Passivity-based Voltage and Frequency Stabilization in AC microgrids

In this paper, we investigate the stability of an islanded AC microgrid (ImG) composed of Distributed generation units (DGUs), dynamic power lines, and loads. Each DGU is equipped with a local controller to ensure desired voltage and frequency in the ImG. Different from the previous works [1], we account for the dynamics of RL power lines and consider generic interconnections of loads and DGUs. The stability of the ImG hinges on the passivity of all components with specific input-output pairs and the properties of the electrical interconnections. For the DGUs, we provide necessary and sufficient conditions for the existence of control gains guaranteeing passivity. In addition, we show that controllers decoupling the direct-and quadrature axis components of the electric variables can never render the DGU passive. Theoretical results are backed by simulation studies.

I. Introduction

Islanded microgrids are spatially-distributed electric systems composed of multiple small subsystems, for example, flexible loads, distributed generation units (DGUs), and storage units, interconnected to each other through an electrical network. A central theme in islanded AC microgrids (ImGs) is to ensure overall voltage and frequency stability through decentralized control of each DGU [2]. Droop-based voltage stabilization is a commonly used decentralized approach but is plagued by load-dependent voltage deviation, propagation of voltage error along resistive transmission lines, and presence of steady state voltage drifts [2]–[6]. Droop-free controllers have also been proposed. However, the voltage and frequency stability is often shown using simplifying assumptions on ImG topology, DGU models, and line dynamics [7], [1], [8].