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

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.