Low-Voltage windings for furnace transformers

Low voltages below 1.5 kV, currents in the range of tens of kiloamperes. Parallel disc construction manages extreme current density with controlled inter-disc insulation and direct oil cooling.

LV-FT

Design Rationale

Low-voltage windings for furnace transformers operate under the most demanding current conditions in industrial transformer technology: voltages below 1.5 kV combined with currents in the range of tens of kiloamperes. This extreme current density makes conventional helical or layer winding constructions unsuitable – the conductor cross-section required to carry the rated current cannot be accommodated in a single helical layer without compromising the winding’s mechanical stability and thermal performance.

The solution adopted for this application is to manufacture the coil as a set of discs with a limited number of turns, arranged axially and connected in parallel. Parallel connection of multiple discs distributes the total current among the disc stack, reducing the current per disc to a manageable level while the axial arrangement preserves the winding’s mechanical rigidity against the very high radial and axial electromagnetic forces generated during furnace operation – including the cyclic short-circuit conditions characteristic of electric arc furnace duty.

Discs are separated by dedicated spacers that simultaneously provide mechanical support, define the inter-disc insulation clearance, and create oil channels for direct cooling of each disc’s conductor surface.

Key Engineering Features

Extreme Current Handling

Parallel disc construction distributes tens of kiloamperes across the axial disc stack, maintaining current density within thermal limits without requiring impractically large individual conductor cross-sections.

Furnace Duty Robustness

Disc geometry provides the mechanical rigidity required to withstand the cyclic radial and axial electromagnetic forces generated during EAF and LF operation, including repetitive short-circuit events inherent to arc furnace duty cycles.

Parallel Disc Architecture

A limited number of turns per disc minimises conductor length per parallel path, reducing resistive losses and simplifying the inter-disc connection geometry – critical for current balancing across the parallel stack.

Dedicated Spacer Cooling

Spacers between discs provide inter-disc insulation clearance and create direct oil cooling channels, ensuring effective heat removal from each disc’s conductor surface under continuous high-current operation.

Technical Note

Current Balancing in Parallel Discs – Equal current distribution among parallel discs is achieved by careful attention to the symmetry of the inter-disc connections (leads and busbars) and, where necessary, by transposing the parallel paths over the winding height. Impedance asymmetry between parallel paths – even at the level of a few microohms – generates circulating currents that overload individual discs and reduce overall winding efficiency. C.E.M.’s design process includes explicit verification of parallel path balance as a standard step for all furnace LV windings.

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