The search for new sources of ancillary services and the projected demand for hydrogen as a medium of energy storage 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 electrical power system dynamics with electrolysers configured as sources of ancillary services. This thesis report provides an overview on implementing practical models of large scale electrolysers (>1 MW) for real time digital simulation and also proposes a control scheme to extend the basic capabilities of large scale electrolysers in order to provide ancillary services to the electrical power system.To illustrate the feasibility of creating such models, case studies are built on a one megawatt-scale plant connected to a high voltage infinite grid via step-down transformers. Real time simulations are performed by using the Real-Time Digital Simulator (RTDS) platform to investigate the response of the electrolyser model to external command signals and power system disturbances. The 1 MWmodel is also scaled to 300 MW with the view to qualitatively assess its performance, with respect to grid code compliance. The results show that the generic 1 MW model in RTDS can replicate the step response profile of a real electrolyser. The results also support the view that high level control schemes are a key enabler for ancillary services using large scale electrolysers. High level control schemes can enable an optimal operation of the electrolyser load on the basis of market and power system conditions.