What Is an Overhead Cable?

Overhead cable (full name overhead insulated cable) is an overhead conductor with an insulation layer and a protective sheath. It is a special cable manufactured similar to the cross-linked cable production process. It is a new power transmission between the overhead conductor and the underground cable. Way ^[1] . Overhead cables are single-core, and can be divided into hard aluminum wire structure, hard-drawn copper wire structure, aluminum alloy wire structure, steel core or aluminum alloy core support structure and self-supporting three-core twisted structure (wire) according to different structures. The core may be hard aluminum or hard copper wire) or the like. It has the main features of high power supply reliability, good power supply security, convenient erection and maintenance, and reasonable economy. Its main technical parameters include weather resistance, insulation level, inner and outer semi-conductive shielding layers. Overhead cables are widely used in power transmission at home and abroad.

Most of the traditional high-voltage power transmission methods use high-voltage overhead lines or high-voltage cable lines as transmission channels, and their respective advantages and disadvantages and their scope of application have been determined ^[1]

The overhead cable is not a common oil-paper-insulated cable or a cross-linked insulated cable directly hung on an overhead pole tower, but a special cable manufactured by a process similar to the cross-linked cable production process.

General overhead cables are single-core, and can be divided into hard aluminum wire structure, hard-drawn copper wire structure, aluminum alloy wire structure, steel core or aluminum alloy core support structure and self-supporting three-core twisted structure according to their structure. (Core can be hard aluminum or hard copper wire) etc. ^[1]

1.High power supply reliability

The use of overhead cables can greatly reduce various types of short-circuit faults (especially flashover failures common to bare overhead conductors). Compared with bare overhead conductors, the failure rate is 4-6 times lower. ^[2]

Weatherability

In addition to the high electrical performance requirements, high-voltage overhead cable insulation also requires good weather resistance ^[4] in order to work safely under long-term sunlight. Existing aerial cable insulation materials are made of polymer insulation material and a certain amount of carbon black. Because the polymer insulation material can block visible light and infrared light that account for more than 94% of energy, making it impossible to penetrate into the polymer, but there is still a small amount (6% of the total energy) of ultra-violet light (commonly known as VA Light) can penetrate into the insulation, crack the polymer under long-term effects, the insulation surface becomes rough or even crack, and eventually the insulation is destroyed. If more than 2% of carbon black is incorporated in the insulation, it can completely prevent the invasion of VA light, but this is not good for the electrical insulation performance of the cable, and will cause the gt B value to rise sharply. To solve this contradiction, the current cable insulation is the best weather-resistant cross-linked insulation formula determined after different formulations and multiple tests. This formula has been successfully trial-produced by some domestic manufacturers. The cross-linked two overhead cables produced with this material It has also been formally put into operation.

Insulation level

When overhead cables are used, because the voltage to ground is not completely added to the cable insulation ^[1] , most of the ground is borne by the air medium, so the insulation performance of overhead cables is higher, and it is obviously better than ordinary Plain greased paper and cross-linked cables. Because the lead of the oil-paper cable and the shielding layer of the cross-linked 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. Therefore, relevant standards in various countries stipulate that the insulation thickness of overhead cables is 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, the long-term voltage of overhead cables will not significantly reduce the insulation quality. Unlike buried cables, long-term operation will produce water trees. When the water trees reach saturation, the cable breakdown voltage will decrease by 1/3 to 1/2. , And overhead cables generally do not produce water trees. Therefore, comprehensively, the insulation performance of overhead cables is high.

Inner and outer semi-conductive shielding layer

1KV low-voltage overhead cables do not require an inner semi-conductive shielding layer because of the low electric field strength on the conductor surface ^[1] . When the overhead cable is close to the ground, the electric field strength on the conductor surface of the 10kV overhead cable is 2 ~ 3kV / mm, and the electric field strength on the conductor surface of the 35kV overhead cable can be as high as about 5kV / mm. This requires the use of an inner semi-conductive layer. Because the conductor and the insulation layer in the cable are not firmly bonded, the two layers will detach from each other under temperature changes. In rainy weather, the detached part is filled with water vapor to reduce its electrical strength. When dry, the electric strength of air can reach 3kV / mm, while the humidity of air is only 0.5-1KV / 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, in addition to causing corona discharge loss, and it is easy to damage the insulating medium. For this reason, aerial cables above 10kV all use a conductor's inner semi-conductive shielding layer.

Split-phase single-core aerial cables do not need to use an outer semi-conductive screen layer, but three-core stranded aerial cables above 10 kV must use outer semi-conductive shielding layers. Because the three-core stranded overhead cable has an uneven external electric field, if there is no outer semi-conductive shielding layer, a strong corona discharge will occur when the air is wet.