Gentlemen, I am in urgent need of technical data on 110KV cables

Installation of 110 kV cables in conduits by Wei Yanping, Beijing Electric Power Design Institute. Abstract: In Beijing, 110 kV cables are mainly installed within tunnels; however, the construction cost of underground power tunnels is high, and the number of tunnels along the main distribution networks in urban areas is already near saturation； At the same time, the cable network is currently expanding rapidly beyond the urban areas. If trench laying is used, the initial investment is too high, and the total number of cables is also limited. This paper discusses the issues that need to be addressed in the conduit-based installation of 110 kV cables, the relevant aspects of the design of conduit shafts used for such installations, as well as an analysis of the economic and technical parameters of these conduit shafts, thereby demonstrating the feasibility of installing 110 kV cables through conduits. Keywords: 110 kV cable laying ； Power conduit well ； Pipes Chinese Library Classification Number: TM757 Document Code: A Article Number: 1003-0867(2006)03-0021-04 At present, there are various methods for laying 110 kV cables in China, such as direct burial, conduit installation, and tunnel installation; in the Beijing area, 110 kV cables are mainly laid within tunnels. Cables laid in tunnels enjoy favorable operating conditions, with a basically constant ambient temperature; they are easy to install and convenient for management and maintenance. However, underground power tunnels are costly to build. If multiple cables are laid along the main road network in urban areas, tunnels can still demonstrate their advantages, and their economic and technical indicators are also reasonable. However, as the tunnels along the main network routes in the city center are nearing saturation, the cable networks are rapidly expanding outside the city area – such as in the areas beyond the third and fourth rings, and even in many economic development zones beyond the fifth ring. The overall planning for these cable network systems is already nearly complete; using tunnels for installation would require excessive upfront investment, and the total amount of cables needed is not large, which makes it uneconomical and results in waste of resources on the part of investors. Therefore, it is very necessary to develop another deployable method suitable for 110 kV cables in the Beijing area. This new method of laying should be cost-effective first and foremost, feasible in terms of construction, operation, and maintenance, and it should also be reusable to reduce the number of times ground excavation is required. Through extensive practice in laying cables in conduit shafts for distribution network projects in recent years, especially with the use of a large number of new types of conduits in the projects related to burying utilities beneath the Third Ring Road, it has been found that laying cables in conduit shafts offers various advantages: the investment required for the conduit shafts themselves is relatively low, multiple cables can be laid once they are constructed, and repeated ground excavations can be avoided. Currently, in regions such as Shanghai and Guangdong in China, many 110 kV cable projects employ conduit installation methods; therefore, there is ample experience in the implementation and operation of 110 kV conduit installation that can be utilized as a reference. 1 Conduit installation of 1110 kV cables 1.1 Current-carrying capacity The method of cable installation has a direct impact on its current-carrying capacity, and the main reason for this is that the thermal resistance of the medium surrounding the cable varies depending on the installation method. For cables in operation, to determine the temperature rise of the cable, it is necessary not only to know the thermal resistance of each component of the cable itself, but also the thermal resistance of the medium surrounding the cable – that is, the temperature difference resulting from the flow of heat from the cable’s surface into the surrounding medium per unit of heat flow. This temperature difference often becomes one of the main factors limiting the cable’s transmission capacity. We can use an example from a real engineering project to compare the current-carrying capacity of the same cable circuit when it is installed in conduits versus when it is installed in tunnels (in air). In this case, the data on the cable’s current-carrying capacity under different installation conditions and at various ambient temperatures were provided by Shandong Cable Factory for the 110 kV cable project between Chaoyangmen and Longfusi. Tables 1 to 3 show the current-carrying capacity of the ZR-YJQ02 64/110 kV 1×800 mm2 cable when arranged horizontally under different conditions. Installation conditions: • Equal spacing for cables: installed in parallel, 200 mm ； •PVC conduit diameter: 180 mm (inner) 200 mm (outer) ； •Cable laying depth: 1000 mm ； •Soil thermal resistance: 1.0 Km/W ； •Three PVC pipes are arranged horizontally. As can be seen from Tables 1 to 3, using conduit for cable installation results in a transmission capacity that is 500–600 A lower than when the cables are installed in the air, which is approximately a 50% reduction. 