XLPE Cable Process Method & Characteristics

Cross-linked cable is short for cross-linked polyethylene (XLPE)  insulated cable. Cross-linked cables are suitable for transmission and distribution lines with power frequency AC voltage of 500KV and below.
At present, most of the high-voltage cables have been used for cross-linked polyethylene insulated cross-linked cables, which usually means that the insulation layer of the cable is made of cross-linked materials. The most commonly used material is cross-linked polyethylene (XLPE).
Processing engineering is the process of using polyethylene (PE) material with linear molecular structure through a specific processing method to form a cross-linked polyethylene with a body-shaped network split line structure. The long-term allowable working temperature is increased from 70°C to 90°C (or higher), and the short-circuit allowable temperature is mainly increased from 140°C to 250°C (or higher). While maintaining its original excellent electrical performance, it has greatly improved Actual performance.

At present, the process methods for producing cross-linked cables in the cable industry are divided into three categories:
The first type of peroxide chemical crosslinking includes saturated steam crosslinking, inert gas crosslinking, molten salt crosslinking, and silicone oil crosslinking;
The second type of silane chemical crosslinking;
The third type of radiation crosslinking.

1. Inert gas cross-linking: Dry chemical cross-linking adopts polyethylene insulating material added with peroxy compound cross-linking agent. After the conductor shielding layer, insulating layer, and insulating shielding layer are extruded through three layers, they are continuously and uniformly filled with high temperature , The sealed cross-linking tube of high-pressure nitrogen completes the cross-linking process. The heat transfer medium is nitrogen (inert gas), the cross-linked polyethylene has excellent electrical properties, and the production range can reach 500KV.
2. Silane chemical cross-linking: warm water cross-linking adopts polyethylene insulating material with silane cross-linking agent, and after extruding the foreign body shielding layer, insulating layer, and insulating shielding layer through the 1+2 extrusion method, the cooled and loaded plate The insulated core is immersed in 85-95℃ hot water for hydrolysis and cross-linking, because wet cross-linking will affect the water content in the insulating layer. Generally, the highest voltage level is only 10KV.
3.Irradiation cross-linking: Physical cross-linking uses modified polyethylene insulating material, and after the extrusion of the different shielding layer, insulating layer, and insulating shielding layer is completed by 1+2 extrusion, the cooled insulated core , The cross-linking process is completed evenly through the irradiation scanning window of the high-energy electron accelerator. No cross-linking agent is added to the radiation cross-linked cable material. During cross-linking, the high-energy electrons generated by the high-energy electron accelerator effectively penetrate the insulating layer, and the cross-linking reaction is generated through energy conversion, because the electrons have high energy. , And evenly pass through the insulating layer, so the formed cross-linked bond has high binding energy and good stability. The physical performance shown is that the heat resistance is better than the chemical cross-linked cable. However, it is mainly limited by the energy level of the accelerator (generally no more than 3. The effective penetration thickness of the OMev electron end is less than 10mm, considering the geometric factors, the voltage level of the production cable can only reach 10KV, and the advantage is below 6KV.

 

XLPE cable characteristics
The aging life of cable insulation material mainly depends on its thermal aging life. It is determined by the speed of thermal oxygen oxidation, thermal cracking, thermal oxidative cracking, polycondensation and other chemical reactions that occur in the insulating material under heat. Therefore, the insulating material The thermal aging life of the cable directly affects the service life of the cable. According to the chemical reaction kinetics derivation and the artificial accelerated thermal aging test (20-30 years), the long-term allowable working temperature of the irradiated cross-linked cable is:
1. If the power cable 0.6/1KV(LV Cable)  is deduced according to the rated working temperature of 105 degrees, its thermal aging life exceeds 60 years.
If deduced according to the rated operating temperature of 90 degrees, its thermal aging life exceeds 100 years.
2. Overhead insulated cable 10KV 122 degrees

When overhead insulated cables are laid in the open air, the environmental and radiation resistance of insulating materials is more important. Irradiation cross-linked insulating materials have to undergo irradiation processing, and they have good radiation resistance. The radiation dose applied in the cross-linking production process leaves a large safety margin from the destruction dose. The radiation damage dose of polyethylene is 1000KGY, and the processing dose is about 200KGY. In addition to the improvement of the special formula, it is still cross-linked by radiation in a relatively wide range, so its performance will be affected by radiation during a long period of early use.

