The progressive phase out of fossil fuel power plants and the integration of variable renewable energy sources, driven by environmental directives striving for the reduction of CO2 emissions, is reshaping how electricity Transmission System Operators (TSOs) plan and operate the power system. The mitigation of uncertainty, which imperils the balance between generation and demand, urges the search of new sources of ancillary services, traditionally provided by bulky synchronous generators. In particular, the coupling of the electricity and gas sectors reveals promising flexibility solutions for power systems through energy conversion and hydrogen storage. The conversion of electrical energy into hydrogen, commonly known as power-to-gas, is enabled by water electrolysis in electrolyzers. Specifically, the Proton Exchange Membrane (PEM) technology can react very quickly to demand variations and thus it holds a strong potential for the procurement of ancillary services. This thesis investigates the viability of the integration of large scale PEM electrolyzer capacity into ancillary services markets in the context of the European initiative “Synergy Action TSO 2020”. The current framework of Dutch frequency balancing markets, voltage control and congestion management is reviewed, as well as the future European plans for the implementation of a harmonized and common frequency market. The conducted research includes the assessment of the technical adequacy of PEM electrolyzers with respect to existing prequalification requirements and the proposal of recommendations for the optimal participation in these markets. To illustrate the value of PEM electrolyzer support for electrical networks, a case study based on a realistic representation of the transmission grid of the Groningen-Drenthe-Overijssel (GDO) area in the north of the Netherlands for the year 2030 is presented. The case study is principally intended to examine the effectiveness of the provision of primary frequency control with PEM electrolyzers. The obtained results highlight how this technology can improve the frequency stability of the power system in comparison with synchronous generators, and more importantly for the future, how it can help alleviate the negative effects attributed to the reduction of inertia in the system.