------[Jinhuanyu Cable] Knowledge
I. Operation condition of cross-linked cable connector
0.6-10KV high-voltage cable is widely used in water conservancy projects and power systems. Its intact joints and accessories are very important for the safe, economical, reliable operation of electromechanical equipment and power supply safety. The cable joints with good design and reasonable construction can be used for a long time in most cases, as proved by actual operation. However, due to the strong current carrying capacity and high current density of cross-linked cables, the requirements for conductor connection quality are more stringent. The mechanical and electrical conditions required for joints are increasingly strict, especially for 6-10KV motor cables. Various joints will be subject to great thermal stress and short circuit current with high intensity and duration. Therefore, the cross-linked cable accessories are not ancillary, let alone secondary components. They are equally important and indispensable components as cables, and are also key products closely related to safe operation. Cross linked cable has been widely used abroad. Although it has been widely used in China for only 10 years, there are still some problems. However, with the development of technology, the matching of accessories, the improvement of quality, and the improvement of process, the trend that cross linked cable has replaced oil paper cable has broad and far-reaching development prospects.
Analysis of the fault causes of the double cross-linked cable joint
There are many types, forms and specifications of cable accessories; The quality is uneven; The technical level of construction personnel varies; Different operation modes and conditions of cable joints lead to different reasons for the failure of cross-linked cable joints. Due to the different media of cross-linked cable and oil paper cable, the causes of joint failure are very different. The failure of oil paper cable joint is mainly caused by insulation, while the failure of cross-linked cable joint is mainly caused by conductor connection. The allowable operating temperature of cross-linked cables is high, which puts forward higher requirements for cable joints, making the joint heating problem more prominent. The contact resistance is too large, the temperature rises faster, and the heating is greater than the heat dissipation, which causes the oxide film of the joint to thicken, and makes the contact resistance larger and the temperature rises faster. Such vicious cycle will destroy the insulation layer of the joint, form a short circuit between phases, and cause explosion and burning. The reasons for the increase of contact resistance are as follows.
1. Poor workmanship
It mainly refers to the construction process of cable joint construction personnel before and after conductor connection.
(1) Poor treatment of the contact surface of the connecting fittings. Whether it is a terminal block or a connecting pipe, due to the influence of production or storage conditions, there are often impurities, burrs and oxide layers on the inner wall of the pipe body. This is a defect that people do not pay attention to, but it has a serious impact on the quality of conductor connection. In particular, a hard and insulating aluminum oxide film is easily formed on the aluminum surface, which makes the connection of aluminum conductors more troublesome than that of copper conductors, and the process technology is much stricter. The main reason for heating of connection (crimping, welding and mechanical connection) is the process technology and sense of responsibility, in addition to the performance factors of machines and materials. If the construction personnel do not understand the connection mechanism and do not operate in strict accordance with the process requirements, the electrical and mechanical strength of the connection will not be reached. The operation proves that the cleaner the contact surface between the crimping hardware and the conductor is, the thinner the oxide film produced and the smaller the contact resistance will be when the temperature of the connector rises.
(2) Conductor damage. The strength of the cross-linked insulation layer is large and it is difficult to peel and cut. During the ring cutting, the construction personnel use an electrician's knife to cut left and right. Sometimes they simply use a hacksaw to cut deep marks, which often leads to damage to the conductor. Although it is not very serious after stripping and cutting, when the wire core is bent and crimping creeps, the conductor damage at the damaged part will be aggravated or broken, which is difficult to find after crimping, and serious heating will be caused due to the reduction of section.
(3) The wire core is not in place when the conductor is connected. When connecting conductors, the insulation stripping length requires that the hole depth of crimping fittings be increased by 5mm. However, because the hole depth of the product is not standard, it is easy to cause insufficient stripping length, or because the string position during crimping causes a gap at the end of the conductor, which is only connected by the wall thickness of the fittings, resulting in increased contact resistance and heat generation.
2. Insufficient pressure. At present, the relevant data only mention the number of pressing pits at each end of the cable connection in the joint manufacturing process and standard drawings, without specifying the crimping area and the crimping height. The construction personnel shall press enough pits as required, but the effect is uncertain. Regardless of the type of pressure connection, the joint resistance is mainly the contact resistance, and the size of the contact resistance is related to the size of the contact force and the actual contact area, and is related to the output tonnage of the crimping tool. The main reasons for insufficient conductor connection pressure are as follows:.
(1) The crimp implement pressure is insufficient. In recent years, there are many manufacturers of crimping machines and tools, with disordered management and no unified standard. Especially for the mechanical crimping pliers produced in recent years, the indentation pit is not only narrow, but also the upper and lower pressing dies cannot match after crimping in place; Some other manufacturers purchase or produce foreign types of crimping clamps. Because of the implementation of foreign standards, they do not adapt to the nominal section of domestic conductors, and it is difficult to ensure the crimping quality.
