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This paper proposes a current-controlled voltage-mode control scheme for dispatchable electronically coupled distributed energy resource (DER) units based on sliding-mode control (SMC) strategy. The proposed control strategy provides fast and stable control on the terminal voltage and frequency of DER unit and ensures protection of the power-electronic interface to external faults. In addition it maintains the quality of the output voltage of the host DER unit in spite of unbalanced and/or distorted load currents. Performance of the proposed SMC strategy is demonstrated through time-domain simulation of a single islanded DER unit, starting from black-start and subjected to various operating scenarios, and it is compared to the performance of a proportional-integral (PI) based control strategy. Also a current-mode control strategy is proposed for DER units based on SMC. The performance of both voltage- and current-mode control strategies are demonstrated in a sample master-slave organized three-unit microgrid. [ABSTRACT FROM AUTHOR]

The small-signal stability of an interconnected source and load can be analyzed using the frequency-response methods of stability analysis. Existing frequency-response methods of stability analysis introduce artificial conservativeness and do not explicitly address stability robustness against load perturbations, that is, by how much the load can change such that the system will remain stable in the small-signal sense. This paper presents a frequency-response method of stability analysis which provides a less conservative stability condition compared to existing methods, in the sense of having less restriction on the control system; further, the proposed method establishes a robust stability margin in terms of perturbations in load parameters. The proposed method is based on μ analysis, and models the source-load dynamic interaction via a closed-loop system of impedances and admittances. The proposed method is used to examine small-signal stability of an islanded subnetwork extracted from a university campus microgrid, composed of a distributed energy resource (DER) unit feeding a load, providing a study with realistic parameter values. Two case studies are presented to show the features of the proposed method, namely, reduced conservativeness and the establishment of a robust stability margin. Another example is provided to show the application of the proposed stability analysis method in a multi-DER-unit network. The findings of the frequency-domain analysis are illustrated through time-domain simulations around the operating point on the system under study. [ABSTRACT FROM AUTHOR]

With the increasing attention to the concept of microgrids, it has become important to identify the requirements that enable an electric distribution network to operate as a microgrid. This paper studies a realistic distribution network to accomplish the following: 1) determine the conditions under which the network can operate as a microgrid and 2) present the control scenarios that allow the islanded operation of the network. In particular, the employment of electronically coupled distributed energy resources with the fault-ride-through and synchronization capabilities is investigated. Another objective of this paper is to propose practical protection strategies that are effective in both operational modes of a microgrid (i.e., the grid-connected mode and the islanded mode), considering the system constraints; the protection strategies provide the microgrid with acceptable reliability against different fault scenarios. The effectiveness of the control and protection strategies is demonstrated and discussed through a comprehensive set of simulation studies conducted in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

This paper proposes a communication-assisted protection strategy implementable by commercially available microprocessor-based relays for the protection of medium-voltage microgrids. Even though the developed protection strategy benefits from communications, it offers a backup protection strategy to manage communication network failures. The paper also introduces the structure of a relay that enables the proposed protection strategy. Comprehensive simulation studies are carried out to verify the effectiveness of the proposed protection strategy under different fault scenarios, in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

This paper proposes an adaptive feedforward compensation that alters the dynamic coupling between a distributed-resource unit and the host microgrid, so that the robustness of the system stability to droop coefficients and network dynamic uncertainties is enhanced. The proposed feedforward strategy preserves the steady-state effect that the conventional droop mechanism exhibits and, therefore, does not compromise the steady-state power sharing regime of the microgrid or the voltage/frequency regulation. The feedforward compensation is adaptive as it is modified periodically according to the system steady-state operating point which, in turn, is estimated through an online recursive least-square estimation technique. This paper presents a discrete-time mathematical model and analytical framework for the proposed feedforward compensation. The effectiveness of the proposed control is demonstrated through time-domain simulation studies, in the PSCAD/EMTDC software environment, conducted on a detailed switched model of a sample two-unit microgrid. [ABSTRACT FROM AUTHOR]

One of the major challenges associated with microgrid protection is to devise an appropriate protection strategy that is effective in the grid-connected as well as islanded mode of operation. This paper proposes a protection strategy based on microprocessor-based relays for low-voltage microgrids. Further, the structure of a new relay enabling the proposed protection strategy is presented. One of the salient feature of the developed protection scheme is that it does not require communications or adaptive protective devices. Moreover, it is to a large extent independent of the fault current magnitude and the mode of operation. Transient time-domain simulation studies are conducted to demonstrate the effectiveness of the proposed protection strategy and its enabling relay, using the PSCAD/EMTDC software package. [ABSTRACT FROM AUTHOR]

This paper proposes a voltage- and frequency-control strategy for the islanded operation of dispatchable electronically coupled distributed-resource units, based on a discrete-time mathematical model which is also valid for variable-frequency operation. The proposed control strategy utilizes a combination of deadbeat and repetitive control to enhance the performance of the control system under unbalanced and/or distorted load currents. Moreover, a new approach is proposed to maintain the effectiveness of the repetitive control under variable-frequency operational scenarios. Furthermore, the control strategy employs feedforward compensation techniques to mitigate the impact of load dynamics on the regulation process. The performance of the proposed control strategy is demonstrated for single- and multiunit islanded networks, through digital time-domain simulation studies in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

This paper proposes a multimode control strategy for a doubly fed induction generator-based wind-power unit, which enables operation in the islanded (autonomous) mode as well as the grid-connected mode. The configuration can be an option for electricity generation in remote communities with favorable wind conditions, but limited or unreliable connection to the grid. In the grid-connected mode, the proposed control enables the wind power unit to operate with or without battery energy storage, as desired. In case that the wind-power unit is augmented with battery energy storage, in the islanded mode, the proposed control strategy enables voltage and frequency regulation for the network, and parallel operation with constant-speed wind-power units, induction motor loads, and passive loads. The proposed control strategy employs a unified controller for all of the foregoing modes of operation and, therefore, relieves the need for switching between different controllers or reconfiguration of the hardware. The effectiveness and robustness of the proposed control strategy are demonstrated for faulted as well as normal operating conditions through simulation studies carried out on a detailed switched model of the system in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

• A power regulation scheme is proposed for grid-connected DER units based on ILC. • The proposed control compensates the harmonic distortion of the grid current • The paper presents the mathematical model , as well as the control design methodology. • A MIMO ILC with perfect power tracking and harmonic rejection performance is proposed. This paper proposes a power regulation scheme for dispatchable grid-connected distributed energy resource (DER) units based on the iterative learning control (ILC) strategy. The proposed control provides fast and stable control of the real and reactive powers that the DER unit delivers to the grid. In addition, it compensates the harmonic distortion of the grid current caused by the local nonlinear loads and delivers a high quality power to the grid. To this end, the paper presents the mathematical model on which the proposed control is based, as well as the control design methodology. A new MIMO iterative learning control with perfect power tracking and harmonic rejection performance as well as minor measurement requirements is proposed. The performance of the proposed ILC strategy is demonstrated through time-domain simulation of a grid-connected DER unit under various operating scenarios, and it is compared with the performance of the conventional control based on the instantaneous power theory (IPT). [ABSTRACT FROM AUTHOR]

Highlights: [•] A communication-aided strategy has been proposed to protect looped microgrids in both modes of operation. [•] The proposed protection strategy does not require protection mode switching and is independent of fault current levels; and type, size, and location of distributed generators. [•] The proposed strategy offers a backup protection in case of communication system failures. [•] The proposed protection strategy can be implemented by commercially available relays. [•] The paper outlines the structure and key modules of the required digital relay. [ABSTRACT FROM AUTHOR]