The insulation layer of the cable is used to isolate the multicore conductor and the conductor and sheath from each other and to ensure a certain electrical voltage strength, which should have a certain heat resistance and stable insulation quality.
The thickness of the insulation layer is related to the working voltage.
Generally speaking, the higher the voltage level, the thicker the insulation layer, but it is not proportional.
Because of the electric field strength, and the same voltage level of the cable, when the conductor cross-sectional area is large, the thickness of the insulation layer can be thinner.
For lower voltage cables, especially paper insulated cables, to ensure that when the cable is bent, the paper layer has a certain mechanical strength, the thickness of the insulation layer with the increase in conductor cross-section and thicken.
Insulation layer materials are mainly oil-immersed cable paper, plastic, and rubber three.
According to the different materials used in the conductor insulation layer, the cable is mainly divided into plastic-insulated cable, rubber-insulated cable, and oil-impregnated paper-insulated cable.
The structure and characteristics of the three cable insulation layers are described below.
Plastic insulation
Plastic insulation mainly includes polyvinyl chloride insulation and cross-linked polyethylene insulation.
The cable insulation layer is made of thermoplastic plastic extrusion and cross-linked by adding cross-linking agent after the extrusion of thermoplastic polyethylene plastic.
This kind of insulation has good electrical performance and water resistance, can resist acid and alkali, and corrosion prevention, it also has the advantages of high allowable working temperature, good mechanical properties, and can manufacture high voltage cables.
Rubber Insulation
The insulation layer of rubber-insulated power cables is styrene-butadiene rubber or synthetic rubber (ethylene propylene rubber, butyl rubber).
The outstanding advantage of this cable is its flexibility and good weldability, which makes it especially suitable for mobile electricity and power supply installations.
However, the rubber insulation will be damaged quickly when it encounters oil.
This is due to the high voltage effect, due to the material problem being susceptible to corona effect cracking.
Therefore, this type of cable is generally used for medium voltage insulated cables, 10kV and below, while synthetic ethylene propylene rubber insulated cables are available up to 35kV.
Oil-impregnated paper insulation
Oil-impregnated paper insulation consists of a combination of cable paper and impregnating agent.
The thickness of ordinary oil-impregnated paper insulation cable paper is 0.08, 0.12, and 0.17 mm.
The impregnating agent is mixed with low-pressure cable oil and rosin, which is called viscous dipping cable oil.
Single-core cable and split-phase lead (aluminum) sheathed cable conductor is round, insulation layer structure for the cable paper tape to concentric multi-layer wrapped into a circle.
I0kV and below multicore cable, the conductor is a semicircular, oval, or fan-shaped, insulation layer structure for the bundle tape type. This structure is in each conductor is wrapped around a part of the insulating paper, several conductors together, and then wrapped around a certain thickness of cable paper, called the total package insulation or with insulation, so that between the conductor and conductor for two times the thickness of the conductor insulation.
Between the conductor and the lead sheath is the thickness of the conductor insulation plus the thickness of the turnkey insulation.
Oil-filled cable oil-impregnated paper insulation requires higher electrical performance, the paper thickness of 0.045, 0.075, 0.125, 0.175 mm, and several other, dielectric loss angle tangent values should be greater than o.0026.
Impregnating agent for the low viscosity of the material oil, oil frequency breakdown strength should not be less than 60kV/2.5 mm.
This greatly improves the electrical strength of the cable insulation under certain oil pressure.
The thickness of the insulation layer of the power cable is determined by the working voltage of the cable and the nominal cross-section of the conductor, which should ensure that it will not be penetrated under the frequency voltage and shock voltage, but also ensure that the insulation will not be mechanically damaged during normal construction.
Oil-impregnated paper insulation has stronger heat resistance than rubber insulation and polyethylene insulation, often operating at temperatures up to 80C, with high electrical strength.
Adhesive oil-impregnated paper insulation is used for 35kV and below, while oil-impregnated paper insulation is used for 60kV and above.
The paper insulation can easily absorb moisture, which greatly reduces the strength of the insulation. Therefore, in addition to removing the moisture and impregnating it, the metal sheath is used to prevent moisture intrusion.
In addition, the bendability of paper insulation is relatively poor, so the minimum allowable bending radius of the cable and the minimum laying temperature is specified.
For these, special attention should be paid during construction to ensure good insulation performance.
PVC insulation has many advantages over oil-impregnated paper insulation, but the dielectric loss of its insulation is larger, about 10~20 times larger than that of oil-impregnated paper insulation.
Moreover, its conductivity rises sharply with the increase in electric field strength, so its application at higher voltages is limited. Under the same conditions, the AC breakdown strength of polyethylene is about 60% higher, and its dielectric loss is only about 0.5% of that of PVC.
Polyethylene-insulated cables are characterized by high insulation performance, low specific gravity, and good resistance to water and chemicals, but their melting point is too low, and they are prone to cracking under mechanical stress.
To take advantage of the good insulation properties of polyethylene and overcome its low melting point disadvantage.
It is possible to cross-link polyethylene by high energy irradiation or chemical methods, so that its molecules change from the original linear structure to a reticular structure, i.e. from thermoplastic to thermosetting, thus improving the heat resistance and thermal stability of polyethylene, which is cross-linked polyethylene.
Its main features are a high softening point, small thermal deformation, high mechanical strength at high temperatures, good anti-heat aging performance, etc. The maximum operating temperature of cross-linked polyethylene cable can reach 90℃, while the allowable temperature in the short circuit reaches 250℃.
Although the cable has very superior electrical properties, there are inevitably micro-porosity, impurities, and some other defects inside the insulation.
In particular, the existence of microporous, so that its water absorption is enhanced, under the action of the high electric field, along the direction of the electric field triggered the "water dendrite" phenomenon, so that the insulation is damaged.
Indeed, the cable in the selection of materials and manufacturing process to control the microporous, impurities, etc. is the main way to reduce the phenomenon of water dendritic state, but in the laying construction of unreasonable construction methods will also lead to the formation of new microporous.
As a result of cable terminals, intermediate joints of poor sealing or cable in the construction of the guillotine is not sealed and into the water, into the moisture, will make the cable in the future operation the possibility of triggering water dendrite discharge, which ZMS cable manufacturers have paid sufficient attention to this.
