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 IĀ²R 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.
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
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