How To Solve The Problem Of Cable Being Damp And Water Intrusion
In actual operation, if water is found in the cable head, we can only saw off a few meters of the front end to see if it is dry inside. If not, continue to saw forward. But if the entire cable has been flooded, we can do nothing. Because according to our current technical strength and existing equipment, it is very difficult to deal with water-infiltrated cables (such as using hot nitrogen pressurized drying). Therefore, power fittings manufacturers recommend that cable water ingress should be mainly prevented, and the solutions are as follows:
Power fittings manufacturers recommend solution 1: Due to the increase in cable insulation thickness, the field strength is reduced, which can prevent the aging of water trees. At the same time, since the 6kV neutral point small current grounding system is single-phase grounded, the cable must withstand 1.73 times the phase voltage and must run for 2 hours as required, it is necessary to thicken the cable insulation layer.
Power fittings manufacturers recommend solution 2: Since impurities and pores in the insulation are the starting point of water trees, the quality of the cable is crucial to preventing the aging of water trees. When purchasing cables, you must choose manufacturers with excellent quality. We conduct strict tests on the samples sent by each manufacturer, and require each manufacturer to bid to select high-quality and low-cost products.
Solution 3 recommended by power fittings manufacturers: Ensure that the cable head is well sealed. For the sawn cable ends, whether they are stacked or laid, they must be sealed with plastic. It is best to use a cable-specific sealing sleeve to prevent moisture from penetrating.
Solution 4 recommended by power fittings manufacturers: After the wires are laid, the cable heads must be made in time. If they cannot be made immediately due to conditions, the cable heads should be sealed and placed overhead.
Solution 5 recommended by power fittings manufacturers: Improving the technical quality of construction personnel and strengthening the management of cable head manufacturing process can effectively prevent water from entering the cable head during the manufacturing process. Practice has proved that once the cable is flooded, the cable head is often the first to show breakdown. Therefore, if the wire head is well made, the overall life of the cable can be extended. For example, when stripping the semiconductor layer of a cable, we must first cut the semiconductor layer in a circular manner according to the specified size, then make several vertical cuts, and then strip the semiconductor along the cuts.
When cutting with a knife, the force must be controlled appropriately. If the cut is too shallow, the semiconductor layer is difficult to strip off. If the cut is too deep, the insulation layer will be damaged, which will give water trees an opportunity to form. When encountering a cable with a semiconductor that cannot be stripped, the semiconductor layer must be scraped off with a glass sheet. This requires the construction personnel to be careful and meticulous. They must not only clean the semiconductor layer, but also try to avoid damaging the main insulation. Finally, the main insulation surface must be polished smooth.
In addition, when making heat shrinkable heads and applying solder, some construction personnel often use a blowtorch to melt the solder directly to save trouble. At this time, the flame will damage the copper shielding layer and the insulation layer. Therefore, it is necessary to prevent such phenomena from happening during on-site production. When the heat shrink material is heated and hardened, it is no longer elastic, which is determined by its material properties. In long-term operation, due to thermal expansion and contraction, a small gap will gradually appear at the cable joint, causing water vapor to invade.
Given that our company has basically eliminated heat shrink heads, and generally uses silicone rubber cold shrink cable accessories produced by 3M. Compared with heat shrink heads, the production process of 3M cold shrink heads is simple and convenient, without fire or soldering. In addition, the silicone rubber cold shrink material has stable performance and elasticity, can be tightly attached to the cable, and will not crack after long-term use, effectively overcoming the shortcomings of heat shrink materials.
Solution 6 recommended by power fittings manufacturers: When laying cables directly buried, we use Single Pole Distributian Blocks as the casing, which is corrosion-resistant, has a smooth inner wall, and has good strength and toughness, so it can greatly reduce the occurrence of cable outer sheath damage.
Solution 7 recommended by power fittings manufacturers: Due to the limitation of conditions, local cable laying adopts direct burial or cable trench. We are located in the coastal area, which is mostly saline-alkali land. In addition, the drainage is not smooth, resulting in water accumulation in the cable trench or cable well. Therefore, in the early planning, the design of the cable trench, culvert and cable well should be coordinated with the civil construction party in time to facilitate the drainage of the cable trench (well). At the same time, the bracket is embedded in the cable trench to support the cable with the bracket. In addition, in view of the current situation of many petrochemical enterprises in the jurisdiction of Shengli Oilfield, the nearby cable trench must have perfect drainage facilities. When designing the cable culvert, it should be as straight as possible, with fewer elbows, so that the cable can be laid easily.
Solution 8 recommended by power fittings manufacturers: When the cable is laid and the cable head is made, a DC withstand voltage leakage test should be carried out according to regulations before it is handed over to the operation management department for formal operation. It can only be put into operation after everything is qualified. When problems with the cable are found during operation, management personnel should strengthen monitoring and deal with them in time. Once a cable fails, it will be very troublesome to deal with it. You need to find the fault point or even replace the entire cable. The system will be hit by short-circuit current, causing unplanned power outages, etc.
Therefore, preventive tests before acceptance are essential. Of course, the DC withstand voltage test is a destructive test, which may have some impact on the life of the cable. Sometimes the cable test data is not ideal, but the cable can smoothly transmit power and run for a long time. Therefore, in the new "Preventive Test Procedures for Power Equipment", it is no longer mandatory to conduct DC withstand voltage tests on cross-linked cables at certain intervals, and only measure insulation resistance, which can simplify the preventive test of cables. Accidents are also common. When such accidents occur, the cable insulation is severely damaged, which can also cause water ingress to the cable.
Studies have shown that after the cable is flooded, water tree aging will occur under the action of the electric field, which will eventually lead to cable breakdown. Water trees are a collection of gaps filled with water with a diameter of 0.1m to several microns. The impurities, pores in the insulation, and the local high electric field formed by the uneven interface between the insulation and the inner and outer semi-conductive layers are the starting point of water trees. The development process of water tree generally takes more than 8 years. The higher the humidity, temperature and voltage, the more ions contained in the water, and the faster the water tree develops.
In addition to water entering the end, the water ingress of cross-linked cables is more due to the damage of cable sheath during cable laying (the technical level of construction personnel in the current power industry is uneven, and barbaric construction is not uncommon), and then the construction site and cross-operation cause cable damage (damage by large-scale engineering machinery is also uncommon).
DC withstand voltage problem of cross-linked cables: Due to the special dielectric properties of cross-linked polyethylene insulation materials, when DC is applied, space charge is easily formed inside the insulation. The high resistance characteristics of cross-linked polyethylene insulation make it difficult for space charge to disappear. Due to the existence of space charge, an additional electric field is generated in certain areas inside the insulation. When the voltage is re-applied, the superposition of electric fields may occur, which may cause cable breakdown. Frequent DC withstand voltage tests will affect the life of the cable and reduce the breakdown voltage. It is recommended to use a series resonance or variable frequency resonance test system.
