Application and Selection Guide of Copper Foil in Dry-Type Transformers
Introduction
Dry-type transformers, with their excellent fire resistance, environmental friendliness, and low maintenance requirements, have become the preferred power distribution equipment in commercial buildings, data centers, industrial plants, and rail transit. Unlike oil-immersed transformers, dry-type transformers rely on air convection for heat dissipation and use solid insulation materials for electrical isolation. Under these operating conditions, copper foil windings exhibit significant technical and economic advantages over traditional round wire windings.
This article systematically introduces the application of copper foil in dry-type transformers, covering material specifications, insulation methods, design considerations, application scenarios, and selection guidelines.
Dry-type transformers are widely used in locations with high fire protection requirements. Regarding winding materials, copper foil winding is the mainstream winding form for medium-to-large capacity dry-type transformers (typically above 1000kVA), featuring excellent electrical performance, good thermal stability, and high mechanical strength.
I. Technical Advantages of Copper Foil Windings in Dry-Type Transformers
1. Reduced Skin and Proximity Effect Losses
At power frequencies (50Hz/60Hz) and higher, the skin and proximity effects significantly increase the AC resistance of round wire windings, leading to increased load losses. Copper foil, as a planar conductor, distributes current more evenly across its cross-section. For dry-type transformers used in special applications such as rectification or frequency conversion, the AC resistance of copper foil windings is much lower than that of round wire windings with the same cross-sectional area, effectively reducing load losses.
2. Excellent Heat Dissipation Performance
Dry-type transformers lack the cooling and insulation effects of transformer oil, making heat dissipation management a key design challenge. Copper foil windings offer the following thermodynamic advantages:
- High Thermal Conductivity: Copper has a thermal conductivity of approximately 400 W/(m·K), allowing heat to be efficiently conducted from the inside of the winding to the surface and then dissipated to the surrounding air through convection.
- Compact Winding Structure: The foil windings are tightly interlayered, minimizing internal air gaps and effectively reducing the risk of hot spot formation.
- Lower Temperature Rise: Actual measurement data shows that, for the same capacity, the hot spot temperature of copper foil windings is 10K to 15K lower than that of round wire windings.
3. Higher Mechanical Strength
Under short-circuit conditions, the windings withstand enormous electrodynamic forces. Copper foil windings are integral planar conductors, without the interlayer gaps of round wire windings, resulting in stronger resistance to electrodynamic deformation. This advantage is particularly significant in resin-cast dry-type transformers—the copper foil windings are completely encapsulated in epoxy resin, forming an integral structure, greatly improving mechanical strength.
4. Compact Structure and High Space Utilization
The radial space occupied by copper foil windings is smaller than that of round wire windings of the same capacity, allowing for a more compact transformer design. This is of great significance for locations with limited installation space (such as underground power distribution rooms, floor-level distribution rooms, and ship cabins).
5. Lower Operating Noise
The compact copper foil winding structure results in low leakage flux and reduced magnetostrictive vibration. Actual measurements show that the sound pressure level of a copper foil-wound dry-type transformer is 3-5 dB(A) lower than that of an equivalent round wire winding design, making it particularly suitable for noise-sensitive commercial buildings, hospitals, and residential areas.
II. Key Specifications for Copper Foil in Dry-Type Transformers
1. Material Purity
Copper foil for dry-type transformers must be made of electrolytically tough copper (ETP copper, Cu-ETP, CW004A), with a copper content of not less than 99.90% and an electrical conductivity of not less than 101% IACS (International Annealed Copper Standard). Impurities such as oxygen and phosphorus increase resistivity, leading to overheating under load. Each batch of products should be accompanied by a material certificate (mill test report).
2. Thickness Range
The thickness of copper foil for dry-type transformers is typically 0.2mm to 3.0mm, selected based on the transformer’s voltage rating and current carrying capacity:
- Low-voltage, high-current windings: 1.0mm to 3.0mm
- High-voltage, low-current windings: 0.2mm to 1.0mm
Thickness tolerance should be controlled within ±0.01mm. Excessive tolerance will affect winding quality and interlayer insulation fit.
3. Width Selection
The copper foil width is determined based on the transformer’s rated capacity, voltage rating, and window geometry. The standard width range is 10mm to 600mm. A width that is too small increases the number of winding layers and time; a width that is too large places demands on winding equipment and operation. The optimal width must comprehensively consider manufacturing efficiency and transformer design constraints.
4. Surface Quality
The copper foil surface must be free of oxide spots, scratches, creases, and wavy edges. Dry-type transformer insulation relies entirely on solid dielectric and air; surface defects can cause partial discharge. A 100% visual inspection should be performed before the product leaves the factory.
