Why Can't High-Voltage Cables Be Buried Underground?
Ultra-high voltage transmission refers to the use of 500 kV-1000 kV voltage levels to transmit electricity. If the 220kV transmission index is 100%, the relative investment per kilometer of UHV transmission, the relative cost per 100 kilometers per kilowatt-hour transmission, and the consumption of metal materials will be significantly reduced, and the utilization rate of the line corridor will be significantly improved. In our daily life, we can often see overhead ultra-high voltage transmission projects, have we thought about such a problem? Why can't they be buried underground like urban underground cables and high-voltage power lines?
Current underground cables are generally of lower voltage levels. The transmission of high voltage lines is usually overhead, mainly due to cost and technical factors.
Underground cables are structurally more complex than overhead lines and have higher technical requirements. It is difficult to manufacture and construct, and the cables are buried underground, so it is difficult to find faults, and it is also difficult to repair and maintain. In terms of cost, the cost of underground cables of the same voltage level is generally 3 to 5 times that of overhead lines.
In particular, our commonly used high-voltage level lines are often used for long-distance transmission. Costs and technical requirements will skyrocket even more if underground cables are used, especially for long-distance transmissions that often traverse complex terrain.
On the other hand, there are also inherently "hard" underground cables themselves. Overhead lines are in good heat dissipation conditions in the air, while the air around underground cables does not flow, making it difficult to dissipate heat, which largely limits the level of power that underground cables can transmit. Importantly, EHV power transmission has not been able to find effective insulating materials for the outer sheath insulation of wires. Therefore, EHV wires are exposed and cannot be buried underground. There are distributed capacitances around the wires, and current can leak through these capacitances, increasing consumption on the one hand. On the other hand, there is also a risk of electric shock if there are animals nearby.
Air is an insulator and earth is a conductor. In the air, as long as the wire is direct, you can have it, but underground, you need to add a layer of insulation outside the wire. Otherwise, the electricity in the wire doesn't go very far, and the leakage will leak the rest. Compared with overhead lines, underground cables have more complex structure, higher technical requirements, and more difficult manufacturing and construction. In addition, the cables are buried underground, so it is difficult to find faults, and maintenance and maintenance are also more difficult. Generally speaking, the cost of underground cables of the same voltage level will be several times or even dozens of times that of overhead high-voltage lines.
Buried EHV cables have both safety and economic concerns. If a fault occurs, the inspection and maintenance of the cable is a very large project and cannot be tossed. Therefore, the current UHV cables should be suspended high in the air.
The outer insulation and protective layer of underground cable conductors are very strict. There is no risk of normal human body contact with the outer sheath of the cable. The laying of cables is also very careful, most of the cables are buried in special cable trenches, cable trenches or cable tunnels, which are well isolated and protected. The depth is generally less than half a meter. The higher the voltage level, the deeper the cable is buried. In addition, every tens of meters on the ground where the cables are buried, there will be a cable project or a cable marker pile as a marker to remind people to pay attention to safety. Therefore, underground cables are generally not dangerous to residents.