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All individual conductors undergo a continuous helical transposition process, cyclically alternating their positions within the bundle at a fixed pitch. Finally, all conductors are uniformly covered with protective insulation—such as cable paper or Nomex insulation paper—to form an integrated wire structure. CTC conductors differ from ordinary composite conductors in that their core features include continuous stranding throughout the entire length, no dead zones in the stranding pattern, and a completely balanced electromagnetic environment. They were originally designed to address issues of circulating current losses, eddy current losses, and uneven temperature rise in the windings of high-voltage, high-capacity transformers.
The CTC conductor consists of three core components: insulated individual wires, a transposition-formed bundle, and an outer layer of integral insulation.
The insulated individual wires are high-purity oxygen-free copper rolled into rectangular flat wires, with their surfaces coated with high-temperature-resistant insulating varnish to ensure electrical isolation between individual wires. The thickness of each individual wire is generally controlled between 0.5 and 2 mm; the thin cross-section minimizes eddy current losses in the individual wires. Multiple insulated individual wires are stacked in two rows, top and bottom, and undergo continuous, orderly, and periodic interleaving via specialized interleaving equipment.
The interleaved and formed wire bundle features a compact structure and uniform arrangement, free from looseness, misalignment, or gap displacement. The outermost layer is wrapped and cured with multiple layers of insulating paper, providing overall insulation protection for the wire bundle against ground and other windings. Some high-end models incorporate a self-adhesive, cured epoxy resin layer to further enhance the overall rigidity and deformation resistance of the wire bundle.
Conventional composite wires simply stack and bundle multiple insulated flat wires without any interleaving structure; the position of each individual wire remains fixed throughout. Since each wire is located in a different radial magnetic field position within the winding, there are significant differences in induced electromotive force, and internal eddy current losses cannot be eliminated.
Conventional composite conductors exhibit uneven current distribution, significant temperature rise variations, and weak mechanical strength against short circuits; they are suitable only for small-capacity medium- and low-voltage equipment. CTC conductors utilize a continuous transposition process, ensuring that each individual wire periodically occupies all positions—inner, outer, upper, and lower—within the bundle. The induced electromotive force, magnetic circuit environment, and heat dissipation conditions are identical for all individual wires, eliminating circulating currents at the source. After forming, CTC conductors exhibit exceptional structural integrity, with tightly wound coils and a stable structure, resulting in mechanical properties that far exceed those of conventional composite conductors. Additionally, CTC conductors achieve higher slot fill rates, lower losses, and more uniform temperature rises, making them suitable for high-voltage, high-capacity, high-end power equipment.
CTC conductors are exclusively used in the windings of various high-voltage, high-capacity, and high-reliability power and electromagnetic equipment. Core applications include main power transformers rated at 110 kV and above, ultra-high-voltage converter transformers, and shunt reactors. They are also widely used in step-up transformers for wind and solar power, traction transformers for rail transit, and large-capacity industrial rectifier transformers. Large dry-type transformers, oil-immersed high-power transformers, and high-frequency high-power reactors all use CTC wire as their core winding material.
This wire is not suitable for small low-voltage distribution transformers; using CTC in low-capacity equipment would result in wasted material costs. Precise application matching allows CTC wire to achieve the optimal balance between performance and cost in high-end power equipment.
| Name | Continuously Transposed wire (CTC Wire) |
| Conductor | Copper and Aluminum |
| Number of strips | 5-80 (odd or even number) |
| Max Overall Dimension | Radial size ( Height): 120mm; (±0.05mm) Axial size ( Width): 26mm (±0.05mm) |
| Single conductor | Thickness(a) : 0.80–3.15mm (± 0.01mm) Width (b) : 2.50 –13.00mm (± 0.01mm) |
| Width(b)/Thickness(a) Ratio of Single conductor | 2.0 ＜ b/a＜ 9.0 |
| Thickness of Acetal Layer | 0.08-0.12mm |
| Thickness of Self –adhesive layer | 0.03-0.05mm |
| Common Wrapping insulation material | Electric kraft paper / High density insulation paper / Thermally upgraded insulation paper / High stretch fiber paper / Nomex paper or according to client’s requirement |
| Standard | IEC, NEMA, GB, JIS, UL |
| Packing | Ply- wooden spool or according to client’s requirement |
| Application | large oil immersed power transformers, reactors and large capacity dry-type transformers. |
Based on the number of individually insulated wires inside, CTC conductors can be divided into three major categories: low-count, medium-count, and high-count. All specifications strictly use an odd number of individual wires to ensure symmetrical transposition and optimal electromagnetic balance.
