Generally speaking, power cables are protected from water during production and must be kept dry internally. Due to the rigorous electric field and voltage testing, cables that have been exposed to water cannot be shipped.
Once water has entered a cable, the electric field can cause water treeing, a aging phenomenon that can lead to cable breakdown. Water trees are collections of water-filled voids ranging in diameter from 0.1m to several microns. Impurities, pores in the insulation, and uneven interfaces between the insulation and the inner and outer semiconducting layers create localized high electric field areas that are the starting point for water treeing. Water treeing typically takes more than eight years to develop, and the higher the humidity, temperature, and voltage, and the more ions in the water, the faster it develops.
Water directly entering low-voltage power cables can cause corrosion of metals like steel and copper strips, degrading insulation performance. Water penetrating the insulation can cause cable breakdown and even explosions, potentially injuring personnel.
Causes of Water Ingress in Cables
1. Storage
Newly purchased cable drums are sealed at both ends with plastic sealing sleeves. However, after a certain amount of cable is used, the remaining cable is wrapped in plastic paper and tied with string. This poor sealing allows moisture to seep into the cable over time.
2. Cable Laying
During cable laying, the plastic-wrapped cable ends can sometimes be immersed in water, allowing water to enter the cable. During cable pulling and conduit threading, the outer sheath can sometimes rupture.
3. After Laying
After laying, cable ends are not promptly fabricated, leaving unsealed cable ends exposed to air or even submerged in water for extended periods, allowing significant moisture to enter the cable.
4. Cable End Fabrication
During cable end fabrication (including terminal ends and intermediate connectors), the end of the cable can sometimes slip into a waterlogged cable well due to carelessness on the part of the fabricator.
5. Cable Operation
During cable operation, if a fault such as a puncture in an intermediate joint occurs, water accumulated in the cable well can enter the cable through the gap. Water can also enter the cable at construction sites if external forces cause cable damage or puncture.
Countermeasures for Water Ingress in Cables
Drying cables after water ingress (such as using pressurized hot nitrogen) is extremely difficult, and the necessary equipment is generally unavailable. In practice, if water ingress occurs in cable R6, we simply cut off the first few meters. If the entire cable is flooded, we are unable to remove it. Therefore, prevention of water ingress in cables should prioritize prevention, using the following measures:
1. Sealing Cable Ends
Sawed-off cable ends, whether stacked or laid, must be sealed with plastic (using cable-specific sealing sleeves) to prevent moisture ingress.
2. Timely Fabrication of Cable Ends
Cable ends must be fabricated promptly after the wiring is laid.
3. When Purchasing Cables
Select manufacturers with proven quality. Because impurities and pores in the insulation are the starting point for water tree formation, cable quality is crucial to preventing water tree degradation.
4. Strengthening the Management of Cable Connector Manufacturing Processes
Once water enters a cable, the cable connector is often the first to experience breakdown. Therefore, well-made connectors can extend the overall life of the cable. For example, when stripping the semiconductor layer of a cable, we make several vertical cuts into the semiconductor layer and then peel it off like peeling sugarcane. However, if the cuts are too deep with a knife, they will damage the insulation layer, creating an opportunity for water tree formation. Furthermore, when soldering, due to a lack of power supply, people often use a blowtorch to melt the solder. In this case, the flame can damage the copper shielding and insulation. To prevent this, the correct solution is to use a UPS, as soldering generally takes only 10 minutes and consumes no more than 500W of power.
5. Using Cold Shrink Cable Connectors
Cold shrink silicone rubber cable accessories are simple and convenient to manufacture, requiring neither a blowtorch nor solder. Silicone rubber cable accessories are flexible and adhere tightly to the cable, overcoming the drawbacks of heat-shrink materials (which lack elasticity and, as the cable expands and contracts, create gaps between the cable and the cable itself, which facilitates the development of water trees).
6. Use cable branching boxes for long cables
For example, for long cables, each approximately 3 km in length, in addition to intermediate joints, one or two cable branching boxes can be used. This prevents water from spreading to other sections of the cable if one section of the cable is flooded. This also facilitates segmented locating in the event of a cable fault.
7. Use 8.7/10kV cable in 10kV systems.
This cable has an insulation thickness of 4.5mm, compared to 3.4mm for 6/10kV cables. The increased cable insulation thickness reduces the field strength and prevents water tree aging. Furthermore, since the 10kV neutral-point low-current grounding system, when single-phase grounded, requires the cable to withstand 1.73 times the phase voltage and operate for 2 hours, thicker cable insulation is necessary.
8. Use PVC plastic double-wall corrugated conduit
This conduit is corrosion-resistant, has a smooth inner wall, and offers excellent strength and toughness. Therefore, it significantly reduces damage to the cable sheath during direct burial.
9. Cable Trench (Pipe) and Cable Shaft Design
Due to limited conditions, our cable installations are all direct burial or trench, with direct burial being the most common method. Our region is located in a coastal, rainy area, and water accumulates year-round in the cable trenches and cable shafts. Because the depth of the trenches and cable shafts often exceeds the depth of the sewers, drainage is difficult. Therefore, planning should include coordination to facilitate drainage of the trenches and shafts. If water accumulation is not possible in the cable shafts, the intermediate joints in the shafts should be supported with brackets. Furthermore, our district is a heavy chemical industry zone, home to numerous chemical companies. During inspections, we discovered that some of the outer sheaths of wires in cable trenches near chemical plants were severely deformed. Therefore, cable trenches near chemical plants must have comprehensive drainage facilities. Furthermore, when designing cable conduits, they should be as straight as possible, with fewer bends, to facilitate cable laying. Furthermore, when constructing cable wells, we divide them into large and small cable wells. Large cable wells are used for pulling cables, coiling cables, and making intermediate joints. In areas where corners are necessary, such as in the middle of a road, where it's not convenient to construct a well, we construct small cable wells. These are simply used to house diverting pulleys during cable laying.
10. Cable Testing: After the cable heads are completed
A high-voltage DC leakage test is performed before commissioning. Subsequently, we only perform pre-tests on cables exiting the substation; other cables are not tested. If a cable exiting the substation fails, the short-circuit current can significantly impact the substation equipment. Therefore, if a cable problem is discovered, we must strengthen operational management and promptly replace it. We believe that post-processing a cable fault and handling a cable fault discovered after testing are equally cumbersome: locating the fault point and even replacing the cable. The disadvantages of the former are unplanned power outages and short-circuit current shocks. The advantages of not testing are that the cable life can be extended (some cables can still operate for a long time even with suboptimal test results, and DC testing increases the likelihood of cable breakdown). The fault point is more obvious and easier to locate. The advantages and disadvantages of the latter are precisely the opposite of the former. Therefore, for cable users who do not perform testing, we focus on ensuring power supply reliability. For example, the 10kV switch stations that supply power to users all utilize dual power supplies and implement automated dispatching. If one incoming cable fails, power is immediately switched to the other cable. In fact, the new "Procedure for Preventive Testing of Power Equipment" no longer requires DC withstand voltage testing at regular intervals for cross-linked cables, only insulation resistance testing, thus further simplifying preventive testing of cables.
