Ultrahigh voltage power transmission refers to the use of 500 kV - 1000 kV voltage level to transport electrical energy. If the 220 kV transmission index is 100%, the relative investment per kilometer of ultra-high voltage transmission, the relative cost of 100 kilometers of electricity transmission per kilowatt-hour and the consumption of metal materials have been greatly reduced, and the utilization rate of the line corridor has been significantly improved.

In daily life, we can often see the overhead ultra-high voltage power transmission project, have you ever considered such a question: why can't you bury all the high voltage wires into the ground like urban underground cables?

At present, the underground cable is generally a low voltage level, and the line transmission with a high voltage level is often an overhead mode, which is mainly a cost and technical factor.

The underground cable is more complex than the overhead line, the technical requirements are high, the manufacturing and construction is difficult, and the cable is buried underground, it is not easy to find the fault, and the maintenance and maintenance are more difficult. In terms of cost, the cost of underground cables of the same voltage level is generally 3 to 5 times higher than that of overhead lines.

In particular, our common high-voltage grade lines are often used for long-distance transmission. If underground cables are used, especially if long-distance transmission often passes through complex terrain, the cost and technical requirements will rise sharply.

On the other hand, it is also the congenital "hard injury" of the underground cable itself. The overhead line has good heat dissipation conditions in the air, while the air around the underground cable does not flow and is difficult to heat, which largely limits the level of power that the underground cable can transmit.

The important thing is that the ultra-high voltage power transmission can not find an effective insulating material to make the outer insulation layer of the wire, so the ultra-high voltage wire is exposed and can not be buried in the ground. There are distributed capacitors around the wire, and the current can be leaked out through these capacitors, which increases the consumption on the one hand; On the other hand, there is also a risk of electrocution if any animals come near.

Air is an insulator, earth is a conductor. In the air, as long as the wire is directly placed there, but in the ground, to add layers of insulation outside the wire, otherwise the electricity in the wire does not go far, the leakage will leak very little.

The underground cable is more complex than the overhead line, the technical requirements are high, the manufacturing and construction is difficult, and the cable is buried underground, it is not easy to find the fault, and the maintenance and maintenance are more difficult. In general, the cost of an underground cable with the same voltage level will be several times or even tens of times that of an aerial high-voltage line.

Ultra-high voltage cable buried underground, both safety problems, but also economic problems, if there is a failure, the inspection and repair of the cable project is very large, can not afford to toss, therefore, the current ultra-high voltage cable should be hung high in the air.

The insulation layer and protective layer of the underground cable conductor are very strict, and the human body will not have any danger in normal contact with the outer skin of the cable. The laying of the cable is also very particular, the cable is mostly buried in the special cable trench, cable tube or cable tunnel, by good safety isolation and protection, the depth is generally less than half a meter, the higher the voltage level of the cable buried deeper.

Moreover, every few tens of meters on the ground where the cable is buried, there will be a cable working well or cable marker pile as a sign to remind people to pay attention to safety. Therefore, underground cables generally do not pose a danger to residents.