1, fault environment:
The underground space in coal mines is small and the air is damp. There is a danger of coal dust and gas explosions. Therefore, the requirements for the intermediate joints of cables are high. The use of heat shrinkable cable intermediate joints, due to the need for open flame heat shrinking, is extremely unsuitable for coal mines. The middle connector of cable made by watering with epoxy resin is large in size, not suitable for laying in the narrow underground space, and some also need to install the grounding system. The cold shrinkable intermediate joint has many advantages, and the installation does not require special tools, which is simple and quick. Short production time. Do not use open flame, safe and reliable. The cable body has a long-term radial pressure, and is wound with a waterproof tape that is breathable with the cable, and has good sealing and waterproof performance to ensure long-term reliable operation. The use of silicone rubber cold shrinkable insulating sleeve, with excellent insulation and high recovery elasticity, so that the inner surface is tightly combined, no electrical breakdown will occur due to the respiration of the cable operation, and the stress part is integrated with the main insulation to effectively solve the problem. The problem of stress concentration at the shield cross-section of the conductive layer of the cable semiconductor ensures reliable insulation and safe operation. The intermediate size of the cable connector is slender and relatively easy to install in the underground roadway relative to the epoxy resin-watered intermediate joint. For this reason, our mine has widely promoted the use of cold shrinkable intermediate joints in 1995. Although the cold shrink joints are good, there are accidents sometimes. Therefore, I will analyze the failure of our latest cold shrink joint and briefly talk about my own opinions.
2, fault conditions
On the night of March 20, 2008, a high-voltage power distribution switch No. 2 in the central substation of a coal mine in Shandong Province experienced a short-circuit tripping fault. The high-voltage switch band was a 240mm2 copper-cored cross-linked polyethylene power cable with four colds in the middle. The shrink cable intermediate connector is the incoming cable of the substation. After the substation has done a good job, it is judged as a cable fault by a megohmmeter. By locating the fault, the fault point is locked at the second cold shrinkable cable intermediate joint at the wellbore. This joint was made during the cable laying and installation in 2001 and was put into operation after the withstand voltage test. It has been running well and the accident has caused a correlation. The attention of department leaders requires analysing the accident and identifying the cause as soon as possible.
3, the reason analysis
With the cooperation of the technical staff of Shenzhen Electric Power Equipment Development Co., Ltd., the cause of the accident was quickly found. The shrink cable connector was simple and convenient for construction on site. The silicone rubber composite insulating sleeve was flexible, so long as the nylon core support rod was pulled out. , can be tightly attached to the XLPE cable, do not need to use a heating tool, to overcome the shortcomings of heat shrinkable material (heat shrinkable attachment is not flexible, in the process of cable expansion and contraction, will appear between the cable body gap). During the installation of our mine, the new process of cold-shrinkable cable intermediate joints was widely promoted. At that time, there were also deficiencies in the mastery of the operation of the new process, so it was necessary to carefully analyze the cause of the fault, find the cause, and learn lessons from it. Measures to improve construction quality and ensure safe operation of cable lines.
We know that the electric field of the cable connector is a distorted electric field, and the main cause of the electric field distortion is: When the connector is made, the copper shielding layer, semiconductor shielding layer, insulation layer, and core of the cable must be stripped and cut in the cable connector core. At the cut-off point of the shielding layer, electrical stress concentration occurs, and the electric field strength is the greatest, which is the weak link of the entire joint. At the same time, poor site conditions will inevitably invade dust, gas, moisture and other impurities. What causes the breakdown of the insulation is the need for further analysis.
For ease of analysis, the faulty cable intermediate joint was sawed off at the site and anatomy was found on the spot.
1) The cable joints are in good condition and no ablation cavity is found
2) Abandoning outer armor and waterproof tape, it was found that there was a significant carbon black.
3) After cutting each phase of the cold-shrinkable silicone rubber insulation sleeve, there are obvious discharge carbonization channels on the surfaces of the two cable insulation layers, which are from the core to the outside of the cable-silicone rubber composite insulation sleeve. It can be seen that the breakdown of the cable is caused by the discharge of the insulating surface at the joint, indicating that there is an air gap at the joint between the shrink sleeve and the cable insulation surface, and the insulation weak point formed by impurities causes a short circuit fault.
As we all know, impurities, air gaps, and moisture are the main causes of creeping discharge of the solid insulating medium. Therefore, the causes of insulation breakdown of the cold-shrinkable joint may be caused by the following aspects: manufacturing process, laying environment, and external force destruction.
