Transformer Insulation Paper Guide | Kraft Paper, Crepe, DDP, TUP

How to Choose Kraft Paper, Crepe Paper, Cable Paper, DDP and Pressboard for Transformer Insulation

Transformer insulation paper is not one single material.

In oil-immersed transformers, different paper-based insulation materials are used in different positions: some are used to wrap conductors, some are used between winding layers, some are used around leads, and some are used as rigid barriers, spacers or oil duct structures.

For transformer manufacturers, choosing the right insulation paper is not only about thickness or price. The real question is:

Where will the material be used inside the transformer, and what function must it provide?

A transformer winding may need several types of insulation paper at the same time, including kraft paper, cable paper, crepe paper, diamond dotted paper, thermally upgraded paper, presspahn paper and transformer pressboard.

This guide explains how these materials are used and how buyers can choose the right transformer insulation paper for winding, wrapping, interlayer insulation, lead insulation and structural insulation applications.

 In real transformer production, the final choice depends on voltage level, oil system, winding structure, temperature class and customer drawings.

1. Kraft Insulation Paper

Kraft insulation paper is one of the most common transformer insulation papers.

It is made from electrical-grade cellulose paper and is widely used in oil-immersed transformer insulation systems. After drying and oil impregnation, kraft paper works together with transformer oil to provide electrical insulation between conductors and winding layers.

Typical applications include:

conductor wrapping;

layer insulation;

winding insulation;

copper or aluminum conductor insulation;

general oil-paper insulation systems.

Kraft paper is usually selected when the transformer design requires a flexible, economical and oil-compatible cellulose insulation material.

For many standard oil-immersed transformers, kraft paper remains a basic insulation material because it is easy to process, easy to wrap and cost-effective.

However, for higher thermal requirements, ordinary kraft paper may not be enough. In that case, thermally upgraded paper should be considered.
Related product: Electrical Insulation Kraft Paper

2. Diamond Dotted Paper

Diamond dotted paper, also called DDP, is used when the transformer winding needs both insulation and bonding.

It is made by coating epoxy resin dots onto electrical insulation paper in a diamond pattern. During transformer manufacturing, the resin softens and cures under heat, helping bond the winding layers together.

Typical applications include:

• winding layer insulation;

• foil winding insulation;

• coil bonding;

• conductor insulation;

• interlayer insulation in oil-immersed transformers.

The diamond pattern is important. It provides bonding points, but still leaves open space for transformer oil penetration.

Compared with ordinary kraft paper, diamond dotted paper provides an additional bonding function. Compared with full-surface adhesive paper, the diamond pattern helps maintain oil channels and reduces the risk of blocked impregnation.
Related product: Diamond Dotted Insulation Paper

3.  Crepe Paper

Crepe paper is used when flat insulation paper cannot wrap the part properly.

Transformer leads, bends, connection points and irregular shapes often need a material that can stretch and conform to the surface. Crepe paper has a wrinkled structure, so it can wrap around curved parts without tearing easily.

Typical applications include:

lead insulation;

connection area wrapping;

winding end insulation;

irregular conductor wrapping;

transformer repair and rewinding work.

The main value of crepe paper is flexibility.

If the insulation position is flat and needs layer separation, kraft paper or cable paper may be more suitable. If the position is curved or difficult to wrap, crepe paper is usually the better choice.

Related product: Electrical Grade Insulation Crepe Paper

4. Thermally Upgraded Paper

Thermally upgraded transformer insulation paper is used when the insulation system needs better resistance to thermal aging.

In oil-immersed transformers, cellulose paper slowly ages under heat, oxygen, moisture and electrical stress. Thermally upgraded paper is treated to slow down this aging process and improve service life under higher temperature conditions.

Typical applications include:

• higher-temperature oil-immersed transformers;

• compact transformer designs;

• distribution transformers with higher load requirements;

• winding insulation requiring improved thermal aging resistance;

• thermally upgraded diamond dotted paper.

