Abstract: the common failure reasons of the cross-linked cable joints and the countermeasures to improve the quality of the joint are analyzed.
Key words: Countermeasures for the failure of the coal mine cross-linked cable joint
With the progress of science and technology, flame retardant cross-linked plastic cable has replaced oil immersed paper insulation cable, and is widely used in 6-10KV high voltage power line in coal mine. The good condition of the joints and accessories of the cross-linked cable in the line has a great influence on the safety, economy, reliable operation and power supply safety of the mechanical and electrical equipment in the coal mine. Because of its strong current carrying capacity and high current density, the quality of conductor connection is more strict. Especially for 6-10KV motor cable, various joints will be subjected to great thermal stress and high intensity and short circuit current of duration.
1、 Common failure reasons of cross-linked cable joints
Due to the variety, form and specification of cable accessories, uneven quality, different technical level of construction personnel, different operation modes and conditions of cable joints, the causes of the failure of the cross-linked cable joints are different. Because the operating temperature of the cross-linked cable is high, the contact resistance at the joint joint will accelerate the temperature rise, which will make the oxide film of the joint thicker, and make the contact resistance larger and the temperature rise faster. This vicious cycle causes the insulation layer of the joint to be damaged, forming a short circuit between phases, causing explosion and burning. The reasons for the increase of contact resistance are as follows.
1. poor process
(l) The contact surface of the connecting hardware is not handled well. Whether it is a terminal or connecting pipe, due to the influence of production or storage conditions, impurities, burr and oxide layer are often found in the inner wall of the pipe body. Especially, the aluminum surface is easy to produce a hard and insulated alumina film, which makes the connection of aluminum conductor more difficult than that of copper conductor, and the strict technology of process is much higher. The main reason for the heating of connection (crimping, welding and mechanical connection) is the key factor of technology and responsibility except for the performance factors of machines and tools and materials. If the construction personnel do not understand the connection mechanism and do not operate strictly according to the process requirements, the electrical and mechanical strength will not be reached at the connection. The operation shows that the cleaner the contact surface between crimping fittings and wires, the thinner the oxide film and the smaller the contact resistance when the joint temperature increases.
(2) Conductor damage. It is difficult to peel the insulation layer of cross-linked cable. When cutting, if it is cut by an electrician knife, sometimes it is simply cut with a steel saw to cut deep marks, which often causes the conductor to be damaged. Although the peeling is not very serious, the conductor damage will be aggravated or broken when the core is bent and crimped. It is difficult to find out after crimping, and the heating is serious due to the reduction of section.
(3) The conductor is not in place when connected. When conducting connection, the insulation stripping length requires 5mm for the hole depth of crimping fittings. However, due to the nonstandard hole depth of the product, it is easy to cause insufficient stripping length, or the gap at the end of the conductor is formed due to the string position during crimping. Only the metal tool wall thickness is used for conducting, which results in the increase of contact resistance and heat generation.
2. insufficient pressure
The number of pits at each end of cable connection is mentioned in the fabrication process and standard drawing of some data, but not the crimping area and crimping depth. The construction personnel can press enough pit quantity according to the requirements, and the effect can not be determined. No matter which type of pressure connection, the contact resistance of the joint is related to the contact force and the actual contact area, and the output tonnage of the crimping tool. The main reasons for the insufficient connection pressure of conductor are as follows
(1) The pressure of the crimping tool is insufficient. In recent years, there are many manufacturers of crimping machines and tools, with disordered management and no unified standards. Especially the mechanical press pliers produced in recent years, the pressure pit is not only narrow, but also the upper and lower pressing dies can not be consistent after the crimping is in place. Some manufacturers use foreign type pliers. Because of the foreign standards, they are not suitable for the nominal section of domestic conductor, so the quality of crimping is difficult to guarantee.
