Insulation structure of oil-immersed transformers

I.the role of transformer insulation
(1) so that the conductor and other parts of the mutual insulation.
(2) can separate different charged parts.
(3) by reasonable insulation arrangement, can improve the uniformity of electric field distribution.
(4) So that the electrical device to obtain a certain electric capacity.
(5) play a mechanical support, fixed and conductive oil circulation and heat dissipation.

II. the transformer insulation classification and requirements 
1. Transformer insulation division

Transformer insulation can be divided into internal and external insulation, internal insulation refers to all parts of the tank insulation, external insulation refers to the upper part of the casing to the ground and the insulation between them. Internal insulation can be divided into two parts: main insulation and longitudinal insulation. The main insulation is the insulation between the winding and the grounded part and between the windings. In the oil-immersed transformer, the main insulation to oil paper barrier insulation structure commonly used.
The main insulation can be divided into graded insulation and full insulation. Graded insulation means that the main insulation level of the transformer's windings near the neutral point is lower than the insulation level of the winding ends. On the contrary, if the first end of the transformer and the end of the same level of insulation is called full insulation. Longitudinal insulation is the same winding between the various parts of the insulation, such as different line between turns, turn-to-turn and turn-to-turn insulation.
2. Transformer insulation requirements

Requirements for transformer insulation is not due to insulation damage within the operating life and affect the normal operation of the transformer. The main requirements are as follows.
(1) can withstand the operation of the overvoltage and normal operating voltage.
(2) can withstand the operation of the short-circuit current, overcurrent and normal operating current.
(3) moisture, aging does not affect the normal operation of the transformer.
3. Transformer insulation materials

Transformer internal main insulating materials are transformer oil, insulating cardboard, cable paper, telephone paper, crumpled paper.
(1) transformer oil.
(2) insulating cardboard. Insulating cardboard is mainly made of unbleached sulphate fibres pressed, there are a large number of pores between the fibres, and thus has a strong air permeability, oil absorption, water absorption, and so on. The life of the paper is greatly improved if it is made of polyamine fibre paper with high heat resistance. For example, as an insulating paper tube, support strip, pads, partitions, corner rings, etc..
(3) Cable paper. As a kind of insulating paper, made of sulphate pulp, in the transformer model DL2-08 and DL2-12 cable paper, its thickness is 0.08mm and 0.12mm. mainly used as the outer surface of the conductor wrapped insulation and coil insulation, lead insulation, etc., which is one of the main insulating materials for oil-immersed transformers.
(4) telephone paper. Made of sulphate pulp. In the transformer using model DH-50 telephone paper. Its thickness is (0.5 ± 5%) mm, rolled into a width of (500 ± 10) mm paper rolls. Mainly used as coil wire insulation and coil end insulation, etc..
(5) Wrinkled paper. It is also used as a kind of insulating paper, cable paper made of sulphate pulp and then processed into the electrical properties in the oil is very good, the performance of the average breakdown voltage is high, the dielectric loss angle of the tangent value is very small. Crumpled paper is mainly used as wrapping for transformer outlets and other places.

It should be noted that the insulating paper and cardboard dielectric coefficient ε is 4 to 5, more than twice as high as the transformer oil dielectric coefficient ε = 2.2. Under the action of an electric field, the field strength shared in the composite insulation is inversely proportional to the dielectric coefficient of the material.

It can be seen that the field strength in the oil gap is much larger than that of the cardboard, so the oil gap becomes the weak link of the oil and paper insulation. When the dielectric coefficient between them is close, the transformer insulation structure size may be reduced, so low dielectric coefficient of the new cardboard is the direction of the current research and development.