An arc furnace transformer feeds the most electrically hostile load in a steel plant. The arc is unstable by nature, the electrodes short to the scrap repeatedly, and the supply sees current swings that would trip a conventional transformer.
What the arc does to the transformer
The load is not a motor or a heater. It is an arc that changes length as the electrode moves and as the scrap collapses, so current and voltage vary rapidly and non-linearly. During meltdown the electrode contacts the charge many times, and each contact is a short circuit that lasts long enough to load the windings mechanically.
Three consequences follow:
- Very high secondary current. Melting power is delivered at low voltage and very high current, which sets the conductor and busbar design.
- Repeated short circuit forces. Every electrode-to-charge contact produces electromagnetic forces between conductors, and the winding has to be braced against them.
- Harmonics and flicker. The arc generates harmonic distortion and voltage flicker on the supply side, which is a grid-connection issue as well as a transformer issue.
Why impedance is set higher than usual
Arc furnace transformers run at noticeably higher impedance than distribution practice, typically in the range of 4.5% to 21% depending on the unit. The reason is arc stability.
Higher impedance limits the short circuit current when the electrodes touch the charge, and it softens the current swings that would otherwise disturb the arc or trip the furnace. The trade-off is reduced voltage regulation and a lower power factor at the furnace, which is why the value is chosen against both the furnace and the grid rather than simply maximised.
The series reactor
Many arc furnace installations include a series reactor for the initial melting period. It lengthens the arc and reduces the current, which cuts electrode consumption and softens the disturbance the furnace imposes on the supply during the noisiest part of the heat. It is commonly built integral with the transformer or supplied as a separate unit, and switched out for refining.
Cooling
Losses at these currents are substantial, and cooling is a defining part of the design. Smaller units use natural or forced air. Larger units use forced oil circulation with forced air over the coolers, or forced oil with water cooling where the heat load is too high for air alone. Where water is used, it is normally through a water-to-oil heat exchanger rather than in direct contact with the oil.
Redundancy is worth specifying on large units. Two coolers each rated for the full losses let the furnace keep running through a cooler failure, and losing a heat to a pump fault costs far more than the second cooler did.
Where they are used
- Electric arc furnace steelmaking, the primary application.
- Ferroalloy production in submerged arc furnaces.
- Silicon metal production.
- Specialty and stainless steel melting.
We build furnace transformers against the process rather than against a generic nameplate. Send the furnace type, capacity and melting cycle alongside the electrical data, and we can come back on the configuration. See the arc furnace transformer range, or contact us.
Related Products
Related Applications
- Transformers for Steel Industry
- Transformers for Electric Furnaces
- Transformers for Industrial Equipment
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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 transformer substation packages pages.
Frequently Asked Questions
How do I approach specifying an arc furnace transformer?
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 makes a furnace transformer different from a distribution transformer?
The duty cycle and the load. Furnace duty brings repeated short-circuit and inrush events, very high secondary currents and frequent tapping under load. Windings, mechanical bracing and tap changer selection are all driven by that duty rather than by steady-state load alone.
How is the secondary current handled at these levels?
By designing the secondary for the current it actually carries, with the busbar or cable terminal arrangement settled at drawing stage. Conductor cross-section, support spacing and cooling are decided together, since the mechanical forces at high current are as significant as the thermal ones.
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.