First, the laying method of the cable
There are several ways to lay cables : direct burial, pipe laying, shallow trench laying, cable trench laying, cable tunnel laying, overhead laying, and other advantages and disadvantages. Generally, urban development planning, existing buildings should be considered. The number of the length of the cable line length and the influence of its surrounding environment. Technically, cable tunneling and cable trenching are convenient for cable construction, maintenance and overhaul. In some developed country cities, public tunnels have been considered in urban planning and construction. Practice has proved that the public tunnel works well, greatly reducing the number of repeated investment and repeated excavation of the road surface. However, the initial investment is huge, and the construction materials consume funds. In China, due to various factors, this laying method is rare. . In contrast, direct-buried laying and shallow-slot laying are economical laying methods. Direct-buried cables are the most economical and widely used electrical laying method, and are used in suburbs and places where vehicles pass less frequently. However, it is not conducive to the maintenance and overhaul of the cable. Once the cable fault is encountered, even if the fault point is measured by the tester, it is extremely inconvenient to re-open the cable trench. Therefore, the choice of cable laying method should be determined according to the actual conditions, according to engineering conditions, environmental characteristics, cable type and quantity, etc., with the development vision, in accordance with the requirements of satisfying operational reliability, easy maintenance and technical and economic rationality.
Second, the cable selection
Commonly used power cables include oil-immersed cables, PVC insulated cables, and cross-linked polyethylene cables, which are extended to different types of special cables depending on the application. At present, with the continuous improvement of production technology and production technology, XLPE cable has become the most widely used cable product. When selecting the cable, it should be selected according to the different environments and conditions used, and the specific situation should be minimized. Crossing various pipe-side railways, roads and communication cables; if direct and shallow trenching is used, cables with steel bars should be considered.
Third, the choice of cable cross-sectional area
The choice of cable cross-sectional area is related to the investment, the loss of the line and the voltage quality, and the service life of the cable. If the cross-sectional area is too small, the voltage quality will drop and the line loss 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 to select the appropriate cross-sectional area so that the power cable meets the core temperature requirements and voltage drop requirements under the maximum operating current, and the thermal stability requirements under the maximum short-circuit current. Because the load forecasting work is difficult and the accuracy is low, when selecting the cable cross-sectional area, it must meet the requirements of the “Technical Guidelines for Urban Medium and Low Voltage Distribution Network Reconstruction†and “Urban Power Network Planning Guidelinesâ€.
When selecting a power cable for a three-phase four-wire low-voltage power grid, the choice of the zero-line cross-sectional area should also be considered. In the public low-voltage network, due to the influence of user factors, the three-phase load balance is difficult to control, in order 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.
Fourth, on cable network and cable network automation
With the continuous promotion and use of power cables in the distribution network, the distribution network can be divided into cable networks and overhead networks (including overhead, cable hybrid networks). The "Implementation and Supplementary Opinions on the Technical Guidelines for the Transformation of Urban Medium and Low Voltage Distribution Networks" also put forward specific requirements for the automation of cable distribution networks. Therefore, when the cable network is used in the regional network of the distribution network, new technologies and new equipment should be adopted according to the requirements of distribution automation. If the conditions are not mature, the supporting equipment should be selected. Considering that there is sufficient room to lay the foundation for an automation solution.
V. Problems that should be paid attention to during the construction of power cables
1, is the eddy current problem caused by high current power cable
In the construction of power cables, steel brackets are used, steel protection tubes are used, and cable cards and overhead laying are used. Any steel (iron) closed loop formed around the power cables may form eddy currents. Especially in high current power cable systems, eddy currents are larger. 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 in the cables.
2. It is a mechanical damage caused by the turning of the power cable.
Due to the large outer diameter of the power cable, transportation and laying are difficult, and the power cable has strict requirements on the turning radius. During the construction of the power cable, if the turning angle is too large, the inside of the conductor may be mechanically damaged, and the mechanical damage may be reduced due to the insulation strength of the cable until a failure occurs. A cable head failure is found during construction. When the cable head is manufactured, three cables are produced. The length of the head is the same. When connected to the equipment, due to the limitation of the terrain, the middle phase cable head is long and becomes arched, and the root of the cable head is damaged and discharged. After taking measures, the connection length of the medium phase cable head is appropriately shortened in the connection of the equipment, so that the three-phase cable head is free from external force, and the operation effect is proved to be good. It can be seen that during the cable construction process, the torque received by the cable should be reduced as much as possible. When the cable turns and the cable is left, the cable is naturally bent to prevent internal mechanical damage.
3, is the power cable moisture-proof problem
Operating experience shows that most of the low-voltage power cable faults are cable intermediate joints and terminal head faults, while the middle joints and terminal head faults are mostly due to poor sealing, moisture intrusion and insulation strength degradation, and medium and low voltage power cables. The network adopts the dendritic power supply mode, and the number of cable terminals is large. Therefore, sealing the cable terminal and the intermediate joint 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.
In the common medium and low voltage power cable network, since the three-phase load is not equal, if a cable with a metal sheath is used, the grounding problem of the metal sheath must be considered, and the grounding at any point of the metal sheath must be ensured. The normal induced voltage must not be greater than 100V. I believe that in the medium and low voltage cable network, the grounding pole (net) should be placed at all cable joints, and the metal sheath should be reliably grounded.
