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Cross-linked cable is the abbreviation of cross-linked polyethylene insulated cable. Cross-linked cable is suitable for power transmission and distribution lines with power frequency AC voltage of 500KV and below. At present, most high-voltage cables are insulated with cross-linked polyethylene.
Cross-linked cable usually refers to the insulation layer of the cable using cross-linked materials. The most commonly used material is cross-linked polyethylene (XLPE).
The process is to use a specific processing method to form a cross-linked polyethylene with a three-dimensional mesh-shaped wire structure from polyethylene (PE) materials with linear molecular structures. The long-term allowable operating temperature is increased from 70¡æ to 90¡æ (or higher), and the short-circuit allowable temperature is increased from 140¡æ to 250¡æ (or higher), which greatly improves the actual performance while maintaining its original excellent electrical properties.
Method for identifying false breakdown of cross-linked cables:
During the process of partial discharge and high voltage test of finished medium voltage cables using series resonance test system, sometimes false breakdown will suddenly occur during the test due to various reasons. How to correctly identify whether the cable itself has a true breakdown or other reasons have caused a false breakdown of the cable is of great significance to improving the efficiency of cable factory test.
The factory partial discharge and high voltage test of medium voltage cross-linked power cables are carried out one by one in the shielding room. At this time, you can consider replacing the insulating core for retesting. If the voltage test passes normally after the core is replaced, it should be determined that the cable has really broken down. Alternatively, if the breakdown voltage shows a trend of gradually decreasing, it can also be determined that the cable has a true breakdown. If the breakdown voltage of the cable is at the same value every time, it should be considered whether the end of the cable has broken down. At this time, the door interlock switch of the partial discharge shielding room door can be closed manually, the shielding room door can be opened, the voltage can be increased, and the breakdown phenomenon can be observed. The breakdown position of the end can be determined quickly. If after investigation, no breakdown occurs at the cable end, multiple insulated wire cores and cables are replaced, and even the cable that has passed the test is retested or retested without load, and the breakdown voltage remains the same, it should be confirmed that the cable has a false breakdown. The equipment should be checked and the test should be conducted after troubleshooting. However, for some long-length single-core medium-voltage cross-linked cables that are inspected, it should be checked whether they have exceeded the allowable load of the test equipment. In this case, detuning tripping may occur during the test. Do not rashly determine that the cable has a breakdown.
There is a special case that needs to be explained here. Sometimes a short circuit occurs inside the cable insulation, and the series resonance test system may not be able to resonate and boost the voltage. This is also a special cable breakdown phenomenon. In this case, just replace the insulated wire core with good insulation for testing to confirm it.
Currently, the cable industry has three types of processing methods for producing cross-linked cables: the first type is peroxide chemical cross-linking, including saturated vapor cross-linking, inert gas cross-linking, molten salt cross-linking, and silicone oil cross-linking. China uses the second type, i.e. dry chemical cross-linking; the second type is silane chemical cross-linking; and the third type is irradiation cross-linking.
Using polyethylene insulation material with peroxide compound cross-linking agent, after the conductor shielding layer - insulation layer - insulation shielding layer is extruded through three-layer co-extrusion, the cross-linking process is completed continuously and evenly through a sealed cross-linking tube filled with high-temperature, high-pressure nitrogen. The heat transfer medium is nitrogen (inert gas), and the cross-linked polyethylene has excellent electrical properties and the production range can reach 500KV.
Use polyethylene insulation material with silane crosslinking agent, and complete the extrusion of conductor shielding layer-insulating layer-insulating shielding layer through 1+2 extrusion method, then immerse the cooled and packaged insulating wire core in 85-95¡æ hot water for hydrolysis and crosslinking, because wet crosslinking will affect the water content in the insulating layer. Generally, the highest voltage level is only 10KV.
A method for applying chemical crosslinking and radiation crosslinking functional master particles
At present, polyethylene cable crosslinking functional master particles are used, and a small amount of this functional master particle can be added to ordinary polyethylene cable material particles, and the extruded cable can become a crosslinked cable.
Cable crosslinking functional master particles are divided into three types: chemical crosslinking function, electron beam irradiation crosslinking function, and ultraviolet irradiation crosslinking function.
