The main insulating materials inside the transformer are transformer oil, insulating cardboard, cable paper, telephone paper, and crepe paper.
(1) Transformer oil.
(2) Insulated cardboard. Insulating cardboard is mainly pressed with unbleached kraft fiber, and there are a large number of pores between the fibers, so it has strong air permeability, oil absorption, water absorption, etc. If polyamide fiber paper with high heat resistance is used, its service life is greatly improved. For example, as insulating paper tubes, stays, pads, partitions, corner rings, etc.
(3) Cable paper. As a type of insulating paper, it is made of sulfate pulp, and cable paper with models DL2-08 and DL2-12 is used in transformers, and its thickness is 0.08mm and 0.12mm. It is mainly used as insulation for wire outer surface wrapping and coil interlayer insulation, lead wrapping insulation, etc., and it is one of the main insulating materials for oil-immersed transformers.
(4) Telephone paper. Made from kraft pulp. Telephone paper with model number DH-50 is used in the transformer. It has a thickness of (0.5±5%) mm and is rolled into a paper roll with a width of (500±10) mm. It is mainly used as coil wire insulation and coil end insulation.
(5) Crepe paper. It is also used as a kind of insulating paper, which is reprocessed from cable paper made of sulfate pulp, and has good electrical properties in oil, which is manifested as a high average breakdown voltage and a small tangent value of the dielectric loss angle. Crepe paper is mainly used for dressing transformer outlets.
It should be noted that the dielectric coefficient of insulating paper and cardboard is ε 4~5, which is more than twice as high as the dielectric coefficient of transformer oil ε=2.2. Under the action of an electric field, the field strength shared in the composite insulation is inversely proportional to the dielectric coefficient of the material.
It can be seen that the field strength in the oil gap is much stronger than that of cardboard, so the oil gap becomes the weak link of the oil-paper insulation. When the dielectric coefficient between them is close, the size of the insulation structure of the transformer may be reduced, so the new type of cardboard with low dielectric coefficient is the current research and development direction.
