Arc Furnace Transformers Explained: How to Specify for EAF and DC Arc Steel Melting Loads

A melt shop engineering team in Southeast Asia sent us a furnace transformer nameplate last spring: 60 MVA, 33 kV primary, 650 V secondary, 8% impedance, and a tap table with 23 positions. Their question was straightforward — could we build the same machine for a second meltshop. The nameplate turned out to be the least useful page in the package. What separates an arc furnace transformer from anything in a distribution catalogue is not printed on the plate at all. It is the duty sitting underneath it.

Steel is moving onto the electric route, and the pace shows up in the numbers. The worldsteel publication World Steel in Figures 2025 puts electric furnace output at 29.1% of global crude steel in 2024. The regional split is what matters to buyers: in the Middle East the electric route already accounts for 95.4% of crude steel, Egypt and Saudi Arabia sit at 100%, and the United States at 71.8%. Every one of those tonnes is melted by an electric arc furnace transformer before it becomes metal. The IEA iron and steel sector page tracks the same shift from the energy side.

 

Why an Arc Furnace Transformer Is Not a Big Distribution Transformer

A distribution transformer is built to carry load and survive a fault. A furnace transformer is built to live inside one. The arc is a controlled short circuit: it extinguishes and re-strikes dozens of times per heat, the scrap column caves in and shorts the electrodes, and bore-in pulls current that a distribution unit would only see under a fault condition. In a melt shop that event is not an anomaly. It is the operating condition, and it happens thousands of times per campaign.

The thermal profile is just as different. A heat runs 35 to 60 minutes and power swings between roughly 20% and full tap within seconds of each other, so winding hot-spot temperature follows the swing. IEC 60076-7, the loading guide for oil-immersed power transformers, treats this duty as its own loading case rather than a variation of normal service.

Parameter	Distribution / standard power transformer	Arc furnace transformer
Secondary voltage	Fixed design voltage, ±2 × 2.5% taps	Wide window, roughly 300–1,100 V over 15–25 positions
Secondary current	Typically under 3 kA	20–100 kA at the busbar
Secondary short circuit	A design event, proven once	Normal operation, thousands of times per campaign
Percentage impedance	4–6%	6–12%, set by fault-current and arc-stability limits
Tap changer	Off-circuit, or a low-duty OLTC	High-duty OLTC, tens of thousands of operations
Cooling	ONAN or ONAF	OFAF or OFWF, often with redundant oil pumps

 

 

The Numbers That Actually Decide the Specification

Five values do the real work: MVA and primary voltage, the secondary voltage window, secondary current, impedance (or total circuit reactance), and temperature rise with its cooling class.

The tap table comes from the furnace vendor, not the transformer supplier, and it encodes the melt curve. Bore-in happens at low voltage and high current because the electrodes are buried in scrap and the arc is effectively short. As the arc stabilises the operator steps up to a longer, higher-voltage arc with better thermal efficiency. A 5 V step on a 600 V secondary looks trivial — under 1% of voltage — yet it is worth several hundred amps of secondary current at the busbar, and the transformer has to deliver every point on that curve at the specified MVA.

Impedance is where quotations diverge most. A furnace unit is normally specified between 6% and 12%, higher than a distribution transformer, because secondary faults are frequent and the winding is the first thing that has to limit them. But percentage impedance is only the transformer’s share of the circuit. The furnace designer works with total circuit reactance — transformer plus busbar, flexible cable, electrode arm and electrode — because that total sets arc stability and furnace power factor. Change the busbar run after the transformer is ordered and you have changed the furnace operating point.

 

Short-Circuit Duty Is the Real Design Driver

Here is the arithmetic that decides how heavy the machine has to be. At 8% impedance a bolted fault on the secondary draws roughly 12.5 times rated secondary current. On a 40 kA unit that is 500 kA at the busbar. Electromagnetic forces scale with the square of current, so the same fault applies on the order of 150 times the nominal mechanical load to the winding, its spacers and its leads.

A distribution transformer is usually tested to prove it survives a short circuit once. A furnace transformer has to survive one as routine duty, so the mechanical design becomes the project: uniform clamping pressure through the winding stack, pre-stressed spacers, work-hardened copper, braced tap leads, and a core frame that does not shift under the axial force of an asymmetric fault. IEC 60076-5 sets the short-circuit withstand requirement and IEEE C57.12.90 defines the test procedure. When a supplier cannot show both the design basis and the test record, the impedance figure on its own tells you very little about how long the unit will last.

In our own design reviews the document that triggers rework is rarely the impedance figure. It is the secondary connection drawing — 40 kA or more through laminated busbar or water-cooled cable, and the support spacing decided on that drawing is what keeps the forces off the winding terminals during a strike.

 

Cooling and Tap Changer Duty in a Melt Shop

Cooling is specified for the worst hour, not the average one. OFAF with redundant oil pumps and oil-flow indicators is ordinary practice on furnace units, and larger ratings move to OFWF. Top-oil temperature lags the arc by minutes, which is why furnace specifications usually add fibre-optic hot-spot probes in the winding.

