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Causes 1. The newly purchased tube-forming cables are sealed with plastic sealing sleeves at both ends. However, after a period of use, the remaining ones are wrapped with plastic paper, and the outside is tied with a rope. The seal is not good. Water vapor will infiltrate the cable.
2. When the cable is laid during cable laying, the cable head wrapped with plastic paper may sometimes be immersed in water to allow the water to enter the cable. When the cable is towed and worn, sometimes the outer sheath ruptures.
3. After being laid, the cable heads are not manufactured in time, so that the unsealed cable ports are exposed to the air for a long period of time, or even immersed in water, so that a large amount of moisture enters the cable.
4. When the cable head is manufactured, when the cable head is manufactured (including the terminal head and the intermediate connector), due to the intention of the production personnel, the cable end sometimes slides into the cable well with water.
5. When the cable is running, when the breakdown of the intermediate connector breaks down, the water in the cable well will enter the cable along the gap; at the construction site, external cables will cause the cable to break or break down, and the cable will also enter the water.
Solution The drying process after the cable is filled with water is very difficult (for example, hot nitrogen is used for pressure drying), and the corresponding equipment is generally not configured. In actual operation, if the cable R6 enters the water, we only saw off the front several meters. If the entire cable has been flooded, we would not be able to make it. Therefore, the prevention of cable flooding should be based on prevention. The following measures should be taken: 1. Cable ends should be sealed off the sawn cable ends. Whether they are stacked or laid, they must be sealed with plastic (using cable-specific sealing sleeves). Prevent moisture infiltration.
2. The cable head should be made in time after laying the wire.
3, when you purchase a cable, you must choose a manufacturer with excellent quality. Since the impurities, pores, etc., in the insulation are the starting points for the occurrence of water trees, the quality of the cable is critical to prevent the aging of the water tree.
4, to strengthen the management of the cable head manufacturing process Once the cable into the water, the first breakdown phenomenon is often the cable head, and thus the wire head is made well, you can extend the cable's overall life. For example, when the cable is peeling off the semiconductor layer, we have several vertical lines on the semiconductor layer, and then strip the semiconductor like stripping sugar cane. However, if you use a knife to draw too much, you will injure the insulating layer and give the water tree an opportunity. In addition, in the case of soldering, since no power source is available, the torch is directly used to melt the solder. At this time, the flame will damage the copper shield and the insulating layer. Therefore, to prevent this phenomenon, the correct method is to configure the UPS because solder The time required is generally only 10 minutes and the power is not more than 500W.
5, the use of shrinkable cable head 3M company's cold-shrinkable silicone rubber cable accessories, production is simple and convenient, no torch, no solder. The silicone rubber cable attachment is elastic and tightly attached to the cable to overcome the shortcomings of the heat shrinkable material (the heat shrinkable material is not elastic, and there is a gap between the cable body and the cable body during the process of thermal expansion and contraction of the cable. Provides convenience for the development of water trees). At present, the 3M company's cold-shrinkable cable accessories are used for the intermediate joints of all major cables of our bureau.
6. The long cable uses several long cables of the cable branch box of our bureau, each length is about 3km. For this cable, in addition to the middle connector, we also use one to two cable branch boxes, once one of the cables enters After water, it will not spread to other sections of cable, but also easy to find in the cable fault segmentation 8.7/10kV grade cable used in 7 and 10kV systems The grade cable insulation thickness of 4.5mm, and 6/10kV grade cable The insulation thickness is 3.4mm. Due to the increase of cable insulation thickness, the field strength is reduced and water tree aging can be prevented. At the same time, since the l0kV neutral current system is single-phase grounded, the cable must withstand 1.73 times the phase voltage and run as required. 2h, therefore, it is necessary to thicken the cable insulation.
8, using PVC plastic double-wall corrugated pipe The pipe corrosion resistance, smooth inner wall, good strength and toughness, so when laying the cable directly buried, can greatly reduce the outer cable sheath damage. 9. Design of cable trenches (tubes) and cable wells Due to the limitation of conditions, our cables are laid in the form of direct burial or cable trenches, and buried directly, and our area belongs to coastal rainy areas, cable trenches or cable wells. Years of stagnant water. Since the depth of the cable trench or the cable well exceeds the depth of the sewer, the drainage is very difficult, so during the planning, coordination should be carried out to facilitate drainage of the cable trench (well). If it is impossible to make water in the cable well, the intermediate joint in the cable well should be supported by a bracket. In addition, our district is a heavy chemical industry zone. There are a large number of chemical companies in the zone. During the inspection, it was found that some of the cables in the cable trench near the chemical plant have been seriously deformed. Therefore, the cable trench near the chemical plant must have Perfect drainage facilities. In addition, when designing the cable pipe, it is necessary to straighten the elbow so as to make the cable easier to lay. At the same time, when the cable well is manufactured, we divide it into large cable wells and small cable wells. Large cable wells can be used to pull cables and coils. In the middle of the road, where it is not convenient to make electric wells but must have corners, we changed to small cable wells, which are used to place diverting pulleys only when laying cables.
10, cable test cable after the completion of production. A high-voltage direct current leakage test was conducted before being put into operation. After that, we only pretested the output cable of the substation, and no other cable was tested. Because once the substation outlet cable fails, the short-circuit current will cause great impact on the substation equipment. Therefore, if it is found that there is a problem with the electric wire, it is necessary to strengthen the timely operation and management exchange. We believe that the post-processing of cable faults, as well as the cables that are found to be faulty after cable testing, are both as much troublesome as troubleshooting: finding fault points or even replacing cables. The disadvantages of the former are: Unplanned blackout, short-circuit current impact advantage is: do not test can extend the life of the cable (some cable test is not ideal, but it can still run for a long time, and after the DC test will increase the cable hit Wearable), the point of failure is obvious, easy to find. The advantages and disadvantages of the latter are exactly the opposite of the former.
Therefore, for cable users who do not test, we focus on the reliability of their power supply. For example, a 10kV switchgear station that supplies power to users uses dual power supplies to automate dispatching. Once an incoming cable fails, it immediately switches to another one. Power cable. In fact, in the new “Preventive Test Regulations for Electrical Equipmentâ€, it is no longer necessary to make DC withstand voltage tests for cross-linked cables for a certain period of time, only to measure the insulation resistance, and thus to simplify the preventive testing of cables.
After the cable enters the water, under the effect of the electric field, water tree aging will occur, and finally lead to cable breakdown. Water trees are collections of voids filled with water ranging from 0.1m to several microns in diameter. A local high electric field site formed by impurities, pores, and insulation that are not in contact with the inner and outer semiconducting layers is the starting point of the water tree. The development process of water trees is generally more than 8 years. The higher the humidity, temperature, and voltage, the more ions in the water, the faster water trees will develop.