What are common Transformer Insulation Materials?

A transformer is an electric device that is tasked with transferring electrical energy from one circuit to another. Transformers are often used for interconnecting a power grid for residential and business buildings, regularly handling high amounts of voltage. With the current traveling through such devices, it is important that they are well insulated for the protection of internal and external components. Insulation failures are one of the most common sources of transformer failures, thus one should know how to properly insulate such devices. In this blog, we will discuss the most common insulating materials used for such assemblies and how they help manage electricity.

Transformer Insulators

When choosing an insulator, it is important that it has properties such as low dielectric loss, high dielectric strength, high volume resistance, and low relative permittivity. As such, the most common types of insulation materials for transformers include electrical grade paper, transformer oil, pressboard, insulating tape, and wood. As each type of material has varying characteristics, they are often used for different parts of the transformer.

Electrical Grade Paper

Electrical grade paper, also known as kraft paper, is a very cheap option that is commonly found in transformer assemblies. Such materials are typically produced from unbleached softwood pulp that undergoes a sulfate process. In order to optimally serve as an insulating material, the electrical grade paper must feature high dielectric strength and should be devoid of any conducting particles. Generally, such paper is used as insulation for winding, condenser core bushings, rectangular copper conductors, and stranded copper cables. Beyond having optimal insulation properties, kraft paper should also feature enough tensile strength to be wrapped over coils without damage.

Transformer Oil

Transformer oil can be considered the most important insulating material, also serving to cool parts and protect them from corrosion. Generally, transformer oil is made up of four compounds, those of which are paraffin, naphthenes, olefins, and aromatics. Saturated paraffin comprises a larger part of the mixture, while olefins are generally avoided.


Pressboard materials are similar to electrical grade paper, serving as layers of paper that are capable of mitigating the flow of electricity. Pressboard materials can be layered with adhesive or through wet stage placement, and they can be placed in various areas of the transformer assembly. Typically, pressboards are implemented at winding ends, the yoke, block washer, coil clamp ring, and other areas.

Insulating Tape

For sections of the transformer that need to be banded with a material that has high dielectric strength, insulating tapes are useful. Generally, insulating tape comes in the form of cotton tapes, woven terylene tape, cotton newar tapes, and polyester resin impregnated weftless glass tapes. With such materials, transformer cores, bolts, and other items may be banded.


Wood used for transformers is often compressed and laminated, produced from wood veneer. After being gummed, stacked, and hot pressed, the wood will feature high dielectric strength, low density, optimal oil absorption, high durability, and high mechanical strength. When placed within a transformer, laminated wood often acts as a yoke clamp, wedge block, cleat and support, core to coil packing, and more.

With the implementation of various types of insulation, transformers and their surrounding components may be protected from the high amounts of current that may pass through such devices. Purchasing Sphere is an online distributor of aircraft parts and other components, and we are your sourcing solution for top quality insulation materials that can benefit the operations of transformers. We invite you to peruse our vast offerings at your leisure, and our team of industry experts is readily on hand 24/7x365 to assist you through the purchasing process as necessary. Get started with a competitive quote for your comparisons today when you fill out and submit an Instant RFQ form on our website.


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