(2) The gap between connecting fittings is large. At present, the connection hardware used by most units of cross-linked cable connectors is still the terminals and crimping tubes produced by using fan type conductors for oil paper cables. Theoretically, the effective cross sections of round and sector cores are the same, but compared with the actual operation, the crimping effects of the two are quite different. Since the conductor of the cross-linked cable is a tightly twisted round core, it cannot achieve sufficient compression force after crimping with a large gap between the conductor and the inner diameter of the commonly used fittings. The contact resistance is inversely proportional to the applied pressure and will therefore cause an increase.
(3) The quality of fake and inferior products is poor. Fake and inferior fittings are not only of impure material and rough appearance, but also easy to crack after pressing, and the specification is inaccurate. The effective section is very different from the correct product, which can not meet the crimping quality requirements at all. Under normal conditions, the operation generates serious heat, and a slight load fluctuation will inevitably lead to failure.
3. The allowable ampacity of cross-linked cable and oil paper cable is insufficient. When the ambient temperature is 25 ℃, the comparison results show that ZQ2-3 × 240 oil paper copper core cable can be replaced by YJV22-3 × 150 cross-linked copper core cable. Because the allowable current carrying capacity of YJV22-3 × 150 cross-linked cable is 476A; The allowable current carrying capacity of ZQ2-3 × 240 oiled paper cable is 420A, which exceeds 56A. ZLQ2-3 × 240 can be replaced by YJLV22-3 × 150, because the current carrying capacity of the cross-linked 3 × 150 aluminum core cable is 364A, while the current carrying capacity of the oiled paper 3 × 240 aluminum core cable is only 320A, and the cross-linked cable still exceeds 44A. If the allowable ampacity is used for calculation, the 150mm2 cross-linked cable is basically the same as the 240mm2 oiled paper cable, or the 150mm2 cross-linked cable can only operate normally when connected with 240mm2 fittings. It can be seen that the insufficient cross section of the connecting fittings will be an important reason for the serious heating of the cross-linked cable connector.
4. Poor heat dissipation. Wrapped joints and various cast joints are not only thicker than the cable cross-linked insulation layer, but also filled with a mixture in the shell, that is, smaller heat shrinkable joints, whose insulation and protective layer are more than double that of the cable body. In this way, no matter what type of joint, it is difficult to dissipate heat. The current insulation materials of various joints have poor heat resistance, and the normal operating temperature of J-20 rubber self-adhesive tape does not exceed 75 ℃; J-30 also reaches 90 ℃; The use conditions of heat shrinkable materials are - 50~100 ℃. When the cable is operating under normal load, the temperature inside the connector can reach 100 ℃. When the cable is fully loaded, the temperature of the cable core can reach 90 ℃, and the connector temperature can reach about 140 ℃. When the temperature rises again, the oxide film at the connector will thicken, and the contact resistance will increase accordingly. Under the effect of a certain power on time, the insulating material of the connector will be carbonized into non insulating material, leading to failure. To sum up, the key factors to reduce contact resistance are to increase the pressure of connection fittings, reduce the operating temperature, clean the surface of connection metal materials, improve the structural dimensions of connection fittings, select high-quality standard accessories, and strict construction technology.
III. Measures to improve the quality of cross-linked cable joints
Due to the different environments and operation modes of cross-linked cable connectors and the different electrical equipment and locations connected, there is a wide choice of cable accessories in terms of material, structure and installation process, but the basic performance of various accessories is the same, so the following measures should be strengthened to improve the connector quality. (1) Cable accessories with advanced technology, mature process, reliable quality and adaptability to the used environment and conditions must be selected. Counterfeit and shoddy products must be resolutely resisted, new technologies, new processes and new products should be tested, summarized and improved constantly, and gradually popularized and applied year by year. (2) The connecting hardware with good material, specification and section meeting the requirements and capable of safe and reliable operation shall be used. For terminal blocks, oil blocking type shall be selected as far as possible, because this type of terminal is generally large in section, which can reduce heating and effectively solve moisture-proof sealing. The connecting pipe shall be made of red copper rod or 1 # aluminum, and the size shall match the diameter of cross-linked cable core. In recent years, the crimping pipes for cross-linked cable joints provided by the accessory factories of Changsha and Shenyang Cable Factory have achieved good results. (3) The crimping machine with large crimping tonnage, good mold matching, sufficient pit area and crimping effect meeting the technical requirements shall be selected. Conduct interface treatment before crimping and apply conductive paste. (4) Train cable technicians who are skilled in technology, conscientious and responsible, and competent in cable construction, installation, operation and maintenance. Improve the construction personnel's understanding of cross-linked cable, enhance their understanding of the characteristics of cross-linked cable accessories, study technology, improve process, formulate construction specifications, strengthen quality control, and ensure safe operation. Due to the short time of popularization and application of cross-linked cable, the variety of cable accessories is disordered, the technical level of construction personnel is different, and the contact force and actual contact area of the connector are changing with the different operating conditions of the connector in operation, so the reasons for the failure of various connectors of cross-linked cable are different. In addition to the heating problem, the sealing problem Joint failures caused by stress problems, connection problems and grounding problems should also be paid attention to.
