This paper presents a stability assessment of an isolated, converter-dominated power system supplying a hydrogen electrolyser load, based on a combined Root Mean Square (RMS) and Electromagnetic Transient (EMT) modelling approach. The objective is to evaluate system-level stability at the pre-feasibility stage, where detailed equipment models are typically unavailable, while systematically validating RMS-based results using higher-fidelity EMT simulations.

A modified IEEE 9-bus system is developed as a 100% inverter-based network comprising grid-forming (GFM) and grid-following (GFL) units representing renewable generation and battery energy storage, together with a hydrogen electrolyser load. Generic converter models with representative control structures, including phase-locked loops, inner current control, plant-level control and current limitation are applied to reflect typical industry practice.

Two alternative modelling approaches for the electrolyser load are investigated. The first represents the load as a simplified aggregated model, while the second adopts a grid-following inverter-based representation emulating an active front-end interface with active power control and potential participation in frequency support. This comparison enables the evaluation of the impact of load modelling assumptions on system dynamics.

A comprehensive set of time-domain simulations is performed to assess system behaviour under disturbances, including load changes, generation outages and network contingencies. The analysis focuses on frequency stability, voltage response, synchronization of GFL units in weak grid scenarios and converter-driven oscillations. Sensitivity studies are conducted to evaluate the influence of key control parameters and to identify stable operating regions.

The RMS results show that system stability is strongly dependent on control architecture, parameter tuning and load representation. Configurations with coordinated inverter-level grid support across all components provide the most robust performance, enabling effective frequency and voltage regulation, together with improved dynamic response.

To address inherent limitations of RMS modelling in representing fast dynamics and nonlinear effects, selected scenarios are further analysed in the EMT domain using consistent model structures and parameter sets. The EMT analysis captures fast control interactions and high-frequency phenomena, enabling validation of RMS results and identification of conditions where discrepancies arise. The proposed combined RMS–EMT framework provides a hierarchical methodology for stability assessment, where RMS simulations are used for system-level screening and sensitivity analysis, while EMT simulations are applied for detailed validation of critical cases. The results demonstrate that RMS modelling, when properly parameterized, is suitable for capturing dominant system-level dynamics, while EMT analysis remains essential for accurate representation of fast converter behaviour.

Overall, the study provides a systematic and practical approach for stability assessment of isolated, inverter-dominated systems with large-scale power-electronic loads, supporting modelling and control design decisions in early project development stages.