Ren Yi official website updated on July 9
Generally speaking, power cables cannot enter water during production, and the inside of the cables must be dry. Because of the severe electric field voltage test, the cables that enter water cannot leave the factory. Once the cable enters the water, under the action of the electric field, water tree aging will occur, leading to cable breakdown. When water penetrates the insulation, it will cause the cable to puncture, or even explode and hurt people.
1. Reasons for water in the cable
1. When storing
For the newly bought tube cable, both ends are sealed with plastic sealing sleeves, but after a section is used, the rest is wrapped in plastic paper, and the outside is tied with a rope. The airtightness is not good. Over time, water vapor will seep in. cable.
2. When laying cables
When the cable is laid, the cable head wrapped in plastic paper is sometimes immersed in water to allow water to enter the cable; when towing and pipe, sometimes the outer sheath is broken.
3. After laying
After laying, the cable head was not made in time, so that the unsealed cable port was exposed to the air for a long time, or even immersed in water, causing a large amount of water vapor to enter the cable.
4. When the cable head is made
During the production of cable heads (including terminal heads and intermediate joints), due to the intention of the production personnel, the cable ends sometimes slip into the cable well with water.
5. When the cable is running
During cable operation, when a fault such as a breakdown of the intermediate connector occurs, the water in the cable well will enter the cable along the gap; on the construction site, the cable will be damaged or punctured by external forces, and water will also enter the cable.
2. Treatment measures for cable ingress
After the cable enters the water, it is very difficult to dry it (such as using hot nitrogen to pressurize it), and generally there is no corresponding equipment. In actual operation, if the cable R6 enters the water, we just saw off the front end a few meters, if the entire cable has entered the water, we are undesirable. Therefore, the prevention of water in the cable should be based on prevention, and the following measures should be adopted:
1. The cable head should be sealed
The sawn off cable ends, whether stacked or laid, should be sealed with plastic (using a special cable gland) to prevent moisture infiltration.
2. Make the cable head in time
After laying the wires, make the cable heads in time.
3. When buying cables
Must choose a manufacturer with excellent quality. Since impurities and pores in the insulation are the starting point of water treeing, the quality of the cable is very important to prevent water tree aging.
4. Strengthen the management of cable head production process
Once the cable enters the water, the cable head is often the earlier breakdown phenomenon, so the wire head is made well to extend the overall life of the cable. For example, when the cable is peeling off the semiconductor layer, we make a few vertical strokes on the semiconductor layer, and then peel off the semiconductor like sugarcane peeling. However, when you use a knife to draw too deeply, it will damage the insulating layer and give opportunities for water trees. In addition, when soldering, because the power source cannot be found, the torch will be used to melt the solder directly. At this time, the flame will damage the copper shielding layer and the insulation layer. Therefore, this phenomenon must be eliminated. The correct way to configure UPS is because the soldering The time required is generally only 10min, and the power is only 500W.
5. Use cold shrinkable cable head
Cold-shrinkable silicone rubber cable accessories, simple and convenient to manufacture, no blowtorch, no soldering. In addition, the silicone rubber cable accessories are elastic and tightly attached to the cable, which overcomes the shortcomings of heat-shrinkable materials. Provides convenience for the development of water trees).
6. Use cable branch box for long cables
For example, several long cables, each with a length of about 3km, for such cables, in addition to making intermediate joints, one or two cable branch boxes can also be used. Once one of the cables enters water, it will not spread to other sections. Cable, and it is also convenient to find in sections when the cable fails.
7. Use 8.7/10kV cable in 10kV system
The insulation thickness of this grade cable is 4.5mm, while that of the 6/10kV grade cable is 3.4mm. Due to the increase of the cable insulation thickness, the field strength is reduced, which can prevent the aging of water trees. At the same time, due to the l0kV neutral point low-current grounding system when single-phase grounding, the cable has to withstand 1.73 times the phase voltage and operate as required 2h, therefore, it is necessary to thicken the cable insulation.
8. Use PVC plastic double-wall corrugated pipe
The tube is corrosion-resistant, smooth inner wall, good strength and toughness, so when the cable is directly buried, it can greatly reduce the damage of the outer sheath of the cable.
9. Design of cable trench (pipe) and cable well
Due to the limitation of conditions, our cables are laid in direct burial or cable trench, and most of them are directly buried. Our area belongs to the coastal rainy area, and there is water accumulated in the cable trench or cable well for many years. Since the depth of the cable trench or the cable well will exceed the depth of the sewer, drainage is very difficult, so in planning, coordination should be carried out to facilitate the drainage of the cable trench (well). If it is impossible to prevent the cable well from accumulating water, the intermediate joints in the cable well should be propped up with brackets. In addition, our district is a heavy chemical industry zone. There are many chemical companies in the zone. During inspections, it was found that some of the outer sheaths of the wires in the cable trenches near the chemical plant have been severely deformed. Therefore, the cable trenches near the chemical plant must have Complete drainage facilities. In addition, when designing the cable piping, try to be as straight as possible and reduce the elbow to make the cable easy to lay. At the same time, when the cable well is made, we divide it into a large cable well and a small cable well. The large cable well can be used to draw cables and coils. , Make the middle joint, and in the middle of the road, where it is not convenient to make the wire well but must have a corner, we changed it to a small cable well, which is only used to place the steering pulley when laying cables.
10. After the test cable head of the cable is made
Do a high-voltage DC leakage test before putting it into operation. After that, we only do a pre-test on the outgoing cables of the substation, and do not test other cables. This is because once the substation outlet cable fails, the short-circuit current will have a great impact on the substation equipment. Therefore, if there is a problem with the wire, it is necessary to strengthen the operation management and timely exchange. We believe that the post-processing of cable faults is the same trouble as the faulty cable found after the cable test: finding the fault point and even replacing the cable. The shortcomings of the former are: unplanned power outages and short-circuit current impact. The advantage is: no test can extend the life of the cable (some cable tests are not ideal, but they can still run for a long time, and the DC test will increase the cable shock. The possibility of wearing), the fault point is obvious and easy to find. The advantages and disadvantages of the latter are just the opposite of the former. Therefore, for cable users who do not conduct experiments, we focus on the reliability of their power supply. For example, the 10kV switch station that supplies power to users uses dual power supplies to realize dispatching automation. Once one incoming cable fails, it will immediately switch to the other. Cable power supply. In fact, in the new "Procedures for Preventive Tests of Power Equipment", the cross-linked cables no longer require DC withstand voltage tests at regular intervals, only the insulation resistance is measured, which can simplify the preventive tests of cables.
Sichuan Renyi Electric Co., Ltd. is registered under the name of Ren Yi, a senior technician for substation maintenance, and uses scientific research, knowledge innovation, publicity, promotion, and teaching that the temperature of electrical contacts (connectors) is lower than the temperature of its conductors (wires). The theory and technology of "thermal electrical contacts" produces and sells registered trademark electrical equipment that does not heat electrical contacts (joints), as a whole with patented products.