Dry Type Transformer Winding Coils Types

07-17 2026 | By:

Selecting the appropriate winding coil is one of the most important decisions in dry-type transformer design. Different winding structures, conductor materials, and insulation technologies directly influence transformer efficiency, thermal performance, mechanical strength, service life, and maintenance requirements.

Modern dry-type transformers generally use one of four winding technologies depending on voltage class, power rating, and application environment.

1. Cast Resin Transformer Coil (CRT)

Best for:

• Medium Voltage Distribution
• Commercial Buildings
• Hospitals
• Airports
• Data Centers
• Renewable Energy Projects

Cast Resin Transformer Coils are the most widely used winding type for modern dry-type transformers. During manufacturing, the high-voltage winding is vacuum-cast with epoxy resin, completely encapsulating the conductor and insulation system.

This process eliminates air voids and significantly improves dielectric strength while protecting the winding from moisture, dust, corrosive gases, and environmental contaminants.

Advantages

• Excellent moisture resistance
• High dielectric strength
• Fire-resistant and self-extinguishing
• Maintenance-free operation
• High mechanical strength
• Outstanding short-circuit withstand capability
• Long service life

Typical Voltage

• 6kV
• 10kV
• 11kV
• 13.8kV
• 20kV
• 35kV

2. Vacuum Pressure Impregnated (VPI) Coil

Instead of casting epoxy around the winding, VPI technology immerses the completed winding into insulating varnish under vacuum and pressure.

After impregnation, the winding is baked at high temperature until the insulation cures.

VPI coils are lighter than cast resin coils and provide excellent cooling performance.

Advantages

• Lower manufacturing cost
• Better heat dissipation
• Lightweight structure
• Easy repair
• High efficiency

Limitations

Compared with epoxy cast coils, VPI windings provide lower resistance to moisture and contamination.

For harsh environments, cast resin technology remains the preferred solution.

3. Copper Foil Winding Coil

Copper foil winding technology is commonly applied to low-voltage windings.

Instead of round conductors, wide copper foil is wound layer by layer with insulation material.

Benefits

• Uniform current distribution
• Reduced eddy current losses
• Lower temperature rise
• Excellent short-circuit strength
• Compact winding structure

Copper foil windings are commonly used in transformers above 630 kVA.

4. Aluminum Foil Winding Coil

Aluminum foil winding follows the same manufacturing principle as copper foil but uses aluminum conductors.

Its primary advantage is weight reduction and cost savings.

Today, aluminum coils are widely used in commercial distribution transformers where cost optimization is important.

Coil Selection Guide

Dry Type Transformer Winding Coil Manufacturing Process

The manufacturing process determines the electrical performance, mechanical strength, dielectric reliability, and service life of the transformer.

Professional manufacturers strictly follow IEC 60076-11 and ISO quality management standards during every production stage.

Step 1 — Raw Material Inspection

Every production cycle begins with incoming material inspection.

Critical materials include:

• Electrolytic Copper
• Electrical Aluminum
• Epoxy Resin
• Glass Fiber
• Nomex® Paper
• DMD Insulation
• Silicon Steel Core Components

Each material undergoes:

• Dimension inspection
• Conductivity testing
• Insulation verification
• Moisture measurement

Step 2 — Coil Winding

Computer-controlled CNC winding machines ensure:

• Accurate turns ratio
• Uniform conductor tension
• Stable winding geometry
• Consistent insulation spacing

The winding accuracy directly affects transformer efficiency and voltage regulation.

Step 3 — Layer Insulation

Each conductor layer is separated using high-temperature insulation materials.

Common insulation systems include:

• Nomex®
• DMD
• Epoxy Glass Cloth
• Polyester Film
• Glass Fiber Mesh

The insulation system prevents turn-to-turn breakdown under high voltage stress.

Step 4 — Vacuum Drying

Before resin casting, coils are dried inside a vacuum chamber.

Purpose:

• Remove moisture
• Remove trapped air
• Improve resin penetration

Vacuum drying significantly reduces the possibility of partial discharge during transformer operation.

Step 5 — Vacuum Epoxy Casting

This is the most critical manufacturing stage.

The winding is placed into a mold.

High-quality epoxy resin is introduced under:

• High Vacuum
• Controlled Temperature
• Controlled Pressure

The resin completely fills every insulation gap.

After curing, the coil becomes a solid insulation system.

Step 6 — Oven Curing

The cast coil enters a temperature-controlled curing oven.

Typical curing temperature:

120–160°C

Purpose:

• Complete resin polymerization
• Increase mechanical strength
• Improve thermal endurance

Step 7 — Machining & Finishing

After curing:

• Flash removal
• Surface polishing
• Terminal machining
• Dimensional inspection

are completed.

Step 8 — Electrical Testing

Each finished winding undergoes comprehensive testing.

Including:

• Partial Discharge Test
• Power Frequency Withstand Voltage Test
• Induced Voltage Test
• Turns Ratio Test
• DC Resistance Test
• Insulation Resistance Test

Only qualified products proceed to transformer assembly.

Copper Transformer Coil vs Aluminum Transformer Coil

One of the most common questions from engineers and buyers is whether copper or aluminum conductors should be selected.

The answer depends on electrical performance, budget, installation conditions, and lifecycle cost.

Copper Transformer Coil

Copper has been the preferred conductor material for decades due to its excellent electrical conductivity and mechanical strength.

Advantages

• Higher conductivity
• Lower resistance
• Smaller conductor size
• Higher short-circuit strength
• Better overload capability
• Longer service life

Typical Applications

• Data Centers
• Hospitals
• Mining
• Petrochemical Plants
• Railway Systems
• Critical Infrastructure

Aluminum Transformer Coil

Aluminum offers an economical alternative for standard commercial applications.

Advantages

• Lower material cost
• Lightweight
• Easy transportation
• Lower total transformer weight

Considerations

Because aluminum has lower conductivity than copper, a larger conductor cross-section is required to carry the same current. Proper connection design is also important to minimize long-term contact resistance.

Copper vs Aluminum Comparison

Selection Tip:
For mission-critical facilities such as hospitals, data centers, rail transit, and industrial plants where reliability and overload capability are priorities, copper windings are generally preferred. Aluminum windings remain a practical choice for many commercial distribution projects where reducing initial investment is an important consideration.

Cast Resin Coil vs VPI Coil

Both Cast Resin and Vacuum Pressure Impregnated (VPI) technologies are widely used in dry-type transformers.

Choosing the correct insulation technology depends on operating environment, maintenance strategy, and project requirements.

Which Technology Should You Choose?

Cast Resin Coils are well suited for environments requiring maximum reliability, fire safety, and resistance to humidity or pollution, such as hospitals, metro systems, data centers, renewable energy facilities, and high-rise buildings.

VPI Coils are often selected for industrial and commercial applications where operating conditions are less demanding and a lower initial cost is preferred.

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