There is a grounded (copper) shielding layer outside the core, and a radial electric field is formed between the conductor core and the shielding layer. That is to say, the electric field of normal cable is only from (copper) conductor to (copper) shielding layer along radius, and there is no electric field (power line) along the axis of core line. The electric field distribution is uniform. When the cable head is made, the shielding layer is stripped, and the original electric field distribution of the cable is changed. The tangential electric field (the power line along the axis of the conductor) which is very harmful to insulation will be produced. The power line stripped of the shielding layer core concentrates at the shielding layer fracture. Then, at the shield layer fracture, it is the most vulnerable part of the cable to break down.
At the fracture of shield layer where the cable is most easily broken down, we adopt the dispersed concentrated power line (electric stress), which has a dielectric constant of 20-30 and a volume resistivity of 108-1012_ cm. The stress control tube (stress tube) is sleeved at the fracture of shield layer to disperse the electric field stress (power line) at the fracture, so as to ensure the reliable operation of the cable. In order to make the cable run reliably, the stress tube is very important in making the cable head. The stress tube is based on not destroying the main insulating layer, so as to achieve the effect of dispersing the electric stress. In the cable body, the outer surface of the core can not be a standard circle, and the distance between the core and the shielding layer will be different. According to the principle of electric field, the electric field intensity will also be large or small, which is also unfavorable to cable insulation. In order to make the electric field inside the cable uniform as far as possible, there is a circular semiconductor layer on the outer surface of the core wire, so that the thickness of the main insulation layer is basically equal, so as to achieve the goal of uniform distribution of the electric field. Outside the main insulating layer, the outer semiconductor layer inside the copper shielding layer is also set to eliminate the unevenness of the copper shielding layer and prevent the uneven electric field. In order to disperse the electric field stress at the break of the shield layer, the contact length between the stress tube and the copper shield layer is not less than 20 mm. Shorter contact length will make the contact surface of the stress tube insufficient, and the power line on the stress tube will not conduct enough (because the length of the stress tube is certain), longer will make the electric field dispersion area (section) smaller and the electric field dispersion insufficient. Generally, it is about 20-25 mm. When making intermediate joints, the main insulating layer must also be stripped off. After the core wires are crimped with copper nozzles, the fillers are used to wrap up (round). There are two ways to make it:
1. Thermal shrinkable sleeve: the main insulating sleeve made of thermal shrinkable material is shrunk (it is recommended to choose the mother row thermal shrinkable sleeve), the main insulating sleeve is shrunk out of the semiconductor tube, then covered with metal shield layer, and finally the outer protective sleeve sleeve.
2. Prefabricated accessories: The materials used are generally silicone rubber or ethylene propylene rubber. It is a hollow cylinder, the inner hole wall is a semiconductor layer, and the main insulating material is outside the semiconductor layer.
Prefabricated installation requirements are higher than heat shrinkable casing, and it is difficult. The aperture of tubular prefabricated parts is 2-5mm smaller than the outer diameter of main insulation layer of cable. The prefabricated pipes of intermediate joints shall be sleeved at both ends outside the main insulating layer of the cable, and the connecting length between each and the main insulating layer shall not be less than 10 mm. There is no need to sharpen the pencil head on the main insulating head of the cable (leaving semiconductor layer on the cable core as far as possible). Copper nozzle surface should be treated smoothly and packed with appropriate amount of filler.
Measures to Improve Electric Field Distribution
There are several ways to improve the electric field distribution at sheath break in power cable joints up to 35 kv.
(1) Expanding bell mouth: pry up the edge of the lead bag at the cut point of the lead bag and form a bell shape. Its edge should be smooth, round and symmetrical.
(2) Reserved package insulation: There is a section of package insulation paper between lead package incision and cable core separation point.
(3) Cut off the semi-conductive paper: Cut the semi-conductive paper below the bell mouth.
(4) wrapping stress cone: insulating tape and conductive metal material are wrapped in a cone, artificially expanding the shielding layer to improve the electric field distribution.
(5) Equipotential method: For dry-packed or cross-linked polyethylene cable head, a metal strip is wrapped on the insulating surface of each core profile and connected together.
(6) Installation of stress control tube: For the cable head with 35 kV or less thermal shrinkable sleeve, first, from the end direction of the copper shield layer of wire core to the insulating profile of wire core, two layers of semiconductor tape are wrapped, and then the corresponding specification of the bending stress tube is sheathed at the end of the copper shield, and the thermal shrinkage forming is carried out.
At present, there are two main types of measures to improve the electric field distribution in medium voltage cable accessories. The first is geometry: the electric field distribution is changed by changing the geometry of the voltage concentration in the cable accessories to reduce the electric field strength, such as wrapped stress cone, prefabricated stress cone, pencil sharpener, bell expander, etc. The second is parametric: a stress control layer made of a certain parameter material is added to the insulation of the copper shield cut off at the end of the cable to change the potential distribution on the surface of the insulation layer, so as to improve the electric field distribution at the end of the cable. For example, common stress control pipes, stress bands, etc.