3 Types of Thermally Upgraded Transformer Insulation Paper

Inside every reliable transformer, insulation matters. The three common thermally upgraded transformer insulation papers are thermally upgraded(TU) kraft paper, thermally upgraded diamond dotted paper(TU DDP) and Dennison paper.

They are all used in oil-immersed transformers, but they are not used for the same purpose.

Thermally upgraded kraft paper is used for heat-resistant insulation and wrapping.

Thermally upgraded diamond dotted paper adds epoxy resin dots for winding layer bonding.

Dennison paper is a thermally upgraded micro-crepe insulation paper used for flexible wrapping, lead insulation and irregular transformer structures.

The simplest way to choose is this:

1. Use thermally upgraded kraft paper when you need better thermal aging resistance without bonding.

2. Use thermally upgraded diamond dotted paper when you need both heat resistance and bonding.

3. Use Dennison paper when you need heat resistance, flexibility, oil absorption and good conformability.

Why Transformers Use Thermally Upgraded Paper

In oil-immersed transformers, cellulose insulation is exposed to heat, oil, electrical stress and long-term aging.

Ordinary kraft paper works well in many standard transformer designs, but when the transformer requires better thermal life, manufacturers often move to thermally upgraded insulation paper.

Thermally upgraded paper is designed to slow down the thermal aging of cellulose insulation. In many grades, nitrogen-containing thermal stabilizing additives are introduced into the paper system.

That is why nitrogen content(1.3-2.6%) is often used as one indicator when checking thermally upgraded transformer insulation paper.

The purpose is not just to increase the temperature number on paper. The real goal is to help the insulation system maintain performance for a longer time inside transformer oil.

Thermally Upgraded Kraft Paper

Thermally upgraded kraft paper is the basic form of thermally upgraded transformer insulation paper.

It is a smooth cellulose insulation paper without resin dots and without a crepe structure.

This paper is selected when transformer manufacturers need better thermal aging resistance, but do not need epoxy bonding.

Main Functions

Thermally upgraded kraft paper provides electrical insulation, better thermal aging resistance, conductor wrapping, winding layer insulation, oil-paper insulation compatibility and longer insulation life compared with ordinary kraft paper

Common Applications

It is commonly used in oil-immersed transformer winding insulation, conductor wrapping, layer insulation, general oil-paper insulation systems and transformer designs requiring improved thermal life.

Thermally Upgraded Diamond Dotted Paper(TU DDP)

Thermally upgraded diamond dotted paper is often called TU DDP. It is made by coating B-stage epoxy resin dots onto thermally upgraded base paper in a diamond pattern.

This gives the material two functions at the same time:

thermal aging resistance from the thermally upgraded paper

winding layer bonding from the epoxy resin dots

During transformer drying and hot pressing, the epoxy resin softens and cures. After curing, it bonds the winding layers and helps improve coil stability.

Main Functions

Thermally upgraded diamond dotted paper provides winding interlayer insulation, coil bonding, improved winding stability, better thermal aging resistance, higher reliability for oil-immersed transformer windings

Common Applications

TU DDP is commonly used in higher-load oil-immersed transformers, transformers requiring longer insulation life, winding interlayer insulation, coil bonding, export transformer projects, transformer designs where thermal aging is a concern.

Dennison Paper

Dennison paper is generally used to describe a thermally upgraded micro-crepe insulation paper for transformers.

It is different from smooth thermally upgraded kraft paper.

It is also different from thermally upgraded diamond dotted paper because it does not use epoxy resin dots for bonding.

The key feature of Dennison paper is its micro-crepe structure.

This structure gives the paper better flexibility, conformability and oil absorption than flat insulation paper.

Main Functions

Dennison paper provides flexible wrapping, good oil absorption, insulation for irregular structures, lead insulation, conductor wrapping support, thermal, aging resistance and better conformability around uneven surfaces

Common Applications

Dennison paper is commonly used for:

transformer lead insulation

conductor wrapping

winding end insulation

irregular insulation areas

oil-immersed transformer insulation systems

positions requiring flexible wrapping and oil absorption

Comparison Table: TU Kraft Paper vs TU DDP vs Dennison Paper
The choice should be based on application position, not only paper thickness or price.

Key Difference 1: Resin Coating

Thermally upgraded kraft paper and Dennison paper do not have resin dots.

Thermally upgraded diamond dotted paper has B-stage epoxy resin dots.

If the transformer winding requires bonding after drying and hot pressing, TU DDP should be considered.

If bonding is not required, thermally upgraded kraft paper or Dennison paper may be more suitable.

Key Difference 2: Paper Structure

Thermally upgraded kraft paper is smooth paper.

Thermally upgraded diamond dotted paper is smooth thermally upgraded paper with epoxy resin dots.

Dennison paper is micro-crepe paper.

This difference affects how the paper behaves during transformer assembly.

Smooth paper is suitable for regular wrapping and layer insulation.

Diamond dotted paper is suitable for bonding winding layers.

Micro-crepe paper is suitable for flexible wrapping around leads, ends and uneven surfaces.

Key Difference 3: Application Position

The three materials are often used in different transformer positions.

A transformer manufacturer may use all three materials in the same transformer, depending on the design.

How to Choose the Right Thermally Upgraded Insulation Paper

Choose Thermally Upgraded Kraft Paper if:

you need better thermal aging resistance

resin bonding is not required

the paper is used for wrapping or layer insulation

the insulation area is relatively flat

you need smooth thermally upgraded insulation paper

Choose Thermally Upgraded Diamond Dotted Paper if:

you need bonding between winding layers

the transformer requires better thermal aging resistance

the winding design needs stronger mechanical stability

higher load or longer design life is required

ordinary DDP is not enough

Choose Dennison Paper if:

the insulation area is not flat

flexible wrapping is required

oil absorption is important

the paper is used around leads, ends or irregular structures

flat paper is too stiff for the application

A simple selection rule is:

TU kraft paper is for heat-resistant insulation. TU DDP is for heat-resistant bonding. Dennison paper is for heat-resistant flexible wrapping.

FAQ

Q1: What is thermally upgraded transformer insulation paper?

A: Thermally upgraded transformer insulation paper is cellulose insulation paper designed for better thermal aging resistance in oil-immersed transformers. It is commonly used when longer insulation life or higher thermal reliability is required.

Q2: What is the difference between thermally upgraded kraft paper and thermally upgraded diamond dotted paper?

A3: Thermally upgraded kraft paper has no resin coating. Thermally upgraded diamond dotted paper has B-stage epoxy resin dots for winding layer bonding.

Q3: Is Dennison paper the same as thermally upgraded kraft paper?

A: No. Dennison paper is a thermally upgraded micro-crepe insulation paper. It has better flexibility and conformability than smooth thermally upgraded kraft paper.

Thermally upgraded kraft paper, thermally upgraded diamond dotted paper and Dennison paper are all used in oil-immersed transformer insulation systems, but each material solves a different problem. For transformer manufacturers, choosing the right paper can improve winding stability, oil-paper insulation performance, thermal aging resistance and long-term reliability.

Xuchang Chengrui Insulation Material Co.,Ltd. supplies thermally upgraded kraft paper, thermally upgraded diamond dotted paper and Dennison paper for oil-immersed transformer applications.

We can provide rolls, slit rolls, sheets and custom specifications according to customer requirements.