High-voltage cable head manufacturing technology

1. The basic requirements of high voltage cable head

The cable terminal is the component that connects the aerial bundled cable(abc) with other electrical equipment. The cable middle is the component that connects the two cables. The cable terminal and the middle are collectively called cable accessories. The cable accessory should be able to operate safely for a long time like the cable body and have the same service life as the aerial bundled cable SANS 1418 standard. A good cable accessory should have the following properties:

Wire core connection is good: mainly because the connection resistance is small and the connection is stable, which can withstand the impact of fault current; after long-term operation, its contact resistance should not be greater than 1.2 times the resistance of the cable core body with the same length; it should have a certain mechanical strength and resistance Vibration and corrosion resistance; in addition, it should be small in size, low in cost, and easy to install on site.

Good insulation performance: The insulation performance of the cable accessories should not be lower than that of the cable body, and the dielectric loss of the insulating materials used should be low. Structurally, the sudden change of the electric field in the cable accessories can be perfected, and there are measures to change the electric field distribution.

2. Principle of electric field distribution

There is a grounded (copper) shielding layer outside each phase core of the high-voltage cable, and a radially distributed electric field is formed between the conductive core and the shielding layer. In other words, the electric field of a normal aerial bundled cable 33-209 is only the electric field from the (copper) wire along the radius to the (copper) shielding layer, and there is no electric field (electric force) in the axial direction of the core wire, and the electric field distribution is uniform.

When making the cable head, the shielding layer is stripped, and the original electric field distribution of the cable is changed, which will produce a tangential electric field (electric force along the axis of the conductor) that is extremely unfavorable to the insulation. The power lines from which the core wires of the shielding layer are stripped are concentrated toward the shielding layer fracture. Then the break of the shielding layer is the most easily broken part of the cable. At the break of the shielding layer where the cable is most likely to break down, we use the concentrated power line (electric stress) to disperse the electric stress control tube (abbreviated as the material with a dielectric constant of 20-30 and a volume resistivity of 108-1012Ω•cm). Stress tube), sleeved at the fracture of the shielding layer to disperse the electric field stress (power line) at the fracture to ensure reliable operation of the cable.

In order to make the cable run reliably, the stress tube is very important in the production of the cable head, and the stress tube can achieve the effect of dispersing electrical stress on the basis of not destroying the main insulation layer. In the cable body, the outer surface of the core wire cannot be a standard circle, and the distance between the core wire and the shielding layer will not be equal. According to the principle of electric field, the electric field strength will also be large, which is also detrimental to the cable insulation. In order to make the electric field uniform within the cable as much as possible, there is a semiconductor layer with a circular outer surface outside the core wire, so that the thickness of the main insulating layer is basically equal, and the electric field is uniformly distributed.

Outside the main insulation layer, the outer semiconductor layer inside the copper shielding layer is also set to eliminate unevenness of the copper shielding layer and prevent uneven electric field.

In order to disperse the electric field stress of the cable at the fracture of the shielding layer as much as possible, the contact length of the stress tube and the copper shielding layer must not be less than 20mm. The length is fixed). If it is longer, the electric field dispersion area (segment) will be reduced and the electric field dispersion will be insufficient. Generally around 20~25mm.

When making intermediate joints, part of the main insulation layer must be stripped off. After the core wire is crimped with a copper tube, it is flattened with filler (round). There are two production methods:

Heat-shrinkable casing: the main insulating casing made of heat-shrinkable material is shrunk, the main insulating casing is externally contracted to the semiconductor tube, and then covered with a metal shield, and finally the outer protective casing.

Prefabricated accessories: The materials used are generally silicone rubber or ethylene propylene rubber. It is a hollow cylinder, the inner hole wall is a semiconductor layer, and the outside of the semiconductor layer is a main insulating material.

Prefabricated installation requires higher and more difficult than heat shrinkable. The hole diameter of the tubular preform is 2~5mm smaller than the outer diameter of the main insulation layer of the cable. The prefabricated pipe of the intermediate joint should be sheathed outside the main insulation layer of the cable at both ends, and the length of each connection with the main insulation layer shall not be less than 10mm. There is no need to sharpen the pencil tip on the main insulation head of the cable (as much as possible to leave the semiconductor layer on the cable core). The surface of the copper pipe should be smooth and packed with proper amount of filler.

Key technical issue: the size of the accessory and the size of the cable to be installed must meet the specified requirements. In addition, it is necessary to use silicone grease to lubricate the interface to facilitate installation, while filling the air gap of the interface to eliminate corona. Prefabricated accessories generally rely on their own rubber elasticity to have a certain sealing effect, and sometimes sealants and elastic clamps can be used to enhance the sealing. The outside of the prefabricated tube is the same as the heat-shrinkable one, the semiconductor layer and the copper shielding layer, the outermost layer is the outer protective layer.