1. Cross-linked functional masterbatch for chemical cross-linked cables, model: DH-125Y,
DH-125Y chemical cross-linked functional masterbatch has the appearance of colorless LLDPE plastic particles. Wire and cable factories only need to purchase common LLDPE polyethylene ordinary plastic particles on the market, such as LLDPE7042, etc. Add 1 kg of DH-125Y functional masterbatch to 25 kg of LLDPE7042 plastic particles, stir them evenly by hand, and directly put them into the extruder for cables and wires to extrude the cables to make cross-linked wires and cables. This functional masterbatch can produce cross-linked wires and cables of 35KV and below.
2. Cross-linked functional masterbatch for electron beam irradiation cross-linked cables, model: DH-125DF
DH-125DF electron beam irradiation cross-linked functional masterbatch has the appearance of colorless LLDPE plastic particles. Add 1 kg of DH-125DF functional masterbatch to the 7042 plastic particles, stir it evenly by hand, and then directly put it into the extruder for placing cables and wires, extrude the cables, and make cross-linked wires and cables by electron beam irradiation. This functional masterbatch can produce cross-linked wires and cables with a long-term working temperature of 125¡ãC.
3. Cross-linked functional masterbatch for UV-irradiated cross-linked cables, model: DH-125ZF,
DH-125ZF UV-irradiated cross-linked functional masterbatch has the appearance of colorless LLDPE plastic particles. Add 1 kg of DH-125ZF functional masterbatch to the 7042 plastic particles, stir it evenly by hand, and then directly put it into the extruder for placing cables and wires, extrude the cables, and make cross-linked wires and cables by UV irradiation. This functional masterbatch can produce cross-linked wires and cables with a long-term working environment temperature of 125¡ãC.
Advantages;
1. Save money:
Reduce costs, cable manufacturers use directly, which is 1500~3000 yuan/ton cheaper than the chemical and irradiation cross-linked polyethylene cable special material particles purchased from the market.
2. Save time;
Cable manufacturers need to inquire, order, produce and transport chemical and irradiation cross-linked polyethylene cable special material particles for a week. When using DH-125 functional masterbatch, after deciding on the production plan, cable production can be carried out directly in 5 minutes.
3. Versatility,
Cable manufacturers can adjust the variety and hardness by themselves: DH-125 functional masterbatch can be added not only to ordinary polyethylene PE particles, but also to ordinary cable material particles that do not have cross-linking properties, so that ordinary plastic cable particles that originally do not have cross-linking properties can be turned into cross-linked plastic particles.
After extrusion of the modified polyethylene insulation material through the 1+2 extrusion method, the cooled insulation core is evenly passed through the radiation scanning window of the high-energy electron accelerator to complete the cross-linking process. No cross-linking agent is added to the radiation cross-linked cable material. During cross-linking, the high-energy electron beam generated by the high-energy electron accelerator effectively penetrates the insulation layer and produces a cross-linking reaction through energy conversion. Because the electrons carry high energy and evenly pass through the insulation layer, the cross-linking bond formed has high binding energy and good stability. The physical properties shown are that the heat resistance is better than that of chemical cross-linked cables. However, due to the limitation of accelerator energy level (generally no more than 3.0Mev, the effective penetration thickness of electron beam is less than 10mm, and considering geometric factors, the voltage level of the produced cable can only reach 10KV, and the advantage is below 6KV.
Cross-linked insulated wires and cables have excellent electrical properties, good operating safety performance and thermal overload mechanical characteristics, as well as simple installation, operation and maintenance.
The cross-linking mechanism of wire and cable insulation materials is to use physical or chemical methods to transform polymer insulation materials from linear molecular structures into three-dimensional network structures, and from thermoplastic materials into thermosetting insulation materials, thereby improving the aging resistance, mechanical properties and environmental resistance of insulation materials. The United States invented cross-linking in the 1950s. Insulated wires and cables were gradually put into use in the 1960s. In the past decade, China has also increasingly widely used cross-linked insulation, which has replaced oil-paper insulation and is gradually replacing PVC plastic insulation.
There are many varieties of cross-linked insulation, which can be divided into two categories based on the cross-linking mechanism, namely physical cross-linking and chemical cross-linking.
