase angle of each inverter and a virtual resistance loop that guarantees equal power sharing among inverters; (ii) a central controller that guarantees synchronization with an external real/fictitious utility, and critical bus voltage restoration. Constant transient and steady-state frequency, active, reactive and harmonic power sharing, and global phase-locked loop resynchronization capability are achieved. Detailed system topology and control architecture are presented in this paper. Also a mathematical model was derived in order to analyze critical parameters effect on system stability. An experimental setup was built in order to validate the proposed control approach under several case-study scenarios. Finally, a conclusion is presented.

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Juan C. Vasquez

Papers

Wind Distributed System Based on Switched Reluctance Generator Using a Bidirectional DC-DC Converter with Sliding Mode Control

VICINITY Platform-based Load Scheduling Method by Considering Smart Parking and Smart Appliance

Internal Model-based Voltage Control for DC Microgrids Under Unknown External Disturbances

Control design of VSIs to enhance transient performance in microgrids

High-performance control of paralleled three-phase inverters for residential microgrid architectures based on online uninterruptable power systems