Seamless Integration of Grid-Forming and Grid-Following Capabilities in a Single VSI Within a Unified Hybrid Control Framework Under Islanded Conditions

Abstract

To achieve seamless integration of grid-forming (GFM) and grid-following (GFL) capabilities in a single voltage-source inverter (VSI) under islanded conditions, this study introduces a unified hybrid control (UHC) scheme with minimal control parameters and measurement sensors. The UHC scheme enables a single voltage-controlled VSI to emulate the behavior of two virtual parallel VSIs, each operating independently as a GFM and GFL inverter, thereby ensuring voltage and frequency stability, as well as reliable power sharing. The proposed UHC approach is designed specifically for islanded microgrids, providing functionalities for voltage-controlled inverters to perform multiple operational tasks simultaneously. These include voltage and frequency regulation, provision of local grid support services, and tracking of reference powers using output current control. The implementation utilizes a dual-loop parallel control strategy based on a proportional-resonant (PR) controller in the αβ reference frame, which operates without requiring a phase-locked loop (PLL) in the islanded operational context. The control structure requires tuning of only three control parameters (two PR controller parameters and one active damping term), while the other parameters represent physical system values. Under the tested islanded operating conditions, VSIs equipped with the UHC scheme demonstrate the ability to mimic virtual inertia, contributing to voltage and frequency stability during sudden load changes, reference power variations, line outages, and short-circuit fault scenarios with appropriate dynamic and transient responses, addressing the specific challenges of islanded microgrid stability and control. Comprehensive simulation studies on single- and multi-inverter systems operating in the islanded mode validate the effectiveness and feasibility of the UHC scheme for practical implementation in power systems.