Frequency stability is vital to power system operation

especially in interconnected power systems (IPS) and smart grids where renewable energy sources and load fluctuations introduce unpredictability. This variability and time delay from decentralised control configurations can impair load frequency control (LFC) and compromise system stability. This study proposes a robust adaptive integral terminal sliding mode controller (RAITSMC) for LFC to address these challenges. The controller mitigates destabilising effects from time delays

parametric uncertainties and nonlinear disturbances. A delay-dependent sliding surface is developed to enhance the system's response to tie-line power and frequency deviations. Perturbations are estimated using an adaptation law

and a decentralised robust control law ensures the system's trajectory remains on the sliding surface with minimal control efforts. The controller's stability is validated via the Lyapunov theorem

and its parameters are optimised using the arithmetic optimisation algorithm. Simulations on the IEEE 10-generator New England 39-bus power system demonstrate significant improvements

including reduced frequency overshoot (48.3%)

undershoot (45.7%) and settling time (37.2%)

along with enhanced robustness under 50% parametric variations. Comparative analyses reveal superior performance across error-based indices

showcasing RAITSMC's potential to ensure a reliable and stable power system operation.