Enhancing Converter-Driven Stability of Solar-PV Grid-Supporting Inverters Under Weak Grid Conditions

Solar photovoltaic (PV)-driven grid-following inverters (GFLI) exhibit stable operation in a strong grid environment but are prone to limited fault ride-through capability, increased oscillations under disturbances, and even instability under weak grid conditions. In the conventional GFLI control strategy, one contributing factor to these limitations is the low stability margin of the PI-based active/reactive power (PQ) controller, particularly under weak-grid conditions. This study addresses these issues by developing an integral terminal sliding mode control (ITSMC)-based PQ controller for the secondary control layer of the grid-supporting GFLI. Unlike traditional linear controllers, the proposed controller offers enhanced robustness under weak grids, finite-time convergence, and inherent post-fault oscillation damping (PFOD) capabilities without additional damping strategies. Furthermore, a simplified small-signal model that incorporates both the primary and secondary control layers, together with grid impedance, is developed to analyze inverter stability under varying grid strengths for both conventional and proposed methods. To enable grid supporting functions, an online short circuit ratio (SCR)-based adaptive method is introduced, ensuring robust performance under extremely weak grids without compromising grid code standards. The proposed method is validated through a hardware-in-the-loop (HIL) experiment and compared with conventional and existing power oscillation damping (POD) methods. Subsequently, a case study is conducted with a multi-inverter IEEE 33- bus distribution network that includes real-world contingency events, demonstrating enhanced stability and compliance with grid-code requirements.