Main technical performance of overhead cables

Weather Resistance In addition to meeting high electrical performance requirements, high-voltage overhead cable insulation also requires excellent weather resistance to ensure safe operation under long-term sunlight irradiation. Existing overhead cable insulation materials are composed of polymer insulation materials and a certain amount of carbon black. Because polymer insulation materials can block visible and infrared light (accounting for over 94% of the energy), preventing them from penetrating the polymer, a small amount (accounting for 6% of the total energy) of ultraviolet light (commonly known as VA light) can still penetrate the insulation. Over a long period, this causes the polymer to decompose, the insulation surface to become rough or even crack, ultimately leading to insulation failure. If more than 2% carbon black is added to the insulation, it can completely prevent the intrusion of VA light, but this is detrimental to the electrical insulation performance of the cable, causing a sharp increase in the gt value. To resolve this contradiction, the optimal weather-resistant cross-linked insulation formula for the cable insulation was determined after multiple trials with different formulations. This formula has been successfully trial-produced by some domestic manufacturers, and cross-linked overhead cables produced using this material have been officially put into operation. Insulation Level: When using overhead cables, the voltage to ground is not entirely applied to the cable insulation; it is largely borne by the air. Therefore, overhead cables have higher insulation performance, significantly better than ordinary paper-insulated cables and cross-linked cables. This is because the lead sheath of paper-insulated cables and the shielding layer of cross-linked cables are usually grounded, so the voltage passing through the cable is entirely applied to the main insulation. In overhead cables, however, the voltage is shared by the main insulation and the air. Therefore, relevant standards in various countries stipulate that the insulation thickness of overhead cables should be thinner than that of ordinary underground cables. Generally, in Europe, America, and Japan, the insulation thickness of overhead cables is 10-20% thinner than that of ordinary cables. Furthermore, the insulation quality of overhead cables does not significantly decrease under long-term voltage exposure, unlike buried cables which develop water treeing over long-term operation. When water treeing reaches saturation, the cable breakdown voltage drops by 1/3 to 1/2, while overhead cables generally do not develop water treeing. Therefore, overall, the insulation performance of overhead cables is superior.

Inner and outer semiconductive shielding layers

1KV low-voltage overhead cables do not require an inner semiconductive shielding layer due to the very low electric field strength on the conductor surface. When the overhead cable is close to the grounding body, the electric field strength on the conductor surface of a 10kV overhead cable is 2~3kV/mm, and the electric field strength on the conductor surface of a 35kV overhead cable can be as high as about 5kV/mm, which necessitates the use of an inner semiconductive layer. Because the conductor and insulation layer in the cable are not firmly bonded, the two layers will separate under temperature changes. In rainy weather, the separated parts are filled with water vapor, which reduces its dielectric strength. The dielectric strength of dry air can reach 3kV/mm, while that of humid air is only 0.5-1KV/mm. Thus, when the cable is running under normal conditions, a corona layer is formed between the conductor surface and the insulation gap, which not only causes corona discharge loss, but also easily damages the insulation medium. Therefore, all countries in the world use an inner semiconductive shielding layer for overhead cables above 10kV. Single-core overhead cables do not require an external semiconductive shielding layer, but three-core laminated overhead cables of 10kV and above must use an external semiconductive shielding layer. This is because the external electric field of a three-core laminated overhead cable is uneven, and without an external semiconductive shielding layer, a strong corona discharge will occur when the air is humid.