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.

Main Components of Overhead Lines

Overhead lines mainly refer to overhead cable, erected on the ground, and are transmission lines that use insulators to fix transmission wires on poles that stand upright on the ground to transmit electrical energy. Overhead lines have low cost, strong mobility, and easy maintenance. However, overhead lines hinder traffic and construction and are easily polluted by debris in the air; moreover, overhead lines may collide with or get too close to trees and other tall facilities or objects, causing electric shocks, short circuits and other accidents.

The main components of overhead transmission lines are Low-voltage cables, insulators, cross arms, low-voltage poles, high-voltage suspension insulators, clamps, high-voltage cables, high-voltage poles,Lightning line.

1.Pole and Tower
The general term for supporting structures that suspend wires through insulators. Pole and tower are the general term for electric poles and iron towers.
The purpose of the tower is to support wires, lightning protection wires and other accessories. In order to keep a certain safe distance between the wires, the wires and the avoidance, the wires and the ground, and the crossing objects.

2. Overhead Wire and Cables
Classification: It can be divided into four types: single-stranded wire, multi-stranded wire, composite multi-stranded wire and insulated wire.
(1). Single-strand Cable: Due to the manufacturing process, when the cross-section increases, the mechanical strength of the single-strand wire decreases. Therefore, the cross-section of the single-strand wire is generally below 10 square millimeters, and currently it is widely used up to 6 square millimeters.
(2). The multi-stranded cable is twisted by multiple thin wires, and the twisting direction of the adjacent layers of the multi-layered wire is opposite to prevent twisting and twisting when paying off. Its advantages are high mechanical strength, flexibility, and flexibility ; And because the surface oxidation resistivity of the strands increases, the current flows along the strands, the skin effect is small, and the resistance is slightly reduced compared with the single-stranded wire of the same cross section.The common type have all aluminum cable (AAC) and all Aluminum Alloy Cable (AAAC ) etc.


(3). Composite multi-stranded wire refers to the multi-stranded wire of two materials. The common type are steel-cored aluminum stranded wire(ACSR cable) and ACAR Cable ,The core part is twisted by steel wire, and the external stranded aluminum wire is integrated. The mechanical properties of aluminum and the electrical properties of aluminum have become the most widely used overhead cable.
(4). Overhead power lines generally use multiple strands of bare conductors, but in recent years, 10kV overhead power distribution lines in urban areas have gradually switched to overhead insulated conductors. The operation proved that it has many advantages, the line failure is obviously reduced, the contradiction between the line and the tree is solved to a certain extent, the maintenance workload is reduced, and the safety and reliability of the line are obviously improved. 10kv mainly has copper core and aluminum core cross-linked polyethylene insulated wires, and low voltage mainly includes JV and JLV.

3. Lightning wire
The lightning protection wire is installed above the wire and is directly grounded for lightning protection to reduce the chance of lightning strikes to the wire, improve the lightning resistance level of the line, reduce the lightning trip rate, and ensure the safe power transmission of the line. Overhead lines have long distances and are exposed to the wilderness, and there are more chances of being struck by lightning. According to the voltage level of the network, the importance of the load, and the number of lightning days and investment conditions in the area, it can be along the whole line or only in the Install lightning protection wires at the entry and exit lines of the substation.

4. Insulator
Insulators, also called porcelain bottles, are used to fix wires and insulate wires and poles. Therefore, the insulator should have sufficient electrical insulation strength and mechanical strength. There are two types of line insulators: high voltage and low voltage. The insulator is a kind of electrical isolation component, its purpose is to insulate between the wire and the wire and between the wire and the ground, support, suspend the wire, and fix it on the cross arm of the tower. The insulator should have good electrical and mechanical properties. . It should also have strong resistance to rain, snow, fog, wind, ice, sudden changes in temperature and the erosion of harmful substances in the atmosphere.

5. Hardware
The role of the fittings: used for suspension, fixation, protection, connection on overhead lines, connecting overhead lines or insulators, and metal devices used to connect the cables on the cable structure of the cable tower.
Basic requirements Electric power fittings are generally made of cast steel and malleable cast iron. It is required that the circuit fittings should have sufficient mechanical strength, and the part of the fittings connecting the conductors should have good electrical properties.