There are several ways to lay cables : direct burial, pipe laying, shallow trench laying, cable trench laying, cable tunnel laying, overhead laying, and other advantages and disadvantages. Generally, urban development planning, existing buildings should be considered. The number of the length of the cable line length and the influence of its surrounding environment. Technically, cable tunneling and cable trenching are convenient for cable construction, maintenance and overhaul. In some developed country cities, public tunnels have been considered in urban planning and construction. Practice has proved that the public tunnel works well, greatly reducing the number of repeated investment and repeated excavation of the road surface. However, the initial investment is huge, and the construction materials consume funds. In China, due to various factors, this laying method is rare. . In contrast, direct-buried laying and shallow-slot laying are economical laying methods. Direct-buried cables are the most economical and widely used electrical laying method, and are used in suburbs and places where vehicles pass less frequently. However, it is not conducive to the maintenance and overhaul of the cable. Once the cable fault is encountered, even if the fault point is measured by the tester, it is extremely inconvenient to re-open the cable trench. Therefore, the choice of cable laying method should be determined according to the actual conditions, according to engineering conditions, environmental characteristics, cable type and quantity, etc., with the development vision, in accordance with the requirements of satisfying operational reliability, easy maintenance and technical and economic rationality.
Second, the cable selection
Commonly used power cables include oil-immersed cables, PVC insulated cables, and cross-linked polyethylene cables, which are extended to different types of special cables depending on the application. At present, with the continuous improvement of production technology and production technology, XLPE cable has become the most widely used cable product. When selecting the cable, it should be selected according to the different environments and conditions used, and the specific situation should be minimized. Crossing various pipe-side railways, roads and communication cables; if direct and shallow trenching is used, cables with steel bars should be considered.
Third, the choice of cable cross-sectional area
The choice of cable cross-sectional area is related to the investment, the loss of the line and the voltage quality, and the service life of the cable. If the cross-sectional area is too small, the voltage quality will drop and the line loss 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 to select the appropriate cross-sectional area so that the power cable meets the core temperature requirements and voltage drop requirements under the maximum operating current, and the thermal stability requirements under the maximum short-circuit current. Because the load forecasting work is difficult and the accuracy is low, when selecting the cable cross-sectional area, it must meet the requirements of the “Technical Guidelines for Urban Medium and Low Voltage Distribution Network Reconstruction†and “Urban Power Network Planning Guidelinesâ€.
When selecting a power cable for a three-phase four-wire low-voltage power grid, the choice of the zero-line cross-sectional area should also be considered. In the public low-voltage network, due to the influence of user factors, the three-phase load balance is difficult to control, in order 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.
Fourth, on cable network and cable network automation
With the continuous promotion and use of power cables in the distribution network, the distribution network can be divided into cable networks and overhead networks (including overhead, cable hybrid networks). The "Implementation and Supplementary Opinions on the Technical Guidelines for the Transformation of Urban Medium and Low Voltage Distribution Networks" also put forward specific requirements for the automation of cable distribution networks. Therefore, when the cable network is used in the regional network of the distribution network, new technologies and new equipment should be adopted according to the requirements of distribution automation. If the conditions are not mature, the supporting equipment should be selected. Considering that there is sufficient room to lay the foundation for an automation solution.
V. Problems that should be paid attention to during the construction of power cables
1, is the eddy current problem caused by high current power cable
In the construction of power cables, steel brackets are used, steel protection tubes are used, and cable cards and overhead laying are used. Any steel (iron) closed loop formed around the power cables may form eddy currents. Especially in high current power cable systems, eddy currents are larger. 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 in the cables.
2. It is a mechanical damage caused by the turning of the power cable.
Due to the large outer diameter of the power cable, transportation and laying are difficult, and the power cable has strict requirements on the turning radius. During the construction of the power cable, if the turning angle is too large, the inside of the conductor may be mechanically damaged, and the mechanical damage may be reduced due to the insulation strength of the cable until a failure occurs. A cable head failure is found during construction. When the cable head is manufactured, three cables are produced. The length of the head is the same. When connected to the equipment, due to the limitation of the terrain, the middle phase cable head is long and becomes arched, and the root of the cable head is damaged and discharged. After taking measures, the connection length of the medium phase cable head is appropriately shortened in the connection of the equipment, so that the three-phase cable head is free from external force, and the operation effect is proved to be good. It can be seen that during the cable construction process, the torque received by the cable should be reduced as much as possible. When the cable turns and the cable is left, the cable is naturally bent to prevent internal mechanical damage.
3, is the power cable moisture-proof problem
Operating experience shows that most of the low-voltage power cable faults are cable intermediate joints and terminal head faults, while the middle joints and terminal head faults are mostly due to poor sealing, moisture intrusion and insulation strength degradation, and medium and low voltage power cables. The network adopts the dendritic power supply mode, and the number of cable terminals is large. Therefore, sealing the cable terminal and the intermediate joint 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.
In the common medium and low voltage power cable network, since the three-phase load is not equal, if a cable with a metal sheath is used, the grounding problem of the metal sheath must be considered, and the grounding at any point of the metal sheath must be ensured. The normal induced voltage must not be greater than 100V. I believe that in the medium and low voltage cable network, the grounding pole (net) should be placed at all cable joints, and the metal sheath should be reliably grounded.
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