Project snapshot
| Sector | Iron and steel — electric arc furnace (EAF) melt shop |
| Region | [Africa] |
| Application | Furnace supply and on-load voltage regulation for an EAF modernization programme |
| Product | Furnace transformer / SZ11 on-load voltage regulating transformer |
| Rating | [ ] MVA |
| Voltage ratio | [ ] kV / [ ] V secondary |
| Impedance voltage | [ ] % |
| Tap changing | On-load tap changer, [ ] steps / ±… % |
| Cooling | ONAF |
| Standard | IEC 60076, short-circuit withstand per IEC 60076-5, OLTC per IEC 60214 |
| Delivered | [Q · 20 ] |
The challenge
An electric arc furnace is one of the most punishing loads a transformer will ever see. Unlike a distribution transformer, which is sized for a relatively steady load profile, a furnace transformer is specified for a duty cycle — and the melt cycle is brutal.
During meltdown, the electrodes repeatedly strike and short against the scrap charge. Each event drives fault current through the windings and generates electromagnetic forces that try to deform them. Over a single heat, that happens hundreds of times. Over a campaign, hundreds of thousands.
At the same time, arc instability means the load swings continuously and violently across the melt. The plant’s existing transformers were struggling on both counts — cumulative mechanical damage from repeated short-circuit forces, and voltage instability under rapid load swings — which was translating into unplanned stoppages during melts, the most expensive kind of downtime in a melt shop.
Three requirements came out of that diagnosis:
Short-circuit withstand. The mechanical design had to survive repeated fault events across the furnace’s service life, not merely pass a single type test.
Voltage stability. Secondary voltage had to be adjustable under load, so the melt power could be controlled through each phase of the cycle without de-energising.
Overload capability. The modernization programme pushed higher throughput through the same melt shop, so the transformers had to carry sustained overload without thermal runaway.
Our solution
Reinforced construction for short-circuit withstand
The furnace transformer was built with oversized windings and bushings dimensioned for the fault levels the melt shop actually produces. Winding conductors, clamping structure and lead exits were sized so that repeated short-circuit forces do not progressively loosen the assembly — the failure mode that ends most furnace transformer lives. Design and verification were aligned to IEC 60076-5, the part of the IEC 60076 series covering ability to withstand short circuit.
This is the defining difference between a furnace transformer and a general-purpose unit, and it is why our electric arc furnace and rectifier transformers are engineered as a separate product line rather than adapted from distribution designs.
On-load voltage regulation through the melt cycle
We supplied an SZ11 on-load voltage regulating transformer alongside the furnace unit. An on-load tap changer lets secondary voltage be adjusted while the furnace stays energised, so melt power can be trimmed for each stage of the cycle — boring, meltdown and refining — instead of being fixed at a compromise setting. The tap changer itself is specified to IEC 60214.
In practice this does two things: it holds voltage stable for the rest of the plant when the arc swings, and it gives the melt shop control over power input without interrupting the heat.
ONAF cooling for continuous overload
The units were specified with ONAF cooling — oil forced, air forced — giving a defined overload envelope rather than a single continuous rating. For a melt shop running back-to-back heats, that headroom is what prevents thermal accumulation across a shift. Our breakdown of transformer cooling classes (ONAN, ONAF, AN, AF) explains how the ratings differ.
Engineered around the duty cycle, not the nameplate
Rather than sizing to a nameplate kVA figure, the transformers were specified against the plant’s actual melt profile: heat duration, number of heats per day, short-circuit frequency and the overload envelope the programme required. That is the only way to get a furnace transformer that lasts a campaign — and it is why the specification table below matters more than the rating line.
Technical specification
| Parameter | Value |
|---|---|
| Rated power | [ ] MVA |
| Primary / secondary voltage | [ ] kV / [ ] V |
| Secondary current | [ ] A |
| Frequency | [50 / 60 Hz] |
| Impedance voltage | [ ] % |
| On-load tap range | [±… %, … steps] |
| Cooling | ONAF |
| Overload duty | |
| Short-circuit withstand | Per IEC 60076-5 |
| Standards | IEC 60076, IEC 60076-5, IEC 60214 |
Testing and verification
Every unit passed routine tests in accordance with IEC 60076 before release: winding resistance, voltage ratio and vector group, no-load loss and current, load loss and impedance voltage, applied and induced voltage withstand, and insulation resistance.
Our guide to transformer routine testing sets out what a unit should pass before shipment, and IEC 60076 vs ANSI/IEEE covers which standard applies to your market.
Result
The modernization programme went ahead with furnace and regulating transformers specified for the melt cycle rather than a nominal rating. Short-circuit withstand and on-load voltage control removed the two failure modes that had been driving unplanned stoppages, and ONAF cooling gave the melt shop the overload headroom the higher throughput demanded — improved melt-cycle reliability with fewer unplanned outages .
Planning an EAF, ladle furnace or induction melting project? Review our electric arc furnace and rectifier transformer solutions, or send us your melt profile and fault levels for a proposal.
Frequently asked questions
How is a furnace transformer different from a standard distribution transformer?
A distribution transformer is sized for a relatively steady load. A furnace transformer is specified for a duty cycle: it delivers very high secondary current at low voltage, is built with a higher impedance to limit fault current, and is mechanically reinforced to survive repeated electrode short circuits — hundreds per heat. Its cooling is also rated for sustained overload rather than continuous nameplate duty.
Why does an EAF transformer need higher impedance?
Impedance limits the magnitude of fault current when the electrodes short against the charge. Higher impedance is what keeps those repeated fault events within what the winding and the upstream network can tolerate, and it stabilises the arc. The trade-off is lower secondary voltage under load, which is one reason on-load tap changing is paired with it.
What does the on-load tap changer actually do during a melt?
It lets secondary voltage be adjusted while the furnace stays energised. Melt power can therefore be reduced during boring, raised for meltdown and trimmed for refining, without interrupting the heat. It also holds voltage steadier for the rest of the plant when the arc swings. Tap changers are specified to IEC 60214.
How is short-circuit withstand verified?
IEC 60076-5 sets out how a transformer’s ability to withstand short circuit is established — either by calculation or by test, depending on rating and what the purchaser specifies. The output is a withstand assessment or test report, which is what a melt shop should ask to see before accepting a furnace transformer.