Thermally upgraded paper is not selected only by price. It is usually selected when the transformer design has a clear thermal requirement.

If the customer drawing or technical specification requires thermally upgraded insulation paper, ordinary kraft paper should not be used as a direct replacement without confirmation.

Related product: Thermally upgraded kraft paper and  Thermally Upgraded Diamond Dotted Paper

7.  Transformer Pressboard

Transformer pressboard is a rigid cellulose insulation board used for structural insulation in oil-immersed transformers.

It is not mainly used for conductor wrapping. It is used where the insulation material must provide mechanical support, dimensional stability and oil-compatible electrical insulation.

Typical applications include pressboard cylinders, angle rings, oil duct strips, spacer blocks, barriers, washers, end insulation parts and support insulation parts.

Transformer pressboard is usually supplied as sheet, strip or machined component. For large transformer manufacturers, pressboard components are often made according to drawings.

If the insulation position requires a rigid structure, transformer pressboard is more suitable than kraft paper or crepe paper.

Related product: Electrical Insulation Pressboard for Transformer

8. Aramid Paper for Special Transformer Insulation

Aramid paper is used in transformer applications where higher thermal resistance or special insulation performance is required.

It is not usually the first choice for standard oil-immersed transformer insulation because the cost is higher than cellulose materials. But for special transformers, compact designs, high-temperature insulation systems or customer-specified designs, aramid paper may be required.

Typical applications include:

high-temperature insulation areas;

special transformer insulation structures;

dry-type transformer insulation;

hybrid insulation systems;

applications replacing or supplementing cellulose insulation.

If the customer specification clearly requires aramid paper or Nomex® paper, ordinary cellulose paper should not be used as a substitute without approval.
Related product: Nomex T410 paper

How to Choose the Right Transformer Insulation Paper

Before ordering transformer insulation paper, buyers should confirm these points:

For transformer insulation materials, thickness and width are only basic information. The application position is more important.

Related Transformer Insulation Materials

Chengrui supplies a range of transformer insulation paper and pressboard materials, including:

Kraft Insulation Paper

Diamond Dotted Paper

Electrical Grade Crepe Paper

Thermally Upgraded Diamond Dotted Paper

Transformer Pressboard

Pressboard Strips for Oil Duct

Transformer Pressboard Cylinders

These materials can be supplied in roll, sheet, strip or customized component form according to transformer production requirements.

FAQ

Q1: What is transformer insulation paper?

A: Transformer insulation paper is electrical-grade paper used inside transformers for conductor wrapping, layer insulation, lead insulation, winding insulation and oil-paper insulation systems.

Q2: Which paper is used for transformer winding insulation?

A: Common materials include kraft paper, diamond dotted paper and thermally upgraded paper. The final choice depends on winding structure, thermal requirement and transformer design.

Q3: What is crepe paper used for in transformers?

A: Crepe paper is used for flexible wrapping around transformer leads, bends, joints and irregular connection areas.

Q4: What is diamond dotted paper used for?

Diamond dotted paper is used for winding layer insulation and coil bonding. The epoxy resin dots bond the winding after heat curing while leaving space for oil penetration.

Q5: What is the difference between transformer insulation paper and pressboard?

Transformer insulation paper is usually flexible and used for wrapping or layer insulation. Pressboard is rigid and used for cylinders, barriers, spacers, oil duct strips and structural insulation parts.

Conclusion

Transformer insulation paper is a group of electrical insulation materials used in different positions inside transformer windings and insulation structures.

Kraft paper are used for conductor wrapping and layer insulation. Crepe paper is used for leads and irregular wrapping. Diamond dotted paper is used when insulation and bonding are both required. Thermally upgraded paper is selected for higher thermal aging resistance.

For buyers, the best way to choose transformer insulation paper is not to start from the material name, but from the application position inside the transformer.

Chengrui supplies transformer insulation paper, diamond dotted paper, thermally upgraded paper, crepe paper, transformer pressboard and customized insulation components for transformer manufacturers and insulation material distributors worldwide.