With the progress of science and technology, flame retardant crosslinked plastic cable has replaced oil-soaked paper insulated cable and is widely used in 6 ~ LOKV high-voltage power lines in coal mines.
In the line, the good condition of the joint and accessories of the cross-linked cable has a great influence on the safety, economy, reliable operation and power supply of the mine mechanical and electrical equipment.
Due to the strong current carrying capacity and high current density, the crosslinked cable has more strict requirements on the quality of conductor connection.
Especially for 6 ~ LOKV motor cables, all kinds of joints will be affected by great thermal stress and short circuit current with high intensity and duration.
I. Common fault causes of crosslinked cable joints
Due to the various types, forms and specifications of cable accessories, uneven quality, the technical level of the construction personnel is different, and the operation mode and conditions of the cable joint are different, the reasons for the failure of the cross-linked cable joint are different.
Due to the high allowable operating temperature of the crosslinked cable, the contact resistance of the joint speeds up the temperature rise, which promotes the thickening of the oxide film of the joint, and makes the contact resistance larger and the temperature rise faster.
Such a vicious cycle, so that the insulation layer of the joint is damaged, the formation of interphase short circuit, causing explosion and burning.
The reasons for the increase in contact resistance are as follows.
1. Poor workmanship
(l) Poor handling of the contact surface of the connecting metal.
Whether terminal or connecting pipe, due to the influence of production or storage conditions, the inner wall of the tube often has impurities, burr and oxide layer, especially the aluminum surface is easy to generate a layer of hard and insulating alumina film, so that the connection of aluminum conductor is more troublesome than the connection of copper conductor, and the rigor of the process technology is much higher.
Causes the connection (pressing, welding and mechanical connection) the main reason for heating in addition to the machine and tools, material performance factors, the key is the process technology and sense of responsibility.
The construction personnel do not understand the connection mechanism, not strictly according to the process requirements of operation, will cause the connection can not reach the electrical and mechanical strength.
The operation proves that the cleaner the contact surface between the pressing metal and the wire, the thinner the oxide film and the smaller the contact resistance will be when the temperature of the joint rises.
(2) conductor damage.
The strength of the insulation layer of the cross-linked cable is larger and it is difficult to strip. When the ring is cut, such as using an electrician's knife, or sometimes simply using a hacksaw ring to cut deep marks, it is often not good to grasp the wire damage.
Although the stripping is not very serious, the bending of the on-line core and the peristalsis of the pressing will cause the conductor damage or fracture at the injured place. The pressing is not easy to find, and the serious fever will be caused by the reduction of the section.
(3) conductor core is not in place when the conductor is connected.
Conductor connection insulation stripping length requirements crimping metal hole depth plus 5mm, but because of the product hole depth is not standard, easy to cause stripping length is not enough, or because of crimping series make the end of the wire to form a gap, only relying on the wall thickness of the metal conduction, resulting in increased contact resistance, heat increase.
2. Not having enough stress
In some materials, the joint manufacturing process and standard drawings only mention the number of pits at each end of the cable connection, but do not specify the area and depth of the crimping.
Construction personnel according to the requirements of enough pressure pit number, how the effect can not be determined.
No matter which type of pressure connection, the size of the contact resistance of the joint is related to the size of the contact force and the actual contact area, and the tonnage of the output of the crimping tool.
The main reasons for insufficient conductor connection pressure are as follows
(1) the pressure of the pressing machine is insufficient.
In recent years, there are many manufacturers of pressing machines and tools, and the management is chaotic. There is no unified standard, especially for the mechanical press pliers produced in recent years, the pit is not only narrow and small, but also the upper and lower pressing dies can not match after the pressure is received.
What still has a few manufacturers to use is foreign type press clamp, because what carry out is foreign standard, do not adapt to with domestic conductor nominal section, press connect quality to assure hard.
(2) the connecting metal gap is large.
