Hydrogen plants based on water electrolysis are expected to play a key role in the decarbonisation of industry and the integration of renewable energy sources. Since electrolyzers operate with DC current, large rectifier systems are required to interface with AC networks. These rectifiers, typically based on thyristors or IGBTs, inject harmonic currents that may significantly affect power quality, particularly in large-scale installations with hundreds of megawatts of electrolyzer capacity. While the harmonic behaviour of individual converters is well documented, there is currently no widely accepted methodology for assessing the cumulative impact of large hydrogen plants on electrical networks. This paper analyses the harmonic interactions produced by multiple electrolyzer rectifiers, with particular focus on harmonic phase-angle coherency, transformer phase-shift cancellation strategies and the limitations of conventional harmonic aggregation methods. The study also evaluates the implications of passive filtering solutions, including resonance effects and filter sizing requirements in large industrial facilities. The work supports the development of more reliable grid integration studies for large-scale hydrogen plants connected to renewable-dominated power systems.