Laying of cables are the following: buried in shallow groove, pipe laying, laying, cable trench laying, cable laying, tunnel overhead cable laying methods have advantages and disadvantages, the general should consider the city development planning, the density of existing buildings laying cable length of the number and the impact of the surrounding environment. More technically, cable tunnel and cable trench laying cable construction, easy maintenance and repair. In some developed countries in the city, city planning and construction, has considered public tunnel. Practice has proved that the running effect of utility tunnel is good, can greatly reduce the duplication of investment and the number of phenomenon repeated excavation of the road, but a huge initial investment, building materials consumption funds, in the domestic, restricted by various factors, the laying method is rare. In contrast, buried and shallow trench laying is laying mode belongs to the economy, the buried cable is the most economical and widely used in electric installation, it is applied to the suburbs and vehicle traffic not too frequent place. But it is not good for the maintenance and repair of the cable, if cable fault, even using the test to measure the point of failure, but also to re dug trenches, very inconvenient. Therefore the cable laying mode selection, combined with the actual situation, according to the engineering conditions, environmental characteristics, cable type and the number of such factors, with the eyes of development, to meet the operational reliability, convenient maintenance requirements and principles of technical and economic reasonable.
Two, the selection of cable
Used in power cables with oil filled cable, PVC insulated cables, XLPE cables, according to the use of different occasions, but also extends to the special different types of cable. At present, with the production technology and production process of continuous improvement, XLPE cable products has become the most widely used, the cable selection, should be based on the use of different environments and conditions, combined with the specific situation of choice, to reduce pass through all kinds of tube side of railway, highway and communication cable; such as the use of buried and shallow trench laying way, should consider the use of cable with steel armor.
Three, the selection of the cable section
The selection of the cable section, related to how much investment, line loss and voltage quality, the service life of the cable. Such as the use of cross-sectional area is too small, will cause the voltage quality, line loss is too large, will make the initial investment is too high. So we should development plan according to the prediction results, select the appropriate load, cross-sectional area, the power cable to meet requirements of cable core temperature maximum working current and voltage drop, thermal stabilization of the maximum short-circuit current. Because the load forecasting work difficulty is high, low accuracy, therefore, selection of cable section, but also meet the requirements of low and medium voltage distribution network in city “transformation” and “technical guidelines for city power network planning guidelines”.
In three-phase four wire power cables of low voltage power network, but also consider the zero line cross sectional area of choice, in public low-voltage network, as affected by the factors of users, the three-phase load balance is difficult to control, to improve the quality of voltage, reduce the loss, the zero line cross section area should be the same with the phase wire cross-sectional area.
Four, a cable network and cable network automation
With the constant promotion and use of electric power cables in the distribution network, the distribution network can be divided into cable network and network (including overhead, overhead cable mixed network). “A city ‘in the low voltage distribution network reconstruction for the implementation of the’ and supplementary comments” for the cable distribution network automation put forward specific requirements. Therefore, the cable network in distribution network of regional network, should be in accordance with the requirements of power distribution automation, adopting new technology, new equipment, has the condition to consider the pilot work automation, the conditions are not ripe also in supporting the selection of equipment, the full room, in order to realize the self foundation dynamic scheme.
FiveShould pay attention to problems in construction, power cable
1, the eddy current problem caused by high current power cable
Power cable in the construction, the steel bracket, a steel protection tube, a cable card and overhead, usually in the form of power cable around the steel (iron) closed loop, are likely to form a vortex, particularly in the large current power cable system, higher eddy current.
In the power cable construction, measures must be taken to make the cable around, cannot form the steel (iron) closed loop, prevent the cable caused by eddy current phenomenon.
2, is the mechanical damage caused by power cable turns
Because the power cable diameter is bigger, transport, installation is difficult, power cable to the requirements of turning radius is strictly. Power cable in the construction, if the turning angle is too large, may make internal conductor by mechanical damage, and mechanical damage because the cable insulation strength decreased, until the emergence of fault, in construction of a cable head fault, in making cable head, three cable head same length, and equipment connected due to topographical constraints in, head of the cable is too long and become the root of arch, cable head damaged discharge. After taking measures, the equipment connection, shorten appropriately in cable connection length of head, make cable head are not affected by external force, the practice proved well. Thus, the cable in the construction process, to minimize the cables are subjected to torsion, in cable turns and Yu left cable, so the cable in the natural bent, put an end to the internal mechanical damage phenomenon.
3, is the electric power cable moistureproof problem
Operating experience shows that, in low voltage power cable fault, most cable middle connector and terminal fault, while the middle connector and terminal fault is mostly due to poor sealing, moisture intrusion caused by insulation strength decreased, while in low voltage power cable network, the dendritic power supply mode, more cable terminal number, so good cable terminals and joints plugging close off is one of the important measures to ensure safe and reliable operation of the cable.
4, is in the low voltage power cables, grounding problem
In the low voltage power cable public, online, because the three-phase load is not equal, therefore, if the metal jacket cables, must consider the grounding metal protecting layer problem, and to ensure that at any point metal protecting layer of non normal induction voltage grounding position shall not be greater than 100V. I think, in the low and medium voltage cable nets, all cable joints shall set grounding (Network), and the metal shield reliably grounded.
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