Seamless corrugated aluminum sleeve

The biggest advantage of corrugated aluminum sheathed cross-linked cables is light weight and large short-circuit thermal stability. In a system with a slightly longer short-circuit current, the standard thickness of the aluminum sleeve can meet the requirements. For example, when the thermal stability is not enough in the calculation, the aluminum sleeve can be thicker to meet the technical requirements. There is no need to increase copper wire shielding. The lead set is simple. The mechanical properties and tightness of the aluminum sleeve extruded by the aluminum press are very good.
The weight of aluminum sheathed cables is much lighter than that of lead sheaths. How to use manual labor to move, lift, carry and move the cables is less labor-intensive than lead sheathed cables, which is very popular with laying workers.

The corrugated aluminum sleeve structure was developed in the 1960s and 1970s. At that time, high-voltage cables were mainly oil-filled aerial bundled cables 7870 standard. The use of corrugated aluminum sleeve in oil-filled cable design has many advantages: the radial reinforcement of the lead sleeve is omitted; the spiral corrugation forms the outer oil passage to significantly reduce the transient pressure drop of the oil-filled cable, which can be released in the circuit design. The length of the fuel supply section can reduce the number of plug joints; the price of aluminum at that time was much lower than the current price, and it was much cheaper than the price of lead and copper. Therefore, corrugated aluminum sleeves have been used to replace lead sleeves in land-based oil-filled cables in some countries, which has great economic and technical benefits. However, the spiral corrugations of the corrugated aluminum sheathed cross-linked cable make the cable do not have the longitudinal water blocking function. In the wire design, if it can also longitudinally block the water and reduce the gap, the electrical performance will be reduced, which seems to outweigh the gain.

The atomic number of aluminum in the periodict able is 13, while lead is 82. Aluminum is a chemically active metal. When encountering external force damage during operation, the outer protective layer is damaged or the outer protective layer is broken on the support, and the aluminum sleeve will soon be corroded and perforated, causing water ingress.

The melting point of aluminum is 658°C, and the mold base temperature is about 500°C when the aluminum press is extruded. The extrusion temperature of the lead sleeve is much higher than that of the lead sleeve. For this reason, there should be a copper wire braided glass ribbon or a semi-conductive resistance hose in the aluminum sleeve to prevent the insulation shield from being burned. There are two types of aluminum pressing machines: continuous pressing and discontinuous pressing. It is not very beneficial to cross-linked cable core to stop for a period of time when adding aluminum ingots discontinuously. Although the new and old aluminum ingots can be combined into one body in the aluminum press, there are still visible seams on the surface of the aluminum sleeve, but it has no effect on the resistance strength of the aluminum sleeve. It should be said that continuous is better than discontinuous.

The aluminum press system is a precision equipment with a high cost, and the production process and process of the aluminum press are also more complicated. The aluminum press has a higher energy consumption and labor consumption to produce the aluminum cover, and the production cost of the metal cover is also higher.

The allowable side pressure of the aerial bundled cable(abc)1418 is related to the type of insulation, not the type of metal sheath. The data recommended in some foreign authoritative manuals are shown in the table below. Although the compressive strength of the corrugated aluminum pipe is higher than that of the lead pipe, the tension of the cable during laying is on the conductor core. The lateral pressure at the bend is transmitted from the conductor to the insulation, metal sheath and sheath, and the corner pulley bears the thrust . Among them, the most susceptible to mechanical force is insulation. It is reasonable to determine the lateral pressure of the cable by the insulation resistance.

Basic cable performance test

1. Inspection method
Routine test: It is an experiment conducted by the manufacturer on all finished cables. Its purpose is to check whether the quality of the product meets the requirements of the technical conditions in order to find accidental defects in the manufacturing process. It is a non-destructive experiment, such as the DC resistance of the wire and the insulation resistance time. And withstand voltage test, partial discharge detection, etc.
Type test: It is the manufacturer who regularly conducts comprehensive performance inspection of the product, especially for a new product before it is finalized for mass production, or the structure, material and main process of a product have changed, which may affect the performance of the cable Time. Pass type test: It can be tested whether the product can meet the requirements of operation, and can be compared with the old product. Such as thermal aging performance of insulation and sheath, long-term stability test of power cable, etc.
Acceptance test: It is an acceptance test performed on the cable after the cable is installed and laid in order to check the installation quality and find possible damage during the construction. Such as withstand voltage test after installation.

2. Test items
2.1 Measurement of DC resistance of wires
The conductive core of the wire and cable mainly transmits electric energy or electric signal. The resistance of the wire is the main indicator of its electrical performance. When the AC voltage is applied, the core resistance is larger due to the skin effect and the adjacent effect surface than when the DC voltage is applied, but the difference between the two is very small when the electric eye frequency is 50Hz. The current standard stipulates That can only require the detection of whether the DC resistance or resistivity of the core exceeds the value specified in the standard. Through this inspection, certain defects in the production process can be found: such as wire breakage or partial single wire breakage; Meet the standard; the length of the product is incorrect, etc. For aerial bundled cables 1418 standard, you can also check whether it will affect the allowable current carrying capacity of wire and cable products during operation.
There are single-arm DC resistance method and double-arm DC bridge method to measure the DC resistance of conductors. The accuracy of the latter is higher than that of the former. The test procedure is also more complicated than the former.

