High current design is not ordinary transformer design scaled up. Above a few thousand amperes, effects that are negligible in normal practice become the constraints that decide the shape of the unit, and they are all fixed at the design stage. Correcting any of them afterwards is expensive or impossible.
What Counts as a High Current Application
High current means large currents at low voltage: thousands of amperes on smaller installations, tens of thousands on the largest. The applications are the ones that convert electricity into chemical or thermal energy rather than into motion or light.
- Electric arc furnaces for steel melting.
- Induction furnaces for metal melting and holding.
- Aluminium smelting potlines.
- Chlor-alkali electrolysis.
- Copper electrorefining and zinc electrowinning.
- Other electrochemical processes, including hydrogen production.
All of them share a common shape: the secondary delivers current into something that is essentially a resistive or electrochemical load, and the voltage is low enough that the current has to be large to deliver the required power.
Conductor and Winding Design
The secondary is where the design pressure sits. Cross-sections are large, and at those sizes the current does not distribute itself evenly through the conductor.
- Skin effect. Current crowds toward the conductor surface as frequency rises, so the effective cross-section carrying current is smaller than the physical one. On a rectifier load with harmonic content, this applies to every harmonic component as well.
- Proximity effect. Adjacent conductors distort each other’s current distribution, pushing current further out of balance. The effect grows as conductors are packed closer together.
- Parallel paths. The answer is multiple parallel conductors rather than one large bar, transposed so that each path links the same flux. Without transposition, one path carries more than its share and becomes the hot spot that limits the whole unit.
- Current density. Set against the cooling actually available, not against a generic value, because the cooling is what removes the heat.
Copper is the usual choice for the secondary, on conductivity and on its behaviour under fault forces. The busbars carrying current to the process are part of the design rather than an afterthought, because they contribute both loss and electromagnetic forces.
Electromagnetic Forces
Force between conductors scales with the square of the current. At short circuit the current multiplies again, so the forces the structure has to resist are two orders of magnitude above the running case.
Windings are braced and clamped against the calculated fault forces rather than against experience, and the low voltage leads are kept short and arranged symmetrically because the forces between heavy parallel leads are significant even at normal load. Support structures are normally verified by finite element analysis, since the forces are not uniform and the weakest bracing point is not always obvious.
Thermal Management and Cooling
The high currents generate I²R losses that are large in absolute terms, and on a rectifier or furnace load they are joined by eddy losses from harmonic current and by stray losses induced in nearby structural steel. The thermal calculation has to include all three.
Cooling escalates with the heat load.
- Forced air for smaller dry type units.
- Forced oil circulation with forced air over the coolers for medium and large oil-immersed units.
- Forced oil with water cooling through a heat exchanger where the heat load exceeds what air can remove.
- Direct water cooling of the secondary busbars on the highest current installations.
Redundancy is worth specifying on continuously loaded units. Two coolers each rated for the full loss allow full-load operation with one out of service, which turns a cooling failure into a maintenance event instead of a production stop.
Stray Losses in Structural Steel
Magnetic flux from heavy secondary leads induces eddy currents in any steel structure close to them. The result is heating in parts that were never intended to carry current, and in a tank or enclosure it can produce a hot spot that damages the surrounding material.
The answer is either to keep steel away from the leads, to use non-magnetic material in the immediate vicinity, or to provide a magnetic shunt that gives the flux a defined path. Which approach is used depends on where the constraint comes from, and it is a design-stage decision because it changes the physical arrangement.
Duty Cycle and Control Interaction
Furnace and electrochemical loads are not steady. Arc furnace duty swings between short circuit during scrap meltdown and light load at the end of a heat, and the transformer has to tolerate both. Electrochemical plants run continuously for years with a largely constant load, so their thermal design is governed by insulation ageing rather than by cyclic stress.
Where the transformer supplies a rectifier, the harmonic spectrum belongs in the specification. Without it, the thermal calculation is based on an assumed waveform, and the assumption is usually optimistic.
What to Send Us
- Secondary current and voltage required.
- Rectifier or furnace topology, and the expected harmonic spectrum.
- Primary voltage and the system short circuit level.
- Duty profile: continuous, cyclic, or with defined overload periods.
- Cooling medium available on site, with flow and temperature if water is used.
- Ambient conditions and any corrosive atmosphere.
See the custom transformer range, the rectifier transformer range, the furnace transformer range, or contact us with the design data.
Where High Current Designs Usually Go Wrong
A high current transformer rarely fails in the winding. It fails at the points where the current leaves the transformer, and in the steel that surrounds the leads. Four places deserve more attention than they usually get:
- Secondary terminations — busbar or cable lug area, contact pressure and the temperature rise allowed at the joint, not just in the winding.
- Parallel windings and split leads — unequal impedance between parallel paths produces circulating current and local hot spots that no amount of extra copper will fix.
- Structural parts near the leads — leakage flux induces current in tank walls, frames and clamps; non-magnetic inserts or shielding are usually cheaper than the repair.
- Cooling air paths — forced air cooling only works if the ducts are actually open; blocked ducts turn an AF rating back into an AN rating without anyone noticing.
We ask for the duty cycle rather than just the nameplate current, because a unit running at 60 % for twenty hours a day is a different design from one that sees full current for two.
Related Products
- Custom Transformer
- Rectifier Transformer
- Furnace Transformer
- Arc Furnace Transformer
- Electric Furnace Transformer
Related Applications
- Transformers for Steel Industry
- Transformers for Electrochemical Industry
- Transformers for Aluminum Industry
- Transformers for Electric Furnaces
- Transformers for Industrial Equipment
Request a Custom Quote
Need a custom transformer for your application? Send us your technical requirements and our engineering team will provide a custom solution within 24-48 hours.
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 and switchgear control and protection pages.
The switching side of the design is covered in more detail in our MV switchgear selection guide, which covers insulation medium, fixed versus withdrawable, and protection and control.
Frequently Asked Questions
How do I approach specifying a transformer for high current applications?
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.