Main Types of Overhead Cable & Wire

Overhead power lines mainly refer to overhead electrical wires, erected on the ground, and are transmission lines that use insulators to fix transmission wires on poles standing upright on the ground to transmit electrical energy.

What type of wire is used for overhead?
The wires used in low-voltage overhead lines are divided into bare wires and insulated wires. According to the structure of the conductor, it can be divided into single-strand conductor, multi-strand conductor and hollow conductor; its common types are AAC/AAAC/ACSR/ACAR.
The bare wire is the main body of the overhead line and is responsible for transmitting electrical energy. Since the wires are erected on the poles, they must often bear the effects of self-weight, wind, rain, ice, snow, harmful gas erosion, and air temperature changes. Therefore, the wire is required not only to have good electrical conductivity, but also to have sufficient mechanical strength and good corrosion resistance.

(1). All Aluminum Conductor (AAC): This bare concentric-lay stranded conductor is constructed with a straight round central wire surrounded by one or more layers of helically layed wires. These wires are of aluminum 1350 and can be provided in different classes of stranding and tempers.

(2). All Aluminum Alloy Conductor (AAAC): This bare concentric-lay-stranded conductor, made from round aluminum alloy 6201 -T81 wires, is constructed with a central core surrounded by one or more layers of helically laid wires.
It was designed to attend the needs of an economic conductor for the applications on aerial circuit that require a larger mechanical resistance than the one of an All Aluminum Conductor (AAC), and a better corrosion resistance than the one produced by the aluminum conductor steel reinforced(ACSR). The conductors of Aluminum Alloy 6201-T81 are harder and have a better resistance to the abrasion than the conductors of aluminum 1350.

(3).Aluminum Conductor Steel Reinforced Conductor (ACSR) : This bare concentric-lay-stranded conductor is made from round aluminum 1350-H19 (extra hard) wires and round zinc-coated or aluminum-coated steel core wire(s) to be used as overhead electrical conductors.Used as bare overhead transmission cable and as primary and secondary distribution cable. ACSR offers optimal strength for line design. Variable steel core stranding enables desired strengthto be achieved without sacrificing ampacity.

(4).Aluminum Conductor Alloy Reinforced Conductor (ACAR)This bare concentric-lay-stranded conductor is made from round aluminum 1350-H19 (extra hard) wires and round aluminum alloy 6201-T81 core wires for use as overhead electrical conductors.It presents a higher mechanical resistance.

Overhead insulated cable is an overhead wire equipped with an insulating layer and a protective sheath. It is a special cable manufactured by a production process similar to that of a cross-linked cable. It is a new transmission method between overhead wires and underground cables.

Aerial bundled cables (also aerial bundled conductors or simply ABC) are overhead power lines using several insulated phase conductors bundled tightly together, usually with a bare neutral conductor. The conductor can be all aluminum, aluminum alloy or aluminum with a steel core, used for overhead power distribution as an alternative to bare conductor.

 

Construction:
ABC cable used for low voltage overhead line transfer, structured by stranded aluminum conductor or aluminum conductor with steel core , both single core and multi-cores ,insulated by UV resistant XLPE.

Overhead ABC Cable Advantage:
ABC cable provide better level of safety and reliability ,lower power losses, easier to install ,less maintenance and operative cost.

  1. High reliability of power supply
    The use of overhead cables can greatly reduce various short-circuit faults (especially the common flashover faults of overhead bare wires). Compared with overhead bare wires, the failure rate is 4-6 times lower.
  2. Good power supply safety
    The use of overhead cables greatly reduces personal injury and death accidents due to electric shock.
  3. Convenient installation and maintenance
    Overhead cables can be erected on any kind of poles and towers, or along walls. Under special circumstances, they can also run through the bushes and be directly fixed on tree poles with hardware. It can be erected on a single circuit or multiple circuits on the same pole without requiring a wide “electrical corridor”.
  4. Reasonable economy
    Although the use of overhead cables is more expensive than the use of overhead bare wires, it is cheaper than ordinary underground cables. Therefore, although the one-time investment is slightly higher for the use of overhead cables, the operating cost will be significantly lower than that of overhead bare conductors based on other factors.

