Cooling is not an accessory on a rectifier transformer. Harmonic currents from the rectifier produce additional losses that a sinusoidal design calculation does not predict, and the duty is usually continuous at or near full load. Getting the cooling wrong is the single most common cause of premature ageing in electrochemical service.
Why harmonic losses change the calculation
The rectifier draws non sinusoidal current. Those harmonic components cause extra eddy current losses in the windings and in structural steel parts, and the effect grows with the delay angle — with thyristor control at large firing angles, commutation is fast and the harmonic content is high. A transformer sized as though it were feeding a sinusoidal load of the same RMS current will run hotter than calculated. The cooling system and the conductor sizing have to be specified against the actual harmonic spectrum.
The cooling designations
- AN — air natural. No fans. Used on smaller dry type units and where noise limits rule out forced cooling.
- AF — air forced. Fans increase the rating of a dry type unit, typically by 40 to 50 per cent over the natural cooled figure.
- ONAN — oil natural, air natural. The baseline for oil immersed units.
- OFAF — oil forced, air forced. Pumps circulate oil through coolers with fans. The standard choice for medium and large electrochemical units.
- OFWF — oil forced, water forced. Oil is cooled by water in a heat exchanger. More compact and quieter than OFAF, chosen where water is available and the room is enclosed.
OFAF against OFWF
OFAF needs a large air cooled cooler bank with good airflow, which usually means a large room or outdoor placement, and it produces fan noise. OFWF needs a cooling water supply with adequate quality and flow, and it introduces a water side failure mode, but the cooler is much smaller and the room can be enclosed. Where a plant already has a cooling water system — most electrochemical plants do — OFWF is usually the better choice. Where water is scarce or its quality is poor, OFAF wins.
Redundancy is not optional
For OFWF, the usual arrangement is two coolers each rated for the full transformer losses, so the unit can stay at full load with one cooler or one pump out of service. For OFAF, dual fans in an active and standby arrangement do the same. Given that an unplanned stop on an electrolysis line costs far more than the redundant equipment, this is easy to justify.
Temperature rise limits
In electrochemical service it is common to specify winding hotspot and top oil temperature rise limits below those of general purpose transformers. A margin of a few kelvin materially slows insulation ageing, and insulation ageing is what ends the life of a continuously loaded rectifier transformer.
Corrosion protection for the cooler
In chlor alkali service the cooler sits in an atmosphere containing chlorine and alkaline mist. Radiators and coolers need heavy duty coating, and corrosion resistant materials for fins and tubes should be specified. This applies equally to OFAF air cooled coolers and to the water side of OFWF heat exchangers.
What to confirm at enquiry stage
- Cooling medium actually available on site, with flow and temperature for water
- Whether the room is enclosed, and its ventilation
- Noise limits at the property boundary
- Whether cooler redundancy is required by the plant standard
- Ambient temperature range and altitude
- The rectifier topology and expected harmonic spectrum
Related equipment
If you are specifying this as part of a larger distribution scheme, the surrounding scope is covered on the industrial power distribution, MV switchgear, switchgear control and protection and rectifier transformers pages.
Frequently Asked Questions
What is the first thing to establish with rectifier transformer cooling?
Start from the load and the environment, then let the equipment follow. Working from a catalogue rating backwards is how projects end up with either a marginal unit or one that costs money sitting under-loaded.
What pulse number should I specify?
It comes from the harmonic limits your supply authority imposes and from the cost of the filters you would otherwise need. A six-pulse arrangement is simplest and produces the most harmonic current; twelve and twenty-four pulse arrangements reduce it at the cost of a more complex transformer.
How do harmonics affect the transformer design?
Harmonic currents raise winding losses and local heating beyond what the fundamental current suggests. The winding configuration, conductor selection and, where the content is high, a K-factor rated design are chosen for the actual current waveform rather than a pure sine wave.
Which standard applies, and is it certified?
The applicable standard and clause set are agreed against your project specification before production starts. Where a project names IEC 60076 for the transformer, we work to those clauses and the routine test report ships with the unit.
What do you need from us to quote?
The single-line diagram or a description of the application, the voltages, the load with its duty, the site conditions, and any standard the project specifies. Send what you have – if something is missing, it can be worked out together rather than delaying the enquiry.