Novel Distributed Secondary Control for Precise Reactive and Harmonic Power Sharing in Islanded Microgrids with Unified Grid-Forming/Following Inverters using Adaptive Instantaneous Circulating Current Compensation

This paper introduces a novel distributed secondary control (SC) strategy for unified hybrid control (UHC)-enabled inverters, seamlessly integrating grid-following (GFL) and grid-forming (GFM) functionalities to enhance stability and resilience in islanded microgrids (IMGs). UHC consolidates GFM capabilities, such as frequency and voltage regulation with essential support services, and GFL capabilities for precise active and reactive power tracking under dynamic conditions, eliminating reliance on phase-locked loops (PLLs). Mismatched feeder impedances and the presence of diverse linear and nonlinear loads hinder accurate reactive and harmonic power sharing among UHC-enabled inverter-based resources (IBRs). The proposed SC strategy addresses these challenges by ensuring proportional sharing of reactive and harmonic power through an adaptive circulating-current minimization mechanism, which dynamically compensates voltage deviations caused by fundamental and harmonic circulating currents without requiring feeder impedance data. A parallel decoupled voltage control structure enables independent and simultaneous sharing of reactive and harmonic power while preserving voltage magnitude and quality. Extensive simulations on an IMG with three UHC-based IBRs validate the robustness and effectiveness of the proposed strategy across diverse operating scenarios. The results demonstrate its practicality for modern decentralized power systems, highlighting its ability to maintain stability, achieve precise power sharing, and provide robust grid support under dynamic and heterogeneous load conditions.