Laying method of overhead ABC cable 
A single conventional laying method. This erection method is to use the current conventional cement poles, iron accessories and ceramic insulator accessories with bare conductors, and erect according to the bare conductor erection method, which is more suitable for the area where the old line is reconstructed and the corridor is sufficient.
A special insulating bracket is used to suspend the wires for single laying. This method can increase the number of circuits erected, save the line corridors, and reduce the cost of the line unit.

Five Main Types of Cable

1.What are the types of cable?
The wire products used to transmit electric (magnetic) energy, information and realize the conversion of electromagnetic energy are collectively referred to as wires and cables. According to different purposes, wires and cables can be divided into five categories:

(1). Bare conductors: products with only conductors without insulation, such as steel core aluminum stranded wire(ACSR cable), aluminum stranded wire, and copper stranded wire. Products are mainly used in suburbs, rural areas, user main lines, switch cabinets, etc.

(2). Winding wire (magnet wire): In the form of a winding, the magnetic line of force is cut in a magnetic field to induce current, or a wire that generates a magnetic field with current. It is mainly used in various motors, instruments and meters. Such as enameled wire.

(3). Power cables: Cable products used to transmit and distribute high-power electrical energy in the backbone of the power system, such as cross-linked polyethylene (xlpe) insulated power cables. The products are mainly used in the transmission of strong electric energy in power generation, distribution, transmission, transformation, and power supply lines, with large current (tens of amperes to several thousand amperes) and high voltage (220V to 500kV and above).

(4). Communication cables and optical cables: cables and optical cables that transmit telephone, telegraph, television, radio, fax, data and other telecommunication information.

(5). Wires and cables for electrical equipment: Wires and cables used to directly transmit electrical energy from the power distribution point of the power system to the power connection lines of various electrical equipment and appliances. Such as control cables, wiring wires, etc.

2. What are the application of Cable?
Power system: The wire and cable products used in the power system mainly include overhead bare wires, busbars (bus bars), power cables (plastic cables, oil-paper cables (basically replaced by plastic power cables), rubber-sheathed cables, overhead insulated cables), Branch cables, electromagnetic wires, electrical equipment wires and cables for power equipment, etc.
Information transmission system: The wires and cables used in the information transmission system mainly include local telephone cables, television cables, electronic cables, radio frequency cables, optical cables, data cables, electromagnetic wires, power communications or other composite cables.
Mechanical equipment, instrumentation system: Except for overhead bare wires, almost all other products have applications, but mainly power cables, electromagnetic wires, data cables, instrument cables, etc.

3. What is the basic structure of wire and cable?

Conductor: An object that conducts current. The specifications of wires and cables are expressed by the cross-section of the conductor.
Insulation:
The insulating material is wrapped on the outside of the conductor in different thicknesses according to the requirements of its withstand voltage.
Protective layer: the part that protects the cable.

Single-core Parallel Type and Multi-core Twisted Type Cable

The advent of power cables has greatly improved the safety of power generation, transmission, transformation, supply, distribution, and use of electricity. Single-core power cables appeared first. As there are more and more occasions for three-phase four-wire power supply, three-phase five-wire power supply and multi-loop power supply in actual use, the requirements for occupied space and laying occasions are also getting higher and higher. When multiple and multi-layer laying are required, and the space occupied and laying conditions are limited, single-core power cables cannot be used conveniently. Therefore, multi-core power cables have been developed and quickly entered the field of power applications, and are accepted and used by the majority of users.

