Hybrid compatible grid forming inverters with coordinated regulation for low inertia and mixed generation grids

Abstract

The rapid displacement of synchronous generators (SGs) by renewable energy sources has resulted in low-inertia power systems that are increasingly vulnerable to frequency instability, poor power-sharing coordination, and limited fault recovery. In this context, this paper proposes a comprehensive control and system-level realization of Hybrid-Compatible Grid-Forming Inverters (HC-GFIs)- a novel inverter framework designed to emulate synchronous generator behavior while enhancing interoperability in mixed-generation systems. The control architecture of the HC-GFIs is designed as a multi-layered cascaded structure incorporating active power-frequency droop control, voltage regulation loops, a current-limiting regulator, and a dynamic current control layer. Additionally, two novel contributions- a saturation-based DC current controller and an AC current regulator- are introduced to overcome known limitations of overcurrent vulnerability and fault ride-through challenges in conventional GFIs. Extensive time-domain simulations were conducted in both the IEEE 9-bus and 39-bus systems to evaluate scalability and dynamic performance. In the 9-bus system, subjected to a 33.33% step load disturbance, HC-GFIs reduced frequency nadir deviations by up to 0.43 Hz and improved settling time by over 90% compared to all-SG systems. Voltage deviation was maintained within 0.02 p.u. with oscillations damped within 5 s, contrasting sharply with the prolonged instability in SG-only networks. In the 39-bus system, under a severe three-phase-to-ground bolted fault, the HC-GFIs maintained voltage regulation near faulted buses and mitigated high RoCoF transients. Furthermore, the proposed HC-GFIs demonstrate compliance with IEEE Std. 2800 − 2022 RoCoF thresholds and outperform SGs in power-sharing, transient damping, and voltage ride-through performance. This study establishes HC-GFIs as a technically robust, scalable, and standards-compliant solution for stabilizing low-inertia grids, offering a critical pathway for enabling the reliable integration of renewable energy resources into future power systems.