1.2 Thermomechanical properties of cables: During operation, cables are subject to extremely large mechanical forces resulting from thermal expansion and contraction caused by changes in the temperature of the conductors. This mechanical force poses a significant threat to safe operation, and therefore must be taken into account during design. Cables are laid in tunnels, and we generally use a serpentine layout to reduce the thrust generated by the expansion of the cable cores. However, when cables are laid in conduits, the limited space available prevents a serpentine layout along the entire length of the cable. Therefore, it is necessary to lay the cable in a wavy pattern near the terminal fittings or connectors, as well as at points where the cable direction changes, in order to create some margin and thus reduce the force exerted by the thermal expansion and contraction of the cable conductors on those terminal fittings or connectors. 1.3 Calculation of cable traction force: In actual engineering design, it is necessary to calculate the traction force on the cable or the allowable pulling length; currently, most cable manufacturers provide the allowable traction force for their cables. Therefore, designers should calculate the maximum allowable pulling length under actual engineering conditions. This length is one of the main factors determining the coil length for cable production. Although some factors cannot be determined at the time of design, by referring to existing data, it is possible to roughly determine the allowable pulling length as well as the appropriate pulling methods, positions, and capacity of the pulling equipment, in order to prevent damage to the cable during pulling. For cross-linked cables, they are mostly laid using a payoff machine to pull the pulling head. The pull head is a sealed socket mounted at the end of the cable; it is the fitting used to transfer the pulling force to the cable conductors when pulling the cable. Under this laying method, the pulling force acts on the core; the tensile strength of the copper core is approximately 240 N/mm2, while the maximum allowable pulling force is 70 N/mm2. Therefore, the pulling force acting on the copper core must not exceed 70 N/mm2 based on its cross-sectional area. In cable routes with bends, when a pulling force acts on the cable on the inside of the bend, the cable is subjected to the component of this pulling force as well as the reaction force, resulting in pressure – this is known as lateral pressure. If this lateral pressure is too high, it can crush the cable. Lateral pressure is the ratio of traction force to bending radius. Generally, the maximum lateral pressure on cross-linked cables during installation is around 3 kN/m. Therefore, during traction, excessive lateral pressure should be avoided at the bent sections to prevent damage to the outer protective layer and thus maintain insulation performance. When calculating the traction force of a cable, cable routes with complex paths are usually broken down into several simplest basic bending types, which are calculated separately. The traction forces of these individual parts are then added together to obtain the total traction force for the entire cable. The following are several commonly used formulas for calculating traction force: Horizontal or vertical traction: T = μWL; Horizontal bending traction: T2 = WRsinh. The formula for calculating lateral force is P = T/R, where T represents the traction force in kg ； 　　 m——coefficient of friction ； 　　 W — Weight of the cable per meter (kg/m) ； 　　 L——Cable length (m) ； 　　 q —— central angle of the curved portion (rad) ； 　　 T1, T2 —— Traction force before bending (kg) ； 　　 R — bending radius of the cable (m) ； 　　 P — lateral pressure (kg/m). As can be seen from the above formulas for traction force and lateral pressure, the magnitude of the traction force is related to the length of the cable reel and the bending radius. If the cable traction force and lateral pressure are required to remain within certain limits, the length of the coil is also restricted. At the same time, when designing cable routes, it is necessary to calculate the pulling force and lateral pressure in advance to prevent the cables from being damaged if these forces exceed the allowable values during installation. 1.4 Cable reel length design: In determining the length of the cable reel used for conduit-mounted cable installation, aside from considerations related to production and transportation, there are two main factors that play a role in practical engineering applications: one is the pulling force factor mentioned earlier ； The other is the factor of well spacing in the connection well chamber. Unlike the equal distribution of coil length in cable tunnel laying design, when cables are laid through pipes, the coil length must not only meet the requirements regarding traction force and lateral pressure but also be determined based on the location of the shafts. That is, the well spacing should be prepared before determining the coil length, which is then determined based on the actual well spacing at the joints. If there is a significant difference in the coil lengths within a set of interconnected coils, it is necessary to determine whether the resulting unbalanced current meets the required standards. 