System-level frequency stability enhancement in solar PV-dominated low-inertia power systems using robust nonlinear control

The rapid integration of inverter-based resources (IBRs) is changing the dynamic behaviour of modern power systems. As synchronous generators are replaced by photovoltaic (PV) inverters, system inertia decreases and frequency stability becomes more difficult to maintain. This paper compares proportional-integral (PI) control and sliding mode control (SMC) for PV inverter regulation in an IEEE 9-bus test system. Synchronous generators are progressively replaced by PV inverters to represent low and high IBR penetration levels. In both cases, active power sharing is provided through frequency-based droop control. Four MATLAB/Simulink scenarios are studied under 10% and 15% load disturbances. The results show that PI control provides smoother and better-damped responses under low IBR penetration. However, its performance deteriorates when penetration level and disturbance magnitude increase. In contrast, SMC improves the frequency nadir and shortens the recovery time under high-penetration and large-disturbance conditions. These findings show that PI and SMC have complementary operating characteristics. PI is more suitable for moderate operating conditions, while SMC provides stronger robustness in low-inertia scenarios. The results provide practical guidance for controller selection in renewable-dominated power systems.