What Insulation Materials Are Used in Oil-Immersed Transformers?

For transformer manufacturers, choosing the right insulation material is not only about voltage rating. It also affects winding stability, drying process, oil impregnation, mechanical strength, heat aging and long-term transformer reliability.
Chengrui Insulation focuses on the main solid insulation materials used in oil-immersed transformers, including:

• Kraft Insulation Paper

• Thermally Upgraded Paper

• Diamond Dotted Paper / DDP

• Crepe Paper and Crepe Paper Tubes

• Transformer Pressboard

• Laminated Densified Wood

• Oil Duct Strips / Spacer Strips

Oil-Immersed Transformer Insulation Materials

1. Kraft Insulation Paper

Basic winding and conductor insulation material

Kraft insulation paper is one of the most widely used flexible insulation papers in oil-immersed transformers.

It is mainly used for wrapping copper or aluminum conductors and for providing insulation between turns, layers and winding sections.

Kraft insulation paper is flexible, easy to wrap and suitable for oil-paper insulation systems after drying and oil impregnation.

For transformer manufacturers, this material is often selected when the winding structure needs a thin, flexible and cost-effective cellulose insulation paper.

2. Thermally Upgraded Paper

Heat-resistant cellulose paper for higher thermal reliability

Thermally upgraded paper is used when ordinary kraft insulation paper does not provide enough thermal aging resistance.

It is still a cellulose-based transformer insulation paper, but it is treated during production to improve heat aging performance. In many transformer applications, it is used as a higher-temperature paper insulation material for winding systems.

The main value of thermally upgraded paper is not simply higher short-term heat resistance. Its real purpose is to help the paper insulation maintain mechanical and electrical performance more stable during long-term transformer operation.

3. Diamond Dotted Paper / DDP

Insulation paper with heat-activated bonding

Diamond Dotted Paper, also called DDP, is a special insulation paper coated with B-stage epoxy resin dots in a diamond pattern.

Before curing, the resin dots remain in a semi-cured state. During transformer winding drying or heating, the resin softens, flows slightly and then cures. After curing, it helps bond winding layers together.

This gives DDP two functions:

electrical insulation and winding bonding.

4. Crepe Paper and Crepe Paper Tubes

Flexible insulation for leads, bends and irregular areas

Crepe paper is different from flat kraft insulation paper.

It has a wrinkled structure, which gives it better stretchability and flexibility. This makes it suitable for transformer positions where flat paper is difficult to wrap smoothly.

In real transformer production, many positions are not flat. Leads bend, joints are uneven, and connection areas often require flexible wrapping. Crepe paper is useful because it can fit these shapes more easily.

Crepe Paper Tubes

Crepe paper tubes are made from crepe insulation paper and used as flexible insulation sleeves.

Chengrui can supply electrical insulation crepe paper, crepe paper tape, crepe paper tubes and semi-conductive black crepe paper according to customer requirements.

5. Transformer Pressboard

Rigid cellulose insulation for barriers, cylinders and structural parts

Transformer pressboard is one of the most important rigid solid insulation materials used in oil-immersed transformers.

Compared with kraft insulation paper, pressboard is thicker, harder and more suitable for structural insulation parts.

6. Laminated Densified Wood

High-strength structural insulation for transformer support parts

Laminated densified wood is used in oil-immersed transformers where the insulation part must provide stronger mechanical support.

It is not ordinary plywood. Transformer laminated densified wood is made for insulation structures that require mechanical strength, dimensional stability and oil-immersed transformer compatibility.

Compared with transformer pressboard, laminated densified wood is often selected for positions with higher mechanical load.

7. Oil Duct Strips / Spacer Strips

Insulation strips that create cooling channels inside windings

Oil duct strips, also called spacer strips, are used to create controlled oil channels inside transformer windings.

They are usually made from transformer pressboard or other suitable insulation materials according to design requirements.

Chengrui can supply pressboard oil duct strips and custom spacer strips according to transformer winding design.

How to Choose the Right Material

The right material depends on where it is used inside the transformer.

Chengrui Products for Oil-Immersed Transformer Insulation

Chengrui supplies a range of solid insulation materials for oil-immersed transformer manufacturers and repair workshops.

Flexible Insulation Papers：

• Kraft Insulation Paper

• Electrical Kraft Paper

• Cable Paper

• Thermally Upgraded Paper

• Crepe Paper

• Crepe Paper Tape

• Crepe Paper Tubes

Bonding Insulation Paper：

• Diamond Dotted Paper

• Single-sided DDP

• Double-sided DDP

• Thermally Upgraded Diamond Dotted Paper

• Diamond Dotted Polyester Film

Rigid Pressboard Materials and Components：

• Transformer Pressboard

• High-Density Pressboard

• Presspahn Paper

• Pressboard Cylinders

• Pressboard Angle Rings

• Pressboard Spacer Blocks

• Pressboard Washers and Discs

Custom Pressboard Insulation Parts：

• Structural Insulation Parts

• Laminated Densified Wood

• Transformer Laminated Wood

• Laminated Wood Support Blocks

• Machined Laminated Wood Parts

• Oil Duct Strips

• Pressboard Spacer Strips

These materials can be supplied in rolls, sheets, slit rolls, cut pieces or machined components according to customer requirements.

FAQ

Q1: What solid insulation materials are used in oil-immersed transformers?

A: Common solid insulation materials include kraft insulation paper, thermally upgraded paper, diamond dotted paper, crepe paper, transformer pressboard, laminated densified wood and oil duct strips.

Q2: What is kraft insulation paper used for in transformers?

A: Kraft insulation paper is mainly used for conductor wrapping, turn insulation, layer insulation and winding insulation in oil-immersed transformers.

Q3: What is the difference between kraft paper and thermally upgraded paper?

A: Both are cellulose insulation papers, but thermally upgraded paper has better heat aging resistance and is used when the transformer design requires higher thermal reliability.

Conclusion

Oil-immersed transformer insulation relies on several solid insulation materials working together.

Kraft insulation paper provides flexible conductor and winding insulation. Thermally upgraded paper improves heat aging resistance. Diamond dotted paper provides both insulation and heat-activated bonding. Crepe paper and crepe paper tubes are used for leads, bends and irregular areas. Transformer pressboard is used for rigid barriers, cylinders, angle rings and spacers. Laminated densified wood provides stronger mechanical support. Oil duct strips create cooling channels and maintain winding spacing.

For transformer manufacturers, the right material depends on the exact insulation position, thickness, mechanical requirement, thermal requirement and processing method.

Chengrui Insulation supplies solid insulation materials for oil-immersed transformers, including kraft insulation paper, thermally upgraded paper, diamond dotted paper, crepe paper, transformer pressboard, laminated densified wood, oil duct strips and custom transformer insulation components.