This paper provides an overview on implementing practical models of large scale electrolysers (>1MW) for real time digital simulation. The search for new sources of ancillary services has aroused considerable interest in the use of large scale electrolysers for power system ancillary services. As the number of large scale electrolysers is projected to grow, it is important that the dynamics of these plants are well understood in order to integrate them successfully. In line with this objective, suitable models must be developed to aid studies of grid dynamics with electrolysers configured as sources of ancillary services. To illustrate the feasibility of creating such models, a case study is built on a MW-scale plant connected to an infinite grid. Real time simulations are performed by using a real-time digital simulation (RTDS) platform to investigate the response of the electrolyser model to basic step commands. The results show that a generic scalable model of large scale electrolysers in RTDS can replicate fairly accurately the response of a real electrolyser.@en