What material is the oil immersed transformer winding?

1. Common Winding Materials & Core Characteristics

(1) Copper Windings

• Material: High-purity oxygen-free copper wire (purity ≥99.95%), with excellent electrical conductivity and thermal conductivity.
• Key Advantages: Low resistivity (0.0172Ω·mm²/m at 20°C), small load loss (energy loss during operation), strong mechanical strength, and good corrosion resistance—extending the transformer’s service life and improving efficiency (meeting Class 1 energy efficiency standards).
• Application: Widely used in high-end, large-capacity transformers (e.g., 110kV/10kV power transformers) or scenarios with high efficiency requirements (e.g., industrial parks, urban power grids).

(2) Aluminum Windings

• Material: High-purity aluminum wire (purity ≥99.7%), with cost advantages.
• Key Characteristics: Higher resistivity (0.0283Ω·mm²/m at 20°C) than copper, leading to larger load loss. To compensate, aluminum windings require a larger cross-sectional area, resulting in larger transformer volume and lighter weight for the same capacity.
• Application: Suitable for cost-sensitive, small-to-medium capacity transformers (e.g., rural distribution transformers ≤500kVA).

(3) Copper-Aluminum Hybrid Windings

• Structure: Copper wire for key parts (e.g., high-voltage windings, joints) and aluminum wire for low-voltage windings—balancing performance and cost.
• Identification Focus: Joint forms (copper-aluminum joints are usually over-wired at terminals) and resistance differences between winding sections.

2. Practical Identification Methods for Winding Materials

(1) Weight Comparison

• Principle: Aluminum has a lower density (2.7g/cm³) than copper (8.96g/cm³). For transformers of the same capacity and structure, aluminum-wound models are 20–30% lighter than copper-wound ones.
• Operation: Use a crane scale to weigh the transformer body (after draining oil) and compare with the manufacturer’s weight data for the same model.

(2) Volume Comparison

• Principle: To achieve the same current-carrying capacity, aluminum windings need a larger cross-sectional area than copper windings. Thus, for the same capacity, aluminum-wound transformers have a 15–25% larger overall volume (especially winding diameter) than copper-wound ones.
• Operation: Measure the transformer’s length, width, and height, or observe the winding diameter through inspection windows—larger volume often indicates aluminum windings.

(3) Nameplate Marking Check (Most Direct Method)

• Standard Regulation: Regular manufacturers (e.g., CHH Power) mark winding materials on the nameplate in accordance with GB/T 1094.1 standards:
  • Copper windings: Marked with model codes like S7, S9, S11 (no “L” in the code).
  • Aluminum windings: Marked with model codes like SL7, SL9 (the letter “L” stands for aluminum winding).
• Operation: Locate the transformer nameplate (usually on the tank side) and check the model code for the presence of “L”.

(4) DC Resistance Measurement (Accurate Quantitative Method)

Method ①: Temperature Coefficient Calculation

• Formula: R₁/R₂ = (x + T₁)/(x + T₂)
  • R₁, R₂: DC resistance of the winding measured at temperatures T₁ and T₂ (unit: °C).
  • x: Material constant (theoretical value: 235 for copper, 225 for aluminum).
• Operation: Measure winding resistance at two different temperatures (e.g., ambient temperature 20°C and after short-term operation at 40°C), substitute into the formula to calculate x—matching 235 indicates copper, 225 indicates aluminum.

Method ②: Resistivity Calculation

• Formula: ρ = (R × S)/L
  • ρ: Resistivity of the winding material.
  • R: Measured DC resistance of the winding (unit: Ω).
  • S: Cross-sectional area of the winding wire (provided by the manufacturer, unit: mm²).
  • L: Total length of the winding wire (provided by the manufacturer, unit: m).
• Operation: Measure winding resistance with a DC resistance tester, calculate ρ, and compare with standard resistivity (copper: 0.0172Ω·mm²/m; aluminum: 0.0283Ω·mm²/m at 20°C). A large deviation from copper’s resistivity indicates non-copper material.

(5) Core-Lifting Inspection (For Copper-Aluminum Hybrid Windings)

• Application: When hybrid windings are suspected (e.g., inconsistent resistance data), disassemble the transformer (drain oil, remove the tank cover) for direct inspection.
• Judgment Criteria:
  • Check coil joints: Copper-aluminum joints are usually “over-wired” at aluminum winding terminals (copper sleeves crimped on aluminum wires).
  • Measure resistance of different winding sections: Significant resistance differences between sections may indicate mixed materials.

3. CHH Power’s Suggestions for Practical Application

• For transformer acceptance or procurement, prioritize nameplate checking + DC resistance measurement to ensure material consistency with contracts.
• Avoid relying solely on weight/volume comparison, as transformer structure (e.g., core type, cooling method) can also affect these indicators.
• For copper-aluminum hybrid windings, confirm the joint quality (e.g., no corrosion, tight crimping) to prevent contact resistance increase and local overheating.