A concept,
The cross-linked cable usually refers to the use of cross-linked material for the insulation layer of the cable.
The most commonly used material is crosslinked polyethylene (XLPE).
The crosslinking process is to make the linear molecular structure of polyethylene (PE) material through a specific processing method, so that it forms the body network of cross-linked polyethylene structure.
The long term allowable working mixing is increased from 700C to 900C(or higher), and the short-circuit allowable temperature is increased from 1400C to 2500C(or higher), which greatly improves the practical performance under the premise of maintaining its original excellent electrical performance.
2. Crosslinking process
At present, the production process of cross-linked cables in the cable industry is divided into three types: the first type of peroxide chemical cross-linking, including saturated vapor cross-linking, inert gas cross-linking, molten salt cross-linking, silicone oil cross-linking, and the second type of dry chemical cross-linking is adopted in China;
The second type of silane chemical crosslinking;
The third type of irradiation crosslinking.
Inert gas crosslinking -- ----- dry chemical crosslinking
The polyethylene insulation material with peroxycompound crosslinking agent is used to complete the conductor shielding layer ---- insulation layer -- the insulation shielding layer is extruded through three layers of co-extrusion, and the crosslinking process is completed continuously and uniformly through the sealed crosslinking pipe filled with high temperature and high pressure nitrogen.
The heat transfer medium is nitrogen (inert gas). The XLPE has excellent electrical properties and the production range can reach 500KV class.
Silane chemical crosslinking -- warm water crosslinking
The polyethylene insulation material with silane cross-linking agent is used to complete the extrusion of foreign shielding layer -- insulating layer -- insulating shielding layer by 1+2 extrusion method. The cooled insulation wire core is immersed in 85-950C hot water for hydrolytic cross-linking, because wet cross-linking will affect the water content in the insulating layer.
Generally, the highest voltage grade is only up to 10kV.
Irradiation crosslinking ---- physical crosslinking
The modified polyethylene insulating material is used to complete the alien shielding layer ---- insulating layer -- insulating shielding layer by 1+2 extrusion method. After the insulation core is cooled, the cross-linking process is completed evenly through the irradiation scanning window of the high-energy electron accelerator.
No crosslinking agent is added to the irradiated cross-linked cable material. In the cross-linking process, the high-energy electron beam generated by the high-energy electron accelerator effectively penetrates the insulation layer and produces the cross-linking reaction through the energy conversion. Because the electrons have a high energy and pass evenly through the insulation layer, the cross-linking bond formed has high binding energy and good stability.
Its physical properties are superior to those of chemical crosslinked cables in heat resistance.
However, due to the limitation of the energy level of the accelerator (generally no more than 3.0MeV, the effective penetration thickness of the electron beam is less than 10mm. Considering the geometric factors, the voltage level of the production cable can only reach 10kV, and the advantage is less than 6kV.
Characteristics of irradiated crosslinked cable
Cable insulation aging life depends mainly on its thermal aging life, it is under the hot insulation materials in the thermal oxygen oxidation and thermal cracking of oxidation and thermal cracking, condensation and decided by the speed of chemical reaction, so the thermal aging of the insulating material life directly affects the service life of the cable, according to chemical reaction dynamics is derived and the artificial accelerated thermal aging test measured (20 to 30 years) irradiation crosslinking cable allowed working temperature for a long time for:
Power cable YJV0.6/1KV1160C
The thermal aging life is more than 60 years at the rated operating temperature of 1050C.
If the rated working temperature is 900C, the thermal aging life is more than 100 years.
Overhead insulated cable
When overhead insulated cables are laid in the open air, the environmental resistance and radiation resistance of insulating materials are more important.
Irradiated cross-linked insulating materials must pass through
The irradiated processing itself has a good radiation resistance, and the radiation dose applied in the cross-linking production process is far from the damage dose, leaving a large safety margin.
The radiation damage dose of polyethylene is 1000kGy, while the processing dose is about 200kGy. With the special formulation improvement, the polyethylene is still in the radiation cross-linking state in a wide range, so its performance will be improved after being exposed to radiation in the long early use process.
Four, commonly used plastic insulated cable performance comparison:
At present, in the cable production, the most commonly used insulating plastics are polyethylene and polyvinyl chloride, polyethylene material has better electrical properties and better crosslinking, so a variety of industrial crosslinking production processes, chemical crosslinking and irradiation crosslinking have been developed.
In addition to the properties in the table below, in the process of production and laying, the insulation layer of the currently commonly used cross-linked cable is characterized by greater hardness and strength (at room temperature), especially more difficult to peel than PVC insulation.
The irradiated cross-linked cable has the best cross-linking performance, the highest cross-linking degree and the highest peel strength.
If the cross-linked cable insulation layer is easy to peel (similar to polyvinyl chloride), it must be insufficient cross-linking or no cross-linking.
In general, the crosslinking degree of the crosslinking cable produced by the warm water crosslinking process is often insufficient. The reason is that the crosslinking degree of this kind of product is relatively low originally, and the crosslinking process is discontinuous and cannot be automatically controlled, which is greatly affected by human factors and is prone to undercrosslinking.