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Knowledge of cable laying in power engineering
Feb 06, 2018

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

1. Cable Laying mode

Cable laying methods are as follows: directly buried laying, pipe laying, shallow groove laying, cable trench laying, cable tunnel laying, overhead laying several ways have advantages and disadvantages, generally consider urban development planning, the density of existing buildings cable line length of the laying of the number of lines and the impact of the surrounding environment. From the technical comparison, the cable tunnel way and the cable trench laying way are convenient for the cable construction, maintenance and overhaul. In some developed countries, urban planning and construction, public tunnels have been considered. Practice has proved that the utility tunnel has a good operation effect, which greatly reduces the repeated investment and repeated excavation of the road surface, but the initial investment is huge, the construction material consumes capital, in the domestic, because of various factors limitation, this kind of laying way is very few. In contrast, direct-buried and shallow-groove laying is an economical way of laying, direct buried cable is the most economical and widely used in the installation of electricity, it is used in suburbs and vehicles not too frequent places. But not conducive to cable maintenance and repair, once the cable failure, even if the use of testing equipment to detect the point of failure, but also to dig open the cable ditch, very inconvenient. Therefore, the choice of cable laying mode, according to the actual situation, in accordance with engineering conditions, environmental characteristics, cable models and quantity and other factors, with the development of the vision, in accordance with the requirements of operational reliability, ease of maintenance and technical and economic reasonable principles to determine.

2. Cable selection

Commonly used power cables have oil-immersed cables, PVC insulated cables, XLPE cables, according to the use of different occasions, and extended to different types of special cable. At present, with the continuous improvement of production technology and production technology, XLPE cable has become the most widely used cable products, in the selection of cable, should be based on the use of different circumstances and conditions, combined with specific circumstances to choose, as far as possible to reduce the crossing of various pipe-side railways, highways and communication cables; Consider using Gagang cables.

3. Selection of sectional area of cable

The selection of sectional area of cable relates to the amount of investment, the loss of line and the voltage quality, and the service life of cable. If the selection section area is small, will cause the voltage quality to drop, the line loss is too big, will make the initial investment too high. Therefore, according to the load forecast result, the development plan, chooses the suitable sectional area, causes the power cable to satisfy the core temperature request and the voltage drop request under the maximum working current, the maximum short-circuit current action thermal stability request. Because of the difficulty and accuracy of load forecasting, it is necessary to meet the requirements of "urban medium and low voltage Distribution network Transformation Technology Guide" and "Urban power grid Planning Guide" When selecting cable section area.

When selecting power cable in three-phase four-wire system, also consider the choice of 0 line section area, in the public Low-voltage network, due to the impact of user factors, three-phase load balance difficult to control, in order to improve voltage quality, reduce line loss, 0 line section area should be the same as the Phase Line section area.

4. 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 networks and aerial networks (including aerial and cable mixed networks). The implementation and supplementary opinions on the technical guidelines for the transformation of urban medium and low voltage distribution networks also provide specific requirements for the automation of cable distribution network. Therefore, in the distribution network in the use of cable networks, should be in accordance with the requirements of distribution automation, the use of new technologies, China's wire and cable network equipment, conditional to consider the automation of the pilot work, conditions are immature also in the matching equipment selection, consider there is ample room, To lay the foundation for the realization of automation scheme.

5. Power cable construction should pay attention to the problems

1), is a large current power cable caused by Eddy current problems

Power cable in the construction, there is the use of steel stent, the use of steel protection pipe, with cable cards and overhead laying, usually in the power cable formed around the steel (iron) closed circuit, it is possible to form Eddy current, especially in the large current power cable system, eddy current is larger. In the construction of power cable, it is necessary to take measures to make the steel (iron) closed loop around the cable to prevent the eddy current caused by the cable.

2) is the mechanical damage caused by the turn of power cable.

Because of the large diameter of power cable, the difficulty of transportation and laying, the requirement of turning radius of power cable is also strict. Power cable in the construction, if the cornering angle is too large, may cause the conductor internal mechanical damage, and mechanical damage due to cable insulation strength down, until the failure, the construction of a cable head found fault, in the cablehead when making, three cable head length consistent, and equipment connectiondue to terrain constraints, The middle phase cable head is long and arched, and the cable head is damaged and discharged. After taking measures, in the connection of the equipment, the length of the cable head should be shortened, so that the three-phase cable head is not affected by external forces, practice shows that the operation effect is good. Thus, the cable construction process, to minimize the torque of the cable, in the cable turn and leave the cable, so that the cable in natural bending, to eliminate the phenomenon of internal mechanical damage.

3), is the power cable moistureproof problem

Operational experience shows that middle and low voltage power cable fault Mostof the cable middle connector and terminal Head fault, while the middle connectorand terminal Head fault is mostly due to poor sealing, moisture intrusion caused by the loss of insulation strength, and medium and low voltage power cable network using dendritic power supply, cable terminal number of more, Therefore, it is one of the important measures to ensure the safe and reliable operation of the cable terminal and the middle joint.

4), is the middle and low voltage power cable grounding problems

In the common medium and Low-voltage power cable network, because the three-phase load is not equal, therefore, if the use of metal protective layer of the cable, must consider the metal layer grounding problem, and to ensure that any point in the metal layer of the normal induction voltage is not greater than 100V. We believe that in the middle and low voltage cable network, all cable joints should be set up grounding poles (nets), and the metal layer of reliable grounding.

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