The growing share of renewable energy sources increases the need for demand-side flexibility. Large-scale water electrolysers can support electric grids while reducing hydrogen production costs. Their participation in ancillary markets offers a dual benefit: grid stabilization and reduced cost for renewable hydrogen.

Existing studies on grid integration of electrolysers often use simplified dynamic models that overlook essential physical characteristics of the stacks. In particular, the influence of balance-of-plant (BoP) equipment, which governs temperature dynamics, is frequently excluded. When BoP modelling is not included, parameters such as the hydrogen-to-oxygen (HTO) ratio during dynamic operation is overlooked, even though it is relevant to operational safety. In this work, we present a Simulink model that integrates power electronics with detailed process dynamics of an electrolyser plant, explicitly including balance of plant (BoP) components. The model is used to evaluate performance of electrolyser plant, applying standardized testing protocols from the QualiGrids project to replicate prequalification requirements for frequency containment reserve (FCR) services.

Simulation results show that the electrolyser plant meets the required KPIs on power stability, ramp duration and initial response time. However, dynamics of HTO ratio evolve more slowly than power responses and has cumulative effect during prolonged operations, highlighting the importance of continuous gas ratio monitoring. This highlights the value of detailed dynamic modelling for safe and reliable operation of electrolyser plants in balancing markets.