Microgrid is a new concept for future energy distribution system that enables renewable energy integration. It generally consists of multiple distributed generators that are usually interfaced to the grid through power inverters. For the islanding operation of ac microgrids, two important tasks are to share the load demand among multiple parallel connected inverters proportionately, and maintain the voltage and frequency stabilities. This paper reviews and categorizes various approaches of power sharing control principles. Simultaneously, the control schemes are graphically illustrated. Moreover, various control approaches are compared in terms of their respective advantages and disadvantages. Finally, this paper presents the future trends.

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Review of Power Sharing Control Strategies for Islanding Operation of AC Microgrids

The objective of this paper is to provide a review of distributed control and management strategies for the next generation power system in the context of microgrids. This paper also identifies future research directions. The next generation power system, also referred to as the smart grid, is distinct from the existing power system due to its extensive use of integrated communication, advanced components such as power electronics, sensing, and measurement, and advanced control technologies. At the same time, the need for increased number of small distributed and renewable energy resources can exceed the capabilities of an available computational resource. Therefore, the recent literature has seen a significant research effort on dividing the control task among different units, which gives rise to the development of several distributed techniques. This paper discusses features and characteristics of these techniques, and identifies challenges and opportunities ahead. The paper also discusses the relationship between distributed control and hierarchical control.

Distributed Control Techniques in Microgrids

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

This paper performs an extensive review on control schemes and architectures applied to dc microgrids (MGs). It covers multilayer hierarchical control schemes, coordinated control strategies, plug-and-play operations, stability and active damping aspects, as well as nonlinear control algorithms. Islanding detection, protection, and MG clusters control are also briefly summarized. All the mentioned issues are discussed with the goal of providing control design guidelines for dc MGs. The future research challenges, from the authors’ point of view, are also provided in the final concluding part.

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Review on Control of DC Microgrids and Multiple Microgrid Clusters

This document is a summary of a report prepared by the IEEE PES Task Force (TF) on Microgrid Stability Definitions, Analysis, and Modeling, IEEE Power and Energy Society, Piscataway, NJ, USA, Tech. Rep. PES-TR66, Apr. 2018, which defines concepts and identifies relevant issues related to stability in microgrids. In this paper, definitions and classification of microgrid stability are presented and discussed, considering pertinent microgrid features such as voltage-frequency dependence, unbalancing, low inertia, and generation intermittency. A few examples are also presented, highlighting some of the stability classes defined in this paper. Further examples, along with discussions on microgrid components modeling and stability analysis tools can be found in the TF report.

Microgrid Stability Definitions, Analysis, and Examples

Several microgrid control strategies are proposed and studied in the literature. However, there are still gaps in improving their transient behavior and studying their stability. This paper uses small-signal analysis to explore the behavior of internal model-based current and voltage controllers by deriving a state-space model and performing eigenvalue and sensitivity analysis on an islanded inverter-based microgrid system. The results are compared with those of the same microgrid but with PI-based current and voltage controllers. Simulation case studies are performed in the PSCAD/EMTDC environment to compare the transient behavior of both methods. Results show that internal model-based controllers have superior eigenvalue patterns that lead to increased stability and improved transient behavior.

Small-Signal Stability Analysis of an Inverter-Based Microgrid With Internal Model-Based Controllers

Power system operates close to its operational limit to achieve a higher level of utilization of the grid infrastructure. As a result, its transient behavior is of great importance to prevent violation of the operation limits. This paper proposes an internal model control (IMC)-based approach for voltage-sourced converters (VSC) to improve their dynamic behavior. This IMC controller is designed based on a detailed model of the VSC. The proposed control approach has superior performance, i.e., faster step response and less overshoot, and higher axes decoupling than the conventional dq-current control method. The proposed IMC-based controller has a straightforward design procedure, and can be implemented using simple PI and PID controllers. Several case studies are presented to evaluate and compare the transient performance of the IMC-based controller with conventional dq current control subsequent to step changes in the set points of direct and quadrature axes current, single-phase fault, and three-phase fault.

Internal Model-Based Current Control of the RL Filter-Based Voltage-Sourced Converter

Microgrid control practice has been dominated by droop control for over a decade. The main reason is that droop control can ensure voltage and frequency stability, and proper sharing of the load among distributed generation units without communication links. However, droop control causes undesirable voltage and frequency deviations. Using a dynamic feedback incorporating washout filters, this letter proposes a novel approach to perform power sharing without any communication links, and without compromising voltage and frequency regulation. Simulation results in PSCAD/EMTDC environment validate the performance of the proposed approach.

Washout Filter-Based Power Sharing

Estimation of electromechanical modes of a power system has attracted attention in the past few decades because it can provide vital information about the stability of the system. In this paper, a new state-space model is proposed for online detection of parameters of a ringdown signal using an extended Kalman filter (EKF). The proposed model can estimate both constant and time-varying parameters. Furthermore, stability analysis of the proposed model is performed to demonstrate the convergence of the parameters. Simulation results confirm the desirable performance of the proposed method for ringdown parameter estimation.

Mehrdad Yazdanian

Papers

Distributed Control Techniques in Microgrids

Small-Signal Stability Analysis of an Inverter-Based Microgrid With Internal Model-Based Controllers

Internal Model-Based Current Control of the RL Filter-Based Voltage-Sourced Converter

Washout Filter-Based Power Sharing

Estimation of Electromechanical Oscillation Parameters Using an Extended Kalman Filter