How to prevent water from entering the cable?
Normal power cables cannot enter water during production. The inside of the cable must be dry. The electric field voltage test will be strictly tested, so the cable that enters the water cannot leave the factory.
After the cable enters the water, under the action of the electric field, the water tree aging phenomenon will occur, resulting in the breakdown of the cable. Water trees are collections of water-filled voids ranging from 0.1 m to several microns in diameter. The impurities, pores in the insulation, and the local high electric field formed by the unevenness of the joint surface between the insulation and the inner and outer semiconducting layers are the starting points of the water tree. The development process of water tree is generally more than 8 years. The higher the humidity, temperature and voltage, the more ions contained in the water, the faster the water tree develops.
Water directly enters the low-voltage power cable, causing corrosion of steel strips, copper strips and other metals, and the insulation performance is reduced. When the water penetrates the insulation, it will cause the cable to break down, or even explode and hurt people.
一、Reasons for water ingress of cables
1. When keeping
The newly purchased cable in a tube is sealed with plastic sealing sleeves at both ends, but after a period of use, the rest is wrapped in plastic paper and tied with a rope on the outside. Penetrate the cable.
2. When laying cables
When the cable is laid, the cable head wrapped with plastic paper is sometimes immersed in water, allowing water to enter the cable; sometimes the outer sheath is broken when pulling and passing through the tube.
3. After laying
After laying, the cable head is not made in time, so that the unsealed cable port is exposed to the air for a long time, or even immersed in water, so that a large amount of water vapor enters the cable.
4. When making the cable head
When the cable head is made (including the terminal head and the intermediate joint), due to the intention of the manufacturer, the cable end sometimes slides into the cable well with accumulated water.
5. When the cable is running
During the operation of the cable, when a fault such as the breakdown of the intermediate joint occurs, the accumulated water in the cable well will enter the cable along the gap; on the construction site, the cable will be damaged or broken due to external force, and the cable will also enter the water.
二、Countermeasures for the treatment of cable water ingress
It is very difficult to dry the cable after entering the water (such as drying with hot nitrogen pressure), and generally there is no corresponding equipment. In actual operation, if the cable R6 gets water, we just saw off the front end a few meters, if the whole cable has got water, we can't do it. Therefore, the prevention of water ingress of cables should be based on prevention, and the following measures should be adopted:
1. The cable head should be sealed
The sawed-off cable ends, whether they are stacked or laid, must be sealed with plastic (using a special cable gland) to prevent moisture from infiltrating.
2. Production of cable head in time
After the wire is laid, the cable head should be made in time.
3. Buy cables
Manufacturers with excellent quality must be selected. Since impurities and pores in the insulation are the starting point for the occurrence of water trees, the quality of the cable is very important to prevent the aging of water trees.
4. Strengthen the management of cable head manufacturing process
Once the cable enters the water, it is often the cable head that breaks down earlier, so the wire head is made well, which can prolong the overall life of the cable. For example, when the cable is peeling off the semiconductor layer, we make a few vertical lines on the semiconductor layer, and then peel off the semiconductor like sugar cane peeling. But when scratching with a knife, if the scratch is too deep, it will damage the insulating layer and bring opportunities for the formation of water trees. In addition, when soldering, because the power supply cannot be found, the solder will be melted directly with a blowtorch. At this time, the flame will damage the copper shielding layer and the insulating layer. Therefore, to prevent this phenomenon, the correct method can be configured with UPS, because the solder The time required is generally only 10min, and the power is only 500W.
5. Adopt cold shrinkable cable head
Cold-shrinkable silicone rubber cable accessories, simple and convenient to make, no blowtorch, no solder. And the silicone rubber cable accessories are elastic and stick tightly to the cable, overcoming the shortcomings of the heat shrinkable material (the heat shrinkable material is not elastic, and there will be a gap between the cable body and the cable body during the process of thermal expansion and contraction of the cable. facilitated the development of water trees).
6. Cable branch boxes are used for long cables
For example, several long cables, each with a length of about 3km, for such cables, in addition to intermediate joints, one or two cable branch boxes can also be used. Once one of the cables enters the water, it will not spread to other sections. cable, and it is also easy to find segments in the event of a cable failure.
7. 8.7/10kV cables are used in the 10kV system
The insulation thickness of this grade cable is 4.5mm, while the insulation thickness of the 6/10kV grade cable is 3.4mm. Due to the increase of the insulation thickness of the cable, the field strength is reduced, which can prevent the aging of the water tree. At the same time, when the single-phase grounding system of the 10kV neutral point small current grounding system is grounded, the cable must bear 1.73 times the phase voltage, and it must be operated as required. 2h, therefore, it is necessary to thicken the cable insulation.
8. Using PVC plastic double-wall corrugated pipe
The pipe is corrosion-resistant, has a smooth inner wall, and has good strength and toughness, so it can greatly reduce the damage of the outer sheath of the cable when the cable is directly buried.
9. Design of cable trench (pipe) and cable well
Due to the limitation of conditions, our cables are laid in the form of 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 cable well will exceed the depth of the sewer, drainage is very difficult. Therefore, coordination should be carried out during planning to facilitate the drainage of the cable trench (well). If it is not possible to prevent water accumulation in the cable well, the intermediate joint in the cable well should be supported by a bracket. In addition, our area is a heavy chemical industry area, and there are many chemical enterprises in the area. During the inspection, it was found that some of the outer sheaths of the wires in the cable trenches near the chemical plant have been seriously deformed. Therefore, the cable trenches near the chemical plant must have Perfect drainage facilities. In addition, when designing the cable pipe, it should be as straight as possible to reduce the elbows, so that the cable can be laid easily; 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 pull cables and coils. , as an intermediate joint, and in the middle of the road where it is not convenient to make a wire well, but there must be a corner, we changed it to a small cable well, which is only used to place the steering pulley when laying the cable.
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 no other cables are tested. Because, once the outgoing cable of the substation fails, the short-circuit current will have a great impact on the equipment of the substation. Therefore, if there is a problem with the wire, it is necessary to strengthen the operation management and replace it in time. We believe that the post-processing of cable faults is the same trouble as the faulty cables found after cable testing: finding the fault point and even replacing the cable. The disadvantage of the former is: unplanned power failure and short-circuit current impact. The advantages are: no test can prolong 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 more 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 do tests, we focus on the reliability of their power supply. For example, the 10kV switch station that supplies power to users adopts dual power supplies to realize dispatch automation. Once one incoming cable fails, it will immediately switch to another. Cable powered.