Structure Analysis of 10kv XLPE Power Cable

XLPE power cable has good electrical and corrosion resistance. It is easy to install and easy to operate and maintain. It is widely used in 10 kV rural distribution networks. However, once it fails, it is difficult to repair and find. Combining the actual operation and work, focus on the analysis of the principle and structure of the cross-linked cable.

1. Analysis of insulation principle of cross-linked polyethylene material (XLPE)
Polyethylene is used as the basic insulating material, and chemical and physical cross-linking methods are used to convert the high molecular compound polyethylene from a linear molecular structure to a three-dimensional network structure of cross-linked polyethylene. It completely maintains the high electrical and physical properties of polyethylene, such as: high breakdown strength, large insulation resistance, low dielectric loss tangent value, etc. At the same time, due to the cross-linking process, it has aging resistance, heat resistance, mechanical properties, The corrosion resistance has been greatly improved.

The XLPE cables used daily are mostly chemical cross-linking method-inert gas cross-linking. The polyethylene material with peroxide (commonly used dicumyl) crosslinking agent is used. After three-layer co-extrusion, it continuously and uniformly passes through a sealed crosslinking tube filled with high temperature and high pressure nitrogen. The peroxide is thermally decomposed to produce Free radicals, free radicals can combine with hydrogen atoms in polyethylene, and polyethylene molecules that have lost hydrogen atoms unite to form cross-linked polyethylene to complete the cross-linking process.

However, XLPE as a polymer also has its inherent defects. The macromolecular solid structure of the polymer makes it easy to accumulate “space charge” inside. Space charge is also called trap charge, that is, the part of the charge that stays in the medium after being trapped. , Can also refer to the polarization charge caused by uneven polarization. The formation of traps is due to pollution in the production process, introduction during mechanical processing or generated during application, but electrode injection is considered to be the main reason for the formation of space charges. The space charge is generally distributed in the medium impurities, physical defects and between polymer molecular chains. Factors such as residual charge in the production process, high electrode injection or polarization caused by impure materials will cause the accumulation of space charge, and these factors are inevitable in actual production. Space charge is very harmful to XLPE cable insulation.

In addition, due to the network molecular structure of XLPE, it has a greater water permeability problem. When the XLPE insulator invades a trace amount of moisture, it will cause the formation of water branches in the insulator, and at the same time cause a high electric field similar to the gas free process, causing insulation damage. The diameter of the water branches is generally only a few microns, and there are many microscopic small water drop gaps. composition. When voltage is applied to the cable, under the action of a strong electric field, water branches will evolve into electrical branches to induce insulation breakdown. It can be seen that the problem of water permeability is the flaw of the XLPE material, and XLPE is the main insulation, so it is necessary to strictly prevent the intrusion of the cable during installation and use.

2. Structural analysis of XLPE power cable

The cross-linked cable is mainly composed of a core conductor, an insulating layer and a protective layer. The core conductor is located in the center, and the insulation shielding layer of the “three-layer co-extrusion” process is on the periphery, which has excellent insulation shielding and heat resistance and heat dissipation performance. The outermost periphery is a protective layer, which is composed of an inner sheath, an armor, and an outer sheath to seal the conductor and the main insulation. This simple structure of cross-linked cable is based on high technology and craftsmanship. Each layer of the structure has special functions and requirements. If a certain layer of structure has a problem, the entire cable will be scrapped. In order to clarify The structural requirements of each layer of the cable, and the principle analysis of each structural part of the cross-linked power  cable are as follows.

(1).  Analysis of core conductor structure

The bare conductor of cable plays the role of transmitting electric energy. When the alternating current passes through the conductor, the skin effect makes the charge density near the surface of the conductor high. Since the electric field intensity on the surface of a conductor is directly proportional to its surface charge density and inversely proportional to the radius of curvature of the conductor surface, the radius of curvature at the edge or tip of the conductor is the smallest, the surface charge density is the largest, the space charge is most likely to accumulate, and the electric field strength is the highest. Local strong electric field discharge is most likely to occur, and this phenomenon is called “edge effect”.