At present, most units of cross-linked cable joint use connecting hardware, or oil paper cable according to the fan type wire production of the terminal and pressure pipe.
Theoretically speaking, the effective cross-section of the circular and fan lines is the same, but from the actual operation comparison, the compression effect of the two is very different.
Because the conductor of the cross-linked cable is tightly twisted round wire core, there is a large gap with the inner diameter of the commonly used metal, and the compression force cannot reach enough after pressing. As the contact resistance is inversely proportional to the applied pressure, the contact resistance increases.
(3) Fake and inferior products are of poor quality.
Fake and shoddy metal is not only impure material, rough appearance, easy to crack after pressing, and the specifications are not allowed, the effective section is very different from the genuine product, can not meet the pressing quality requirements, under normal circumstances, the operation of heating is serious, the load is slightly fluctuating fault is bound to occur.
3. Insufficient cross section
When the ambient temperature is 25℃, the allowable carrying capacity of the cross-linked cable and the oil-paper cable are compared. The allowable carrying capacity of the ZQ2-3X240 oil-paper copper core cable is 420A, and the allowable carrying capacity of the YJV22-3X150 cross-linked copper core cable is 476A. Therefore, it can replace the ZQ2-3X240 oil-paper copper core cable.
The former can be replaced by the latter.
If the cross-linked 3X150 aluminum core cable is 364A, and the oil-paper 3X240 aluminum core cable is 320A, exceeding 44A, the allowable carrying capacity of the two is basically the same, or the L50mm2 cross-linked cable can be normally operated by using 240mm2 metal connection.
It can be seen that the insufficient cross section of the connecting metal is also an important reason for the serious heating of the crosslinked cable joint.
4. Poor heat dissipation
For the wrapped joint and all kinds of casting joint, the insulation is thicker than the cable cross-linked insulation layer, and the shell is also injected with a mixture, is the smallest type of thermal shrinkage joint, the insulation and protective layer is more than doubled than the cable body.
So no matter what type of joint there are heat dissipation problems.
The current insulation of various joints is poor in heat resistance. The normal working temperature of J-20 rubber self-adhesive tape is not more than 75℃, and J-30 is only up to 90℃. The use condition of heat shrinkable material is -50 -- lOO℃.
When the cable is running at normal load, the joint temperature can reach LOO ℃;
When the cable is full load, the temperature of the cable core reaches 90℃, and the temperature of the joint reaches about 140℃.
When the temperature rises again, the oxide film at the joint thickens, and the contact resistance increases accordingly. After a certain power on time, the insulating material of the joint is carbonized as non-insulating material, leading to the failure.
II. Countermeasures to improve the quality of crosslinked cable joints
(1) Selection of advanced technology, mature process, reliable quality, can adapt to the environment and conditions of the cable accessories.
Fake and shoddy products must be resolutely resisted, new technology, new process, new products should focus on the test, constantly summarize and improve, gradually popularize and apply year by year.
(2) the use of fine material, specifications and section meet the requirements, can be safe and reliable operation of the connecting metal.
For the terminals, should be used as far as possible plugging type, because the terminal section is generally larger, and can effectively solve the moisture-proof seal.
The connecting pipe shall be made of copper rod or L # aluminum car, and the specifications and sizes shall be matched with the diameter of the cross-linked cable core.
In recent years, the pressure nozzles for cross-linked cable joints provided by Changsha and Shenyang Cable Factory Accesory Factory have been used well.
(3) Choose the pressing machine with large tonnage, good mold match, sufficient pit area and pressing effect that can meet the technical requirements.
Do a good job of interface treatment before pressing, and apply conductive paste.
(4) Train cable technicians who are skilled in technology, skilled in process, serious and responsible in work, and capable of cable construction, installation, operation and maintenance.
To improve the understanding of the construction personnel to the XL cable, enhance the understanding of the XL cable accessories characteristics, improve the process, formulate construction specifications, strengthen the quality control.
Ensure safe operation