Electric Arc Furnace Transformer

Overview

An electric arc furnace transformer steps utility voltage down to the few hundred volts and tens of kiloamps that an arc furnace needs to melt steel scrap. It is one of the most heavily stressed transformers in industry. The arc is an unstable, non-linear load: it draws harmonic-rich current, it short circuits every time an electrode bores into cold scrap, and the demand swings from a near short circuit at melt-in to a steady refining arc within one heat. A furnace transformer therefore carries far more conductor copper on its secondary than a distribution transformer of the same rating, has a wide-range on-load tap changer to set arc power, and is braced internally for repeated electromagnetic shocks.

Ratings commonly run from 40 MVA on a small meltshop to over 150 MVA on a modern ultra-high-power furnace. The primary is usually 33 kV class, fed from a dedicated furnace bus. The secondary delivers 200 to 1,500 V and 30 to 120 kA per phase. Because the secondary voltage must change while the furnace is energised, the on-load tap changer is a defining feature, often with thirty or more taps so the operator can pour maximum power into cold scrap and then back off to a soft refining arc.

Core and magnetic circuit

The Magnetic Core is a three-limb stacked design built from grain-oriented silicon-steel laminations of 0.23 to 0.30 mm thickness, laser-scribed to cut loss, and assembled with mitred step-lap joints to reduce no-load loss and noise. Flux density is held lower than in a utility transformer because the harmonic content of furnace current would otherwise drive the core into saturation. Each limb carries the concentric windings; the yokes close the magnetic path top and bottom. A clamping frame tied by insulated tie rods compresses the stack against the axial forces of a short circuit, and magnetic flux shunts on the tank wall divert stray flux so the tank does not overheat. The core is earthed at one point through a removable link so insulation tests can be made.

Windings and secondary bus

The high-voltage winding is a disc-wound coil of paper-insulated transposed copper, with a separate regulating section whose taps the OLTC selects. The low-voltage winding is the unusual part: it has very few turns and must pass tens of kiloamps, so it is built from paralleled copper sheets or water-cooled hollow conductors rather than ordinary wire. Heavy clamp rings resist the radial forces that try to blow the coils apart on every electrode short. Current leaves the winding through flexible copper links into the water-cooled secondary bus. That bus includes the delta-closing connection and the electrode terminal palms; many sections are hollow water-cooled bus bars fed from a cooling manifold, since at these currents even short copper runs dissipate large heat. A flow switch trips the furnace if cooling water is lost.

Insulation, tank and cooling

The dielectric system follows standard oil-paper practice but is rated for the thermal cycling of furnace duty. Mineral insulating oil both insulates and cools; kraft paper and pressboard barriers form the major insulation between windings, with static rings grading the impulse field at the coil ends. The active part sits in a stiffened steel tank. Furnace transformers usually run a forced cooling class: oil pumps push oil through oil-to-water coolers (OFWF) or through radiators with fans (OFAF), because the load factor is high and the heat to reject is large.

Tap changer and protection

The on-load tap changer is the operator's power control. Its diverter switch makes and breaks load current in tens of milliseconds while transition resistors limit the circulating current between taps; the selector pre-positions the next tap. Protection follows utility practice: a Buchholz relay watches for gassing and oil surge, a pressure-relief device and sudden-pressure relay guard against internal arcing, and winding temperature indicators stage the cooling and alarm on overload. Bushing current transformers feed differential and overcurrent relays. An online dissolved-gas monitor tracks fault gases between oil samples.

Standards, variants and use

Furnace transformers are designed to IEC 60076 and IEEE C57.12.00, with the application guidance of IEC 60076-22-7 for arc-furnace duty. Variants include the booster reactor built into or beside the transformer to stabilise the arc, the delta-on-electrode arrangement that closes the secondary delta at the furnace itself to cut secondary impedance, and DC arc furnace rectifier transformers that feed a thyristor bridge instead of an AC arc. The conservator with its air cell and silica-gel breather keeps moisture out of the oil. All auxiliaries land in the marshalling cabinet, which wires the cooling, tap changer, and protection to the meltshop control system. These transformers are found in every electric steelmaking plant, ferroalloy smelter, and many foundries, where they are typically the single largest electrical asset in the shop.