The main insulation materials inside the transformer include transformer oil, insulation cardboard, cable paper, telephone paper, and corrugated paper.

(1) Transformer oil.

(2) Insulated cardboard. Insulated cardboard is mainly made by pressing unbleached sulfate fibers, which have a large number of pores between the fibers, thus possessing strong breathability, oil absorption, water absorption, etc. If high heat-resistant polyamide fiber paper is used, its lifespan is greatly improved. For example, as insulation paper tubes, braces, pads, partitions, corner rings, etc.

(3) Cable paper. As a type of insulation paper, it is made of sulfate pulp and used in transformers with cable paper models DL2-08 and DL2-12, with thicknesses of 0.08mm and 0.12mm. It is mainly used for insulation of the outer surface of wires, insulation between coil layers, insulation of lead wrapping, etc. It is one of the main insulation materials for oil immersed transformers.

(4) Phone paper. Made from sulfate pulp. Use telephone paper model DH-50 in the transformer. Its thickness is (0.5 ± 5%) mm, rolled into a paper roll with a width of (500 ± 10) mm. Mainly used for insulation of coil wires and end insulation of coils.

(5) Wrinkle paper. It is also a type of insulation paper, made from cable paper made from sulfate pulp and further processed. It has good electrical performance in oil, manifested by high average breakdown voltage and small tangent value of dielectric loss angle. Wrinkle paper is mainly used for wrapping transformer output lines and other areas.

It should be noted that the dielectric constant ε of insulating paper and cardboard is 4-5, which is more than twice as high as the dielectric constant ε of transformer oil, which is 2.2. Under the action of an electric field, the field strength shared in composite insulation is inversely proportional to the dielectric constant of the material.

From this, it can be seen that the field strength in the oil gap is much greater than that in the cardboard, making the oil gap a weak link in the oil paper insulation. When the dielectric constant between them is close, the insulation structure size of the transformer may shrink, so the new type of cardboard with low dielectric constant is currently the direction of research and development.