1.Busway
With the rapid growth of power cable usage, even if single-core power cables are used in actual use, the joints and branches shall be stripped and insulated on site, and the branches or joints shall be crimped before using epoxy resin insulation The method of material encapsulation treatment still has disadvantages such as large site occupation, long construction time, high cost, multiple equipment, high technical requirements, and high difficulty, especially the joints or branches after the completion of the on-site construction, and their insulation Strength, reliability, and consistency are difficult to guarantee. Therefore, the busway was developed, and after the development was put into the market, it was quickly accepted and used by a large number of users.
With the increase in user usage, it is found that the bus duct also has some defects, such as too many parts connected by * screws, complicated installation and construction, and large maintenance and high maintenance costs. In the process of operation, it often encounters the influence of electromagnetic vibration, thermal expansion and contraction, expansion coefficient, external force and other factors, which will cause the loosening of the screw. If a screw is loose, there will be heat and high temperature at the fault point, which will affect the stability of the entire bus duct. In particular, the improper use of the five-wire bus duct will also cause the contact resistance of the PE wire to increase. It violates the basic requirements for the continuity of the PE wire that is clearly stipulated in the building electrical design code and construction code. However, bus ducts still have their own advantages in the case of large capacity. Because when the current reaches thousands of amperes, if a cable is used, even a single-core cable must be laid in multiple, otherwise the corresponding large current capacity will not be reached, and the busway will show its own advantages at this time.

2. Prefabricated branch cable
With the development of technology and the increasing market demand, prefabricated branch cables have been developed and developed from single-core prefabricated branch cables to multi-core prefabricated branch cables, which also include flame-retardant, fire-resistant, and armored prefabricated branch cables. That is, in the factory, in accordance with the cable specifications, models, cross-sections and specific locations of the branches specified in the architectural design drawings, special production equipment and molds are used on the professional production line, which can be completed in one time. Its advantage lies in the high insulation strength, the encapsulated injection-molded branch joint connector and the outer insulating sheath of the cable are tightly bonded together, and it has excellent water tightness and air tightness. Because it is made of factory-specific equipment and molds, It has excellent reliability and consistency, and it is extremely convenient for users to install and use. When installing vertically, only the main cable can be evenly fastened to the corresponding bracket. In horizontal, pre-buried, overhead, tunnel, airport runway, port Construction and installation under environmental conditions such as docks, mines, and modern industrial plants are simpler and more convenient. The requirements for site, equipment, technical level of construction personnel, and costs during the installation and construction process are much lower than those for handling cable branches or cables at the construction site. Busway: It saves a lot of maintenance costs during operation and reduces the power outage time. In some cases, it can achieve the effect of maintenance-free. In the case of small cable shafts and cable channels, it can show its unique advantages. Therefore, branch cables are ideal products to replace bus ducts in medium and small capacity power supply occasions.

3. Stranded and twisted prefabricated branch cables
With the acceptance of prefabricated branch cables by the majority of users, a variety of prefabricated branch cables have entered the market under the situation of rapid increase in usage. This situation has brought a certain degree of selection to building electrical designers. difficult. The choice of stranded type, also called twisted type, branch cable is introduced below.

4. Multi-core prefabricated branch cable
The multi-core power cable is the core wire conductors are individually insulated, and are collectively and parallelly enclosed in the same outer sheath. Inside the outer sheath of the entire power cable, whether it is armored or unarmored, all power cable core wires that have been independently insulated and encapsulated are parallel and tightly encapsulated in the outer insulation. Set of interior. During the entire encapsulation process of the outer insulating sheath, no part or any power cable is allowed to “cross”, “displace”, or “twist” inside the sheath. Of course, “twisting” is not allowed. If any of the above phenomena occurs, the entire power cable will be judged as “unqualified” and not allowed to leave the factory. Therefore, the multi-core power cable itself is a qualified product that uses special production equipment in a professional production plant and strictly follows the relevant standards. Regardless of the distribution of inductance and capacitance and the basic requirement that the vector sum of any part is equal to zero, good consistency can be guaranteed. This cannot be achieved by any “stranded conductor type” or “twisted type”, and is a basic requirement that must be guaranteed on the power distribution circuit. Therefore, prefabricated multi-core branch cables made of qualified multi-core power cables can ensure that various technical parameters and basic requirements will not be destroyed, and can ensure the stability and reliability of operation. Of course, compared with single-core prefabricated branch cables, multi-core prefabricated branch cables are much more complicated and difficult regardless of the required technical level, the complexity of the manufacturing process, and the production equipment and investment.

