Knowledge of Cable Laying in Electric Power Engineering

The operation quality, safety and reliability of the power supply system are not only related to the quality of the wire and cable itself, but also related to the construction quality of the cable accessories and lines.

1. Cable laying method

Cable laying methods have the following types: directly buried laying, pipe laying, shallow channel laying, cable trench laying, cable tunnel laying, overhead laying several ways have advantages and disadvantages, generally to consider the urban development planning, the density cable line length of existing buildings and the impact of the surrounding environment. From the technical comparison, the cable tunnel mode and the cable trench laying mode are convenient for the cable construction, maintenance and overhaul. In some cities in developed countries, public tunnels have been taken into account in urban planning and construction. Practice has proved that the operation effect of the public tunnel is good, which greatly reduces the number of repeated investment and the phenomenon of repeated road excavation. However, the initial investment is huge, and the building materials cost money. In China, due to the limitation of various factors, this laying method is very rare.In contrast, direct buried laying and shallow trough laying are economical laying methods. Direct buried cable is the most economical and widely laid way, which is used in the suburbs and areas where vehicle traffic is not very frequent. But it is not conducive to the maintenance and overhaul of the cable. Once the cable fault is encountered, even if using the tester to measure the fault point, it is necessary to dig the cable trench again, which is very inconvenient. Therefore, the choice of cable laying mode should be combined with the actual situation, according to the engineering conditions, environmental characteristics, cable model and quantity and other factors, with the perspective of development, in accordance with the requirements of operation reliability, convenience of maintenance and the principle of technical and economic rationality.

2. Cable type selection

Commonly used power cables are oil-immersed cable, PVC insulated cable, cross-linked polyethylene cable, etc., according to the use of different occasions, and extended for 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 products, in the cable selection, should be based on the different environment and conditions used, combined with the specific situation, to minimize the crossing of various pipe edge railway, highway and communication cable; such as directly buried and shallow channel laying mode, should consider the use of steel armored cable.

3. Selection of cable cross-sectional area

The choice of the cable cross-sectional area is related to the amount of investment, the line loss and voltage quality, and the service life of the cable. If the cross-sectional area is small, it will lead to the decrease of voltage quality and the line loss is too large, which will make the initial investment too high. Therefore, according to the load prediction results, the development planning, the appropriate cross-sectional area should be selected, so that the power cable can meet the cable core temperature requirements and voltage drop requirements under the maximum working current, and the thermal stability requirements under the action of the maximum short-circuit current. Due to the high difficulty and low accuracy of load forecasting, when selecting the cable cross-off area, it should also meet the requirements of the Technical Guidelines for Urban Power Network Planning Guidelines.

When the three-phase four-wire low-voltage grid chooses power cable, the choice of zero-line cross-sectional area should be considered. In the public low-voltage network, due to the great influence of user factors, the three-phase load balance is difficult to control. In order to improve the voltage quality and reduce the line loss, the zero-line cross-sectional area should be the same as the phase line.

4. Automation of cable network and cable network

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 and cable hybrid network). The Implementation and Supplementary Opinions on the Technical Guidelines for the Transformation of Urban Medium and Low-voltage Distribution Network also put forward specific requirements for the automation of cable distribution network. Therefore, when the cable network is adopted in the distribution network area, new technology and new equipment should be adopted according to the requirements of distribution automation. Where conditions permit, the automation pilot work should be considered. If the conditions are not mature, and sufficient room should be considered in the selection of supporting equipment, so as to lay a foundation for the realization of the automation scheme.

5. Problems that should be paid attention to in power cable construction

1) It is the eddy current problem caused by the high-current power cable

In the construction of power cable, there are steel support, steel protection pipe, cable card and overhead laying, all the formation of steel (iron) closed loop around the power cable, are likely to form eddy current, especially in the high current power cable system, eddy current is larger. During the construction of power cable, measures must be taken to make the steel (iron) closed loop cannot be formed around the cable to prevent the cable from causing eddy current phenomenon.

2) It is the mechanical damage problem caused by the turning of the power cable

Due to the large outer diameter of the power cable, it is difficult to transport and lay, and the requirements of the turning radius of the power cable are also relatively strict. Power cable in the construction, if the turning Angle is too large, may make the conductor internal mechanical damage, and mechanical damage due to the cable insulation strength decline, until the fault, construction found a cable head fault, in the cable head, three cable head length, and equipment connection due to the limitation of terrain, medium phase cable head long and become the arch, cable head root damaged discharge. Later, measures are taken to shorten the connection length of the medium-phase cable head appropriately, so that the three-phase cable head is not subject to external forces, and the practice has proved that the operation effect is good. It can be seen that in the process of cable construction, the torque of the cable should be reduced as far as possible. When the cable turns and leaves the cable, let the cable be in natural bending, and put an end to the internal mechanical damage phenomenon.

3) It is the power cable moisture-proof problem

Operation experience shows that most of the medium and low voltage power cable fault for cable intermediate connector and terminal fault, and the middle joint and terminal fault is mostly due to poor sealing, moisture intrusion caused insulation strength drop, and medium and low voltage power cable network using dendritic power supply, cable terminal number, so the good cable terminal and intermediate joint plugging seal is one of the important measures to ensure the safe and reliable operation of the cable.

4) The grounding problem of the medium and low voltage power cables

In the public medium and low voltage power cable network, because the three-phase load is not equal, if the cable with metal sheath is used, the grounding problem of the metal sheath must be considered, and the normal induction voltage at any point non-connection of the metal sheath shall not be greater than 100V. We believe that in the medium and low voltage cable network, the grounding poles (net) at all cable joints shall be set, and the metal sheath shall be reliably grounded.