Primary and Secondary Transformer Windings Explained: Copper vs Aluminium Comparison

 Primary and Secondary Transformer Windings Explained: Copper vs Aluminium Comparison

1. Transformer Windings Overview

Transformers consist of primary and secondary windings, which are the conductive coils responsible for transferring energy via electromagnetic induction.

• Primary Winding: Connected to the input voltage source.
• Secondary Winding: Connected to the load, where the transformed voltage is delivered.

 

2. Turns Ratio (Primary to Secondary)

The voltage transformation depends on the ratio of the number of turns in the primary and secondary windings:

Example:

• A 11 kV / 415 V transformer
• 

This means 26.5 turns of primary for each turn of secondary.

Figures:

Transformer Rating	Primary Turns	Secondary Turns	Ratio
11 kV / 415 V	2650	100	26.5
33 kV / 11 kV	3300	1100	3
132 kV / 33 kV	13200	3300	4

Note: Turns ratio is proportional to the rated voltage and inversely proportional to the rated current.

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3. Copper vs Aluminium Windings

Copper Windings:

• High electrical conductivity (≈ 58 MS/m)
• High mechanical strength and thermal endurance
• Preferred for HV and LV windings in most transformers

Aluminium Windings:

• Conductivity ≈ 61% of copper → requires ~1.56× cross-sectional area for same current
• Lighter and cheaper than copper
• Higher thermal expansion → more mechanical stress
• Lower tensile strength → can sag under high temperature

Impact of Using Aluminium:

• Higher resistive loss (I²R) due to larger cross-section
• Slightly lower efficiency (~0.5–1% loss increase for power transformers)
• May need reinforced insulation due to higher operating temperature

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4. Insulation of Windings

Transformer windings require robust insulation to prevent short circuits and withstand voltage stress.

Primary Winding Insulation (High Voltage):

• Materials: Paper, Nomex, Pressboard, Epoxy resin
• Typical Ratings:
• Up to 33 kV → Kraft paper + oil
• 66–132 kV → Pressboard + oil
• HV dry-type → Epoxy-mica composite

Secondary Winding Insulation (Low Voltage):

• Materials: Polyester, Nomex, enamel-coated wire
• Typical Ratings:
• 415–11 kV → Enamel insulation + oil/paper
• Dry-type LV → Epoxy resin

Example:

Winding Type	Insulation Material	Max Voltage Rating
HV Primary	Kraft Paper + Oil	33 kV
HV Primary	Pressboard + Oil	132 kV
LV Secondary	Enamel-coated Wire	415 V – 11 kV
Dry-type LV	Epoxy resin	415 V

Insulation thickness and design depend on voltage level, creepage distance, and temperature rise.

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5. Practical Notes & Engineering Considerations

• Always calculate temperature rise based on conductor and insulation material.
• Aluminium windings require larger conductor size and careful clamping to prevent vibration.
• Ensure creep distance and insulation strength meet IS/IEC standards.
• Turns ratio should account for tap changer adjustments to regulate voltage.

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