Conclusion
In summary, the use of “twisted” or “twisted” prefabricated branch cables in the three-phase three-wire, three-phase four-wire, and three-phase five-wire power supply loops is used as the power supply loop to ensure the safety of its operation. And stability are extremely disadvantageous. The author also consulted related materials, regulations, specifications, standards such as IEC, NEC, etc., which clearly stipulated the distance between cables and cables when laying in parallel, and the correction coefficient of the current carrying capacity of cables at various distances. The author believes: This is also enough to explain the use of power cables. The series of processes from design to laying must fully consider the distribution of capacitance and inductance during operation, electromagnetic attraction, repulsion, vibration, etc.; the temperature rise during operation (capacitance And the distribution of inductance, electromagnetic attraction, repulsion, vibration, etc. are all part of the’temperature rise’) and heat dissipation environment and conditions. Therefore, only stipulating the different correction factors for the ampacity at different distances is sufficient to explain the nature and essence of the problem. What’s more, these are facts proved by theory and practice

Cable Broken Core Detection Method

The broken core fault point has the following methods to detect:

1.1. Induction method
It can be used with induction pen and digital multimeter; suitable for cables without metal armor and steel tape shielding; attention should be paid to prevent electric shock, the test place and the terminal connection when the equipment is rewinded.

specific methods:
1) Hang the conductor core of the cable in the air, and ensure that it will not cause road and electric shock accidents; at the same time, ensure that the cable is as far away as possible from the grounding body (such as the ground, equipment, etc.);
2) Select a good insulation core in the cable, connect the 220VAC phase wire (live wire), not the ground wire;
3) If you use an induction pen, touch your finger to the induction contact on the pen, and test whether the pen is normal outside the insulating layer of the charged body. If you use a digital multimeter, put the multimeter in the 20 or 200mV range, put a thinner plastic insulating sleeve on the red test lead, and hold the black test lead with your hand; test it outside the insulating layer of the clearly charged body, and take the reading; then remove it The charged body is far away, and read; compare the difference between the two readings, usually on the charged body, there should be a higher reading, such as 0.4mV, while the farther away from the charged body is lower, such as 0.15mV; remember this feature, you can Start testing.
4) Test close to the cable along the cable. When the indicator light of the induction pen is dimmed or the reading of the multimeter drops significantly, the change is the breakpoint.
5) The test is completed, pay attention to discharge!  

 

1.2. Capacitance method

When there is a metal armor layer such as copper or steel tape armored cable outside the cable, the induction method cannot be used for detection. At this time, the capacitance method is used; it is suitable for all cables; when using the capacitance method, first understand the principle of capacitance testing-when testing capacitance, The AC/pulse signal is used in the test loop, that is, measuring the AC partial voltage or charging and discharging the capacitor (two mutually insulated metal poles) to test the accumulated electricity on the capacitor and convert it to the reading of the capacitance.

Capacitance method may affect the accuracy due to the inductance formed by winding the cable into a circle, the insulating cores being wound together, the resistance of improper conductors (such as steel strips), and the stray capacitance between conductors; among them The inductance is very small and can be ignored; the resistance has little effect on the measured capacitance, but the difference in capacitance between the bare conductor and the steel strip and the unconnected capacitance is not big, and it can be ignored; but the stray capacitance has a greater impact, so I did an experiment : The capacitance between the intact core and the steel strip is 117nF. Connect the other cores to the steel strip. The measured result is still 117nF, while there is 72nF between the two cores.

