Issues to be noted during the construction of power cables
1. Cable laying method
Is there a direct burial method for cable laying? Laying through pipes? Shallow trench laying? Cable trench laying? Cable tunnel laying? How many methods are there, such as overhead laying? Technically speaking, the cable tunnel method and cable trench laying method are preferred because they are convenient for cable construction? Maintenance and overhaul? In some developed cities, public tunnels have been considered during urban planning and construction?
Practice has proven that the operation of public tunnels is effective, greatly reducing the number of repeated investments and the phenomenon of repeated excavation of road surfaces. However, the initial investment is huge? Due to various factors, this laying method is extremely rare in China? Compared to other methods, direct burial and shallow trench laying are economical laying methods, but they are not conducive to cable maintenance and repair. Once a cable failure occurs, even if the fault point is detected by a tester, the cable trench must be dug again, which is extremely inconvenient? Therefore, the choice of cable laying method should be based on the actual situation and engineering conditions? Environmental characteristics? From a developmental perspective, considering factors such as cable model and quantity, in order to meet operational reliability requirements? Are the requirements for easy maintenance and the principles of technical and economic rationality determined?
2. Cable selection
What are the commonly used power cables, oil immersed cables? Polyvinyl chloride insulated cable? Cross linked polyethylene cables, etc., can be extended into different types of special cables according to different usage scenarios? At present, with the continuous improvement of production technology and production processes, cross-linked polyethylene cables have become a commonly used cable product. When selecting cables, the selection should be based on different environments and conditions of use, combined with specific situations. For example, when using direct burial and shallow trench laying methods, steel armored cables should be considered?
3. Selection of cable cross-sectional area
The choice of cable cross-sectional area is related to the amount of investment? What are the losses and voltage quality of the circuit? What is the service life of the cable? Will choosing a smaller cross-sectional area lead to a decrease in voltage quality? Excessive line loss, and in severe cases, even cable overheating and burnout; Will a large cross-sectional area result in excessive initial investment? Therefore, based on the load forecasting results, with a developmental perspective, the appropriate cross-sectional area should be selected to ensure that the power cable meets the requirements of cable core temperature and voltage drop under working current, as well as the thermal stability requirements under short-circuit current? Due to the high difficulty of load forecasting work? Due to low accuracy, when selecting cable cross-sectional area, it is necessary to meet the requirements of the "Technical Guidelines for Urban Medium and Low Voltage Distribution Network Transformation" and the "Guidelines for Urban Power Network Planning"?
When selecting power cables in a three-phase four wire low-voltage power grid, the selection of the cross-sectional area of the neutral line should also be considered. In a public low-voltage network, due to the significant influence of user factors, it is difficult to control the balance of three-phase loads. In order to improve voltage quality and reduce line loss, the cross-sectional area of the neutral line should be the same as that of the phase line?
4. Regarding Cable Networks 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 network and overhead network (including overhead cable hybrid network)? The Implementation and Supplementary Opinions on the Technical Guidelines for the Renovation of Urban Medium and Low Voltage Distribution Networks also put forward specific requirements for the automation of cable distribution networks? Therefore, when using cable networks in distribution network areas, new technologies should be adopted in accordance with the requirements of distribution automation? For new equipment, if conditions permit, automation pilot work should be considered. For equipment selection that is not yet mature, sufficient room should be considered to lay the foundation for implementing automation solutions?
2、 Issues to be noted in the construction of power cables
1. Eddy current problem caused by high current power cables
During the construction of power cables, some use steel supports, some use steel protective tubes, and some use cable clamps and overhead laying. Any steel (iron) closed circuit formed around the power cable may generate eddy currents, especially in high current power cable systems where eddy currents are even greater?
There used to be a section of approximately 0.4km 10kV overhead cable in a certain place, which used steel stranded wire as the overhead support and cable clamps for postal and telecommunications to fix the cable. Shortly after being put into operation, a grounding fault occurred. After inspection, it was found that the cable clamp and steel stranded wire formed a closed eddy current circuit, which burned out the insulation layer of the cable after heating up, causing the grounding fault? After analysis and testing, it was found that after isolation with an insulation layer (such as stripped cable insulation) at the junction of the cable clamp and the steel strand, there was no longer any eddy current phenomenon. It has been running normally for many years without similar faults? From this, it can be seen that measures must be taken during the construction of power cables to prevent the formation of a steel (iron) closed loop around the cable and prevent the occurrence of eddy currents caused by the cable?
2. Mechanical damage caused by turning of power cables
Due to the large outer diameter of power cables, transportation? Is it difficult to lay and has strict requirements for the turning radius of power cables? During the construction of power cables, if the turning angle is too large, it may cause mechanical damage to the inside of the conductor, which cannot be seen due to being covered by the cable insulation layer. Even if the circuit resistance is measured, insulation and leakage tests are difficult to detect defects. During operation, overheating at the damaged area will reduce the insulation strength of the cable until a fault occurs? The author has found that the reason for multiple cable head failures is that during the production of cable heads, the lengths of the three cable heads were the same. However, due to terrain limitations, the middle phase cable head was too long and became arched when connected to the equipment, causing damage and discharge at the root of the cable head? Afterwards, measures were taken to shorten the length of the middle phase cable head connection appropriately according to the connection of different devices, so that the three-phase cable heads are not subjected to external forces. Practice has proven that the operation effect is good? From this, it can be seen that during the cable construction process, it is necessary to minimize the torque on the cable as much as possible. When turning and leaving room for the cable, the cable should be naturally bent to prevent internal mechanical damage?
3. Moisture prevention issues of power cables
Operating experience shows that? The majority of low-voltage power cable failures are caused by faults in the middle joints and terminals of the cable, while the faults in the middle joints and terminals are mostly due to poor sealing and moisture intrusion, resulting in a decrease in insulation level? Low voltage power cable networks often use dendritic power supply methods, with a large number of cable terminals. Therefore, ensuring the sealing and sealing of cable terminals and intermediate joints is one of the important measures to ensure the safe and reliable operation of cables?
4. Grounding problem of medium and low voltage power cables
In public medium and low voltage power cable networks, due to the unequal three-phase loads, if cables with metal protective layers are used, the grounding problem of the metal protective layer must be considered, and the normal induced voltage at any point of the metal protective layer that is not grounded must not exceed 100V. In my opinion, in medium and low voltage power cables, the grounding problem of the metal protective layer must be considered, and the normal induced voltage at any point of the metal protective layer that is not grounded must not exceed 100V? In low-voltage cable networks, grounding electrodes (grids) should be installed at all cable joints, and the metal protective layer should be reliably grounded?