2 Buried Pipe Design 2.1 Cross-Section of Buried Pipes Considering the one-time investment and the cable capacity ratio, the cross-section of the buried pipes can be designed with around 28 holes; a layout of 4×7 f200 is considered reasonable. Pipes with this cross-section can accommodate up to 8 or 6 110 kV cables, as well as several 10 kV cables, once installed. It can be seen that its cable arrangement capacity is not much different from that of the tunnel. 2.2 Grounding Device The installation of the grounding device in the pipe well depends on the location of the cable connections. Unlike tunnels where grounding flat bars are installed along the entire length, in pipe shafts, grounding devices need to be installed only at the junction shafts, ensuring that the required grounding resistance value is met; this value generally should not exceed 0.5 Ω. Each joint chamber should be connected to several sets of grounding electrodes based on its length, while the remaining construction trenchs can have a closed grounding grid installed around them according to actual conditions. If it is considered that a ground grid using a single connection well is insufficient, grounding cables can be installed between the connection wells; laying such grounding cables can occupy one pipe hole. 2.3 Shaft Chamber Setup In shaft works, there are two types of shaft chambers based on their function: joint shaft chambers and construction setting-out shafts. As the name implies, a junction well is located at the junction of cable lines, and it is used for the fabrication, installation, and fixation of various cable joints. Construction shafts should also be installed between the connection wells; their function is to use some kind of machinery or manual labor in the several construction alignment wells located between the connection wells to lay cables, thereby carrying out the construction of the cable lines. The cross-section of the well can be designed as 2m×2m, and its length should vary depending on its function. The main function of the joint well chamber is to manufacture and install joints and secure them therein. For 110 kV single-core cables, there are three joints per set for each cable circuit; considering the size of the joints and ease of installation, the spacing between joints is 5 meters, and this distance should be increased appropriately in the case of joints on large-cross-section cables. Based on the typical length of 110 kV cables, it is advisable to provide a joint shaft every 300–500 meters. Based on a 28×f200 cross-section, the length of the junction well chamber should be around 30 m. The junction well should be equipped with appropriate supports or hangers, as well as the embedded components required to fix the joints. Furthermore, the junction between the pipe and the well should be sealed, and the well chamber should be made waterproof. The junction well should be equipped with a grounding device; grounding flat bars on both sides of the well chamber should be connected to the grounding grid, so as to enable the grounding cable at the junction to be grounded safely and reliably. Considering the pulling force that the cable can withstand and the pulling capacity of the laying machinery, a construction laying shaft can generally be placed every 50 m along the straight sections of the cable. Additionally, a construction setting-out well should be installed at the corner. Since the construction alignment well is used only for laying cables, its length can be designed to be around 6 m. The wellbore should be equipped with appropriate brackets or hangers and other accessories for securing cables. 3 Comparison of Types and Properties of Cable Conduits At present, there are a variety of cable conduits with operational experience in China, the main ones being: RMDP power cable protection conduits, seamless galvanized steel pipes, sepiolite fiber cement pipes, ordinary composite fiberglass reinforced plastic cable conduits, and DBS alkali-free glass fiber quartz cable conduits. Steel pipes can only be used in three-core cable projects. Cables of 110 kV and above are mostly single-core cables, and steel pipes cannot be used due to eddy current losses. Through the widespread use of RMDP and DBS pipes in the underground construction projects along the Third Ring Road, we have accumulated considerable experience, which provides a solid basis for selecting pipes. This article provides a detailed introduction only to the RMDP power cable protection conduit and the DBS alkali-free glass fiber quartz cable conduit, as these two types of power protection conduits are relatively new products that possess notable advantages over older types such as asbestos and cement conduits. 