The energy transition is accelerating the deployment of large-scale hydrogen electrolysis plants to support the decarbonization of hard-to-abate sectors such as steel, chemicals and e-fuels. As hydrogen developers increasingly require power system studies to optimize electrical designs and demonstrate grid-code compliance, accurate dynamic models of electrolyzer rectifier systems have become essential. This need is further reinforced by the growing interest in partially or fully off-grid configurations coupled with renewable energy sources, where weak-grid conditions may pose significant stability challenges. This paper presents a detailed electromagnetic transient (EMT) model of a 20 MW alkaline electrolyzer supplied through a multi-winding transformer and a thyristor-based rectifier. Developed in a MATLAB-based environment, the model includes transformer topology, rectifier behaviour, electrolyzer electrical characteristics, and control systems. Based on the EMT reference model, a Root Mean Square (RMS) representation suitable for large-scale power system studies is proposed and implemented in PSSE. A comparison between EMT and RMS simulations shows that the proposed RMS representation reproduces the main steady-state and dynamic trends of the EMT reference model within the investigated operating range.