For the convenience of description, suppose the cable is a 2-core steel tape armored cable with 1 core with 1 break point; the specific methods are as follows:

1) Hang all insulated cable core conductors and armor layers at both ends of the cable;
2) Measure the capacitance value of the intact insulating core and the broken core insulating core to the steel strip (or the third intact insulating core) at both ends, and record the value; at this time, the corresponding intact insulating core is measured at both ends The capacitance value should be very close; the sum of the capacitance value at both ends of the same broken core should be slightly greater than the capacitance value of the intact insulated core at the same position, it means there is only one breakpoint, or multiple breakpoints but very close; if both ends of the same broken core The sum of the capacitance values ​​is less than the capacitance value of the intact insulating core at the same position, indicating that there are at least two breakpoints;
Note: Theoretically, if there is only one breakpoint or multiple breakpoints but very close, the sum of the capacitance values ​​at both ends should be greater than the capacitance value of the intact insulating core at the same position. The amount varies with different cables. See the following theoretical analysis.
3) Compare and calculate the capacitance value of the broken core insulation core and the intact insulation core, and obtain the lengths of the two ends respectively. At this time, the length may be different from the actual length. The next step is to re-calibrate; but the two-core unarmored cable cannot be Make corrections.
4) If the sum of the calculated length is greater than the actual length, the extra length will be negative, and if it is less than the actual length, it will be positive; then use the capacitance value of the broken core insulating core to allocate the difference and divide the long segment to correct the long segment , The short segment is corrected for the short segment (see the following theoretical analysis), and the actual position of the breakpoint is obtained.

Development of Submarine Cables

In 1988, a transoceanic submarine optical cable (TAT-8) system was installed between the United States, Britain and France, with a total length of 6,700 kilometers.This optical cable contains 3 pairs of optical fibers, each pair has a transmission rate of 280Mb/s, and the relay station distance is 67 kilometers. This is the first communication submarine cable across the Atlantic Ocean, marking the arrival of the era of submarine cable. In 1989, the submarine optical cable (13,200 kilometers in length) across the Pacific was also successfully constructed. Since then, submarine optical cables have replaced concentric cables in the field of intercontinental submarine cables across the ocean, and submarine cables have no longer been laid between oceans.
Optical fiber has large transmission capacity and long distance between relay stations, which is suitable for long-distance communication under the sea. Optical fibers used in submarine optical cables have higher requirements than optical fibers used in terrestrial optical cables; they require low loss, high strength, long manufacturing length, and long relay distances of optical cables, generally above 50 kilometers.

The transmission performance of optical fibers is required Will not change within 25 years. In terms of the structure of the submarine optical cable: it is required to withstand strong pressure and tension, especially for deep-sea optical cables (optical cables laid on the seabed at a depth of more than 1,000 meters). In addition to the weight of the optical cable itself, sea waves must be added to the laying and maintenance operations. The dynamic stress added to the optical cable, under such a large load condition, the strain of the optical cable must be limited to 0.7-0.8%; the structure of the submarine optical cable requires strong and light materials, but light metal aluminum cannot be used because aluminum and sea water will Hydrogen is generated in response to electrochemistry, and hydrogen molecules diffuse into the glass material of the optical fiber, which increases the loss of the optical fiber. Therefore, the submarine optical cable must not only prevent hydrogen from being generated inside, but also prevent hydrogen from penetrating into the optical cable from the outside. For this reason, in the early 1990s, a carbon- or titanium-coated optical fiber was developed to prevent hydrogen penetration and chemical corrosion. The optical fiber connector is also required to be high-strength, requiring the connection to maintain the strength of the original optical fiber and the surface of the original optical fiber from damage.

According to the above requirements and characteristics, the basic structure of the submarine optical cable is to spirally wrap the optical fiber after one or two coating treatments around the center and around the reinforcing member (made of steel wire). The structure of several typical deep-sea optical cables: deep-sea optical cables, the optical fiber is arranged in a spiral U-shaped grooved plastic skeleton, and the groove is filled with grease or elastic plastic to form the core. The core is wrapped with high-strength steel stay wire. During the wrapping process, all the gaps should be filled with waterproof material. Then a layer of copper tape is wrapped around the steel wire and the seam is welded to make the steel wire and the copper tube form a resistance A combination of compression and tension, this copper tube is also a conductor that carries a remote current. A layer of polyethylene sheath should be added on the outside of the steel wire and copper pipe. Such a tight multi-core cable structure is to protect the optical fiber, prevent breakage and prevent the intrusion of seawater, and also to withstand huge tension and pressure during laying, recovery and repair.