If impurities invade the inner structure of the cable, the impurities will form tips and cause partial discharges, which will eventually lead to breakdown. This is another important reason why no impurities can penetrate into the cable structure.

In order to avoid the “edge effect” harm to the cable insulation, try to make the conductor electric field uniform and reduce the insulation requirements, we should make the conductor into a geometric shape with the largest radius of curvature-round, and make the conductor surface as smooth as possible to avoid The sharp electric field is strong. For this reason, the cross-linked cable core is a multi-core compact round stranded wire. It can be seen that during cable laying and installation, it is necessary to strictly avoid the behavior of damaging the flatness of the internal structure of the cable and destroying the uniform electric field.

(2). “Three-layer co-extrusion” analysis of the inner and outer semi-conductive layer and the main insulating layer

The semiconducting layer is a polymer material with a higher dielectric constant (it is a conductor under a high electric field), which makes up for the stranded cores that cannot be completely rounded, and the uneven electric field on the surface of the cores is uniform. At the same time, it can prevent manufacturing The local high electric field caused by the accidental stab of the core and the introduction of external impurities during the process. It contains substances that quickly capture moisture, which can effectively block moisture from intruding into the insulating layer from inside and outside, prevent moisture from spreading along the core, and prevent the generation of water branches. In addition, the inherent thermal resistance of the polymer can play a role in thermal shielding of separate temperature and increase the current carrying capacity of the cable.

The “three-layer co-extrusion” process is to tightly extrude the inner and outer semiconducting layers and the insulating layer, so that the three layers are tightly combined. This process avoids the local high electric field caused by the intrusion of external impurities (air, moisture, foreign particles, etc.), makes the electric field uniform and smooth, thereby increasing the initial free discharge voltage and greatly improving the insulation strength.

(3). Copper shield and armor

Between the outer semiconducting layer and the inner lining layer, two layers of annealed copper tape are spirally covered to form a cylindrical concentric conductor layer. This is the copper shielding layer, which has good contact with the shielded semiconducting layer, and For the equipotential. During installation, both ends of the copper shielding tape are grounded, so that the outer semi-conductive layer of the cable is always at zero potential, thereby ensuring that the electric field is evenly distributed in the longitudinal direction. When the cable fails, the protective device will act quickly through the ground current or short-circuit current on the copper shielding layer, thereby protecting the non-faulty part of the cable. Note that the length of the copper shielding layer not in contact with the outer semiconducting layer shall not exceed 2 cm after calculation.

The main function of armoring is to increase the longitudinal mechanical stress of the cable, reduce the influence of mechanical force on the cable, and at the same time, it also plays a role in uniform electric field and protection through fault current.

When the cable is subjected to insulation breakdown, lightning strike, operating overvoltage or large fault current flowing in the core, the induced voltage of the metal sheath may cause the inner sheath to break down and cause arcing, and the fault current at this time is not enough When the relay protection is activated, the breakdown phenomenon will gradually increase until the metal sheath is burnt into a hole, which further increases the fault. In order to eliminate this hazard, the metal sheath must be grounded at the terminal.

Power Cable Fire Protection Technology

According to incomplete statistics, there have been more than 100 major accidents caused by cable fires spreading across the country in the past ten years, and more than 320,000 meters of cables have been burned. The restoration and reconstruction work is expensive and time-consuming, and the loss of power supply alone amounts to more than 100. 100 million yuan. The cable fire accident has its special hazards, that is, the failure of the control circuit causes the accident to expand and even damage the main equipment, and it is difficult to repair, and production cannot be resumed for a long time.

Power cables are an important part of the normal operation of power plants and substations. The widespread distribution of cables in power plants and substations, the flammability of cables, the serial ductility of cable fires, and the seriousness of the consequences of cable fires make the fire protection of cables highly valued by power departments, firefighting agencies and scientific research institutions. In view of the frequent occurrence of cable fire accidents, it is necessary to discuss the fire safety measures for cables.