In cable production recently, the most commonly used insulating plastics are polyethylene and polyvinyl chloride. Among them, polyethylene materials have better electrical properties and better crosslinking properties. Therefore, a variety of industrial crosslinking production processes have been developed. Cross-linking and radial cross-linking In the process of production and laying, the insulation layer of the currently commonly used cross-linked cables has a higher hardness and strength (at room temperature), especially more difficult to peel off than PVC insulation. Because the spoke cross-linked cable has the best cross-linking performance and the highest degree of cross-linking, relatively speaking, the peel strength is also the highest. If the peeling of the cross-linked cable insulation layer is relatively easy (similar to polyvinyl chloride), it must be insufficient or no cross-linking. Under normal circumstances, the cross-linked cable produced by the warm water cross-linking process has more cases where the degree of cross-linking is not enough. The reason is that the degree of cross-linking of such products is relatively low, and the cross-linking process is not continuous and cannot be automatically controlled. Affected by human factors, under-crosslinking easily occurs.

What is XLPE cable?

What is XLPE cable?

The cross-linked polyethylene(XLPE) insulated cable uses peroxide cross-linking method to change the polyethylene molecule from a linear molecular structure to a three-dimensional network structure, and from a thermoplastic material to a thermosetting material. The working temperature is increased from 70°C to 90°C. Improve the current carrying capacity of the cable;

What is the advantages of XLPE cable ?
1. Heat resistance: MPE with a net-like three-dimensional structure has excellent heat resistance. It will not decompose and carbonize below 300°C. The long-term working temperature can reach 90°C and the thermal life can reach 40 years.
2. Insulation performance: XLPE maintains the original good insulation properties of PE, and the insulation resistance is further increased. The tangent of the media loss angle is very small, and it is not greatly affected by temperature.
3. Mechanical properties: due to the establishment of new chemical bonds between macromolecules, the hardness, stiffness, wear resistance and impact resistance of XLPE have been improved, thus making up for the shortcomings of PE  cable being vulnerable to environmental stress and cracking;
4. Chemical resistance: XLPE has strong acid and alkali resistance and oil resistance. Its combustion products are mainly water and carbon dioxide, which are less harmful to the environment and meet the requirements of modern fire safety.

what’s the XLPE useage ?

Cross-linked polyethylene insulated cables are suitable for power distribution networks, industrial installations or other fields that require large-capacity power consumption. They are used for fixed laying on power transmission and distribution lines with AC 50Hz and rated voltages of 6kV to 35kV. The main function is to transmit electrical energy.
Cross-linked polyethylene insulated flame-retardant cables have the function of preventing fire from spreading in the cable circuit, can avoid the expansion of fire accidents and reduce losses, and are suitable for places with high cable laying density such as subways, tunnels, and high-rise buildings.

Is XLPE low smoke ?

The halogen-free low-smoke flame-retardant XLPE insulated cable( LSOH cable) has excellent flame-retardant properties, and the sheath material does not contain halogen, which can ensure that only a small amount of toxic and corrosive gases are released during combustion, and it has the characteristics of less smoke. An environmentally-friendly new product, with superior electrical properties, heat resistance, chemical resistance, environmental stress cracking resistance, aging resistance, and long service life.

Precautions for Xlpe Cables Inspection

In recent years, silane cross-linked polyethylene cable material (hereinafter referred to as XLPE) has become the leading material for low-voltage cross-linked (xlpe) insulated cable insulation because of its simple manufacturing equipment, mature technology, convenient operation, and low overall cost.

At present, the commonly used XLPE is the two-step XLPE. When the cable factory produces the insulated core, the polyethylene (PE) grafted with silane and the catalyst masterbatch are mixed in a certain proportion and extruded in a common extruder. Then the cross-linking is completed in hot water or steam; another one-step method XLPE is made by the cable material manufacturer, which mixes all the raw materials together in a special method according to the ratio, and the cable factory directly completes the grafting and grafting in one step in the extruder. Extrude the insulated core, and then complete the cross-linking under natural conditions. The common point of these two types of XLPE is that no special extrusion equipment is needed, and the cross-linking process is relatively simple. As long as the raw materials and process conditions meet the requirements, it can be made into an insoluble and infusible thermosetting plastic. Compared with thermoplastic PE, its heat-resistant deformation and mechanical properties at high temperature, environmental stress cracking, aging resistance, chemical resistance, etc. have been improved or improved, while the electrical properties remain basically unchanged, and the long-term work of the cable The temperature is increased from the original 70°C to 90°C, thereby improving the short-term current withstand capability of the cable. In summary, XLPE low voltage cables have become the main products of cable manufacturers in recent years.

