The operation quality, safety and reliability of power supply system are not only related to the quality of wires and cables, but also to the construction quality of cable accessories and lines.
1. Cable laying method
The cable laying methods are as follows: directly buried laying, pipe laying, shallow trench laying, cable trench laying, cable tunnel laying and overhead laying, which have advantages and disadvantages. Generally, urban development planning, density of existing buildings, length of cable lines, and influence of surrounding environment should be considered. From the technical comparison, the cable tunnel mode and cable trench laying mode are convenient for cable construction, maintenance and repair. In some developed countries, public tunnels have been considered in urban planning and construction. Practice has proved that the operation effect of public tunnel is good, which greatly reduces the number of repeated investment and repeated excavation of pavement. However, the initial investment is huge and the construction materials consume funds. In China, due to various factors, this kind of laying method is very few. In contrast, direct buried cable laying and shallow trench laying are economic laying methods. Direct buried cable is the most economical and widely used electric laying method. It is used in suburban areas and places with less frequent vehicle traffic. However, it is not conducive to the maintenance and repair of the cable. Once the cable fault occurs, even if the fault point is detected by the tester, the cable trench should be excavated again, which is very inconvenient. Therefore, the selection of cable laying mode should be determined according to the actual situation, engineering conditions, environmental characteristics, cable model and quantity, etc., with the development perspective, in accordance with the requirements of operation reliability, easy maintenance and reasonable technology and economy.
2. Cable selection
The commonly used power cables include oil immersed cables, PVC insulated cables, XLPE cables, etc. according to different applications, they are extended to different types of special cables. At present, with the continuous improvement of production technology and production process, XLPE cable has become the most widely used cable product. In cable selection, the cable should be selected according to the different environment and conditions, combined with the specific situation, so as to minimize crossing all kinds of pipeline side railway, highway and communication cables; if the direct buried and shallow trench laying methods are adopted, the steel armored cable should be considered.
3. Selection of cable section area
The selection of cable cross-section area is related to investment, line loss, voltage quality, cable service life, etc. If the cross-sectional area is too small, the voltage quality will be reduced, the line loss will be too large, and the initial investment will be too high. Therefore, according to the results of load forecasting, development planning and appropriate cross-sectional area should be selected to make the power cable meet the requirements of core temperature and voltage drop under the maximum working current and the thermal stability requirements under the maximum short-circuit current. Due to the high difficulty and low accuracy of load forecasting, it is necessary to meet the requirements of “technical guidelines for urban medium and low voltage distribution network transformation” and “guidelines for urban power network planning” when selecting cable cross-section area.
When selecting power cables in three-phase four wire low-voltage power grid, the selection of zero line cross-section area should be considered. In public low-voltage network, due to the influence of user factors, it is difficult to control the three-phase load balance. In order to improve the voltage quality and reduce the line loss, the cross-sectional area of the zero line should be the same as that of the phase line.
4. About cable network and cable network automation
With the continuous promotion and use of power cable in distribution network, distribution network can be divided into cable network and overhead network (including overhead and cable hybrid network). The implementation and supplementary opinions on technical guidelines for urban medium and low voltage distribution network transformation also put forward specific requirements for cable distribution network automation. Therefore, when cable network is adopted in distribution network area network, new technology and new equipment shall be adopted according to the requirements of distribution automation. If conditions permit, automation pilot work shall be considered; if conditions are not mature, sufficient space shall be considered in the selection of supporting equipment, so as to lay a foundation for realizing automation scheme.
5. Problems needing attention in power cable construction
- Is the eddy current problem caused by high current power cable
In the construction of power cables, some use steel supports, some use steel protective tubes, some use cable clamps and overhead laying. All steel (iron) closed circuits around power cables may form eddy current, especially in high current power cable systems. In the construction of power cable, measures must be taken to prevent the formation of steel (iron) closed loop around the cable to prevent the occurrence of eddy current caused by the cable.
- Is the mechanical damage caused by the turning of power cable
Due to the large outer diameter of power cables, it is difficult to transport and lay them, and the requirements for turning radius of power cables are also quite strict. In the construction of power cable, if the turning angle is too large, the conductor may be damaged mechanically. The mechanical damage is caused by the decrease of the insulation strength of the cable until failure occurs. A cable head fault is found in the construction. When the cable head is made, the length of the three cable heads is the same. When connecting with the equipment, due to the terrain restriction, the middle phase cable head is too long and becomes an arch, and the root of the cable head is damaged and discharged. After that, measures were taken to shorten the connection length of the middle phase cable head in the connection of the equipment, so that the three-phase cable head was free from external force. The practice proved that the operation effect was good. It can be seen that in the process of cable construction, the torsion of the cable should be reduced as much as possible. When the cable turns and the cable is reserved, the cable should be in natural bending to prevent internal mechanical damage.
- It is the moisture-proof problem of power cable
Operation experience shows that most of the faults of medium and low voltage power cables are the faults of intermediate joints and terminal heads of cables, while the faults of intermediate joints and terminal heads are mostly caused by poor sealing and moisture intrusion, and the medium and low voltage power cable networks mostly adopt tree shaped power supply mode, and the number of cable terminals is large, Therefore, it is one of the important measures to ensure the safe and reliable operation of the cable that the cable terminal head and intermediate joint are sealed.
- It is the grounding problem of medium and low voltage power cables
In the public medium and low voltage power cable network, because the three-phase load is not equal, therefore, if the cable with metal sheath is used, the grounding problem of metal sheath must be considered, and the normal induced voltage at any point of the metal sheath that is not grounded shall not be greater than 100V. We believe that in the medium and low voltage cable network, all cable joints should be set with grounding electrode (net), and the metal sheath should be reliably grounded.