5. Edge Treatment
The cut edges of copper foil must be deburred and rounded. Sharp burrs may penetrate interlayer insulation under winding or short-circuit electrodynamic forces, leading to inter-turn faults. Dedicated deburring and rounding equipment should be used during manufacturing.
III. Copper Foil Winding Insulation Methods
1. Resin Cast Insulation
Resin casting is the most common insulation method for dry-type transformers. After the copper foil winding is completed, it is placed in a vacuum chamber for epoxy resin vacuum casting. The cured resin forms a dense, monolithic structure with excellent dielectric strength, mechanical stiffness, and moisture and dust resistance. Cast resin transformers are widely used in commercial buildings, industrial facilities, and infrastructure projects.
In this application, the copper foil surface is typically coated with a layer of polyester imide or polyamide imide varnish to provide inter-turn insulation before resin casting.
2. NOMEX Paper Insulation
In NOMEX-insulated dry-type transformers, the copper foil windings are wrapped with NOMEX paper (aramid insulating paper) to achieve interlayer insulation. This structure eliminates the resin casting process, resulting in better heat dissipation and stronger overload capacity. NOMEX-insulated transformers are commonly used in special applications such as rail transit and shipbuilding where high overload capacity and maintainability are required.
3. Varnish Impregnation
After the copper foil windings are wound, they are impregnated with insulating varnish (such as polyester imide varnish or polyamide-imide varnish), which cures to form an insulation layer. This process is low-cost and simple, but the insulation class is relatively low, and it is mostly used in small-capacity dry-type transformers.
LP Industry provides copper foil products with pre-coated insulation layers according to customer needs. The most common coating is polyester imide or polyamide-imide varnish, with thermal class up to 180 (Class H), 200, and 220.
IV. Design Considerations for Dry-Type Transformers with Copper Foil Windings
1. Current Density Selection
Dry-type transformers have inferior heat dissipation compared to oil-immersed transformers, so the current density must be selected appropriately. Typical current densities for copper foil windings are 2.0 A/mm² to 3.0 A/mm²:
- Natural Air Cooling (AN): 2.0 A/mm² to 2.5 A/mm²
- Forced Air Cooling (AF): 2.5 A/mm² to 3.0 A/mm²
Lower current densities increase the winding cross-sectional area and transformer size, but reduce load losses and temperature rise.
2. Air Duct Design
Air duct design is crucial to the thermal performance of dry-type transformers. Axial and radial air ducts must be properly arranged in the copper foil windings to ensure smooth exhaust of hot air. The number and location of air ducts should be optimized based on the transformer’s thermal model and cooling requirements.
3. Insulation Coordination
Dry-type transformers rely entirely on solid dielectrics and air gaps for insulation, making insulation coordination design crucial. Sufficient safety margins must be provided for interlayer insulation, end insulation, and phase-to-phase insulation to withstand lightning impulses, switching impulses, and power frequency overvoltages.
4. Temperature Rise Control
Dry-type transformers have stricter temperature rise limits than oil-immersed transformers:
- Class F insulation: Winding temperature rise limit 100K
- Class H insulation: Winding temperature rise limit 125K
Design practice recommends controlling the actual temperature rise to 80% or less of the standard limit. Hot spot temperature is a critical parameter; localized overheating accelerates insulation aging and shortens the transformer’s service life.
5. Short-Circuit Withstand Capability
Short-circuit withstand capability is a key challenge in dry-type transformer design. Although copper foil windings have good inherent mechanical strength, the design still requires short-circuit current testing verification according to IEC 60076-5 or relevant national standards. The design should consider the most severe short-circuit conditions to ensure that the windings do not deform and the insulation is not damaged.
V. Copper Foil vs. Round Wire: Selection Guide
Whether to use copper foil or round wire for dry-type transformer windings depends on the following factors:
1. Transformer Capacity
- Below 500kVA: Round wire windings are less expensive and technically meet most application requirements.
- 500kVA to 1000kVA: Both options are feasible; copper foil has a significant performance advantage with a moderate cost increase.
- Above 1000kVA: Copper foil winding is the preferred option, offering advantages such as low loss, compact structure, and low noise.
2. Application Environment
- Commercial Buildings, Data Centers, Hospitals: Copper foil winding is recommended for lower noise, smaller size, and higher reliability.
- General Industrial Applications: If cost is the primary consideration and performance requirements are moderate, round wire can also meet the needs.
