Large-scale hydrogen electrolyzers for hard-to-abate industries, such as steel industry, have the potential to be an essential tool of demand response in low inertial power systems with high shares of renewable energies. Their flexibility comes from the possibility to store hydrogen, decoupling electric consumption from hydrogen demand. Therefore, they can help in the integration of more renewable energies by the provision of grid services, such as frequency regulation. Alkaline electrolyzers (AELs) are the most mature and cost effective technology for large-scale hydrogen applications. However, their slow dynamics do not allow a fast response. Therefore, their combination with energy storage systems (ESSs) into hybrid hydrogen systems (HHSs) enhances their flexibility and fast response for frequency regulation. Supercapacitors (SCs) are suitable ESS technology in this application due to the high power and low energy required. A decentralized dynamic power sharing control is proposed for an AEL/SC HHS to provide frequency regulation with scalability. The control strategy respects the slow dynamics of the AEL, while the use of the SC is optimized by the automatic recovery of the dc bus voltage and SC state of charge (SoC). The decentralized approach of the control strategy enables easy expansion of the system, essential for large-scale hydrogen systems. The effectiveness of the method in large-scale power systems, as well as its scalability is shown in simulation results. The control strategy is validated with experimental results.