2.2 Test of insulation resistance
Insulation resistance is an important indicator reflecting the insulation characteristics of wire and cable products. It is closely related to the product’s electrical strength, dielectric loss, and the gradual deterioration of insulating materials under working conditions. For communication cables, low insulation resistance between wires will also increase loop attenuation, crosstalk between loops, and long-distance power supply leakage on conductive cores. Therefore, insulation resistance should be higher than the specified value.
Defects in the process can be found by measuring the insulation resistance, such as the insulation is dry and impermeable or the sheath is damaged and damp; the insulation is contaminated and conductive impurities are mixed in; the insulation layer is cracked due to various reasons. In the operation of aerial bundled cables(abc) sans1418, the insulation resistance and leakage current are often tested as the main basis for continued safe operation.
At present, in addition to the ohmmeter (shaking meter), the current measurement of the insulation resistance of wires and cables is the galvanometer comparison method and the high resistance meter method (voltage-current method).

Analyze how to distinguish the quality of optical cables

1. Ointment. The ointment mainly includes fiber paste and aerial bundled cable(abc) 7870. Under normal circumstances, the fiber paste should fill the entire loose tube, and the cable paste should fill every gap of the optical cable core under pressure. Nowadays, there are ways to fill the fiber paste half full or less. For the cable paste, some just apply a layer on the outside of the cable core, and some are not filled between the two ends of the optical cable. This will make the optical fiber not well protected, affect the transmission performance such as optical fiber attenuation, and the poor waterproof performance can not reach the national standard. Once the optical cable accidentally sees water, the entire link will seep and be scrapped. Under normal circumstances, even if there is an accidental seepage, you only need to repair the part of the seepage. (The national standard requires water blocking performance: three meters of fiber optic cable, one meter of water column pressure, no water seepage for 24 hours.) If you use poor ointment, the above problems will also occur, and it may be due to poor thixotropy of the ointment , The optical fiber will cause microbending loss, and the transmission characteristics of the entire link will be unqualified; if the grease is acidic, it will react with the metal material in the optical aerial bundled cable SANS 1418 to precipitate hydrogen molecules, and the attenuation of the optical fiber will increase rapidly when it encounters H, causing the entire The link is interrupted for transmission.

2. Sheath. The fiber optic cable sheath must not only adapt to many different and complex climate environments, but also ensure long-term (at least 25 years) stability. The optical cable sheath must not only have a certain strength, low thermal deformation, abrasion, water permeability, thermal shrinkage and friction coefficient, but also strong resistance to environmental stress and good material processing performance. Although the sheath material that is rarely used or poorly used can pass the factory acceptance, cracking and water seepage will occur after a period of use due to quality defects. It will be more serious if recycled plastic is used to replace the high-quality polyethylene sheath material. The fiber optic cable made of high-quality sheathing material has a smooth, bright, uniform thickness and no bubbles after the cable is formed. Otherwise, the fiber optic cable skin will be rough, and because there are many impurities in the raw material, you can find that the fiber optic cable skin is very small. Wow, and because of its thin thickness, the overall outer diameter of the optical cable will be much smaller than that of a high-quality optical cable. Indoor optical cables are generally made of high-quality flame-retardant polyvinyl chloride. They should have a smooth, bright appearance, good flexibility, and easy peeling; otherwise, they will have poor skin finish and easily adhere to tight-fitting fibers and aramid.

3. Steel belt and aluminum belt. The steel strips and aluminum strips in the optical cable are mainly used to protect the optical fiber from mechanical side pressure, moisture resistance, etc., and better optical cables generally use chrome-plated steel strips. Inferior fiber optic cables use ordinary iron sheet or black sheet (uncoated steel strip) with rust-proof treatment on only one side instead of chrome-plated steel strip. It is easy to separate from the sheath to form a comprehensive bonding layer, and the moisture barrier performance is also very poor; some use tin-plated steel strips instead of chrome-plated steel strips. The surface of the tin-plated steel strips, bubbles, etc. are inevitable. Therefore, it is prone to corrosion under humid atmosphere and surface condensation or water immersion conditions, especially under acidic conditions. The tin-plated layer has poor heat resistance, and the melting point is only 232 degrees Celsius. In the application, due to the high temperature when the sheath is extruded, the peel strength is uncertain, which affects the moisture resistance of the optical aerial bundled cable(abc) SANS 1418 standard. The melting point of chromium is 1900 degrees Celsius, and its chemical properties are very stable. It will not rust when placed in the air or immersed in water at room temperature. It has very good corrosion resistance. Because the surface is easily oxidized to form a passivation layer, it has good environmental resistance. For aluminum tapes, unqualified hot-sticking film-coated aluminum tapes are generally used instead of casting-coated aluminum tapes, which will also affect the performance of the optical cable.