EN 
KR 
ES 
AR 
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.
| Material Name | Main Application | Typical Location in Windings |
|---|---|---|
| Transformer Kraft Paper | Conductor wrapping, turn-to-turn and layer-to-layer insulation | Winding conductors, between winding layers |
| Thermally Upgraded Kraft Paper | Improved resistance to thermal aging and extended insulation life | Winding conductors, between winding layers |
| Diamond Dotted Paper (DDP) | Layer insulation, bonding, and winding fixation | Between winding layers |
| Transformer Crepe Paper | Insulation for leads, winding ends, and curved areas | Winding ends, leads |
| Transformer Pressboard | Main insulation, oil ducts, spacers, and structural insulation | Between windings, winding ends, support structures |
| Aramid Paper / Nomex Paper | High-temperature insulation | High-temperature windings, localized high-temperature areas |
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
Transformer Kraft Paper is one of the core materials used in the oil-paper insulation system of oil-immersed transformers. Its main functions include:
Simply understood:
Copper/Aluminum Conductor → Wrapped with Kraft Paper → Wound into the Transformer Winding
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.
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:
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.
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:
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
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:
ZTelec Group is a professional provider of electrical insulation material solutions. For different types of oil-immersed transformers, voltage levels, and application requirements, we 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 ZTelec Group. 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.
