In electric power distribution systems, the oil‑immersed transformer is one of the most critical pieces of equipment. The choice of winding material – copper or aluminium – directly affects the transformer’s performance, cost, and service life. As a Chinese enterprise deeply engaged in the power equipment sector, Xinhong Electrical provides this in‑depth analysis of the respective advantages of copper and aluminium windings in oil‑immersed transformers, helping you make a more informed selection decision.
China is one of the world’s largest manufacturing bases for transformers, and oil immersed transformer China products are well known globally for their high cost‑performance ratio and mature technology. Whether copper‑wound or aluminium‑wound, Chinese oil immersed transformer manufacturers can supply high‑quality products that meet international standards such as IEC and ANSI.
Copper is the best conductive metal after silver. At 20°C, copper has a resistivity of approximately 1.72×10⁻⁸ Ω·m, while aluminium has about 2.82×10⁻⁸ Ω·m. Copper’s electrical conductivity is roughly 1.6 times that of aluminium. This means that for the same transmitted current, a copper winding has lower resistance, resulting in lower copper losses (load losses).
For example, a 1000 kVA oil‑immersed transformer with copper windings has a full‑load loss of about 15 kW, whereas an aluminium‑wound unit of the same rating has about 25 kW – a reduction of 30‑40% with copper.
Oil‑immersed transformers rely on circulating transformer oil for cooling. Copper’s thermal conductivity is about 401 W/(m·K), much higher than aluminium’s 237 W/(m·K). Copper windings transfer heat more quickly to the transformer oil, effectively preventing local overheating. This excellent thermal performance gives copper‑wound oil‑immersed transformers a stronger short‑term overload capacity – typically 1.2 to 1.5 times the rated load.
Copper has a tensile strength of 220‑300 MPa (annealed), while aluminium has only 100‑150 MPa. Copper windings are mechanically stronger, ensuring more uniform winding density during manufacture and reducing leakage flux. Under extreme fault conditions such as short circuits, copper windings can withstand 25 kA for 3 seconds without deformation, whereas aluminium windings typically handle only 15‑20 kA.
Copper windings offer superior mechanical and thermal stability, with a design life of 25‑30 years, compared to 15‑20 years for aluminium. Copper is chemically more stable than aluminium, particularly in oil‑immersed environments, where it is less prone to oxidation or electrochemical corrosion. Copper‑wound transformers also operate with lower noise, thanks to copper’s lower coefficient of thermal expansion, which reduces thermal mechanical stress on the coils.
Because of copper’s lower resistivity, the cross‑sectional area of a copper conductor is only about 60% of that of an aluminium conductor for the same resistance. This allows copper‑wound transformers to be smaller and lighter. For the same capacity, a copper‑wound oil‑immersed transformer can have a footprint reduction of 15‑20% compared to conventional designs.
Xinhong Electrical manufactures copper‑wound oil‑immersed transformers using oxygen‑free copper magnet wire and optimised core designs, achieving industry‑leading performance in energy efficiency, reliability, and service life.
Aluminium’s market price is much lower than copper’s. A copper‑wound transformer typically costs 20‑50% more than an aluminium‑wound equivalent. For medium‑sized transformers (400‑1600 kVA), conventional aluminium windings cost roughly RMB 50,000‑120,000 per unit, while copper windings cost RMB 80,000‑180,000. For budget‑constrained projects, aluminium windings are undoubtedly the more economical choice.
Aluminium has a density of only 2.70 g/cm³, while copper is 8.96 g/cm³ – aluminium is about 69% lighter. In a 1000 kVA transformer of the same capacity, copper windings weigh about 1500 kg, whereas aluminium windings weigh only about 1000 kg. Lighter weight means lower transportation costs and simpler installation requirements.
In oil‑immersed transformers, aluminium’s oxidation resistance is better than copper’s. Aluminium naturally forms a dense oxide film on its surface, providing additional protection. Moreover, in cast resin transformer applications, aluminium’s coefficient of thermal expansion is closer to that of epoxy resin, so the epoxy is less likely to crack during thermal expansion and contraction of the coil.
