1. The insulation performance of the cable accessories should not be lower than that of the cable body. The dielectric loss of the insulating materials used should be low. In the structure, the sudden change of the electric field in the cable accessories can be improved, and there are measures to change the electric field distribution.
2. Principle of electric field distribution
The high-voltage cable has a grounded (copper) shielding layer outside each core, and a radially distributed electric field is formed between the conductive core and the shielding layer. That is to say, the electric field of the normal cable has only the electric power line from the (copper) wire along the radius to the (copper) shield layer, and there is no electric field (power line) in the axial direction of the core wire, and the electric field distribution is uniform.
3, the most easily broken breakdown of the shield, we take the concentrated power line (electrical stress), with a dielectric constant of 20 ~ 30, volume resistivity 
The electric stress control tube (referred to as stress tube) made of Ω·cm material is placed at the fracture of the shielding layer to disperse the electric field stress (power line) at the fracture to ensure reliable operation of the cable.
4. In order to make the cable run reliably, the stress tube is very important in the production of the cable head, and the stress tube can achieve the effect of dispersing the electrical stress without destroying the main insulation layer. In the cable body, the outer surface of the core wire may not be a standard circle, and the distance between the core wire and the shielding layer may be unequal. According to the electric field principle, the electric field strength may also be small, which is also disadvantageous for cable insulation. In order to make the electric field inside the cable as uniform as possible, the outer surface of the core wire has a circular semiconductor layer on the outer surface, so that the thickness of the main insulating layer is substantially equal, and the electric field is uniformly distributed.
Outside the main insulating layer, the outer semiconducting layer in the copper shielding layer is also provided to eliminate the unevenness of the copper shielding layer and prevent the electric field from being uneven.