1. Create a good operating environment to avoid accelerated aging and damage of cable insulation
Cable trenches and cable tunnels must have good drainage facilities, such as shallow drainage ditches and water collection wells, and can effectively hold water. If necessary, install automatic start and stop pumping devices to prevent water accumulation and keep the interior dry. The longitudinal drainage slope of cable trenches and tunnels should not be less than 1% to 2%, and should be at least greater than 0.5% to prevent water, corrosive gas or liquid, and flammable liquid or gas from entering. Cable trench, cable tunnel. The cable tunnel should be naturally ventilated, but when the normal load of the cable causes the air temperature in the tunnel to be higher than 40℃~50℃, a combination of natural exhaust and mechanical exhaust can be used for ventilation. The fan of the ventilation system should be interlocked with the fire detector to ensure that the air supply can be automatically stopped in the event of a fire in the tunnel. The cable tunnel shall not be used as the air inlet of the ventilation system.

 

Avoid placing the cable fire door in a normally closed state, using a fire barrier to completely seal the cable, and filling and sealing all gaps in the cable trench cover, which affect the ventilation and heat dissipation of the cable. Moreover, completely enclosing the cable also makes normal inspection of the cable impossible, and the cable failure cannot be found in time.

In addition, there must be complete facilities for preventing the intrusion of rats and snakes to prevent small animals from destroying the insulation cable and causing accidents.

2. Strengthen the preventive test of cables
The preventive test of cables should not only look at the qualified and unqualified test data, but also compare and analyze the data. It can be compared with the test data of the same cable or with the historical test data of this cable to explore the law of the test data. For example, when the DC withstand voltage test is performed, if the measured leakage current value rises faster with the increase of the test voltage value or the increase of the pressure time, or the value increases more than the same cable, or if the The measured data shows an obvious upward trend, or the leakage current imbalance coefficient between the three is relatively large, etc., which should be carefully analyzed. If it is not caused by improper test method, the test voltage can be appropriately increased or the test time can be extended to determine whether the cable meets the conditions for continued operation.

3. It is necessary to strengthen the management and operation monitoring of the production quality of cable heads
According to statistics, cable fire and explosion accidents caused by cable head failure account for about 70% of the total cable accidents. Therefore, it is necessary to strictly control the quality of the materials and workmanship of the cable head. It is required that the service life of the made cable head should not be less than the service life of the cable. The rated voltage level and insulation level of the connector shall not be lower than the rated voltage level and insulation level of the connected cable. The withstand voltage between the insulation pads on both sides of the insulation head shall not be less than 2 times the insulation level of the cable sheath. The form of the connector should be compatible with the environmental conditions set, and should not affect the flow capacity of the cable. Within the range of 2 to 3 meters on both sides of the cable head, fire-resistant tapes shall be used for fire-resisting and flame-retardant cable treatment.

Generally speaking, the cable head is the weak link of the cable insulation, so strengthening the monitoring and management of the cable head is an important part of cable fire protection. The terminal cable head must not be placed in the cable trench, cable tunnel, cable trough box, or cable interlayer. The intermediate cable heads placed in the cable trenches, cable tunnels, cable trough boxes, and cable interlayers must be registered and monitored using a variety of monitoring equipment. When the cable head is found to have abnormal temperature rise or smell or smoke, exit the operation as soon as possible to avoid the cable head from catching fire during operation.

There should be sufficient safety length between the middle cable heads. Two or more cable heads should not be placed in the same position. Tight sealing measures should be taken between the cable heads and other cables.

4. Prevent other equipment from catching fire and igniting the cable
Oil-filled electrical equipment, coal conveying, pulverizing systems, and steam engine oil systems can be ignited by cables. Corresponding measures must be taken for them; the cable trench cover near the oil-filled electrical equipment should be sealed to prevent the oil from flowing into the cable trench to ignite the cable when the equipment fails and catches fire. The dust accumulation on the electric blanket near the coal conveying and pulverizing system should be cleaned regularly to prevent the spontaneous combustion of the pulverized coal from igniting the cable. The cables facing the explosion-proof door of the pulverizing system must be packed in a fire-proof box to prevent the fire-proof door from igniting the cable. The cables under the head of the turbine should be packed in a fire-proof trough box.

