The primary and secondary coils of a transformer are the key components in performing its basic function of transforming voltage and current. Materials are used to insulate the primary and secondary coils. In transformers, in addition to the primary and secondary coils, there are several other important components and accessories.
The insulating material
is one of the most critical components of a transformer. Sufficient insulation between different active parts of the transformer is necessary for its safe operation. Adequate insulation is not only necessary to isolate coils from one another, or from the core and tank, but also ensures the safety of the transformer against accidental overvoltages.
The insulation system in a transformer can be categorized as follows.
• Major insulation: – between core and low-voltage (LV) winding; – between LV and high-voltage (HV) winding; – between top and bottom of winding and yoke; – between HV and tank; – bushings.
• Minor insulation: – between conductors; – between turns; – between layers; – between laminations; – between joints and connections.
The insulation material
commonly used between the grounded core and the LV coil to ground, and also between HV and LV coils, is oil-impregnated solid pressboard. Solid insulation, including pressboard or paper, can have small internal air voids. This reduces the insulating strength of the solid insulation as well as reducing its heat dissipation capacity. When transformer oil is used to impregnate solid insulation, the voids are filled with oil, resulting in an improvement of both the insulation strength and the heat dissipation capacity of the solid insulation. In larger transformers, cellulose-based paper tape is usually wrapped over individual conductors. Layer-to-layer or disc-to-disc insulation is mostly provided by oil-impregnated Kraft paper or even thick pressboard or transformer board in case of higher-rating transformers.