Droop‐free hierarchical control strategy for inverter‐based AC microgrids

Abstract

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.

Distributed Control Strategies for Microgrids: An Overview

“…As a result of the advances in secondary control, a new fully distributed approach called droop-free has been proposed [108], [199], [200] for AC-MGs, which rely only on a sparse communication network between neighbouring DER units and can replace a conventional droop scheme.…”

Distributed Control for a Cost-based Droop-free Microgrid

“…i (e ω i (t)) where e P i (t) and e ω i (t) are tracking error terms for power-sharing and frequency restoration, respectively [12], [19]. For simplicity, the droop-free strategy reported in [14], [20] is adopted, where m i = 0 causing no steady-state frequency deviations so no frequency restoration loop is required, i.e. δω i (t) = δω 1…”

“…i (e P i (t)). Differently from [14], [20], the power variable is changed for an IC variable, so the converters' phase angle dynamics is the following:…”

A Linear Quadratic Regulator With Optimal Reference Tracking for Three-Phase Inverter-Based Islanded Microgrids

“…These methods are aimed to improve closed-loop stability, performance, and robustness [22], [23]. Some important contributions related to optimal control methods for VSI power sharing control have been developed [24]- [29]. Most of these approaches rely on the use of an LPF inside the control loop to regulate active and reactive power.…”

“…Also, the fuzzy-based controllers do not allow to perform stability or robustness analysis to the closed-loop system. In [29], a droop-free controller using a nonlinear optimization method is used to achieve optimal power dispatch, adequate power sharing, and minimization of the voltage in the AC bus. In addition, the authors propose a novel model for optimizing their control objectives.…”