Transformer insulation can be divided into internal insulation and external insulation. Internal insulation refers to the insulation of each part within the tank, while external insulation refers to the insulation between the upper part of the bushing and ground, and between bushings themselves. Internal insulation can be further divided into main insulation and longitudinal insulation. Main insulation is the insulation between the windings and the grounding part, as well as the insulation between windings. In oil-immersed transformers, the oil-paper barrier insulation structure is most commonly used for main insulation.
Main insulation can be further divided into graded insulation and full insulation. Graded insulation means that the main insulation level of the transformer windings near the neutral point is lower than the insulation level at the winding ends. Conversely, if the insulation level at the beginning and end of the transformer is the same, it is called full insulation. Longitudinal insulation is the insulation between different parts of the same winding, such as the insulation between different turns, between turns, and between turns.