5. Use measures such as sealing, blocking, coating, isolation, and wrapping to prevent the cable from spreading

Fire-retardant paint has the characteristics of thin coating, does not affect normal heat dissipation, and can play a good heat insulation and flame retardant effect, but it also has many defects.
Therefore, it is not advisable to apply a large number of fire-retardant coatings to the cable to be flame-retardant. Instead, a large number of methods of sealing, blocking and isolation should be used. When using sealing, blocking, and isolation methods, the following issues should be noted:

(1). Use the methods of sealing, blocking and separating to ensure that the fire of a single cable does not extend to multiple cables. Cables enter cable trenches, cable tunnels, and cable trough boxes; the nozzles of the cable interlayer should be tightly fireproofed to prevent a single cable or a small number of cables from igniting a large number of cables. A fire wall and fire door shall be installed every 60 to 100 meters in cable trenches and cable tunnels. Fire barriers shall be arranged in layers in the shafts. Fire barriers shall be arranged between power cables and control cables. The control cables shall be fully fireproofed. Flame-retardant cables are used to ensure that the main equipment can safely stop running in any emergency. Important cable channels should be equipped with automatic alarm and automatic fire extinguishing devices, such as water spray and water mist fire extinguishing devices, to achieve early detection and early fire fighting.

(2). The tightness and thickness of the fire-proof sealing must be guaranteed. If the fire-proof sealing is not tight, the sealing effect will be lost, especially where the cables are concentrated, it is best to use soft blocking materials to ensure tight sealing. During maintenance and inspection, the damaged blockage should be restored in time. The thickness of the blocking material is not enough, and the fire will pass through the blocking material and burn after the cable catches fire. The thickness of the plugging material should be proportional to the number of cables on the plugging surface, the more the number of cables, the thicker the plugging. The fire door should have a device that automatically closes after a fire occurs.

(3). The fire blocking layer must have sufficient mechanical strength. Because the cable catches fire, especially the electrical short circuit, it will cause the rapid expansion of the air to produce a certain impulse, destroy the fireproof sealing layer with low mechanical strength, and make the fireproof sealing useless.

Cable selection method of generic cabling system

Generic cabling is one of the key links of information network system. In order to ensure the high-speed and stable operation of information network system, it is particularly important to select high-quality and high-performance cables.

The cable selection should be based on the system requirements, technical performance, investment estimates and other factors, but in the wiring system, it should first determine the type of cable used and the structure of wiring (unshielded cable, shielded cable, optical cable or mixed use). Cables usually use Aluminum conductors with insulation and one or more layers of plastic sheath. The cable usually consists of 2 to 3600 pairs. Large logarithmic cables are usually used in backbone cabling systems and are suitable for voice and low rate data transmission. The maximum lengths of these cables for trunk and horizontal (hub to desktop) cabling systems are specified in the latest GB 50311-2007. It should be noted that these maximum length limits apply to all media. They do not consider the impact of performance differences due to different cable types and protocol types used in the network. In fact, the maximum cable length will depend on the application of the system, the type of network and the quality of the cable.

Method of cable selection in generic cabling system

Since it is a building, it will definitely use a lot of communication cables. As the nervous system of intelligent building, generic cabling system is the key part and infrastructure of intelligent building. It has a very close relationship with the planning and design, construction, installation and maintenance of construction engineering. It is like an information superhighway in the building. People can communicate and communicate conveniently, quickly and effectively. It can be said that the generic cabling system connects the communication, computer and various facilities and equipment in the intelligent building to form a complete set of whole, so as to meet the requirements of highly intelligent.

But then we have to consider a problem: fire prevention. Once a fire breaks out in the building, the heat and gas released by these cables will become a major safety hazard. Looking at several large-scale fire accidents in China in recent years, many of them are due to the fact that the victims can’t escape. The burning of cables emits poisonous acid gas. In addition, a lot of heat and smoke are released from the combustion, which makes it difficult for the victims to breathe and lead to tragedies. Therefore, in addition to considering the performance, fire protection and environmental protection are also very important reference factors when we select cables for generic cabling system.

