1. The basic requirements of high voltage cable head
The cable terminal is the component that connects the aerial bundled cable(abc) with other electrical equipment. The cable middle is the component that connects the two cables. The cable terminal and the middle are collectively called cable accessories. The cable accessory should be able to operate safely for a long time like the cable body and have the same service life as the aerial bundled cable SANS 1418 standard. A good cable accessory should have the following properties:
Wire core connection is good: mainly because the connection resistance is small and the connection is stable, which can withstand the impact of fault current; after long-term operation, its contact resistance should not be greater than 1.2 times the resistance of the cable core body with the same length; it should have a certain mechanical strength and resistance Vibration and corrosion resistance; in addition, it should be small in size, low in cost, and easy to install on site.
Good insulation performance: The insulation performance of the cable accessories should not be lower than that of the cable body, and the dielectric loss of the insulating materials used should be low. Structurally, the sudden change of the electric field in the cable accessories can be perfected, and there are measures to change the electric field distribution.
2. Principle of electric field distribution
There is a grounded (copper) shielding layer outside each phase core of the high-voltage cable, 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 aerial bundled cable 33-209 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 toward the shielding layer fracture. 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 break down, we use the concentrated power line (electric stress) to disperse the electric stress control tube (abbreviated as the material with a dielectric constant of 20-30 and a volume resistivity of 108-1012Ω•cm). Stress tube), 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.
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 electrical stress on the basis of not destroying 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, and the electric field is uniformly distributed.
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 must 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 flattened with filler (round). There are two production methods:
Heat-shrinkable casing: the main insulating casing made of heat-shrinkable material is shrunk, the main insulating casing is externally contracted to the semiconductor tube, and then covered with a metal shield, and finally the outer protective casing.
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. 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 at both ends, 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 (as much as possible 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.
Key technical issue: the size of the accessory and the size of the cable to be installed must meet the specified requirements. In addition, it is necessary to use silicone grease to lubricate the interface to facilitate installation, while filling the air gap of the interface to eliminate corona. Prefabricated accessories generally rely on their own rubber elasticity to have a certain sealing effect, and sometimes sealants and elastic clamps can be used to enhance the sealing. The outside of the prefabricated tube is the same as the heat-shrinkable one, the semiconductor layer and the copper shielding layer, the outermost layer is the outer protective layer.