This guide explains how transformer copper windings are designed and manufactured, the main winding types used in industrial applications, the role of Continuously Transposed Conductor (CTC) in high-performance transformers, the insulation systems that protect the copper from electrical and thermal failure, and how CEM Engineering’s in-house winding shop provides a controlled, quality-assured capability for new windings in refurbishment and new build programmes.

Why copper windings? The choice of conductor material

Copper windings is the dominant conductor material in power transformer for well-established reasons: its electrical conductivity is exceeded only by silver; its thermal conductivity allows efficient heat removal from the winding; its mechanical strength supports the winding structure under the electromagnetic forces of short-circuit events; and it is readily available in a wide range of conductor configurations round wire, flat bar, and continuously transposed cable.

Aluminium is used in some distribution transformer windings as a lower-cost alternative. For the industrial transformer applications that CEM Engineering specialises in EAF, LF, and rectifier transformers, copper is the universal choice. The higher conductivity of copper reduces winding cross-section for a given current-carrying capacity, reducing the transformer’s physical size. More importantly, copper’s superior mechanical strength is essential in applications where the winding must withstand repeated short-circuit forces without deformation.

In a 100 MVA EAF transformer, the electromagnetic repulsive forces on the winding conductors during a bolted short circuit can exceed 1,000 tonnes. The winding must absorb these forces across thousands of events over a 25-year service life. Conductor material, winding geometry, and clamping design must all be engineered for this reality from the outset.

Main winding types in industrial transformers

Power transformer windings are classified primarily by their geometric construction. The main types used in industrial transformers are documented in technical literature including the Electrical Engineering Portal’s comprehensive winding construction guide. The choice of winding type depends on the voltage level, current magnitude, and mechanical requirements of the specific application.

Disc winding

Disc windings, also called continuous disc windings, consist of a series of flat, pancake-shaped coils (discs) wound in horizontal planes and connected in series, alternating between inside and outside crossover points. EEP’s winding construction guide explains that each disc comprises multiple turns wound over other turns, with the crossovers alternating between inside and outside. The axial stack of discs forms the complete winding assembly.

Disc windings are used primarily for high-voltage windings, the HV winding of EAF, LF, and rectifier transformers. Their key mechanical advantage is that the disc geometry distributes the axial electromagnetic forces during short circuits across a large number of discrete disc units, each supported by spacers and a robust clamping structure. This makes disc windings inherently resistant to axial deformation under short-circuit loading, a critical property for industrial furnace transformer service.

The interleaved disc winding is a variant that improves the voltage distribution across the winding during transient overvoltage events (lightning impulse, switching surge) by increasing the series capacitance between adjacent turns. This is particularly important for HV windings at the highest voltage classes.

Helical winding

Helical windings consist of one or more insulated conductors wound continuously in a helix along the length of the winding cylinder, with axial spacers inserted between adjacent turns. EEP describes a helical winding as resembling a corkscrew, with turns separated by spacers and transpositions included where multiple parallel conductors are used to minimise circulating currents between strands.

Helical windings are used for high-current, low-voltage applications, the LV winding of EAF and rectifier transformers, where secondary currents can reach tens of thousands of amperes at voltages of a few hundred volts. The helical geometry accommodates the very large conductor cross-section required for these extreme currents, and the relatively few turns in a low-voltage winding make the helical construction practical.

For very high current LV windings, such as those in large EAF transformers or aluminium smelter rectifier units, the LV winding may be constructed using multiple parallel helical layers, or using flat copper bar conductors in a foil winding configuration.

Layer winding

Layer windings consist of conductors wound in concentric cylindrical layers, with insulation between layers. They are used for high-voltage windings at lower power ratings, and for certain intermediate-voltage windings in multi-winding rectifier transformer designs. Layer windings are simpler to manufacture than disc windings but have different short-circuit force distribution characteristics that must be carefully analysed in the mechanical design.

Continuously transposed conductor (CTC): the key technology for high-performance windings

For the most demanding winding applications, high-current disc windings in large power transformers and the LV windings of furnace transformers, the standard conductor choice is Continuously Transposed Conductor (CTC). APAR’s technical overview of CTC explains that CTC consists of multiple rectangular enamelled copper strands, continuously transposed in a precise pattern and covered with suitable insulation material, ensuring that each strand occupies every possible position within the conductor cross-section during one transposition cycle.

