Main Insulation Structure Of Oil Immersed Transformer

1. Between the winding and the iron core
The iron core includes a core column and an iron yoke, which are in a grounded state during operation. The winding near the core column and the core column serve as the main insulation between the winding and the ground, with an insulating paper tube and a cylindrical iron core. Between the outer diameter of the paper tube and the inner diameter of the winding, a support strip is used to create a certain thickness of oil gap insulation. When the voltage is high, it can be constructed by repeatedly using paper tube supports.
At the upper and lower ends of each phase winding, between the winding and the upper steel pressure plate and the lower iron yoke, there is a main insulation at the winding end, called iron yoke insulation. The structure of the yoke insulation can have a thickness that can be adjusted as needed. When the voltage is high, several layers of paper rings and pads can be placed alternately. Between the winding and the iron yoke insulation, end coils are also placed to provide insulation at the ends.
The distribution of the electric field at the end of the winding is extremely uneven. To improve its distribution, electrostatic screens are placed at the ends of 110kV and above. In addition to improving the distribution of the end electric field and making it uniform, electrostatic screens can also improve the initial voltage distribution under the action of impulse voltage. In addition, a certain number of positive and negative angle rings are placed at the end to divide the oil gap into several sections, which also plays a role in evenly distributing the electric field.
2. Between windings
Paper tube oil gap insulation has been widely used for the main insulation between windings of different voltage levels in the same phase, or between windings of different voltage levels in different phases. In large capacity ultra-high voltage transformers, thin paper tube small oil gap structures are commonly used.
3. Between the winding and the casing
The outermost winding and the oil tank form the main insulation between the winding and the box shell. When the voltage is 110kV or below, the insulation mainly relies on the thickness of the insulating oil; When the voltage is 220kV or above, add cardboard screens to strengthen the main insulation between the ground.
4. Insulation of outgoing lines
The thickness of corrugated paper wrapped around the winding ends varies depending on the voltage level. The higher the voltage level, the thicker the corrugated paper. Wrap corrugated paper of appropriate thickness near the winding end, slightly away from the winding, using bare cable or metal hard busbar, and then weld a multi-layer soft copper wire directly to the porcelain bushing.
5. Insulation of tap changer
The operating rod of the tap changer also serves as the main insulation between the high and medium voltage windings and the ground, because one end of the operating rod is connected to the high and medium voltage conductive parts, while the other end is installed on the box shell, which is grounded. The material of the operating rod is mostly made of phenolic insulation paper tube, but it can also be made of dried wood and coated with protective paint on the surface. The tap changer is installed on an insulating bracket, and the conductive part forms the main insulation between the insulating bracket and the ground. The main insulation is made of southern wood or phenolic cardboard.
6. External main insulation of transformer
The insulation sleeve of a transformer is used to lead the high and low voltage leads inside the transformer to the outside of the oil tank. It not only serves as insulation for the leads to the ground, but also plays a role in fixing the leads. Therefore, it must have the electrical strength and sufficient mechanical strength specified in the manufacturing standards.
The conductor in the porcelain bushing of a transformer is one of the current carrying components, which passes through the load current for a long time during the operation of the transformer, and passes through the short-circuit current during a short circuit. Therefore, the porcelain bushing must have good thermal stability. The main structure of the insulation sleeve depends on the voltage level.