This work presents a novel, physically based RMS model of a large-scale electrolyzer, developed to be used in power system

stability studies. This model is made open-source to ensure its long-term development and improvement. It combines electrical

and electrochemical behaviors by simulating a tap-changing transformer, a thyristor rectifier and an electrolyzer cell. A successful

validation of this model for the dynamics of interest is performed against a generic EMT model. The model is then used in a

prospective study of the French power system on a real large-scale test, highlighting its value as it clearly demonstrates the role

of the smoothing inductance, and provides more reliable outcomes compared to simplified parametric models used until now.