Even with such tight protection, in the late 1980s, there were instances where the polyethylene insulators of deep-sea optical cables were bitten by sharks causing power failures. The remote power supply of the submarine cable system is very important, and the repeaters along the submarine cable rely on the remote power supply of the landing station. The digital repeater used in the submarine optical cable has many functions, and the power consumption is several times larger than that of the analog repeater of the submarine cable. The power supply requires high reliability and cannot be interrupted. Therefore, in areas where sharks are infested, two layers of steel tape and a layer of polyethylene outer sheath should be added to the outside of the submarine optical cable.

In the 1990s, submarine optical cables and satellite communications have become the main means of contemporary intercontinental communications. From 1989 to the end of 1998, my country has successively participated in the construction and investment of 18 international submarine optical cables. The first international submarine cable system to land in China was the China-Japan (C-J) submarine cable system built in December 1993. In February 1996, the China-Korea submarine optical cable was completed and opened, and landed in Qingdao, China and Taian, South Korea, with a total length of 549 kilometers; in November 1997, China’s participation in the construction of the ball submarine optical cable system (FLAG) was completed and put into operation. The intercontinental optical cable systems landed in China were landed in 12 countries and regions including the United Kingdom, Egypt, India, Thailand, and Japan. The total length is more than 27,000 kilometers, of which the Chinese section is 622 kilometers. The Asia-Europe submarine optical cable system jointly initiated by the company is under construction. The system connects Asia, Europe and Oceania. The system has landed in 33 countries and regions. The total length is 38,000 kilometers. It is the longest submarine optical cable in the world. 8-wavelength wavelength division multiplexing technology, the design capacity of the backbone route is as high as 40Gb/s, landed in China’s Shanghai and Shantou, completed and opened at the end of 1999.

The volume of intercontinental communications undertaken by submarine optical cables has increased year by year, and has exceeded that of satellite communications, becoming the main force of modern intercontinental communications.

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.

How to Choose Photovoltaic Generation Cable?

Part 1:Relevant standards and specifications for cable selection
1. DL/T5044-2004 People’s Republic of China Electric Power Industry Standard
2. GB50217 Power Engineering Cable Design Code

Part 2:Main technical requirements

1. Two-core cables should be used for the DC power supply loop, and single-core cables can be used when needed.
2. Polyvinyl chloride insulated cables are not suitable for high temperature above 100℃ or low temperature below -20℃.
3. When laying cables directly, steel tape armored cables should be used when the cables are under heavy pressure or are in danger of mechanical damage.
4. The temperature of the cable core under the maximum working current shall not exceed the service life of the cable. 5. Determine the ambient temperature for the continuous allowable current carrying capacity of the cable. If the cable is laid in the air or in the cable trench, the highest temperature of the hottest month should be used. average value.

Part 3. The selection of cable path should meet the following requirements
1. Avoid the cables suffering from mechanical external force, overheating, corrosion and other hazards.
2. Make the cable shorter when meeting safety requirements.
3. Easy to install and maintain.
4. Avoid the place to be excavated and the cable shall meet the requirements of the allowable bending radius of the cable in any way of laying and all path conditions of the cable.

Part4. Cable types, characteristics and laying methods of photovoltaic power generation system

1. Special photovoltaic cables
The cables from the string to the combiner box generally use photovoltaic special PV cables PV1-F1*4mm2. The photovoltaic cable has a simple structure, and the polyolefin insulation material it uses has excellent heat resistance, cold resistance, oil resistance, and ultraviolet resistance. It can be used under harsh environmental conditions and has a certain mechanical strength.

2. Power cable
1) Steel tape armored cable flame-retardant cross-linked cable ZRC-YJV22
Widely used: connecting cables from combiner boxes to DC cabinets, DC cabinets to inverters, inverters to transformers, transformers to power distribution devices, and power distribution devices to power grids.
2) Power cable NH-VV
NH-VV copper core PVC insulated PVC sheathed fire-resistant power cable.

3. Control cable
ZRC-KVVP copper core PVC insulated PVC sheathed braided shielded control cable. It is suitable for control, monitoring circuit and protection circuit of AC rated voltage 450/750V and below.

