Selecting a rectifier transformer is not a matter of picking a kVA figure from a table. The rating follows from the DC process it has to feed, and every other parameter — pulse number, phase shift, impedance, cooling — falls out of that decision. This guide walks through the sequence we use when an enquiry lands.
Step 1: Fix the DC output first
Start with what the process needs, not with what the grid can give. You need the DC voltage at the electrolyser or drive terminals and the DC current it will draw at full production. Everything upstream is derived from those two numbers.
For context on magnitude: a single chlor alkali electrolysis unit typically runs around 40 kA DC at roughly 280 V, paired with a transformer in the 12 MVA class. Aluminium potline rectifier groups generally sit between 25 kA and 100 kA per unit, with newer integrated designs reaching 100 kA and older installations at 25 or 32 kA. At the top of the market, specialist manufacturers publish figures up to 180 kA DC and 180 MVA. Our own designs cover secondary currents up to 10,000 A, which sits in the middle of that band and suits potline sections, plating lines, water treatment electrolysers and general electrochemical duty.
Step 2: Choose the pulse number
- 6 pulse: the simplest arrangement. Cheapest, but leaves significant 5th and 7th harmonics on the grid. Acceptable on a strong grid where harmonic limits are loose.
- 12 pulse: the industry baseline. Two windings with a 30 degree phase shift cancel the 5th and 7th, leaving the 11th and 13th as the dominant residuals.
- 24, 36, 48 pulse: achieved by paralleling multiple phase shifted units. Used where grid harmonic limits are tight or where DC ripple affects product quality, as it does in ion exchange membrane chlor alkali cells.
The pulse number is not just a power quality decision. Higher pulse counts reduce DC ripple, and in membrane cell electrolysis ripple shortens membrane life. That is why large chlor alkali plants pay for 24 pulse and above.
Step 3: Set the phase shift and winding arrangement
The phase shift is produced by the winding connection, usually an extended delta on the grid side, with split windings on the valve side producing the required AC phase difference. Manufacturing tolerance on the shift angle matters: when units are paralleled, an error in the shift angle shows up as unequal current sharing between them. Good practice holds the phase angle error to a fraction of a degree and keeps current sharing deviation between parallel units within a few percent.
Step 4: Pick the impedance
Rectifier transformers run at higher impedance than distribution units — commonly in the 10% to 18% band for electrochemical service. The impedance limits the commutation short circuit surge and smooths the DC waveform, at the cost of some regulation. Lower impedance gives better voltage regulation but higher fault current; the right value is a compromise set by the rectifier manufacturer’s requirements and the short circuit level of the plant.
Step 5: Specify the voltage regulation
Electrolysis load changes as cells age and as production rates vary. On load tap changing with a wide regulating range — typically on the order of 70% to 105% of nominal — lets the DC voltage be trimmed without stopping the line. Pay attention to the tap changer duty: chlor alkali service operates the tap changer far more often than general duty equipment, so a dedicated oil compartment and wear resistant contacts are worth specifying.
Step 6: Choose the cooling
- ONAN / AN: natural convection. Used on small units and where noise limits rule out fans.
- OFAF: forced oil, forced air. The common choice for medium and large electrochemical units.
- OFWF: forced oil, forced water. More compact and quieter, chosen where water is available and where the rectifier room is enclosed and poorly ventilated.
Whichever you choose, specify redundancy. On OFWF, two coolers each rated for the full transformer losses let the unit stay at full load if one cooler or its pump fails. On OFAF, dual fans in an active and standby arrangement do the same job.
Step 7: Deal with harmonics in the design, not afterwards
Harmonics from the rectifier cause extra eddy current losses in the windings and in structural parts. If the transformer is designed as though it were feeding a sinusoidal load, those losses appear as unexpected hot spots. The cooling system and the conductor sizing have to account for the harmonic spectrum, which is why the rectifier topology belongs on the transformer enquiry, not in a separate document.
Step 8: Confirm the environment and the standards
Chlor alkali and plating environments are corrosive. Chlorine gas and alkaline mist attack paint, seals and bushings, so the specification should call for heavy duty coating, acid and alkali resistant seals and corrosion resistant bushings, plus positive pressure ventilation in the rectifier room. On the electrical side, confirm which standard applies — IEC 60076 in most export markets, GB 1094 for domestic projects, or a customer specific specification — and whether type test evidence or witness testing is required.
What to send us
DC voltage, DC current, pulse number, duty cycle and overload profile, ambient conditions, harmonic limits, available cooling medium, applicable standards, and quantity. With those we can return a rating, an outline and a test schedule. Without them, any quote is a guess.