At the present stage of China's cable engineering design mainly refer to GB 50217-2007 and DL/T 5221-2005 standards, GHFB type high voltage power cable protection of single wall corrugated pipe DL/T 5221-2005 requirements for the minimum bending radius of XLPE insulated power cable, 12D single core, three core 10D (D= cable diameter measured, but different manufacturers bend) according to the different radius of medium voltage cable is slightly different. Theoretically speaking, there are 6 and below direct buried modes of laying, 16 or less tunnel laying methods, and 21 or below laying of pipe or cable trench. However, the above data are all ideal conditions, in the construction should be based on the requirements of different products and the real geography and related environment. This paper mainly discusses three types of land laying methods, such as direct burial laying, pipe laying and tunnel laying.
Land laying mode of three medium voltage XLPE insulated power cables
1. Direct burial and laying
At present, a wide range of laying in our country is directly buried, because the direct burial laying has the obvious advantages of low cost and simple laying procedure. Before choosing direct burial, the construction company must know the properties of the product and communicate with the cable supplier, such as the type of the sheath, whether it has termite, insect and other functions, and the bending radius recommended by the cable supplier, such as whether it has armour, etc. It is determined that after being buried directly, it is necessary to conduct on-site investigation and verification according to the construction design diagram, determine the trend of the cable, and try to avoid high temperature areas (such as urban heating systems, factories, etc.), bad soil or multi rocky areas (such as debris flow lots). In general, the buried depth of XLPE insulated power cable should be below the permafrost layer, and there is a great difference between the South and the north. Before laying cables, the 100-120mm thickness of fine sand shall be laid in the cable channel, then the cement or concrete protective cover will be added. The width of the cover plate is recommended to be larger than the diameter of the cable 100-120mm. Then backfill, back fill the cable sheath to cope with no corrosion, the backfilling process is better to add a layer of color Mini warning belt. After the completion of the backfilling, it is suggested that the warning should be done at the junction of the cable joints, the bend or the other lines, so as to prevent the man-made destruction from the outside world.
The way of directly buried XLPE insulated cable is only suitable for areas where no or little termite, far away from heat source, flat terrain, dense traffic, no acid base or chemical corrosion, no underground operation and so on. Therefore, we must do a good job of geological survey before choosing directly buried and laying. In general, the cable with 10kV and below and with the armoured layer can be directly buried and laid for actual laying.
2. Pipe laying
The so-called pipe laying is the way to install the cable in a safe and orderly arrangement of pipelines when the number of cables has more roots. The layout of pipe laying requires that the inner diameter of the pipe is greater than or equal to 1.5 times of the actual outer diameter of the cable. The same requirement is laid out below the permafrost layer. The row pipe should have a slope greater than 0.2% for the working well direction, and the clearance between each row shall be greater than 20mm, so as to ensure good heat dissipation. When installing the cable, it is necessary to ensure that the entrance of the pipe is smooth. There is no sharp or sundries in the tube so as not to be bruised when the cable is installed. XLPE insulated cable working temperature is 90 DEG C, selection of pipe material, to select materials with good mechanical properties, thermal performance, the main pipe material is vinylon cement pipe, double wall corrugated pipe, plastic pipe, flame retardant PVC-C and fusion PE drawing etc..
Exhaust pipe laying the advantages of easy maintenance, and other cable reserved space, small occupied area, the cable well can be adjusted to a larger space, has good fireproof performance, strong mechanical resistance, generally applicable to the cross line more, such as highway and railway traffic in busy areas, this is not recommended with the installation of armoured cable the way. Pipe laying has obvious shortcomings, compared with buried pipe laying, installation of high cost, poor heat dissipation, easy to cause the XLPE insulated cables of long service life of small (usually XLPE insulated cables for the design life of 30 years, according to statistics, if the pipe laying properly, will make the service life of the cable XLPE reduce 35%--40%). In addition, there are a lot of pipe laying handover heads, such as water leakage, small animals invading and tearing and other hidden dangers, and its cost of maintenance is high.
3. Tunnel laying
The so-called cable tunnel laying, generally refers to accommodate 21 or more cable locations, and equipped with inspection and maintenance channels, laying a cable in a fully enclosed tunnel. The number of cables can be increased and reduced at any time in the tunnel. It does not need two times to break earth and work, and has strong resistance to external force, and it is very convenient for inspection and maintenance. When laying the tunnel, we first need to know the voltage level of the cable, the diameter of the cable and the performance of the insulation sheath material, and then we can determine the distance between the cables arranged. A lighting system and drainage system in the cable tunnel, and a double ventilation system of mechanical ventilation and natural ventilation, reduce the humidity of the air in the tunnel, the tunnel and smoke alarm system, automatic fire extinguisher and fire hydrant, some cable tunnel is also equipped with a temperature control system, on-line monitoring system, anti-theft system etc..
Although the cable laying tunnel has many advantages, but the construction investment, cost of the tunnel is very high, and long construction period, many construction units to give up the cable tunnel in order to save cost and reduce the installation completion period. At this stage, tunnel laying is not popular in China, but in Japan, South Korea and other developed countries, tunnel laying has been widely used. Some developed countries have adopted the combination of the three methods mentioned above, GHFB square type high pressure