The tap changer lives in a different world from utility practice. An operator may move the tap five to fifteen times in a single heat, and a two-shift melt shop runs eight to twelve heats a day — 10,000 to 30,000 operations a year is ordinary. Furnace units are therefore specified with high-duty, usually vacuum-type diverter switches, and the operation counter, not the calendar, drives the maintenance plan. IEC 60214-1 is the standard the hardware is built and tested to.

The mechanics of tap changer wear are the same ones we set out in our guide to OLTC and off-circuit tap changers, with one difference: in a melt shop the clock runs about ten times faster, and a diverter switch that is never inspected is a scheduled outage waiting for a date.

 

 

Harmonics, Flicker, and What the Transformer Cannot Fix

The arc is a non-linear, continuously changing load. It injects harmonics through the second, third, fifth and seventh orders plus interharmonics, and it modulates busbar voltage at frequencies the eye reads as flicker. Transformer design helps at the margin — a delta winding traps triplen harmonics and flux-density headroom keeps additional losses and noise under control — but no transformer specification removes flicker. That work belongs to an SVC or STATCOM on the furnace busbar, and to a power-quality study done before the transformer is ordered rather than after commissioning.

DC arc furnaces shift the problem instead of deleting it. The arc is fed by a thyristor rectifier through an oil immersed transformer built as a rectifier transformer, so the harmonic spectrum becomes the characteristic 6-pulse or 12-pulse pattern the filter is designed around. The transformer is then specified against a defined harmonic current spectrum rather than a flicker envelope.

 

What We Verify Before a Furnace Transformer Leaves the Factory

Ryan Electric has manufactured transformers since 2007 from a 120,000 m² plant with more than 180 sets of production and test equipment and 37 patents, and we have been an Eaton joint venture partner since 2023. Furnace-class units are engineered oil immersed transformer designs built to the customer’s tap table rather than to a catalogue rating, which is also why our engineers ask for the furnace vendor’s document set before a price is quoted.

The test sequence runs further than the routine list: turns ratio measured at every tap position, winding resistance per phase, insulation resistance and polarisation index, applied and induced-voltage withstand, partial discharge measurement, impedance measured per tap rather than at nominal only, and oil testing for breakdown voltage, moisture and dissolved gas. Where the specification requires it, temperature rise is demonstrated at the governing tap with core and winding temperatures recorded, not just the top-oil rise. For projects bound for the United States and Canada, UL and CSA certification is applied through a third-party inspector rather than by our own quality department, and the laboratory work sits under CNAS accreditation.

The most common problem we see on furnace enquiries is not a rating at all. It is a tap table that does not match the electrode control system, and it stays invisible until hot commissioning — when the furnace is standing idle and the schedule is already tight.

 

Specifying an Arc Furnace Transformer: The RFQ Checklist

Put these items in the enquiry and the technical comparison becomes a fair fight:

• Primary voltage, system frequency (50 or 60 Hz) and MVA at every tap position

• The furnace vendor’s tap table: voltage and current at each position

• Electrode diameter, arc voltage range and the secondary circuit layout

• Impedance target, or the total circuit reactance the furnace designer is working to

• Maximum prospective fault current at the furnace breaker

• Ambient temperature and altitude at the melt shop — 45 °C to 50 °C is normal in the Gulf and in parts of Southeast Asia

• Cooling class (ONAN, OFAF or OFWF) and whether redundant oil pumps are required

• Tap changer type and the expected number of operations per year

• Cyclic rating: RMS current per heat and the number of heats per day

• Tests required — heat run, partial discharge, short-circuit evidence, tap sequence run

• Interface drawings: HV bushings or cable box, LV busbar box orientation, marshalling box position, cable entry direction

• Compliance basis: the IEC 60076 series or IEEE C57, plus whatever certification the destination market requires

One procurement note carries more weight than the rest. A furnace transformer is an engineered unit, so it sits on the critical path next to the furnace shell and the electrode control system. Freezing the tap table early costs nothing; expediting a winding after the melt shop date is fixed is where transformer lead times turn into commissioning delays, and protection scope is cheaper to settle against a protection specification than after the marshalling box is built.

Send us the furnace vendor’s tap table, the electrode diameter and the ambient conditions, and our engineers will come back with an arc furnace transformer recommendation — impedance, tap range, cooling class and the loss figure at each end of the range — plus the tap positions at which temperature rise and impedance will be guaranteed. That conversation takes a week at most, and it is considerably cheaper than a furnace sitting cold.

About the Author: This article was written by the engineering team at Ryan Electric (Jiangsu Ryan Electric Co., Ltd.), a transformer manufacturer founded in 2007 and an Eaton joint venture partner since 2023. Our engineers support UL-, CSA- and IEEE-compliant projects across North America, Southeast Asia, the Middle East and Africa, and our test laboratory operates under CNAS accreditation. For furnace transformer selection, tap table review or a quotation, contact our engineering desk.