Hydrogen power plants based on electrolysis are expected to play a key role in the electrification and decarbonization of industry. These plants base their operation process in the electrolysis of water to separate oxygen and hydrogen. This process is performed in DC, so to feed it from the three-phase AC network, a rectifier converter is necessary to transform the current through. These rectifiers can be based on either thyristors or IGBTs, and while each type of device has its own advantages and disadvantages, both inject harmonic currents into the AC network.

The effects of the harmonic injections of a particular device and the possible corrective measures have been widely studied. Still, the impact of hundreds of MW of electrolyzer rectifiers in a power plant and their impact on the external grid do not have a defined methodology at this stage. IEC 61000-3-6 proposes the methodology of the summation law based on the harmonic order. However, this methodology is based on empirical analysis for regular converters, and there are concerns about whether this is applicable to rectifiers used in hydrogen power plants.

A key aspect is the coherency of the phase angle of the harmonics injected by the rectifiers. Some harmonics are injected with similar phase angle regardless of the point of operation of the rectifier as they are very linked to the fundamental frequency. On the other hand, some others present a random behavior. To cancel the harmonics with high phase coherency, three-winding transformers are used to increase the number of pulses of the studied set of converters. However, the cancellation is not perfect, and when hundreds of MW in converters are installed, a slight unbalance can lead to high current distortion. On top of this, when the pulse number is increased, the distortion in certain specific orders increases, so additional filtering solutions are needed. Installing additional passive filters introduces resonances at lower orders that could match with the not-perfectly cancelled harmonics, which leads to more filters needed. Meanwhile, active filters technology at MV might not be mature enough to handle harmonic mitigation of this magnitude.

The proposed paper analyzes all these effects, and the drawbacks of the classical filtering solutions and methodologies. Finally, the paper also compares the harmonic-injection information provided by the rectifier manufacturer with the distortion obtained from a proper EMT model of the electrolyzer to find differences and determine if the models match with the available information.

This work results from a collaboration between Hitachi Energy Power Consulting, a hydrogen project developer, and a rectifier manufacturer, and supports more reliable grid integration studies for large-scale electrolyzer installations connected to renewable-dominated power systems.