In a transformer, we need to provide the insulation between the core and LV winding and also between the LV winding and HV winding. The insulation comprises of backlight and paper. The insulation system consists of paper wrapping including backlight cylinders, wood spacers, and end packings.
The copper conductors of windings are wrapped with paper. Then the paper-wrapped conductors are lashed with cotton strands. Because it gives some mechanical strength to the paper insulation. The paper insulation is more suitable for circular windings. A number of backlight cylinders or barrels separates the high voltage winding from the low voltage winding. This arrangement also creates ducks for passing circulating oil. The end turns of the windings are more heavily insulated in a transformer.
The paper has a high tendency of absorbing oil. Therefore, it requires varnishing. If the insulated winding does not go to the proper treatment it may shrink after a few months of service. Therefore it may cause loosening the winding assembly. Therefore, during electrical faults in the transformer, the loosen portion of the winding may have a significant mechanical displacement. This is because a significant mechanical thrust accompanies with a high electrical current.