4. Communication cable
1) DJYVRP2-22 polyethylene insulated PVC sheathed copper wire braided shielded armored computer special soft cable, suitable for electronic computers and automation connecting cables with high anti-interference requirements for rated voltages of 500V and below.
2) RVVP copper core PVC insulated PVC sheathed insulated and shielded flexible cable RVVP, also known as electrical connection anti-interference flexible cable, is suitable for alarm, security and other communication cables that require interference prevention and safe and efficient data transmission.

5. RF cable
Solid PE insulated PVC sheathed radio frequency coaxial cable SYV.

Cable Insulation Resistance Decreases Due to Moisture

The insulation resistance of directly buried power cables is reduced and the phenomenon of cable line failures often occurs, especially when the cable insulation is damp, it is easy to cause the insulation resistance of directly buried power cables to decrease and substandard

1. The cable raw materials are damp

The raw materials used for cable insulation and sheathing are mainly plastic and rubber materials, and many kinds of materials with special functions are derived from this modification. When manufacturing materials, material manufacturers go through the processes of compounding, mixing, granulation, cooling and drying, as well as during the transportation and storage of the materials, dampness of varying degrees often occurs, resulting in varying degrees of material content. Moisture. Therefore, the cable manufacturer must dry the material before squeezing the material on the cable conductor. The extrusion unit is equipped with a material drying device to prevent the extruded insulation layer and sheath from happening. No defects such as bubbles and blisters, and no bubbles on the surface. This is the rigid process regulation of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.

2. the cable manufacturing process is damp

In the insulation extrusion process, the insulation layer is scratched, causing holes or degumming of the insulation layer, and the insulation core enters the cooling water tank, which causes the insulation resistance to drop. Or when the protective layer is squeezed, the protective layer is damaged and water enters, so that the insulating layer is damp and the insulation resistance decreases. When manufacturing a multi-core cable, even if the insulation layer is extruded intact, when the insulated core is twisted into a cable, and when the sheath is extruded, damage may occur and the water may enter and become damp, so the finished cable cannot pass the factory withstand voltage test .

3. Moisture during cable construction

In the process of direct buried cable construction, if the cable trench is excavated, the cable burying operation, the cable intermediate joint and the terminal joint are not made standardly, it is very likely to damage the cable sheath and insulation layer. If the soil is wet or the cable trench is filled with water, water in the cable will definitely occur. After the insulation is damp, the surface resistance of the cable insulation will decrease and the surface leakage current will increase, and the insulation resistance will decrease, which will also cause the electric field distortion between the conductor and the insulation layer. The uneven electric field distribution in the insulation will cause free discharge inside the insulation and even lead to cable breakdown. After-sales service practice has proved that more than 95% of the direct-buried cable insulation resistance drop accidents are caused by improper construction.

 

As we all know, water and damp in the process of cable manufacturing and laying operation are the main factors that endanger the electrical performance and service life of the cable. Both cable manufacturers and users attach great importance to this.
Practical experience has proved that the main reasons for the ingress of water and moisture in the cable are as follows.

1) Material purity
If the cable insulation material is mixed with impurities, especially metal impurities, even the pigments of different colors used by China Wire and Cable manufacturer  will directly affect the electrical performance of the insulation and reduce the insulation resistance. The reason is that the non-metallic impurities in the insulating layer will absorb moisture when the cable is damp, and form numerous conductive points; second, the m

etallic impurities in the insulating layer are directly conductive points. Under the combined action of the conductor operating temperature and the external ambient temperature, these conductive points form conductive channels in the insulating layer, resulting in a decrease in insulation resistance and an increase in leakage current, which in turn leads to insulation breakdown.

2) The material is damp
If the cable insulation material has been damp and is not dried before being squeezed on the conductor, there will be quality defects such as a large number of pores in the insulation layer, an unsmooth extrusion surface, reduced mechanical strength, and even cracking. Therefore, when extruding the cable insulation layer, the cable manufacturer must dry the material. When extruding low-smoke and halogen-free materials, more attention should be paid to drying. These are the basic technical knowledge of cable manufacturers.