A high current rectifier transformer is defined by its secondary. Where a conventional transformer is described by its voltage ratio, this one is described by how much current it can push through a very short, very thick conductor.
What the currents look like
Aluminium smelting and chlor-alkali production need DC at low voltage and enormous current. Secondary currents run into the thousands of amperes on smaller installations and tens of thousands on the largest potlines. At those levels several design effects that are negligible in ordinary practice become the governing constraints.
Design problems that only appear at high current
- Skin and proximity effect. Current crowds toward the surface of a conductor as frequency rises, and adjacent conductors push the current distribution further out of balance. The answer is multiple parallel conductors, transposed so they share current evenly.
- Stray losses in steelwork. Magnetic flux from heavy secondary leads induces eddy currents in nearby structural steel, which heats parts that are not designed to carry current at all.
- Electromagnetic forces. Force between conductors scales with the square of the current. The bracing has to be designed against the short circuit case, not the running case.
- Heat concentration. Losses are concentrated in the secondary and its leads, so the cooling has to be sized against the whole unit rather than the core and winding alone.
All four are design-stage problems. Once the unit is built, correcting any of them is expensive or impossible.
Conductor and busbar design
The secondary is built from large cross-section copper arranged as multiple parallel paths. Where the current is high enough that air cooling of the leads becomes impractical, the busbars are water-cooled. The low voltage leads to the rectifier are kept short and arranged symmetrically, which limits both the losses and the forces between them.
Cooling
Forced oil circulation with forced air over the coolers is the standard arrangement for medium and large units. Where the heat load exceeds what air can remove, forced oil with water cooling through a heat exchanger is used. Direct water cooling of the secondary busbars is applied on the highest current installations.
Redundancy is worth specifying on continuously loaded units. Two coolers each rated for the full losses allow operation at full load with one cooler out of service.
What to send us
- Secondary current and voltage required.
- Rectifier topology and the expected harmonic spectrum.
- Primary voltage and system short circuit level.
- Cooling medium available on site, with flow and temperature for water.
- Ambient conditions, including any corrosive atmosphere.
- Duty: continuous or cyclic, and the expected load profile.
See the rectifier transformer range, or contact us with the requirements.
Related Products
Related Applications
- Transformers for Electrochemical Industry
- Transformers for Aluminum Industry
- Transformers for Industrial Equipment
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How this is packaged on real projects
On most sites the unit described here is not installed on its own. It goes into an assembly that includes the incoming switching, protection and outgoing feeders, which you can see on the industrial power distribution, MV switchgear and switchgear control and protection pages.
Frequently Asked Questions
What should be confirmed before ordering a high current rectifier transformer?
System voltage and frequency, fault level at the point of connection, the load with its duty, and the site conditions including ambient range and indoor or outdoor placement. With those settled the rest of the specification follows.
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.