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The insulation of the transformer can be divided into two parts: main insulation and longitudinal insulation. Main insulation refers to the insulation between windings to ground, between phases and between phases, in phase and between different voltage levels. Longitudinal insulation refers to the same winding of the same voltage level, the insulation between different parts, such as interlayer, turn, insulation between the winding and the electrostatic screen. The main insulating materials inside the transformer are transformer oil, insulated crepe paper, insulated crepe paper tube, and crepe paper. In the past, yellow paint strips, tapes, etc. have been used, and they have now been replaced by new materials.

The longitudinal insulation of the transformer includes three parts: turn-to-turn insulation, interlayer insulation, and inter-segment insulation. When designing the longitudinal insulation, we need to consider the various voltages and their gradient distribution acting on the longitudinal insulation, the process degree in the process of manufacturing the windings of the transformer, the interaction between the windings under special conditions, the influence of the longitudinal insulation and the effect of the size of the oil gap between the segments on the heat dissipation and so on. We must also consider these aspects:

1. Inter-turn insulation. The windings of oil-type transformers are generally wound by cable paper. Because paper is used as the 匝 insulation of the transformer winding, because the dielectric constant of the paper is not much different from that of the oil, the electric field distribution can be made uniform, but we must also pay attention to the fact that the data cannot be selected according to the complete breakdown of the oil gap. We must keep enough thickness for the insulation thickness of the crucible.

2. Insulation between layers and between segments. Interlayer insulation is primarily suitable for cylindrical windings. When the working voltage between the two layers is high, the interlayer insulation must be thicker, which not only increases the radial size of the transformer winding, but also is not conducive to heat dissipation, so that the temperature of the transformer winding increases.

3. Longitudinal insulation structure of oil-type transformer: transformer of 35 kV and below; total insulation of transformer of 110 kV or more.

It is understood that the insulation technology of transformers at home and abroad continues to develop, and the oil passage between the segments of the transformer winding has been extended to less than six millimeters. The winding height of the transformer is reduced, and the technical and economic indicators of the transformer are correspondingly improved.

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