1) There may be the following reasons for the production process
(1) When stripping the semiconductor shielding layer of cable, the tool marks are too deep, so that the surface of the main insulation layer is flawed and air gaps are likely to exist.
(2) After the semiconductor layer of the cable is stripped, it is not removed. The semiconductor is left on the main insulating layer, or it does not follow the process requirements during cleaning. Scrubbing back and forth causes hidden trouble and flashover discharge.
(3) After the crimping of the cable core, the connecting pipe is deformed by the tip and corners, causing electric field distortion, local field concentration, and tip discharge.
(4) Cold-shrinkable silicone rubber bushings are prefabricated accessories that must be matched to the cable cross-section. If there is no careful inspection of the attachment prior to making the joint, it will cause the shrinkage to be tight and cannot guarantee the interface pressure, resulting in impurities invading the air gap or getting wet.
(5) When the cold shrinkable head was made, after the silicone rubber insulating sleeve was shrunk, the two ports were not subjected to any sealing treatment, which was an important reason for the intrusion of moisture.
(6) When making cold shrink fittings, because the three-phase cold shrinkable insulating sleeve is in the center position, due to unevenness, there will be wrinkles in the wraparound waterproof tape, causing wraps to be tight, which will also lead to moisture in the joints. One of the important reasons.
(7) The improper size of semiconductor shielding and the failure to recover the copper shield are the main causes of short-circuit faults.
2) Laying the Environment and Destroying External Forces
(1) The underground environment is wet, and the main road often flushes with water and dust
(2) Access to data and working logs It is learned that the cable has undergone many rectifications and landings to meet the standardization of mine quality.
(3) After recollection, this cable connector became blisters in the process of dropping the ground.
Based on the above analysis, the following conclusions can be drawn: Due to the cable's landing, relocation, and movement, the middle box of the cable may be damaged. The flushing of the big lane and the blisters can make the cold shrinkable cable middle joint or the other outer sheath of the cable immersed in the water and immersed in the cold shrinkable cable middle joint, together with the manufacturing process of the cold shrinkable middle joint. The occurrence of an accident
4. Lessons Learned and Countermeasures
In order to absorb lessons learned from accidents and prevent the occurrence of similar accidents, it is also necessary to:
1) To strengthen the responsibility of the work, we must strengthen the protection of the middle connector of the cable when moving and moving the cable to avoid the force.
2) Protect the area where water and water may scour, avoid watering
3) The cable must not be flooded
4) The production of cables must be done strictly in accordance with the process requirements
5) After the cable insulation is stripped, carefully polish the surface of the main insulation layer with a fine sandpaper to make it smooth without any marks or semiconductor residue. The cleaning insulating layer must be directed from the core to the semiconductor shield layer with a cleaning solvent. Do not clean the surface of the main insulating layer with cleaning paper that has touched the semiconductor shield.
6) After the wire core is crimped, use a trowel or sandpaper to carefully grind to remove edges and corners, and note that metal dust must not remain on the surface of the insulating layer.
7) During the process of making cable joints, special care should be taken to keep it clean. At the same time, the production time should be shortened as much as possible. The longer the cable is exposed and exposed to air, the more likely it is to invade impurities, moisture, gas, dust, etc. , thereby affecting the quality of the joints. Therefore, it is required that all preparation work be done well before construction to ensure uninterrupted production at one go.
8) Before making the cold shrink fittings, we must carefully check whether the cable accessories and the cables are matched. In this way, we can strictly control the interference of the cold shrinkable silicon rubber insulating sleeves, ensure that they have enough grip force, make the interface contact tightly, and there is no air gap. .
9) The semiconductor self-adhesive tape is wrapped around the two ports of the silicone rubber composite insulating sleeve of each phase after shrinking. In this way, the semiconductor layer outside the silicone rubber sheath can be in good contact with the semiconductor shielding layer outside the cable, and can also play the role of axial water and moisture proof.
10) Wrapping the self-adhesive waterproof tape is the key link of the moisture-proof seal of the cold-shrinkable joint. Before wrapping the self-adhesive waterproof tape, thoroughly clean and grind the outer jacket of the cable at both ends of the cable head, and then use the half overlap method from the joint. One end is wrapped around the other, and then it is wrapped in the opposite direction from this end to the start end, wrapping two layers. After each layer is wrapped, it is necessary to hold it tightly with both hands in order to make it better bonded. Wrap around must be appropriate, so that it can be wrapped tightly and seamlessly.
11) When the armor is finally wrapped around, it must be properly energized to make it more tightly wound so as to increase the strength of the cold-shrinkable cable in the middle of the street, thus preventing the destruction of the external force.