This continuous transposition equalises the magnetic environment for all strands within the conductor bundle, which has two critical effects: it eliminates circulating currents between parallel strands (which would cause additional losses and uneven current distribution), and it minimises eddy current losses in the conductor cross-section. The result, as documented by APAR, is enhanced transformer efficiency, lower heat generation, and improved performance stability under high electrical and thermal stresses.

As noted in the IEEE transformer manufacturing process documentation, CTC conductors are used in combination with different insulation systems depending on the mechanical requirements: standard thermally upgraded paper for normal service, epoxy-bonded CTC for high short-circuit force applications, and Nomex-insulated CTC for the most thermally demanding designs. In industrial furnace transformer service, where short-circuit forces are extreme, epoxy-bonded CTC is the standard choice for the highest-stress winding positions.

Winding Insulation Systems

The copper conductor is the electrical component of the winding. The insulation system is what determines its life. In oil-immersed transformers, the insulation system consists of two complementary elements:

• Solid insulation: cellulose paper (kraft paper or thermally upgraded paper such as Nomex) wrapped around individual conductors and placed between winding turns, discs, and layers. The paper provides the primary electrical insulation between conductors at different potential, and also absorbs and retains the transformer oil, contributing to the overall dielectric strength of the insulation system.
• Liquid insulation: transformer oil (mineral oil, natural ester, or synthetic ester) impregnates the paper insulation and fills the spaces between conductors, providing additional dielectric strength, cooling by convection, and protection of the paper from moisture and oxidation.

The combination of paper and oil achieves dielectric strengths that neither material could provide alone, and this synergy is why the oil-paper insulation system has dominated power transformer design for more than a century. The insulation system ages thermally over time: the paper undergoes chemical degradation (hydrolysis and pyrolysis) that reduces its mechanical strength. The Degree of Polymerisation (DP) test measures this degradation, a DP below 200 indicates severely aged insulation approaching end of life.

Short-circuit withstand: the mechanical design challenge

The most demanding mechanical requirement in winding design is short-circuit withstand, the ability to absorb the large electromagnetic forces generated by a bolted short circuit without deformation, turn-to-turn contact, or structural failure. In industrial transformer service, this requirement is particularly severe: EAF transformers experience thousands of partial and bolted short-circuit events during a normal service life.

The electromagnetic forces during a short circuit have two components: axial forces (acting along the transformer’s vertical axis, tending to compress or expand the winding axially) and radial forces (acting across the winding’s horizontal plane, tending to push the outer winding outward and compress the inner winding inward). Both must be addressed in the winding design.

Axial force management relies on the clamping system, the steel plates and tie-rods that hold the winding assembly under defined compression. In disc windings, the axial spacers between discs must be correctly dimensioned and the clamping pressure maintained throughout the transformer’s service life. The Royal SMIT independent clamping system, which CEM Engineering references as a design benchmark, addresses this by maintaining winding clamping pressure independently of core clamping, preventing the progressive loosening that occurs in systems where core and winding clamping are coupled.

Radial force management is addressed in the conductor selection and winding geometry: the use of high-strength, epoxy-bonded CTC increases the mechanical rigidity of the winding assembly, resisting the radial deformation that can lead to conductor-to-conductor contact and inter-turn short circuits.

CEM Engineering’s in-house winding shop

CEM Engineering operates an in-house winding shop that produces custom copper windings for industrial transformer refurbishment and new build programmes. This capability is a significant differentiator in the industrial transformer service market, where most service organisations depend entirely on external manufacturers for replacement windings, introducing lead time, quality control, and specification compliance risks that in-house capability eliminates.

Our winding shop produces disc windings and helical windings for EAF, LF, and rectifier transformer applications, using copper conductors including CTC in standard and epoxy-bonded configurations. The facility operates under our ISO 9001-certified quality management system, with full traceability of conductor materials, insulation materials, and dimensional checks throughout the winding process.

The practical benefits for clients are significant:

• Reduced lead times: in-house production eliminates the long lead times associated with sourcing replacement windings from external manufacturers, which can extend a refurbishment outage by weeks or months.
• Quality control: we specify, inspect, and accept the conductor materials and insulation components that go into every winding we produce. There is no dependence on a third party’s quality system.
• Engineering continuity: the same engineering team that diagnoses the fault, defines the refurbishment scope, and oversees the return-to-service testing also specifies and reviews the new winding design. This end-to-end engineering continuity prevents the specification gaps that occur when winding design is separated from service engineering.
• Flexibility for custom designs: industrial transformers are custom-engineered products. Replacement windings must match the specific geometry, conductor specification, and insulation design of the original — or incorporate improvements to address identified failure modes. Our in-house capability allows this customisation without the constraints of a catalogue-based external supplier.

