Introduction to the application of heat shrinkable tubing in solving high-voltage cable faults: Each phase core of the high-voltage cable has a grounded (copper) shielding layer, and a radially distributed electric field is formed between the conductive core and the shielding layer. In other words, the electric field of a normal cable is only the electric field from the (copper) wire along the radius to the (copper) shielding layer, and there is no electric field (electric force) in the axial direction of the core wire, and the electric field distribution is uniform. When making the cable head, the shielding layer is stripped, and the original electric field distribution of the cable is changed, which will produce a tangential electric field (electric force along the axis of the conductor) that is extremely unfavorable to the insulation. The power lines from which the core wires of the shielding layer are stripped are concentrated at the fracture of the shielding layer. Then the break of the shielding layer is the most easily broken part of the cable.
At the break of the shielding layer where the cable is most likely to be broken down, we use the concentrated power line (electric stress) to disperse the electric stress control tube made of material with a dielectric constant of 20-30 and a volume resistivity of 108-1012ω?cm (abbreviated as stress Pipe), sleeved 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. 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 electrical stress on the basis of not damaging the main insulation layer. In the cable body, the outer surface of the core wire cannot be a standard circle, and the distance between the core wire and the shielding layer will not be equal. According to the principle of electric field, the electric field strength will also be large, which is also detrimental to the cable insulation. In order to make the electric field uniform within the cable as much as possible, there is a semiconductor layer with a circular outer surface outside the core wire, so that the thickness of the main insulating layer is basically equal to achieve the purpose of uniform electric field distribution. Outside the main insulation layer, the outer semiconductor layer inside the copper shielding layer is also set to eliminate unevenness of the copper shielding layer and prevent uneven electric field. In order to disperse the electric field stress of the cable at the fracture of the shielding layer as much as possible, the contact length of the stress tube and the copper shielding layer should not be less than 20mm. The length is fixed), if it is longer, the electric field dispersion area (segment) will be reduced and the electric field dispersion will be insufficient. Generally around 20~25mm. When making intermediate joints, part of the main insulation layer must be stripped off. After the core wire is crimped with a copper tube, it is flat (round) with filler. There are two production methods:
1. Heat-shrinkable tubing: the main insulating tubing made of heat-shrinkable material is shrunk (recommended to use busbar heat-shrinkable tubing), the main insulating tubing is externally shrinking the semiconductor tube, and then covered with a metal shield, and finally the outer sheath .
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 outside of the semiconductor layer is a main insulating material.
Prefabricated installation requires higher and more difficult than heat shrinkable tubing. The hole diameter of the tubular preform is 2~5mm smaller than the outer diameter of the main insulation layer of the cable. The prefabricated pipe of the intermediate joint should be sheathed outside the main insulation layer of the cable, and the length of each connection with the main insulation layer shall not be less than 10mm. There is no need to sharpen the pencil tip on the main insulation head of the cable (try to leave the semiconductor layer on the cable core). The surface of the copper pipe should be smooth and packed with proper amount of filler.
Measures to improve electric field distribution
1. In 35kv and below power cable joints, there are several ways to improve the electric field distribution at the disconnection of the sheath
(1) Expansion bell mouth: Pry up the edge of the lead bag at the cut point of the lead bag to make it into a horn shape. The edge should be smooth, round and symmetrical.
(2) Reserve turnkey insulation: a piece of turnkey insulation paper is left between the cut of the lead bag and the separation point of the cable core.
(3) Cut off the semi-conductive paper: Cut the semi-conductive paper below the bell mouth.
(4) Wrap the stress cone: wrap it into a cone with insulating tape and conductive metal material, and artificially expand the shielding layer to improve the electric field distribution.
(5) Equipotential method: For dry-wrapped or cross-linked polyethylene cable heads, wrap a piece of metal tape on the insulating surface of each core and connect them together.
(6) Install stress control tube: For 35kv and below heat shrinkable casing cable head, firstly wrap 2 layers of semiconductor tape from the end of the core copper shielding layer through the semiconductor tape to the core insulation profile, and then fold the corresponding specifications to the stress The tube is sheathed at the end of the copper shield and heat-shrinked.
2. At present, there are two main types of measures to improve electric field distribution in medium voltage cable accessories. The first is geometry: it changes the electric field distribution by changing the geometric shape of the voltage concentration in the cable accessories, and reduces the electric field strength there, such as the wrap stress cone, prefabricated stress cone, pencil sharpener, expansion bell mouth, etc.