The TSO 2020 project has published a stability analysis of an international electricity system connected to regional and local sustainable gas systems. The report overiews an analysis of the current regulations for ancillary services provision. Next, it presents the development of a model of a 1-MW electrolyser equipped with frequency and voltage controllers to support the stability of the local transmission network. The performance of this electrolyser in a power system was simulated and analysed by using a reduced size model of the electrical transmission network in Veendam-Zuidwending. The electrolyser model was then scaled up to represent a large 300-MW electrolyser. A test model of the northern part of the Dutch transmission network was developed and various simulations were performed to study the impact of a large-scale electrolyser in an interconnected power system. The study was completed by the development of control approaches that enable electrolysers and multi-energy hubs to participate in the provision of electrical ancillary services.