Oil-Immersed Transformer Winding Insulation Paper

Paper Insulation Materials Required for Oil-Immersed Tr […]

Paper Insulation Materials Required for Oil-Immersed Transformer Windings

In the winding system of an oil-immersed transformer, paper-based insulation materials play a very important role. They are not all used simply for wrapping conductors. Different materials perform different electrical and mechanical functions, including turn-to-turn insulation, layer-to-layer insulation, end insulation, lead insulation, and the electrical and mechanical functions of the oil-paper insulation system.

I. Common Paper Insulation Materials for Oil-Immersed Transformer Windings

For oil-immersed power transformers, the five types of materials that are particularly important to understand are:

Kraft Paper → Thermally Upgraded Kraft Paper → DDP → Crepe Paper → Pressboard

1. Transformer Kraft Paper — Basic Winding Paper Insulation

Transformer Kraft Paper is one of the core materials used in the oil-paper insulation system of oil-immersed transformers. Its main functions include:

Conductor wrapping
Turn-to-turn insulation
Layer-to-layer insulation
Local electrical isolation within the winding

Simply understood:Copper/Aluminum Conductor → Wrapped with Kraft Paper → Wound into the Transformer Winding

2. Thermally Upgraded Kraft Paper — Thermally Stable Insulation Paper

Thermally Upgraded Kraft Paper is essentially part of the same oil-paper insulation system, but it is specially treated to provide improved resistance to thermal aging.

It is mainly used in: High-temperature operating transformers, windings requiring a longer insulation life, oil-immersed power transformers, and special high-load transformers.

3. Diamond Dotted Paper (DDP) — Important for Winding Layer Insulation

Diamond Dotted Paper (DDP) is a specialized composite insulation material widely used in the oil-immersed transformer industry. It is based on high-purity electrical Kraft insulation paper, with regularly arranged diamond-shaped B-stage semi-cured modified epoxy resin dots applied to one or both surfaces through a precision coating process. It combines two key functions: electrical insulation and bonding/fixation of transformer windings, making it an important material for power transformer winding manufacturing.

DDP can be simply understood as:

Kraft Paper Substrate + Diamond-Shaped Resin Adhesive Dots

When heated, the resin dots provide a bonding effect. Therefore, DDP not only provides electrical insulation but can also help with:

Layer insulation
Winding fixation
Improved mechanical stability of windings
Reduced movement between winding layers during transformer operation

It is particularly suitable for layer-to-layer insulation in transformer windings.

Therefore:Kraft Paper focuses mainly on “electrical insulation and conductor wrapping,” while DDP adds the functions of “layer bonding and winding fixation.”

This is why DDP is considered a relatively independent product category among transformer insulation materials.

4. Transformer Crepe Paper — Mainly for Curved and Irregular Areas

One of the major differences between Transformer Crepe Paper and ordinary Kraft Paper is that Crepe Paper provides better elongation and flexibility.

Therefore, it is particularly suitable for:

Transformer leads
Outlet sections
Winding ends
Curved areas
Irregular structures
Conductor connection areas

For example, a rigid insulation board is not suitable for wrapping a curved transformer lead, while Crepe Paper can conform to the shape of the lead during wrapping.

A simple way to remember it:

Straight and regular conductors → Kraft Paper

Curved and irregular leads/end areas → Crepe Paper

5. Transformer Pressboard

Transformer Pressboard is another very important insulation material used in oil-immersed transformers.

The main difference between Pressboard and Kraft Paper is:

Kraft Paper: Thin, flexible, and suitable for wrapping

Pressboard: Thicker, more rigid, and capable of providing structural support

It is commonly used for:

• Main insulation between windings
• Insulation between high-voltage and low-voltage windings
• Winding end insulation
• Insulation cylinders
• Spacers
• Oil duct structures
• Barriers
• Various insulation structural components

II. Transformer Insulation Material Solutions

Xujue Electrical is a professional electrical insulation materials manufacturer with more than 65 years of manufacturing experience, dedicated to providing reliable insulation materials and comprehensive solutions for power transformers.

For different types of oil-immersed transformers, voltage levels, and application requirements, we can provide a wide range of insulation materials, including Transformer Kraft Paper, Diamond Dotted Paper (DDP), Transformer Crepe Paper, Transformer Pressboard, Electrical Laminated Wood, and other related products. We can also assist customers in selecting the appropriate combination of materials according to their specific requirements.

If you are looking for a complete range of insulation materials for oil-immersed transformers, welcome to contact Xujue Electrical. Based on your transformer type, material specifications, and specific application requirements, we can provide detailed product specifications, samples for testing, and quotations, helping you complete material selection and procurement more efficiently.