The insulation layer of the cable is used to isolate multi-core conductors and between conductors and sheaths, and to ensure a certain electrical withstand voltage strength. It should have certain heat resistance and stable insulation quality.
The insulation layer of the cable is used to isolate multi-core conductors and between conductors and sheaths, and to ensure a certain electrical withstand voltage strength. It should have certain heat resistance and stable insulation quality.
The thickness of the insulation layer is related to the operating voltage. Generally speaking, the higher the voltage level, the thicker the insulation layer, but not proportionally. Because from the perspective of electric field strength, when the cross-sectional area of the conductor is large for cables with the same voltage level, the thickness of the insulation layer can be thinner. For cables with lower voltage levels, especially oil-impregnated paper insulated cables with lower voltage levels, in order to ensure that the paper layer has a certain mechanical strength when the cable is bent, the thickness of the insulation layer becomes thicker as the conductor cross-section increases.
There are three main materials for the insulation layer: oil-impregnated cable paper, plastic and rubber. According to the different materials used in the conductor insulation layer, cables are mainly divided into plastic insulated cables, rubber insulated cables and oil-impregnated paper insulated cables. The structures and characteristics of the three types of cable insulation layers are described below.
(1) Plastic insulation
There are two main types of plastic insulation: polyvinyl chloride insulation and cross-linked polyethylene insulation. The cable insulation layer is made of thermoplastic extrusion and cross-linking after extrusion and cross-linking of thermoplastic polyethylene plastic with added cross-linking agent. This kind of insulation has good electrical properties and water resistance, can resist acid, alkali, and corrosion. It also has the advantages of high operating temperature, good mechanical properties, and can be used to manufacture high-voltage cables.
PVC insulation has many advantages over oil-impregnated paper insulation, but its dielectric loss is larger, about 10 to 20 times greater than that of oil-impregnated paper insulation, and its conductance (ion) increases with the increase of electric field strength. rises sharply, so applications at higher voltages are limited. In this regard, the insulation performance of polyethylene is greatly improved compared to PVC. Under the same conditions, the AC breakdown strength of polyethylene is increased by about 60%, and its dielectric loss is only about 0.5% of that of PVC. Polyethylene insulated cable has the characteristics of high insulation performance, small specific gravity, good water and chemical resistance, etc. However, its melting point is too low and it is easy to crack under the action of mechanical stress. In order to take advantage of the good insulation properties of polyethylene and overcome its shortcomings of low melting point, high-energy radiation or chemical methods are used to cross-link polyethylene, so that its molecules change from the original linear structure to a network structure, that is, from a thermoplastic It becomes thermosetting, thus improving the heat resistance and thermal stability of polyZene. This is cross-linked polyethylene. Its main features are high softening point, small thermal deformation, high mechanical strength at high temperatures, good thermal aging resistance, etc.: The maximum operating temperature of cross-linked polyethylene cable can reach 90°C. The allowable temperature during short circuit is up to 250°C. .
Although cross-linked polyethylene cables have very superior electrical properties, there are inevitably micropores, impurities and other defects inside the insulation. In particular, the presence of micropores makes it more water absorbent and affects the performance of high electric fields. Under the electric field, the "water tree" phenomenon is caused along the direction of the electric field, thereby destroying the insulation. It is true that efforts to control micropores and impurities in cable material selection and manufacturing processes are the main way to reduce the occurrence of water tree phenomena, but unreasonable construction methods during laying construction can also lead to the formation of new micropores. Due to poor sealing of cable terminals and intermediate joints or the ingress of water or moisture due to unsealed cable ends during construction, the cable may cause water branch discharge in future operations, and sufficient attention should be paid to this.
(2) Rubber insulation
The insulation layer of rubber insulated power cables is styrene-butadiene rubber or synthetic rubber (ethylene-propylene rubber, butyl rubber). The outstanding advantages of this kind of cable are that it is soft and flexible, and is especially suitable for mobile electricity and power supply devices. However, rubber insulation will be damaged quickly when it encounters oil: under the action of high voltage, it is susceptible to cracks due to corona. Therefore, this kind of cable is generally used for voltage levels of 10kV and below, while synthetic ethylene-propylene rubber insulated cables can be used for voltage levels of 35kV.
(3) Oil-impregnated paper insulation
Oil-impregnated paper insulation is composed of cable paper and impregnating agent. The thickness of ordinary oil-impregnated paper insulated cable paper is 0.08, 0.12, and 0.17mm; the impregnating agent is made of low-voltage cable oil and rosin, which is called viscous impregnated cable oil. The conductors of single-core cables and split-phase lead (aluminum) sheathed cables are round, and the insulation layer structure is a cable paper tape wrapped in concentric multi-layers to form a round shape. For multi-core cables of I0kV and below, the conductors are semicircular, oval or fan-shaped, and the insulation layer structure is tie-down. In this structure, each conductor is wrapped with a portion of insulating paper (called conductor insulation), then several conductors are twisted together, and then wrapped with a certain thickness of cable paper (called turnkey insulation or tape insulation). , so that between the conductor and the conductor is twice the conductor insulation thickness; between the conductor and the lead sheath is the conductor insulation thickness plus the overall insulation thickness.
The oil-impregnated paper insulation of oil-filled cables requires higher electrical performance. The thickness of the paper is 0.045, 0.075, 0.125, 0.175mm, etc., and the tangent value of the dielectric loss angle should be greater than o.0026. The impregnating agent is low-viscosity oil (insulating oil), and the power frequency breakdown strength of the oil should not be less than 60kV/2.5mm. In this way, under the action of a certain oil pressure, the electrical strength of the cable insulation is greatly improved.
The thickness of the power cable insulation layer is determined according to the working voltage of the cable and the nominal cross-section of the conductor. It must not only ensure that it will not be broken down under power frequency voltage and impulse voltage, but also ensure that the insulation of the cable will not be mechanically affected during normal construction. damage.
Oil-impregnated paper insulation has stronger heat resistance than rubber insulation and polyethylene insulation. Its regular operating temperature can reach 80C and its electrical strength is high. Viscous oil-impregnated paper insulation is used for voltage levels 35kV and below. Oil-impregnated paper insulation supplemented by oil pressure is used for voltage levels of 60kV and above. Paper insulation easily absorbs moisture, which greatly reduces the insulation strength. Therefore, in addition to removing the moisture contained in it and impregnating it during manufacturing, a metal sheath is also used to prevent moisture intrusion. In addition, the flexibility of paper insulation is relatively poor. Therefore, the minimum allowable bending radius and the minimum laying temperature of the cable are stipulated. For these, special attention must be paid during construction to ensure good insulation performance.