The power cables used in the previous construction are mainly YJLV22 seriesCross linked aluminum core cableYJV22 seriesCross linked copper core cableMainly (10KV), compared with overhead lines and old-fashioned oil filled cables, this type of cable has good insulation performance, allows high working temperatures (up to 90 ℃), and has good mechanical strength. The extensive application of cables not only improves power supply reliability, but also beautifies urban appearance, which is worth promoting. However, during the long-term construction process, combined with a large amount of feedback from the operating unit, we have found that the phenomenon of cables getting damp and water entering has become a potential hazard affecting the safe operation of cables.

Reasons and hazards of cable moisture and water ingress

1) The newly added whole cable is sealed with plastic sealing sleeves at both ends when it leaves the factory. However, after using up a section according to the actual situation at the construction site, the remaining part is simply wrapped with plastic cloth to cover the fracture. Due to poor sealing and outdoor placement, over time, it is inevitable that moisture will seep into the cable.

2) When laying cables, it is necessary to frequently cross roads, bridges, culverts, etc. Due to weather or other reasons, a lot of water often accumulates in the cable trench. During the laying process, it is inevitable that the cable head will be immersed in water, and water will enter the cable due to loose or damaged plastic wrapping; In addition, during traction and threading, sometimes the outer sheath or even steel armor may be scratched, which is particularly prominent when using mechanical traction.

3) After the cable laying was completed, due to the limitations of on-site construction conditions, the cable head could not be made in a timely manner, resulting in the unsealed cable fracture being exposed to air for a long time, and even immersed in water, causing a large amount of water vapor to enter the cable.

4) During the production process of cable heads (including terminal heads and intermediate joints), due to the negligence of construction personnel, newly processed cable heads may sometimes accidentally fall into the accumulated water on site.

5) In the normal operation of the cable, if a breakdown or other fault occurs for some reason, the accumulated water in the cable trench will enter the interior of the cable along the fault point; In civil construction, especially in construction sites that use large construction machinery, cable damage or breakdown accidents caused by various human factors are not uncommon. When such accidents occur, the insulation of the cable is severely damaged, which can also cause water to enter the cable.

Research has shown that after water enters the cable, water tree aging occurs under the action of an electric field, which eventually leads to cable breakdown. Water trees are a collection of voids filled with water with a diameter ranging from 0.1m to a few micrometers. The impurities, pores, and uneven bonding surfaces between the insulation and the inner and outer semiconducting layers that form local high electric field sites are the starting points for the occurrence of water trees. The development process of water trees generally takes more than 8 years. The higher the humidity, temperature, and voltage, the more ions are contained in the water, and the faster the development of water trees.