How to lay power cables
1. Laying methods of cables There are the following types of laying methods for cables: Direct burial, laying through pipes, laying in shallow trenches, laying cable trenches, laying cables, laying cables, and laying overheads have several advantages and disadvantages. Generally speaking, urban development must be considered. Planning, the density of the existing building's cable lengths and the impact of its surrounding environment. From a technical comparison, the cable tunneling method and the laying of cable trenches are convenient for the construction, maintenance and overhaul of cables. In some developed country cities, public tunnels have been considered when planning and constructing cities. Practice has proved that public tunnels work well, greatly reducing the number of repeated investments and the phenomenon of repeatedly excavating roads. However, the initial investment is huge and construction materials consume funds. In China, due to limitations of various factors, this type of laying is extremely rare. . In contrast, direct laying and shallow trough laying are economical laying methods, and buried cables are the most economical and widely used electric laying methods. They are used in places where suburban and vehicle traffic are less frequent. But it is not conducive to the maintenance and overhaul of the cable. Once a cable fault is encountered, even if the tester is used to measure the fault, it is necessary to reopen the cable trench, which is inconvenient. Therefore, the choice of cable laying methods must be based on actual conditions and based on factors such as engineering conditions, environmental characteristics, cable types and quantities, and development perspectives, and should be determined in accordance with the requirements of meeting operational reliability, ease of maintenance, and technical and economical rationality.
2. Cable selection The commonly used power cables are oil-impregnated cables, PVC insulated cables, cross-linked polyethylene cables, etc., which are extended to different types of special cables depending on the occasion. At present, with the continuous improvement of production technology and production technology, cross-linked polyethylene cables have become the most widely used cable products. When selecting cables, they should be selected according to different conditions and conditions to minimize Through various pipe-edge railways, highways and communication cables; when using direct burial and shallow troughs, the use of steel cables should be considered.
3. Selection of cable cross-sectional area The choice of cable cross-sectional area relates to the number of investments, the loss and voltage quality of the line, and the service life of the cable. If the cross-sectional area is selected to be too small, the quality of the voltage will drop, and the loss of the line will be too large, which will make the initial investment too high. Therefore, according to the load forecasting results, the development plan should be selected and the appropriate cross-sectional area should be selected so that the power cable can meet the core temperature requirements and voltage drop requirements under the maximum operating current, and the thermal stability requirements under the action of the maximum short-circuit current. Because of the high difficulty and low accuracy of the load forecasting work, when selecting the cable cross-sectional area, it is also necessary to meet the requirements of the “Technical Guidelines for Urban Medium and Low Voltage Distribution Network Transformation” and “Guidelines for Urban Power Network Planning”.
When choosing a power cable for a three-phase four-wire low-voltage power grid, the selection of the zero-line cross-sectional area must also be considered. In a public low-voltage network, three-phase load balance is difficult to control due to the large influence of user factors, so as to improve the voltage quality and reduce Line loss, the cross-sectional area of the zero line should be the same as the cross-sectional area of the phase line.
4. Problems to be noticed in the construction of power cables (1) The eddy current problems caused by high-current power cables In the construction of power cables, steel brackets are used, steel protection pipes are used, and cable cards and overhead laying are used. Anything that forms a steel (iron) closed loop around a power cable can create eddy currents, especially in high-current power cable systems. In the construction of power cables, measures must be taken to prevent the formation of steel (iron) closed loops around the cables to prevent eddy currents from occurring.
(2) It is the mechanical damage caused by the turning of the power cable. Because the outer diameter of the power cable is relatively large, it is difficult to transport and lay the power cable. The power cable has strict requirements for the turning radius. When the power cable is under construction, if the turning angle is too large, the conductor may be mechanically damaged, and the mechanical damage is reduced due to the insulation strength of the cable until a fault occurs. A cable head failure is found during the construction. When the cable head is manufactured, three cables are used. The length of the head is the same. When the device is connected with the equipment, due to the topography limitation, the middle cable head becomes long and arched, and the root of the cable head is damaged and discharged. After taking measures, in the connection of the device, the length of the connection of the intermediate phase cable head is appropriately shortened so that the three-phase cable heads are not subjected to external forces. Practice has proved that the operation effect is good. It can be seen that during the construction of the cable, the torsion of the cable should be reduced as much as possible. When the cable is turned and the cable is left to stay, the cable is naturally bent to prevent internal mechanical damage.
(3) The operation experience of the moisture-proof cable of the power cable shows that most of the faults of the medium- and low-voltage power cables are faults in the middle and end of the cable, while the faults in the middle connector and the terminal are mostly caused by poor sealing and intrusion of moisture. The intensity decreases, while the medium- and low-voltage power cable networks mostly use dendritic power supply methods, and the number of cable terminal heads is large. Therefore, shutting off the good cable terminal header and the middle connector plugging and sealing is one of the important measures to ensure the safe and reliable operation of the cable.
(4) It is the grounding problem of medium- and low-voltage power cables on public and low-voltage power cable networks. Because the three-phase loads are not equal, therefore, if a cable with a metal sheath is used, the grounding of the metal sheath must be considered and Ensure that the normal induced voltage at any point of the metal sheath that is not grounded is not greater than 100V. We believe that in the medium and low voltage cable networks, grounding poles (nets) should be provided at all cable joints and the metal sheathing should be reliably grounded.
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