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Knowledge of cable laying in power engineering
- May 18, 2017 -

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

1. Cable Laying mode

The laying of cables is the following: direct buried laying, pipe laying, shallow slot laying, cable trench laying, cable tunnel laying, overhead laying several methods have advantages and disadvantages, generally consider urban development planning, the existing building density cable line length laying the number of bars and the impact of the surrounding environment. From the technical comparison, the cable tunnel mode and cable trench laying mode are convenient for the construction, maintenance and overhaul of cables. Public tunnels have been considered in urban planning and construction in some developed countries. The practice proves that the public tunnel works well, greatly reduces the repeated investment times and the repeated excavation of the pavement phenomenon, but the initial investment is enormous, the construction material consumption funds, in the domestic, due to various factors of restriction, the laying method is very few. In comparison, direct-buried and shallow-groove laying is an economical way of laying, direct buried cables are the most economical and widespread use of electricity-laying methods, it is used in suburban and vehicle traffic is not too frequent places. But it is not conducive to the maintenance and repair of cables, once encountered in a cable failure, even if the use of tester to detect the point of failure, also need to reopen the cable trench, very inconvenient. Therefore, the choice of cable laying mode, to the actual situation, according to the engineering conditions, environmental characteristics, cable model and quantity and other factors, with the development of the vision, according to meet the operational reliability, ease of maintenance requirements and technical and economic reasonable principles.

2. Cable selection

Commonly used power cables are oil-immersed cables, PVC insulated cables, XLPE cables, etc., according to the use of different occasions, and extend to different types of special cables. At present, with the production technology and the continuous improvement of production technology, cross-linked polyethylene cable has become the most widely used cable products, in the cable selection, should be based on the use of different circumstances and conditions, combined with the specific circumstances to choose to minimize cross-border rail, highway and communication cables, such as the use of direct buried and shallow groove laying, should consider using Gagang cables.

3. Selection of cable section area

The choice of cable section area relates to how much investment, the loss of line and the quality of voltage, the service life of cables, etc. If the selection of small area, will lead to a decline in voltage quality, the line loss too large, will make the initial investment too high. Therefore, according to the load forecasting results, development planning, select the appropriate cross-section area, so that the power cable to meet the maximum operating current under the core temperature requirements and voltage drop requirements, the maximum short-circuit current under the effect of thermal stability requirements. Because the workload forecasting is difficult and accurate, it is also necessary to satisfy the requirements of the Technology Guide for the transformation of urban low-voltage distribution network and the guide of Urban Power network planning when selecting the cable section.
When the power cable is selected in three-phase four-line low voltage grid, also consider the choice of 0 line section area, in the public low-voltage network, because of the user factors, the three-phase load balance is difficult to control, to improve the voltage quality, reduce line loss, 0-line cross-sectional area should be the same as the Phase line section.

4. About cable network and cable network automation

With the continuous popularization and use of power cable in distribution network, distribution network can be divided into cable network and overhead network (including overhead, cable mixing network). The implementation of the technical guide for the renovation of the urban low-voltage distribution network and the supplementary opinion also put forward specific requirements for the automation of cable distribution networks. Therefore, in the distribution network in the area of the use of cable network, should be in accordance with the requirements of distribution automation, the use of new technologies, equipment, the conditional need to consider automated pilot work, immature conditions also in the selection of equipment, consider there is ample room for the realization of automation program lay the foundation.

5. Problems needing attention in power cable construction

1, is the eddy current problem caused by the large electric power cable

Power cables in construction, there is a steel bracket, the use of steel protection tubes, the use of cable cards and overhead laying, all the power cable around the formation of steel (iron) closed circuit, there is a potential to form Eddy current, especially in the large current power cable system, the vortex is larger. In the construction of power cables, measures must be taken to make the cable not formed around the steel (iron) closed loop, to prevent the cable caused Eddy current phenomena.

2. Mechanical damage caused by turning of power cables

Because of the larger diameter of power cable, the transportation and laying of the power cables are difficult, the requirements of the turning radius are more stringent. In the construction of power cables, if the angle is too large, the conductor may be subjected to mechanical damage, and mechanical damage due to the cable insulation strength decreased, until the failure, the construction found a cable head failure, in the production of cable head, three cable head length of the same, and equipment connected with the terrain limited by the length of the cable head and become arched, the cable head of the root damage discharge. After taking measures, in the connection of equipment, appropriately shorten the connection length of the medium-phase cable head, so that the three-phase cable head are not subjected to external forces, the practice proves that the operation is good. Thus, the cable construction process, to minimize the torque of cables, cable cornering and margin cables, so that the cable in natural bending, to eliminate the phenomenon of internal machinery damage.

3, is the power cable moistureproof problem
The operation experience shows that the fault of middle and low voltage power cable is most of the cable joints and terminal head faults, and the intermediate joints and terminal head faults are mostly due to poor sealing. The tidal intrusion caused the insulation strength decreased, while the medium and low voltage power cable network adopts the branch-like power supply mode, the number of cable terminals are more, so the good cable terminal head and intermediate joints plugging seal is one of the important measures to ensure the safe and reliable operation of cables.

4, is the middle and low voltage power cable grounding problem
In the public, low-voltage power cable, because the three-phase load is not equal, so if the use of metal retaining cables, must consider the metal retaining layer grounding problem, and to ensure that the metal retaining layer at any point of non-grounding the normal induction voltage must not be greater than 100V. We believe that in the medium and low voltage cable network, all cable joints should be set ground pole (nets), and the metal retaining layer reliably grounding.