Second harmonic voltage injection-based self impedance estimation for effective decoupled droop control in a microgrid

Abstract

In an islanded microgrid (IMG), droop control effectively shares real and reactive power demands among distributed generators (DGs), thereby regulating the frequency and voltage in high X/R ratio networks. However, there is a strong coupling between the real power voltage and reactive power frequency in medium- and low-voltage microgrids due to the low X/R ratio. For effective droop control, the coupling between the real power voltage and reactive power frequency should be eliminated through decoupling factors. Existing methods in the literature on decoupled droop control do not consider the effect of changing network conditions. This study proposes a decoupling method for improved power sharing among parallel-operated inverter-based DGs. Each DG injects a second-harmonic voltage, based on which a second-harmonic self-impedance is calculated at the DG terminal. With certain assumptions, the fundamental self-impedance is computed and used to determine the decoupling factors. The proposed method was implemented on two test cases and compared with existing droop methods. In comparison, the power-sharing ratio is close to the actual value of the proposed method. The simulation results clearly demonstrate that the proposed method improves power-sharing accuracy despite varying the load and network topology.