The winding refurbishment process

A transformer winding refurbishment follows a defined sequence of steps that must be executed with precision to restore the unit to its original, or improved, performance level:

• Fault analysis: DGA review, visual inspection, and electrical testing (winding resistance, insulation resistance, FRA) to characterise the fault and define the refurbishment scope.
• Tank opening and active part extraction: the transformer is drained of oil, the tank cover removed, and the active part (core and winding assembly) extracted for inspection.
• Winding disassembly and damage assessment: the fault-affected winding(s) are removed and inspected. Clamping pressure measurement, conductor condition assessment, and insulation sampling define the full extent of degradation.
• New winding manufacture: replacement windings are produced in our winding shop to the refined design specification, incorporating any improvements identified during fault analysis.
• Assembly and vacuum drying: the new winding assembly is fitted to the core, the active part reassembled, and the complete unit subjected to vacuum drying to remove moisture from the insulation system before oil filling.
• Oil impregnation and filling: the dried active part is impregnated under vacuum with new transformer oil, filling the insulation system and removing residual air.
• Factory acceptance testing: the refurbished transformer is tested to the relevant IEC 60076 test programme, including winding resistance, ratio and vector group, no-load and load loss measurement, impulse and power frequency dielectric tests, and heat run.

Why choose CEM Engineering’s winding shop

Our winding shop exists because we believe that industrial transformer refurbishment should be managed end-to-end by engineers who understand the application. We are not a general-purpose transformer repair workshop. We produce windings specifically for the EAF, LF, and rectifier transformer applications that are our engineering core, units where the winding design, conductor specification, and mechanical design must be calibrated for the most demanding operating conditions in industrial electrical service.

When you engage CEM Engineering for a transformer refurbishment programme, you get engineering expertise, in-house winding production, ISO 9001-certified quality processes, and a continuous chain of responsibility from fault diagnosis through return to service.

Contact our team to discuss a winding refurbishment assessment for your industrial transformer.

FAQ – copper windings

What is the difference between a disc winding and a helical winding in a transformer?

A disc winding consists of a series of flat pancake-shaped coils stacked vertically, used primarily for high-voltage windings. A helical winding consists of one or more conductors wound in a continuous helix along the winding cylinder, used primarily for high-current, low-voltage windings. EAF and rectifier transformers typically use disc windings on the HV side and helical windings on the LV side.

What is Continuously Transposed Conductor (CTC) and why is it used?

CTC is a conductor cable consisting of multiple rectangular copper strands continuously transposed in a precise pattern, ensuring that each strand occupies every possible position within the cross-section during one transposition cycle. This eliminates circulating currents between parallel strands and minimises eddy current losses, key advantages for high-current winding applications. APAR’s technical overview notes that CTC has become integral to modern transformer design for demanding power applications.

How does a transformer winding fail?

Transformer winding failures typically result from: inter-turn or inter-layer short circuits caused by insulation breakdown from thermal ageing, mechanical damage from short-circuit forces, or moisture ingress; conductor deformation from excessive electromagnetic forces during repeated short-circuit events without adequate mechanical support; or connection failures at winding leads and terminals due to thermal cycling and vibration. Regular DGA monitoring and periodic winding condition inspection are the primary tools for detecting developing faults before they cause failure.

What is the winding drying process and why is it critical?

Transformer winding insulation, cellulose paper, absorbs moisture from the atmosphere during manufacture and repair. Moisture dramatically reduces the dielectric strength of the oil-paper insulation system and accelerates thermal ageing. Before oil filling, all new or repaired windings must be thoroughly dried under vacuum to remove this moisture. The drying process is one of the most critical quality steps in transformer manufacturing and refurbishment, inadequate drying is a leading cause of premature dielectric failure in new or refurbished units.

Can CEM Engineering produce replacement windings for transformers from other manufacturers?

Yes. Our winding shop produces replacement disc and helical windings for EAF, LF, and rectifier transformers regardless of the original manufacturer. We work from the original design documentation where available, or from dimensional measurements and conductor specification derived from inspection of the removed winding, incorporating design improvements where the fault analysis identifies weaknesses in the original design.

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