Hydrogen and power-to-X |
Grid-friendly rectifiers for hydrogen electrolysers
Without a robust, stable power supply electrolysers for hydrogen production cannot operate. And, there are risks to the grid if the power management equipment at the electrolyser park is inadequately specified. With so much at stake, selection of the most appropriate power management technology is crucial
Stephen B. Harrison sbh4 consulting
INEOS Electrochemical Solutions, Sany and HydoTech. Thyristor rectifiers are the only technology that can provide the combination of high current at low voltage that is required by this generation of electrolysers.
High current density alkaline water electrolysis Source: sbh4 GmbH
Despite their robustness, thyristor rectifiers have severe limitations regarding grid compliance. Their power quality is low, characterised by a poor power factor and high harmonic content. Grid compliance requires integrating complicated multi- pulse systems (18-pulse or 24-pulse) or external filter banks, adding to the capital cost, complicating the design and increasing system size.
Power management equipment represents around 20% of the equipment capital cost of a typical electrolytic hydrogen scheme. This cost is generally shared equally between the switch gear, transformers and rectifiers. Transformers cascade high voltage electricity from the transmission grid to the lower voltage required by electrolysers. Rectifiers convert the alternating current that is used in the grid to the DC required. Harmonic filters ensure that the electrolyser park does not disrupt the grid. Power factor correction may also be required.
Transformers – on the critical path Transformers for electrolyser parks are no different from those used in conventional applications. However, the demand for this equipment is high as there is an increasing level of electrification in many countries. On the other hand, transformer assembly and component manufacturing is dominated by incumbent firms with a high degree of inertia, which rely on aged factories with extensive use of highly skilled manual production techniques, which are difficult to scale rapidly. Another major bottleneck in transformer supply is sourcing the high-tech ceramic insulators that connect the power cables to the transformer.
Transformer supply has simply not kept up with demand. The result, is that the delivery lead time can range from 18 to 36
months and transformers will often be on the critical path for equipment procurement, construction and commissioning. To mitigate the risk of delays in bringing electrolytic hydrogen projects to commercial operation, specifying and ordering the transformer must be done early in the project timeline.
Rectifiers – a range of technology options
In contrast to transformers, which have barely evolved in recent decades, rectifier technology has advanced significantly. In part, this has been driven by the increased use of PV solar and battery energy storage systems (BESS), both of which employ DC power. There is now a range of rectifier technology classifications available on a commercial basis. They offer technology selection options for electrical engineers when specifying power management
equipment for electrolyser schemes. ● Thyristor-based SCRs
Thyristor-based silicon-controlled rectifiers (SCRs) have been used for decades to power electrolysers used for chlorine and sodium hydroxide production in the chlor- alkali industry. More recently, they have become the default choice for advanced alkaline electrolyser systems for hydrogen production. High current density advanced alkaline electrolysers, traditionally the domain of thyssenkrupp nucera, are also being offered by challengers such as
32 | January/February 2026 |
www.modernpowersystems.com
Thyristor rectifiers are phase- controlled, semi-controlled devices. They leave a large ripple in the DC current flowing to the electrolyser. This means the electrolyser oscillates around its point of maximum efficiency. Thyristor rectifiers themselves are less expensive than more modern technologies. However, when the cost of the additional conditioning equipment is considered, the total capex will be similar to more advanced technologies. Furthermore, the lower efficiency of thyristor rectifiers increases their lifetime cost of ownership significantly versus more efficient modern technologies.
● Diode and IGBT topology Passive diode rectifiers represent the simplest solution for AC/DC conversion. However, they have a potential for introducing severe harmonic distortion into the grid because they are based on passive components without internal control circuitry. Modern diode rectifiers can include an active component to manage the DC output. A common configuration incorporates a buck-type chopper utilising insulated gate bipolar transistor (IGBT) switches after the diode rectifier. The combined diode and IGBT rectifier topology achieves high efficiency and low harmonics and has resulted in IGBT rectifiers becoming the default choice for modern PV solar parks and lithium-ion BESS.
Compared with thyristor rectifiers, IGBT technology reduces the DC ripple supplied
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