Heating reason of power cable in operation

When the power cable passes through a certain load current, it will be heated. With the increase of the load current, the cable surface temperature will be higher. If it is not handled in time, the consequences can be imagined. For example: PVC cable, the core temperature of 70 degrees is considered as the upper limit, and the surface temperature will be 5-10 degrees lower. Therefore, the cable surface temperature below 60 ℃ is basically safe. Considering the power supply maintenance, of course, the lower the temperature, the better.

The heating reasons of power cable in operation are as follows:

1. Cable conductor resistance does not meet the requirements, resulting in cable heating in operation.

2. Improper selection of cable leads to too small conductor cross-section of the cable used and overload phenomenon in operation. After long-term use, the imbalance of heating and heat dissipation of the cable causes heating phenomenon.

3. If it is too close to the cable, it may cause heat dissipation when the cable is too close to the cable.

4. Poor joint manufacturing technology and loose crimping will lead to excessive contact resistance at the joint and also lead to cable heating.

 

5. The insulation performance of cable phase to phase is not good, resulting in low insulation resistance and heating phenomenon during operation.

6. The partial sheath damage of armored cable will cause slow damage to the insulation performance after water inflow, resulting in the gradual decrease of insulation resistance, and also the heating phenomenon during the operation of the cable.

If the cause is not found out and the fault is eliminated in time, the insulation thermal breakdown will occur after the power cable is continuously energized. This will lead to phase to phase short-circuit tripping of cables, which may cause fire in serious cases.

Discussion on environmental protection requirements of PVC cables and wires

The use of non environmental protection cables is strictly prohibited in developed countries, and China also attaches great importance to this field. China’s relevant laws and regulations clearly require the use of PVC wires and cables in important buildings. Halogen free and low smoke crosslinked PVC wires and cables must be used to avoid heavy smoke and chlorine gas causing casualties in case of fire.

Environmental protection cable will effectively drive the development of cable industry, enhance the competitiveness of cable manufacturer, and make the cable industry more and more stable on the road of sustainable development in green practice and innovation. In recent years, China’s cable enterprises have gradually made efforts to environmental protection cables and low smoke and halogen-free cables. As far as possible, they do not contain heavy metals such as lead, cadmium, mercury, bromine flame retardants, harmful halogen gases, corrosive gases, less heat during combustion, and no soil pollution. Environmental protection cables will be more favored by the market, and will be recruited by power grid companies It is easier to get high order in the bid.

The user’s requirements for environment-friendly wires and cables can be summarized as follows:

1. Low content of lead and heavy metals is required

It is well known that lead and some heavy metals will have adverse effects on human health. In PVC formulation, additives such as lead, cadmium, barium and other heavy metal compounds are usually needed. However, since the 1990s, some developed countries and large electric companies have successively formulated laws and regulations. In the PVC wires and cables used, there are eight kinds of heavy metal content shall not be greater than the regulations. These new regulations not only bring new business opportunities to wire and cable manufacturers, but also bring serious challenges to PVC cable manufacturer.

2. Low smoke and low hydrogen oxide are required

Good flame retardant performance is a very obvious advantage of PVC wire and cable. However, when ordinary flame retardant PVC cable is on fire, it will release thick black smoke and a large amount of HCI gas, which will cause “secondary disaster” after the fire, and seriously affect personnel evacuation and fire rescue work. Therefore, in addition to the flame retardant requirements for wires and cables, the requirements of “low smoke” and “low HCI” will be put forward in some applications.

3. Low toxicity or non toxicity is required

In addition to lead-free and heavy metal free, non-toxic (or low-toxic) cables are required for some household appliances, medical and health appliances and children’s electric toys that may come into contact with food and blood products. This means that all constituent materials used in PVC formulations should be non-toxic. There are a lot of plasticizers in cable materials, but the non-toxic plasticizers must be used to manufacture non-toxic materials. Therefore, the requirements of this PVC material are higher than that of lead-free and heavy metal free materials, and the price is naturally more expensive.

4. Other prohibited items

At present, the regulations being reviewed by the EU impose restrictions on the following items to ensure that wires and cables meet the requirements of environmental protection. For example: 1) asbestos; 2) biphenyl and its ethers, polybrominated phenol; 3) polychlorinated biphenyls; 4) short chain (C10-C13) chlorinated paraffin.

