High-voltage overhead cable insulation, in addition to meeting high electrical performance requirements, also requires good weatherability in order to work safely under sunlight irradiation for long periods of time. The existing overhead cable insulation material is formed by blending a polymeric insulating material with a certain amount of carbon black.
2. Insulation level
When an overhead cable is used, since the voltage to ground is not completely imposed on the cable insulation, most of the ground voltage is borne by the air medium. As a result, the insulation performance of the overhead cable is higher, and it is obviously better than ordinary ordinary paper and Cross-linked cables. Because the leaded package of the oil-paper cable and the shielding layer of the cross-over cable are usually grounded, the voltage passed by the cable is completely applied to the main insulation, while the overhead cable is shared by the main insulation and the air medium. At the same time, under the influence of long-term voltage, the insulation quality of the overhead cable will not drop significantly. Unlike buried cables, water trees will be generated during long-term operation. When the water trees reach saturation, the breakdown voltage of the cable will decrease by 1/3 to 1/2. While overhead cables generally do not produce water trees. Therefore, overall, the insulation performance of overhead cables is higher.
3. Inner and outer semi-conductive shields
Low-voltage overhead cables do not require the use of an inner semi-conductive shield due to the low electric field on the conductor surface. Because the conductor and the insulating layer in the cable are not firmly bonded, the two layers will be separated from each other under the change of temperature. In the rainy weather, the dissashed part is filled with water vapor, which reduces the electric strength of the cable. When the cable is operated under normal conditions, a corona is formed between the surface of the conductor and the insulation gap, which not only causes the loss of the corona discharge, but also easily destroys the insulating medium. For this reason, in the countries of the world, the overhead cable of more than 10kV adopts the inner semi-conductive shield layer of the conductor. Phase-separated single-core overhead cables do not need to use external semi-conducting screens. However, semi-conducting screens must be used for triple-core overhead cables above 10 kV. Because the triple-stranded overhead cable does not have uniform external electric field, if there is no outer semi-conductive shielding layer, strong corona discharge will be generated when the air is wet.