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Composite combination conductors differ from the two mainstream types of winding conductors and have clearly defined application boundaries:
First, compared to single large-cross-sectional-area flat conductors: the multi-wire parallel structure results in lower eddy current losses and lower winding temperature rise under equivalent current-carrying conditions, making it suitable for medium- to large-capacity transformers and reactors;
Second, compared to CTC (Continuous Transposition) conductors: Since there is no periodic continuous transposition process, production line equipment investment is lower and material costs are more economical; however, loop current losses within the wire bundle cannot be completely eliminated, making it suitable only for medium- and low-voltage, medium-capacity power equipment with moderate loss requirements; This product is positioned as a mid- to high-end, cost-effective conductor material for windings. It is widely used in applications such as distribution transformers, conventional industrial power transformers, and general-purpose reactors, and serves as the mainstream standardized material of choice for windings in traditional oil-immersed power equipment.
The industry standardized mass production arrangement scheme is 2, 3 and 4 single-line single-sided side-by-side structures, corresponding to equipment of different capacity grades, and the parameters and adaptation scenarios are clearly divided:
Two-wire side-by-side structure (2-strand) Two insulated flat wires are flat and juxtaposed, the overall thickness of the harness is small, the flexibility is excellent, and the winding processing difficulty is the lowest.It is mainly suitable for small and medium-sized distribution transformers of 10kV and below, small capacity reactive power compensation reactors, moderate current carrying requirements for single equipment windings, and no strict restrictions on additional losses.
Three-wire side-by-side structure (3-strand) Three insulated flat wires are regularly arranged in parallel, the total conductive cross-sectional area is medium, the eddy current loss and manufacturing cost are balanced, which is the general type of 20kV ~ 110kV conventional oil-immersed power transformer, and the market application ratio is the highest.
Four-wire side-by-side structure (4-strand) Four single wires are connected in parallel synchronously, the total conductive cross-section area of the harness is the largest, and it can carry continuous large rated current. It is suitable for large-capacity shunt reactors, high-power industrial rectifier transformers and step-up transformers in the factory area.
All three types of standard structures are horizontally arranged side by side, the cross section of the harness is regular and uniform, and the core slot wall is completely attached when winding, no local air gap accumulation, and the heat dissipation channel is uniform and stable.
| Name | Composite conductor |
| Conductor | flat copper/aluminum wire, paper/film-wrapped flat copper/aluminum wire, enameled flat copper/aluminum wire |
| Combination method | Two strands side-by-side (2-strand) Three strands side-by-side (3-strand) Four strands side-by-side (4-strand) Arrangement can be customized according to customer requirements |
| Single-strand Dimension | Single-strand thickness (mm): 1.00–8.00; Single-strand width (mm): 4.00–25.00; Single-strand width-to-thickness ratio ≤20 |
| Number of Conductors | 2~4 |
| Wrapped insulation material | Power cable paper, high-density insulating paper, heat-resistant insulating paper, high-elongation fiber paper, aramid paper (Nomex), PET film, etc. |
| Application | Oil-immersed transformers, power transformers, reactors and distribution transformers |
The internal conductive strands in composite stranded wires are divided into three major standardized categories. The conductor base materials fall into two main metal systems—copper and aluminum—to meet different design requirements for capacity, cost, and loss.
Bare Flat Copper / Bare Flat Aluminum Wires: The conductor consists of rectangular flat metal strips without surface insulation. During use, electrical isolation between individual wires is achieved through interlayer insulating paper, with the outer layer uniformly wrapped in insulating paper. This structure has the lowest cost and is commonly used in small power distribution transformers and low-loss dry-type reactors; equipment manufacturers must incorporate a layered insulation isolation process during the winding operation.
Paper/Film-Wrapped Flat Copper/Aluminum Single-Wire: Each flat conductor is pre-wrapped with a thin layer of insulating paper or PET film, providing independent insulation protection for the individual wire. Once the wire bundle is assembled, no additional interlayer insulation medium is required, simplifying the winding process. This type is suitable for oil-immersed power transformers rated at 110 kV and below.
Enameled Flat Copper / Flat Aluminum Single-Wire: The conductor surface is continuously coated with a high-temperature-resistant insulating varnish film. The varnish film is dense and free of pinholes, offering high dielectric strength and stable insulation consistency. The insulation of each individual wire exhibits excellent resistance to partial discharge and is commonly used in high-power filter reactors and industrial rectifier transformers subject to frequent load fluctuations and high alternating magnetic field strengths.
Copper base material (oxygen-free TU1/TU2 copper) features high electrical conductivity, low DC loss, and strong mechanical toughness; it is resistant to cracking during bending and forming, making it the preferred base material for high-voltage, large-capacity oil-immersed main transformers and high-power reactors; Aluminum substrates are lightweight and have lower raw material procurement costs. While their electrical conductivity is lower than that of copper, they require a larger single-wire cross-sectional area to meet the same current-carrying requirements. They are primarily used in small- and medium-sized urban and rural distribution transformers and indoor dry-type distribution equipment. All three types of insulated single wires can be freely combined with either copper or aluminum substrates, allowing engineers to flexibly select the appropriate configuration based on equipment loss specifications and cost budgets.
Mandatory constraints on the geometric dimensions of single wires; all mass-produced specifications must adhere to the following parameter ranges:
Single-wire thickness: 1.00 mm to 8.00 mm;
Single-wire width: 4.00 mm to 25.00 mm;
Upper limit for single-wire width-to-thickness ratio: The ratio of wire width to thickness must be ≤20.
A width-to-thickness ratio of ≤20 is a core engineering design threshold, carrying two key technical implications:
First, at the forming process level: For flat conductors with a width-to-thickness ratio exceeding 20, the surface insulation is prone to tensile cracking during bending and winding, and the copper material is subject to internal stress deformation, significantly increasing the defect rate of winding insulation;
Second, from the perspective of electromagnetic losses: when the width-to-thickness ratio of flat wire is too high, the eddy current concentration effect on one side is significantly enhanced under an alternating leakage magnetic field, causing additional winding losses to rise sharply and making it impossible to meet transformer energy efficiency standards. Product specifications can be categorized into two application-oriented types: narrow-thick single-wire types focus on reducing eddy current losses and are suitable for high-current, high-power equipment; thin-wide single-wire types focus on improving the slot fill rate in the core and are suitable for compact, miniaturized transformer designs.
