Stability and power sharing in microgrids

Abstract

Motivated by environmental, economic and technological aspects, the penetration of renewable energy sources into the electrical networks is increasing worldwide. This fact requires a paradigmatic change in power system operation. One solution to facilitate this change are microgrids. In the present work, the problems of frequency stability, voltage stability and power sharing in microgrids are considered. More precisely, control concepts that address the aforementioned problems are investigated. The main contributions of the present work comprise: (i) A generic modular model of an uncontrolled microgrid is derived. (ii) A consensus-based distributed voltage control (DVC) is proposed, which guarantees a desired reactive power distribution in steady-state. In contrast with other control strategies available thus far, the control presented in this work only requires distributed communication among generation units, i.e., no central computing nor communication unit is needed. (iii) Conditions for local asymptotic stability of several microgrid configurations are derived. The considered networks comprise inverter-based microgrids operated with frequency and voltage droop control, as well as microgrids operated with frequency droop control and the proposed DVC. The conditions are established via converse Lyapunov theorems in combination with tools from linear algebra, as well as port-Hamiltonian systems. Most conditions are derived under the assumption of dominantly inductive power lines. (iv) Conditions are given under which the frequency droop control, respectively the proposed DVC, solve the problem of active, respectively reactive, power sharing in microgrids with dominantly inductive power lines. The claims are established by combining the aforementioned stability results with a design criterion for the controller gains and setpoints of the frequency droop control, respectively with the inherent properties of the DVC. (v) The analysis is validated via simulation on a microgrid based on the CIGRE benchmark medium voltage distribution network.