(2) The gap between the connecting fittings is large. Now, the connecting fittings used by most units of the cross-linked cable joints are terminals and pressure connection pipes produced by fan type wires of oil paper cables. Theoretically, the effective section of circular and fan core is the same, but the effect of crimping is quite different from the actual operation. Because the conductor of cross-linked cable is a tightly twisted round core, there is a large gap with the inner diameter of commonly used fittings, and the compression force can not be reached after crimping. Because the contact resistance is inversely proportional to the applied pressure, the contact resistance increases.
(3) The quality of fake and inferior products is poor. Fake and inferior fittings are not only of impure material, rough appearance, easy to crack after pressing, but also have inaccurate specifications. The effective section is very different from the genuine ones, which can not meet the requirements of crimping quality. Under normal conditions, the operation and heating are serious, and the load fluctuation will inevitably lead to failure.
3. insufficient section
The allowable carrying capacity of cross-linked cable and oil paper cable is compared at 25 ℃ in the ambient temperature. The allowable capacity of zq2-3x240 oil paper copper core cable is 420a, and that of yjv22-3x150 cross-linked copper core cable is 476a. Therefore, zq2-3x240 oil paper copper core cable can be replaced. The former can be replaced by the latter. If the cross-linked 3x150 is aluminum core cable, its carrying capacity is 364a, while that of oil paper 4x240 aluminum core cable is 320A, exceeding 44a, and the allowable carrying capacity of both is basically the same, or l50mm2 cross-linked cable can be connected with 240mm2 metal fittings to operate normally. It can be seen that the insufficient cross-section of the connecting hardware is also an important reason for the serious heating of the cross-linked cable joint.
4. poor heat dissipation
For the wrapped joint and various cast-in-place joints, the insulation is thicker than the cross-linked insulation layer of the cable, and a mixture is also noted in the shell, which is the smallest type of heat shrinkable joint, and the insulation and protection layer of the joint is more than twice as much as that of the cable body. In this way, there are heat dissipation problems in any type of joint. The thermal resistance of the insulation materials of various joints is poor at present. The normal working temperature of j-20 rubber self adhesive tape is not more than
Causes and Countermeasures of cross linked cable joint failure in coal mine
75 ℃, and the j-30 can reach 90 ℃ and the service conditions of the heat shrinkable material are -50-lioo ℃. When the cable is under normal load, the joint temperature can reach OO ℃; When the cable is full of load, the temperature of cable core reaches 90 ℃, and the temperature of joint will reach 140 ℃; When the temperature increases again, the oxide film at the joint thickens and the contact resistance increases. After a certain power on time, the insulation material of the joint carbonizes into non insulation, which leads to the failure.
2、 Countermeasures to improve the quality of cross-linked cable joints
(1) Select cable accessories with advanced technology, mature technology, reliable quality and can adapt to the environment and conditions. We must resist the fake and inferior products firmly. We should test the new technology, new technology and new products, and constantly summarize and improve them, and gradually promote and apply them year by year.
(2) The connection fittings with good material, specification and section meet the requirements and can operate safely and reliably. For terminal, oil blocking type should be selected as far as possible, because the general section of the terminal is large and can effectively solve the moisture-proof seal. The connecting pipe shall be made of copper rod or l × aluminum car. The specification and size shall be matched with the diameter of cross-linked cable core. In recent years, the pressure connection of the cross-linked cable joint provided by Changsha and Shenyang cable factory accessory factory has good effect.
(3) The crimping machines with large crimping tonnage, good die matching, sufficient pit area and crimping effect can meet the technical requirements are selected. Do well the interface treatment before crimping and apply conductive paste.
(4) Train cable technicians with good technology, skilled technology, conscientious and responsible work and are competent for cable construction, installation and operation and maintenance. Improve the construction personnel's understanding of cross-linked cable, enhance the understanding of the characteristics of the accessories of the cross-linked cable, improve the process, formulate construction specifications, strengthen quality control, and ensure safe operation.