Detailed explanation of insulation power cable laying methods in developed countries

In 2011, China's total output value of wires and cables surpassed that of the United States and became the world's largest manufacturer of wires and cables. However, it must be soberly recognized that China's cable production capacity has seriously exceeded its capacity. According to statistics, if all cable companies in China start construction comprehensively, it can meet the demand of the entire Asia and Oceania. At the same time as the serious production capacity of cables, cable breakdown caused by improper laying of XLPE insulated power cables accounts for a considerable proportion of cable operation failures. Therefore, it is urgent to comprehensively understand and recognize the scientific laying methods of XLPE power cables. This article first introduces three main land installation and laying methods for medium voltage XLPE insulated power cables.

At present, the design of cable engineering in China mainly refers to relevant standards such as GB 50217-2007 and DL/T 5221-2005. DL/T 5221-2005 requires the small bending radius of XLPE insulated medium voltage power cables to be laid, with a single core of 12D and a three core of 10D (D=measured outer diameter of the cable). However, different manufacturers have slightly different bending radii for different types of medium voltage cables. In theory, 6 or fewer cables should be laid directly, 16 or fewer should be laid in tunnels, and 21 or fewer should be laid in ducts or cable trenches. However, the above data are all in an ideal state, and construction should be based on different product requirements and the actual geographical and related environment. This article mainly discusses three land laying methods: direct burial laying, pipe laying, and tunnel laying.

2. Three types of land laying methods for medium voltage XLPE insulated power cables

2.1 Direct burial laying

The widely used laying method in China is direct burial laying, which has obvious advantages such as low cost and simple laying procedures. Before choosing direct burial laying, the construction unit must understand the properties of the product and communicate with the cable supplier, such as the type of sheath (whether it has anti termite, insect and other functions), the bending radius recommended by the cable supplier (such as whether it is armored, etc.), etc. After determining that it can be buried directly, on-site survey and verification should be carried out according to the construction design drawings to determine the direction of the cable. It is required to avoid high-temperature areas (such as urban heating systems, factories, etc.), areas with poor soil quality or rocky terrain (such as areas prone to mudslides, etc.) as much as possible. In general, the direct burial depth of XLPE insulated power cables should be below the frozen soil layer, with significant differences between the north and south. Before cable laying, fine sand with a thickness of 100-120mm should be laid in the cable channel, and then covered with cement or concrete protective cover plate. It is recommended that the width of the cover plate be greater than the cable diameter of 100-120mm. Then backfilling should be carried out, and the backfill soil should be non corrosive to the cable sheath. During the backfilling process, an additional layer of colored mini warning tape should be added. After backfilling is completed, it is recommended to provide warning treatment at cable joints, bends, or intersections with other lines to prevent external human damage.

The direct burial laying method of XLPE insulated cables is only suitable for areas with no or few termites, away from heat sources, flat terrain, low traffic density, no acid, alkali or chemical corrosion, and no underground operations. So before choosing direct burial, it is necessary to do a good geographical survey. Under normal circumstances, cables with armor layers of 10kV and below can be directly buried depending on the actual installation situation.

2.2 Pipe laying

The so-called pipe laying refers to the installation of cables in underground pipelines that have been securely placed and arranged in an orderly manner when there are a large number of cable laying wires. The installation of cable ducts requires that the inner diameter of the ducts be greater than or equal to 1.5 times the actual outer diameter of the cable. It is also required that the laying depth be below the frozen soil layer. The ducts should have a slope of more than 0.2% towards the working well direction, and the gap between each duct should be greater than 20mm to ensure good heat dissipation. When installing cables, ensure that the entrance of the conduit is smooth and there are no sharp edges or debris inside the conduit to prevent scratches during cable installation. The working temperature of XLPE insulated cables is 90 ℃. When selecting conduit materials, materials with good mechanical and heat dissipation properties should be chosen. Currently, the main conduit materials are vinylon cement pipes, double wall corrugated pipes, flame-retardant PVC-C, coated steel pipes, and hot-melt PE pull pipes.

The advantages of pipe laying are that it is easy to maintain and can reserve space for other cables. It occupies a small area and has a large adjustable space for cable shafts. It has excellent fire resistance and strong mechanical resistance, and is generally suitable for areas with many intersecting lines, highways, railways, and other busy traffic. It is not recommended to choose this installation method for armored cables. The disadvantages of pipe laying are also obvious. Compared with direct burial laying, pipe laying has higher installation costs and poorer heat dissipation performance, which can easily reduce the long-term service life of XLPE insulated cables (generally, XLPE insulated cables are designed to have a service life of 30 years. According to statistics, if pipe laying is not handled properly, it can reduce the service life of XLPE cables by 35% -40%). In addition, there are many joints in the laying of pipes, which pose hidden dangers such as water leakage and the invasion and biting of small animals, resulting in high maintenance costs.