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Overhead high voltage insulated cable
High voltage overhead cable insulation, in addition to having to meet the requirements of high electrical performance, but also requires a good weathering resistance insulation, in addition to having to meet the requirements of high electrical performance, but also requires a good weathering resistance , in order to work safely in the long-term sunlight irradiation.The existing overhead cable insulation material is made up of polymer insulation material and a certain amount of carbon black.Because of high polymer insulating materials can avoid energy accounted for more than 94% of visible light and infrared light, make its cannot penetrate into polymers to inside, but there are still a small amount (6%) of the total energy of uv light (commonly known as the VA) can penetrate inside insulation, make polymer pyrolysis under the long-term effects, insulation surface becomes rough and even cracking, eventually make insulation damage.If more than 2% carbon black is included in the insulation, VA light invasion can be completely prevented, but this is unfavorable for the electrical insulation performance of cables, which will sharply increase the value of gt b.In order to solve this contradiction, the current cable insulation is the best weathering crosslinking insulation formula determined after different formulas and repeated tests. The formula has been successfully piloted by some domestic manufacturers, and the crosslinking two aerial cables produced with this material have been formally put into operation.
When the overhead cable is used, since the ground voltage is not completely added to the cable insulation , most of the ground voltage is borne by the air medium, so the insulation performance of overhead cable is higher, and it is obviously better than the ordinary oilpaper and cross-linked cable.Since the lead coating of oil-paper cables and the shielding layer of cross-linked cables are usually grounded, the voltage through the cables is completely added to the main insulation, while the overhead cables are jointly borne by the main insulation and the air medium.Therefore, the relevant standards of all countries stipulate that the insulation thickness of overhead cables should be thinner than that of ordinary cables. Generally, the insulation thickness of overhead cables in Europe, America and Japan is 10-20% thinner than that of ordinary cables.At the same time, under the action of long-term voltage, insulation quality of overhead cables will not significantly decline, unlike the water tree generated when buried cables run for a long time. When the water tree reaches saturation, the breakdown voltage of cables will drop by 1/3 to 1/2, while overhead cables generally will not produce water tree.Therefore, from a comprehensive perspective, the insulation performance of overhead cables is relatively high.
Inner and outer semi-conductive shielding layer
As the electric field intensity on the conductor surface is very low for 1KV low-voltage overhead cable, internal semi-conductive shielding layer is not needed .When the overhead cable is close to the grounding body, the electric field intensity on the conductor surface of 10kV overhead cable is 2~3kV/mm, and the electric field intensity on the conductor surface of 35kV overhead cable can be up to about 5kV/mm, which requires the use of an inner semi-conductive layer.Because the conductor and insulating layer in the cable are not firmly bonded, the two layers will detach from each other under the change of temperature. In rainy weather, the detaching part is full of water vapor, which reduces its electrical strength.When dry, the electrical strength of air can reach 3kV/mm, while wet air is only 0.51kv /mm.In this way, when the cable runs under normal conditions, a layer of corona is formed between the conductor surface and the insulation gap, which not only causes corona discharge loss, but also easily damages the insulation medium.For this reason, all overhead cables above 10kV adopt inner semi-conductive shielding layer of conductor.
Phase split-phase single-core overhead cable does not need to use the outer semi-conductive screen osmolar layer, but more than 10kV three-core grain overhead cable must use the outer semi-conductive shielding layer.Because the external electric field of three-core striated overhead cable is uneven, if there is no external semi-conductive shielding layer, strong corona discharge will be generated when the air is moist.