Low-strand CTC cables contain 5, 7, or 9 individual strands. They feature a simple structure, are easy to manufacture, and offer moderate costs. They are primarily used in 110 kV small- and medium-sized main transformers, standard dry-type transformers, conventional reactors, and other equipment.
Medium-strand CTC cables contain 15 or 21 individual strands. They offer excellent loss control and balanced overall performance. They are suitable for 220 kV and 330 kV high-voltage, high-capacity power transformers, as well as main transformers in wind and solar power step-up substations.
High-strand-count CTC conductors consist of 27, 37, or 49 individual wires, with extremely low losses due to the fine division of strands. They are specifically used in 500 kV, 750 kV, and 1,000 kV ultra-high-voltage converter transformers and ultra-high-capacity hub transformers.
Based on differences in the outer insulation material, CTC conductors can be divided into two categories: paper-insulated CTC and composite-insulated CTC.
Paper-insulated CTC is wrapped in high-purity cellulose cable paper, offering stable insulation performance, strong oil resistance, and low cost. It is the mainstream choice for oil-immersed power transformers, accounting for over 90% of the market and representing the most mature technology.
Composite-insulated CTC is wrapped in Nomex aramid paper and polyimide composite film, providing exceptional high-temperature resistance. Its heat resistance rating can reach Class 200 or higher, and its resistance to aging, moisture, and breakdown is superior to that of ordinary paper-insulated CTC. It is primarily used in dry-type transformers, industrial transformers operating under high-temperature conditions, and power equipment in special environments such as high-altitude and high-humidity regions.
Based on whether the wire harness has self-adhesive curing capability, CTC wires are classified into standard non-self-adhesive types and self-adhesive curing types.
Standard non-self-adhesive CTC wires rely solely on transposition molding and outer paper wrapping for fixation, with a single-wire, non-bonded structure. They offer good structural flexibility and ease of winding, meeting the mechanical strength and stability requirements for conventional transformers. They are suitable for conventional power grid transmission and distribution transformer equipment operating under stable conditions without frequent short-circuit impacts.
Self-adhesive curing CTC conductors feature an integrated epoxy resin bonding layer; after heat curing, the individual wires form a monolithic, rigid structure. This significantly enhances resistance to short-circuit deformation, structural stability, and vibration tolerance, offering the highest reliability rating. They are specifically used in ultra-high-voltage converter transformers, rail transit traction transformers, and grid-connected transformers for renewable energy systems subject to frequent voltage fluctuations.
Based on equipment cooling methods and operating conditions, CTC conductors can be divided into two main types: oil-cooled and air-cooled.
Oil-cooled CTC conductors use specialized oil-resistant insulation paper and oil-resistant paint coatings that do not age or swell even after long-term immersion in insulating oil. The lay gaps are uniform and unobstructed, ensuring smooth flow of insulating oil and high heat dissipation efficiency, making them suitable for all types of oil-immersed transformers.
Air-cooled CTC features a high-temperature-resistant composite insulation structure with a higher heat resistance rating, making it suitable for dry, oil-free operating conditions. The spacing between wire bundles is optimally designed to enhance convective heat dissipation and prevent heat buildup in dry environments. It is widely used in urban indoor dry-type transformers, explosion-proof dry-type transformers for industrial and mining applications, and high-frequency, high-power reactors.