1. Chemical cross-linking: Chemical cross-linking is divided into two methods: high-temperature cross-linking and low-temperature cross-linking.
(1) High-temperature cross-linking is also called peroxide cross-linking. Generally, organic peroxides are used as cross-linking agents. Under the action of heat, they decompose to generate active free radicals. These free radicals create active points on the polymer carbon chain and produce CC cross-linking bonds to form a three-dimensional network structure.
High-temperature cross-linking includes steam cross-linking and dry cross-linking. In terms of process form, most foreign cross-linked cables in the 1960s used steam cross-linking technology. Since steam cross-linking increases the moisture content in the insulation and the insulation quality is poor, it has now been completely eliminated. Since the 1970s, foreign countries have generally used dry cross-linking technology, using high-pressure vulcanization pipes and rapid heating methods for cross-linking.
(2) Low-temperature cross-linking is also called warm water cross-linking or silane cross-linking. The cable is cross-linked in warm water at 70-90¡ãC. After absorbing water, the cross-linking agent in the insulation, silane, reacts linearly to form a network cross-linked structure.
2. Physical cross-linking: also known as irradiation cross-linking, it is divided into two methods: ¦Ã-ray cross-linking and electron beam cross-linking.
(1) Due to the low dose rate of ¦Ã-ray cross-linking, it cannot penetrate the core wire of the cable during the irradiation process. Therefore, it is currently only used in the cross-linking of heat shrinkable materials, and ¦Ã-ray cross-linking is generally not used in the production of wires and cables.
(2) Electron beam cross-linking, using an electron accelerator in conjunction with a beam irradiation device, uses a high-energy electron beam (generally between 1.0-3.0 MeV) to irradiate the insulation layer of wires and cables, inducing the polymer material to produce free radicals, forming CC cross-linking bonds, and generating a three-dimensional network structure.
Performance comparison of commonly used insulating cables:
Currently, the most commonly used insulating plastics in cable production are polyethylene and polyvinyl chloride, among which polyethylene materials have better electrical properties and better cross-linking properties. Therefore, a variety of industrial cross-linking production processes have been developed, including chemical cross-linking and radiation cross-linking. In addition to the properties in the table below, during the production and laying process, the insulation layer of the commonly used cross-linked cables currently exhibits greater hardness and strength (at room temperature), especially more difficult to strip than polyvinyl chloride insulation. Since radiation cross-linked cables have the best cross-linking performance and the highest degree of cross-linking, their peeling strength is also relatively the highest. If the cross-linked cable insulation layer is easy to strip (similar to polyvinyl chloride), it must be that the degree of cross-linking is insufficient or there is no cross-linking. Under normal circumstances, cross-linked cables produced by warm water cross-linking process often have insufficient cross-linking. The reason is that the degree of cross-linking of such products is relatively low, and the cross-linking process is non-continuous and cannot be automatically controlled. It is greatly affected by human factors and is prone to under-cross-linking.
Characteristics of irradiated cross-linked cables:
The aging life of cable insulation materials mainly depends on their thermal aging life, which is determined by the speed of chemical reactions such as thermal oxidation, thermal cracking, thermal oxidation cracking, and polycondensation that occur in the insulation materials under hot working. Therefore, the thermal aging life of the insulation material directly affects the service life of the cable. According to the derivation of chemical reaction kinetics and the artificial accelerated thermal aging test (20-30 years), the long-term allowable operating temperature of irradiated cross-linked cables is:
Power cable YJV 0.6/1KV
If deduced at the rated operating temperature of 105 degrees, its thermal aging life exceeds 60 years.
If deduced based on the rated working temperature of 90 degrees, its thermal aging life exceeds 100 years.
Overhead insulated cables
Overhead insulated cables are laid in the open air, so the environmental resistance and radiation resistance of the insulating materials are even more important. Radiation cross-linked insulating materials must undergo irradiation processing, and they themselves have good radiation resistance. The irradiation dose applied during the cross-linking production process has a large safety margin from its destructive dose. The radiation destructive dose of polyethylene is 1000KGY, while the processing dose is about 200KGY. In addition, due to the improvement of the special formula, it is still in a radiation cross-linked state within a fairly wide range, so its performance will be improved during the long initial use process.
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