3. Life Cycle Cost Analysis
Copper foil wound transformers may have slightly higher initial costs due to their demanding manufacturing process. However, their lower load losses significantly reduce operating costs over the transformer’s service life. A life cycle cost analysis over a 15-20 year period shows that copper foil wound transformers are more economical for medium to large capacity units.
VI. Common Application Scenarios
1. Commercial Buildings
Power distribution rooms in office buildings, shopping malls, and hotels are typically located in basements or on floors, requiring strict fire protection and low noise levels. Copper foil wound dry-type transformers are the standard choice for these applications.
2. Data Centers
Data centers have extremely high requirements for power supply reliability. UPS systems and power distribution networks extensively use dry-type transformers, and copper foil windings, with their low losses and high reliability, have become the preferred winding technology for data center applications.
3. Rail Transit
Rail transit systems such as subways, high-speed rail, and light rail have extremely strict fire protection requirements for platforms and tunnels. Dry-type transformers are standard, with copper foil windings providing the mechanical strength and seismic resistance required for rail transit applications.
4. Industrial Facilities
Industrial facility power distribution systems operate under conditions of high ambient temperature and large load fluctuations. Copper foil-wound dry-type transformers are maintenance-free, have excellent heat dissipation, and are suitable for harsh industrial environments.
5. New Energy Projects
Photovoltaic power plants and wind farms use dry-type transformers for voltage boosting and power distribution. High-efficiency copper foil-wound transformers help improve overall system efficiency and power generation.
6. Marine and Offshore Platforms
Marine and offshore platform dry-type transformers require small size, light weight, and fire and explosion protection. Copper foil-wound transformers have a compact structure and are ideal for marine and offshore platform applications.
VII. Quality Control and Testing
1. Raw Material Incoming Inspection
Incoming copper foil inspection items include: chemical composition analysis, conductivity testing, tensile strength measurement, elongation testing, thickness tolerance verification, and surface quality assessment. Non-conforming materials are rejected and returned.
2. Winding Process Monitoring
During the winding process, the following parameters are monitored and controlled: foil tension, interlayer insulation quality, inter-turn insulation quality, and winding tightness.
3. Finished Product Inspection
After the transformer is assembled, comprehensive factory tests are conducted according to IEC 60076 and relevant national standards:
- Turns ratio test
- DC resistance measurement
- Insulation resistance test
- Dielectric loss factor (tan δ) test
- Power frequency withstand voltage test
- Induced withstand voltage test
- Partial discharge test
- Temperature rise test
- Short-circuit withstand capability test
Only products that pass all tests can leave the factory.
LP Industry conducts strict quality inspections at every stage from raw material intake to finished product shipment, and each batch of products is accompanied by a complete test report.
VIII. Selection Recommendations
- Verify Material Certificates: The purity of copper directly affects conductivity and resistivity. Purchase through authorized channels and require a material certificate for each batch.
- Check Thickness Tolerances: Excessive tolerances affect interlayer insulation quality. Tolerances should be strictly controlled within ±0.01mm.
- Confirm Insulation Requirements: Resin-cast and NOMEX paper dry-type transformers have different copper foil insulation requirements; confirm specific specifications with the transformer manufacturer.
- Assess Supplier Production Capacity: Copper foil winding requires specialized winding equipment. Select suppliers with complete production processes and proven experience.
- Review Quality Certifications: Suppliers holding ISO 9001, ISO 14001, and related product certifications demonstrate standardized quality management systems.
- Confirm Delivery Lead Times: Dry-type transformer projects typically have strict schedules; supplier delivery capability is an important consideration.
IX. LP Industry Copper Foil Products
Zhengzhou LP Industry Co., Ltd. has specialized in the electromagnetic wire industry for 30 years and is a source manufacturer of copper foil, aluminum foil, and enameled wire. Copper foil product specifications are as follows:
- Material: Electrolytic copper (ETP), Cu content ≥ 99.9%
- Thickness: 0.2mm to 3.0mm, customizable
- Width: 10mm to 600mm, customizable
- Conductivity: ≥ 101% IACS
- Surface Treatment: Deburring, edge rounding
- Insulation Coating: Polyester imide or polyamide-imide varnish, optional
- Thermal Class: Class F (155°C), Class H (180°C), 200, 220
Products are exported to over 50 countries worldwide. Quality certifications include ISO 9001, ISO 14001, ISO 45001, and UL, REACH, RoHS compliance.
Standard specifications are available from stock, shipped within 15 days; custom specifications delivered within 30 days.
For inquiries, please contact: office@cnlpzz.com or WhatsApp: +86-19337889070. Our technical team can recommend suitable copper foil specifications based on your application needs.
This article is original content from LP Industry. Reproduction requires authorization.