As a third-party inspection agency, the type of cables inspected is increasing year by year. How to accurately provide the test results of the thermal extension and aging performance of this type of product? Inspectors are faced with some special circumstances. The following is an analysis:

First, the problem of abnormal thermal extension of XLPE insulation. When testing, the author often finds that the elongation rate of XLPE cable insulation under load in the 200℃ thermal extension test greatly exceeds the requirements of the standard, or the sample is put into the oven and melted in a short time. If the test is repeated immediately with the original sample, The reproducibility of the results is very good. If you follow the routine, as long as the test method is correct and the sampling is correct, a conclusion can be drawn based on the test results. However, for XLPE, doing so may be very risky. Because the cross-linking process of XLPE is a slow chemical change process related to temperature, humidity, time, insulation thickness and other factors, especially the naturally cross-linked XLPE insulation material is affected by the above factors to complete the cross-linking. There will be a big difference in time, and it is entirely possible that the natural cross-linking has not been completed within the prescribed test period. Once the natural cross-linking is completed over time, its performance may meet the requirements of national standards. For such cases, the author believes that under the premise of reflecting the current situation of the sample, we should not rush to determine, but should provide the sample with a condition to promote crosslinking-soaking in hot water at 90°C±2°C Do the hot extension test after 4 to 5 hours. Practice has proved that the test results at this time can be used as a basis for judgment. It is worth mentioning that individual manufacturers are pursuing commercial profits one-sidedly, using the similar characteristics of PE and XLPE to pretend to be XLPE, and PE cable will not produce cross-linking changes no matter what conditions are provided to promote cross-linking. In terms of performance, it does not meet the requirements of XLPE at all. This is the same reason that rocks cannot hatch chicks. This requires inspectors to have the ability to identify true and false, good and bad XLPE. In fact, through observation and work accumulation, we can distinguish whether the tested sample is under-crosslinked, inferior XLPE or PE is used according to the fusing time and fusing point of the sample after it is placed in the oven. However, as a third-party inspector, you cannot draw conclusions based on experience alone, and must make judgments based on real data.

Second, the problem of the change rate of XLPE heat aging test exceeding the standard. During testing, if you get the sample, prepare it immediately, and put it in the oven for aging as usual, the tensile strength and elongation at break will often exceed the standard after aging, and you must be cautious in judging this result. This phenomenon is not completely caused by poor aging performance, it may be because XLPE has not been completely cross-linked (from the time curve of XLPE cable material thermal elongation with warm water, it can be seen that when the thermal elongation is qualified, it does not represent the sample Completely cross-linked), and after being put into the aging box, XLPE is still completing its cross-linking process, which leads to an increase in tensile strength, a decrease in elongation at break, and a final change rate exceeding the standard. Due to the long time to complete the aging, it will be troublesome to discover the problem once the test is over. Therefore, it is necessary to thoroughly crosslink the sample before performing the aging test.

In summary, it can be seen that special factors should be considered to determine the thermal elongation and thermal aging performance of XLPE. Personnel engaged in third-party inspections can neither make a hasty conclusion about the test results, because doing so will risk misjudgeting qualified products as unqualified; nor can they avoid these two tests because it is difficult to draw conclusions. This may cause substandard products or counterfeit products to be missed. Therefore, it is necessary to exclude the possibility that the sample has not been cross-linked or completely cross-linked before performing the above two tests. We advocate the use of scientific and reasonable test methods to provide fair and reliable test results.

Treatment of Damp XLPE Power Cable

In urban power grid renovation projects, cables, especially XLPE insulated power cables, have been widely used. However, due to the particularity of the cable, there are special requirements for the installation, operation and maintenance of the cable. Moisture or water ingress of the cable reduces the insulation resistance of the cable, which is an important aspect of the operation of the cable line.

Causes and hazards of damp and water in cables:

(1) When the new cable is shipped from the factory, both ends of the cable are sealed with plastic sealing sleeves. However, after a section is used according to the actual situation at the construction site, the remaining part is simply wrapped with plastic cloth. Placed in the open air and poorly sealed, over time, water vapor will inevitably seep into the cable.

(2) When laying cables, they need to cross roads, bridges and culverts frequently. Due to weather or other reasons, a lot of water often accumulates in the cable trenches. During the laying process, it is inevitable that the cable heads will be immersed in water. , Because the plastic cloth is not wrapped tightly or damaged, so that water enters the cable; in addition, the outer sheath or even the steel armor cable is sometimes scratched when pulling and threading. This phenomenon is particularly prominent when using mechanical traction.

(3) After the cable is laid, the cable head cannot be made in time due to the constraints of the site construction conditions, so that the unsealed cable fracture is exposed to the air for a long time, or even immersed in water, causing a large amount of water vapor to enter the cable.

(4) In the process of making cable heads (including terminal heads and intermediate joints), due to the negligence of the construction personnel, the newly processed cable ends may accidentally fall into the stagnant water on site.

(5) In the normal operation of the cable, if a breakdown or other fault occurs for some reason, the water in the cable trench will enter the cable along the fault point; in civil construction, especially when using large construction machinery It is not uncommon for cables to be damaged or broken down due to various human factors on construction sites in China. When such an accident occurs, the cable insulation is severely damaged, and water can enter the cable.

After the cable enters the water, under the action of the electric field, the aging phenomenon will occur, and finally the cable will break down.