Aluminium‑wound transformers are not a new technology – they have been on the market for decades and are technologically mature. Modern transformer manufacturers, through careful design and sizing optimisation, can achieve efficiency levels comparable to copper‑wound transformers. Aluminium windings are particularly suitable for projects with tight budgets, such as solar power plants.
| Characteristic | Copper Winding | Aluminium Winding |
|---|---|---|
| Conductivity (IACS) | 100% | 61‑65% |
| Resistivity (20°C) | 1.72×10⁻⁸ Ω·m | 2.65‑2.82×10⁻⁸ Ω·m |
| Thermal conductivity | 401 W/(m·K) | 237 W/(m·K) |
| Density | 8.96 g/cm³ | 2.70 g/cm³ |
| Tensile strength | 220‑300 MPa | 100‑150 MPa |
| Design life | 25‑30 years | 15‑20 years |
| Cost difference | Baseline (+20‑50%) | 20‑50% lower |
| Volume / weight | Smaller, heavier | Larger, lighter |
The choice between copper and aluminium windings should be based on the specific technical requirements and budget of each project. If you prioritise maximum efficiency, longest life, and smallest size, copper is the better choice. If cost and weight are the primary concerns, aluminium is a viable alternative.
Beyond winding material, the choice of transformer type is equally critical. The question of oil immersed transformer vs cast resin transformer is central to many project decisions.
In an oil‑immersed transformer, the core and windings are submerged in insulating oil (mineral oil or natural ester). Key advantages include:
Excellent cooling efficiency – oil circulation provides high thermal inertia and strong overload capability.
Higher power density – more compact for the same capacity.
Lower losses – particularly efficient at large ratings.
Longer service life – typically 25‑30 years.
Oil‑immersed transformers are widely used in outdoor substations, factories, industrial parks, and renewable energy projects. Xinhong Electrical also offers integrated prefabricated substation solutions (from professional prefabricated substation manufacturers) that combine oil‑immersed transformers in compact package substations – ideal for urban core areas and industrial sites with limited space.
A cast resin transformer (also called a Dry transformer) has windings encapsulated in epoxy resin and relies on natural air cooling or forced air cooling. Core advantages include:
Oil‑free safety – no risk of oil leakage, lower fire hazard.
Low maintenance – no oil testing or filtration required.
Indoor suitability – perfect for hospitals, airports, shopping malls, basements, and other indoor locations with high fire safety requirements.
Environmental compliance – no liquid insulating medium, no risk of environmental contamination.
The decision between oil‑immersed and dry‑type transformers depends on the specific application:
Outdoor, large‑capacity, high‑load scenarios – prefer oil‑immersed transformers for their better cooling efficiency and higher power density.
Indoor, fire‑sensitive, limited‑maintenance environments – prefer dry‑type (cast resin) transformers.
Environmentally sensitive areas – dry‑type transformers have no oil leakage risk, offering a clear advantage.
As one of China’s leading power equipment manufacturers, Xinhong Electrical is dedicated to providing high‑quality oil immersed transformer products to customers worldwide. Our product range includes:
Copper‑wound and aluminium‑wound oil‑immersed power transformers
Fully sealed oil‑immersed distribution transformers
Prefabricated substations (prefabricated substation)
Dry‑type transformers (Dry transformer)
Our oil‑immersed transformers feature high‑permeability silicon steel cores, oxygen‑free copper or high‑grade aluminium windings, and fully sealed tank designs, offering low losses, low noise, and high efficiency. They comply with international standards including IEC 60076 and ANSI C57.12, and are widely used in industrial plants, residential communities, commercial complexes, renewable energy stations, and public utility power transmission and distribution.
As professional oil immersed transformer manufacturers, Xinhong Electrical provides a one‑stop service from design and manufacturing to after‑sales support. Whether you need a high‑performance copper‑wound solution or a cost‑effective aluminium‑wound option, we can tailor the optimal power solution for your project.
Both copper and aluminium windings have their unique strengths – copper excels in conductivity, heat dissipation, and longevity, while aluminium offers cost savings and lighter weight. Oil‑immersed transformers and dry‑type (cast resin) transformers each have their own suitable applications. The right choice should be based on a comprehensive evaluation of the project’s technical requirements, installation environment, budget, and long‑term operating costs.
Xinhong Electrical, with extensive industry experience, strict quality control, and a professional technical team, is committed to providing the highest quality power equipment and services to customers around the world. For more product information or technical support, please feel free to contact us at any time.
