Choosing the right rectifier to maximise green hydrogen production

What is the role of the rectifier in electrolysis and green hydrogen production?

Green hydrogen production depends on an electrolyser fed with green electricity. How much electricity it takes, measured as kWh consumed per kg of hydrogen, depends on the electrolyser and on the power supply that feeds it. This article focuses on that power supply: the AC-DC converter, or rectifier.

There are two ways to supply an electrolyser: direct DC-DC conversion from a solar array, or, more commonly, an AC-DC converter on a grid-tied solar network. A comparison of the three converter technologies, thyristor (SCR), diode + IGBT chopper and IGBT active front end (AFE), is covered in a separate article.

Drawing on over 35 years of experience, this article addresses MW-scale rectifiers. SMPS suits ratings of 3–5 kW and single IGBT converters suit ratings under 300 kW, but MW-scale plants, built in multiples of about 5 MW, need a different approach.

What are the selection criteria for hydrogen rectifiers and power supplies?

• Use of the MV transformer for the AC-DC converter, and its efficiency
• Input line current harmonics and the point of common coupling (PCC)
• Input line power factor (PF)
• AC-DC converter technology and efficiency
• AC ripple in the DC output vis-à-vis electrolyser behaviour
• Overall AC (MV) to DC efficiency and how it is measured
• Air- vs water-cooled rectifiers
• Enclosures

Using the MV transformer directly for AC-DC converters, and efficiency

Conventionally, 400/415/440 V LV AC from the first transformer (TX1) was fed to the converter through an LV converter transformer, so the efficiencies of two transformers in series were multiplied together. The preferred design avoids the separate LV converter transformer.

IS 1180 Part-1 and national electricity authority guidelines (March 2023) restrict the size of MV-to-LV transformers, generally to 5000 kVA at 33 kV.

Transformer peak efficiency is typically declared at 99 to 99.2%, but this figure is usually calculated rather than measured. Measuring it is affected by harmonics, by instrument accuracy (instruments must be 5–10 times more accurate than the quantity measured) and by the need for a purely resistive load bank. A practical figure of 98.5% at best is recommended. This applies to all three converter technologies.

Input line current harmonics and the point of common coupling (PCC)

Harmonics matter because the supply comes through solar or wind inverters with the grid also present. IEEE 519-2014, amended as IEEE 519-2022, uses the PCC as the point of measurement.

Specifying 3% THD on a grid that already carries 5–7% voltage THD ignores source impedance: any converter can only restrict the current THD it generates; it cannot improve the existing grid quality.

Type tests at MW scale are impractical, so testing at lower ratings and extrapolating is suggested. Typical input current THD by technology:

• Thyristor converter with 24-pulse effect: 3.5–5.5%
• Diode bridge converter with 24-pulse effect: 3–5%
• IGBT AFE converter: 3–4%

An IGBT/AFE converter at MW scale, above 350 V with currents in the thousands of amps, has not yet been proven over 2–3 years of operation.

Input line power factor (PF)

• Diode + chopper converters: a maximum of 0.93–0.96, stable across a wide DC voltage variation.
• IGBT/AFE converters: about 0.98–0.99 throughout the range.

Additional L&C filters add both cost and losses.

AC-DC converter efficiency (rectifier efficiency in green hydrogen)

Conduction loss per thyristor is P = I_T(RMS)² × r_T + V_T0 × I_T(AV), where V_T0 is the threshold voltage and r_T the slope resistance. Theoretical efficiency is 98.9% at low voltage and high current, and 99.5% at high voltage. In practice, once busbars, joints, chokes and fuses are included, efficiency is about 98.5% at best.

For a diode rectifier followed by a buck chopper, with stage efficiencies of 98.8% and 98.5%, the combined efficiency is 98.8 × 98.5 = 97.3%. An IGBT/AFE converter gives an average efficiency of about 97.3%.

Switching converter peak efficiencies of about 99.2% apply only under test-bench conditions.

AC ripple in the DC output vis-à-vis electrolyser behaviour

Ripple has two components, frequency and amplitude. Frequency has no effect on electrolyser performance, but amplitude can:

• Reduce H₂ production by up to 4% per kWh consumed
• Affect the life of electrolyser materials

Ripple is typically 1–2% RMS in SCR converters and under 1% RMS in chopper-based types. We suggest specifying ripple below 1–1.5% RMS of the maximum DC voltage, subject to the electrolyser manufacturer's input. Research using impedance spectroscopy is still at laboratory stage.

Measuring overall AC to DC efficiency in MW transformer-rectifiers

• Transformer efficiency is calculated under ideal conditions.
• Losses in busbars, joints, heatsinks, chokes and auxiliaries affect end-to-end efficiency between the input (X1) and output (X2) points.
• Measuring to 1% accuracy needs meters, including current sensors, with an accuracy of at least 0.5%.
• Declared efficiencies are generally accepted, but site verification and penalties are contested.

Overall efficiency = transformer efficiency × (transformer-to-panel and auxiliary loss efficiency) × (converter efficiency including the DC filter). For a thyristor converter this gives 98.5 × 99 × 98 = 95.5%. A reasonable figure to specify is above 94.5%.

Moreover, most project requirements do not make clear who bears the onus of measuring rectifier efficiency at site, or how, given how sensitive the measurement is to the environment and grid quality.

Air- vs water-cooled rectifiers in green hydrogen production

• Up to 4000 A per single thyristor bridge, air cooling is preferred for its simplicity and reliability.
• Water cooling needs pumps, flow meters, piping and maintenance, and the pumps draw power continuously.
• Air cooling needs a proper air inlet and exhaust arrangement in the room.

For very high single-bridge ratings, water cooling is the only option unless the bridges are split.

Enclosures and degree of protection: outdoor or indoor rectifiers in green hydrogen?

Enclosures are generally IP55, but India's dust can choke the filters. DC terminals usually exit from the bottom. Outdoor, container-type rectifiers may suit large plants, with these advantages:

• Flexible mechanical layout
• Ease of maintenance
• No air-conditioning needed
• Good air inlet and outlet
• Stackable for future expansion

A container does not have to be an ISO container, so costs need not rise.

Conclusion

The transformer-rectifier deserves close attention. Harmonics and power factor need a practical assessment against Indian grid conditions, and electrolyser manufacturers should be involved in specifying ripple. Proven, reliable technology is preferable to newer options until those are established, and efficiency should be judged on practical rather than textbook figures, with clarity on who measures it at site and how.

Air-cooled enclosures are the more reliable solution for green hydrogen applications, with special cases assessed separately. We welcome your comments at comms@energiaa.in.

Statcon Energiaa engineering team