With more and more attention to environmental issues, many countries have formulated a variety of environmental protection policies, so people prefer to use green cable. Low smoke halogen-free flame retardant cable material is one of the most popular environmental protection cables. Although China’s market demand for green cable is growing, compared with developed countries, China still has a long way to go in terms of relevant policy formulation and mandatory application. China should improve the environmental protection cable standards as soon as possible.

Treatment method of PVC power cable affected by damp

Cables, especially PVC insulated power cables, have been widely used in urban power grid reconstruction projects. However, due to the particularity of the cable, the installation, operation and maintenance of the cable have special requirements. Cable insulation resistance is reduced by damp or water intrusion, which is an important aspect of cable line operation accidents.

Cause and harm of cable damp and water inflow

PVC power cable affected by damp  (1) When the new cable is delivered from the factory, both ends of the cable are sealed with plastic sealing sleeve. However, after a section of cable is used at the construction site according to the actual situation, the remaining part is simply wrapped with plastic cloth. As it is usually placed in the open air and the sealing is not good, water vapor will inevitably seep into the cable after a long time.

(2) During power cable laying, it is necessary to often cross roads, bridges and culverts. Due to weather or other reasons, a lot of water is accumulated in the cable trench. During the laying process, it is inevitable that the cable head will be immersed in the water, and the water will enter the cable due to loose wrapping or damage of plastic cloth. In addition, the outer sheath and even steel armor will sometimes occur during traction and pipe threading Scrape phenomenon, when using mechanical traction, this phenomenon is particularly prominent.

(3) After the completion of cable laying, due to the limitation of site construction conditions, the cable head can not be made in time, so that the cable fracture without sealing treatment is exposed to the air for a long time, even immersed in water, so that a large amount of water vapor enters the cable.

(4) During the manufacturing process of cable head (including terminal head and intermediate joint), due to the negligence of construction personnel, the newly treated cable end sometimes falls into the water on site.

(5) In the normal operation of the cable, if the breakdown and other faults occur for some reason, the water in the cable trench will enter the cable along the fault point; in the civil construction, especially in the construction site using large construction machinery, the cable damage or breakdown accidents caused by various human factors are also common. When this kind of accident occurs, the cable insulation will be seriously damaged, which will also cause cable water intrusion.

 

After the cable enters water, the aging phenomenon will occur under the action of electric field, which will eventually lead to cable breakdown.

The global subsea power cable market will grow at a compound annual rate of over 4% from 2020 to 2024

The market for subsea power cables is expected to grow at a compound annual rate of more than 4 percent from 2020-2024, driven largely by surging demand for electricity, according to Technavio.

According to the report, 47 percent of the market growth in the forecast period will come from Europe, with Italy, Denmark and Finland being important markets for European subsea power cables.

Europe is the largest subsea power cable market in 2019, and government support and growing concern about wind power will significantly drive growth in the subsea power cable market in the region during the forecast period.

Offshore wind turbines can produce more energy using fewer turbines than conventional ones. As a result, the Asia-Pacific region will enjoy rapid growth during the forecast period. Thanks to heavy investment in offshore wind projects over the past few years, China and India will account for most of the region’s share of the forecast period.

According to the IEA, demand for electricity will rise by 40 per cent by 2024. Similarly, growing environmental concerns will force economies to use renewable energy to generate electricity. As a result, the number of renewable projects such as wind and tidal power projects is increasing in most countries, which in turn will increase the demand for undersea power cables or submarine cables. The growing demand for intercountry and island connections will be one of the main drivers of market growth. Subsea power grid interconnection projects in France – UK, Belgium – UK and Ireland – UK are under construction.

On the other hand, the growing demand for high-voltage direct current (HVDC) power cables will also drive the market growth. These HVDC cables are more and more popular compared with traditional cables because they are safe and reliable and can transmit power over long distances. These factors will drive the global subsea power cable market to grow at a compound annual rate of over 4% over the forecast period.

From the manufacturers’ point of view, Prysman, Nexans, Sumitomo Electric, Lenny Cable, Ankart Cable, Tengura Cable, Guhe Electrician, LS cable will continue to dominate the global subsea power cable market in the forecast period.