3.1 RMDP Power Cable Protection Conduit: The RMDP power cable protection conduit is made from CPVC and PVC resins as the main raw materials, with stabilizers, lubricants, modifiers, etc. added; it is formed through high-speed kneading and extrusion processes. Its production technology and formula are advanced and rational. This pipe material features low self-weight, a high Vicat temperature, strong impact resistance, good flame-retardant properties, and excellent corrosion resistance; it eliminates the need for concrete, protective layers, and road closures during pipeline installation. The successful development of this pipe material can replace existing outdated materials and construction methods in power cable trenching projects. This product offers advantages over cement asbestos pipes and other products in terms of safety, ease of installation, durability, absence of toxins, and no environmental pollution; it thus provides both economic and social benefits. The features of RMDP products can be summarized as follows: • It has advantages such as heat resistance, pressure resistance, corrosion resistance, and aging resistance (with a service life of over 40 years). •The RMDP cable conduit consists of components such as connectors, waterproof seals, and brackets. It features a rational design and easy construction; no concrete protective layer is required. The scaffolding uses modular connections, which shortens the construction period. It contains no radioactive carcinogens, offering significant economic and social benefits. •The amount of earth to be excavated is small. The weight of pipes is only 1/4 that of steel pipes and 1/10 that of concrete pipes; the construction process is simple, resulting in reduced labor costs. •The RMDP cable conduit completely overcomes the poor weather resistance of ordinary PVC pipes. Its strength allows it to replace steel pipes, and it overcomes the problems of corrosion in steel pipes as well as the excessive temperature rise in single-core cables caused by the formation of closed magnetic circuits, which can lead to damage. Construction features of RMDP cable conduits: • The standard length of cable conduits is 4 m, and they are connected using socket-type interfaces. In principle, they are laid directly underground, usually at a depth of at least 1 m below the ground surface. •The groove width should be sufficient to provide the minimum clearance required after the pipes are connected, the foundation is constructed, and backfilling is completed. Generally, the digging width of the trench is 0.8–0.85 m smaller than that of the trench for laying double-asbestos-cement pipes, which helps to reduce the amount of excavation work required. •During foundation construction, it is necessary to take into account that the pipes must withstand heavy loads such as soil pressure and wheel loads. If the foundation is not level, it can cause the pipes to bend, leading to excessive local loads; therefore, care should be taken to level the bottom of the trench so that the pipe supports are even. If the soil is soft, it is recommended to lay sand under the pipe or a layer of concrete with a thickness of 100 mm. In the case of underground streams or small rivers, the silt should first be removed, and a reinforced concrete base layer with a thickness of 200 mm should be placed at the bottom of the pipe; the concrete grade should be C20. •When installing pipe supports at the standard length for power cable conduits, the spacing between the pipe supports is 1.5 m. The pipe supports are connected using dovetail pins, while the pipes are connected with plug-type joints. To facilitate the insertion of the pipes, a small amount of soapy water or a specialized lubricant can be applied to the inside of the rubber ring and on the outside of the entire plug. The surface of the tube should be marked with a line indicating the insertion length, so that it can be verified after the tube is inserted. The development of the RMDP power cable conductor has been recognized; its various physical and chemical performance indicators meet or exceed those of similar foreign products. The widespread use of this product allows for the elimination of outdated and harmful materials as well as construction methods, contributing to the improvement of urban aesthetics, traffic conditions, and the environment. At the same time, it has received positive feedback in the application of cable engineering in the distribution network sector; its use in the laying of 110 kV cables is technically mature. 3.2 DBS alkali-free fiber glass quartz cable conduit: The DBS alkali-free fiber glass quartz cable conduit is a new product developed on the basis of existing composite fiberglass reinforced plastic cable conduits. It features a scientifically sound structural design, and its overall performance is superior to that of other cable conduits. The DBS cable conduit offers the following advantages in terms of application: • This product is manufactured using a new process, with tension-controlled winding technology on an automated programmable winding machine to achieve one-time formation; as a result, its structure is more compact and its quality is stable and reliable. •Thanks to the new technology that uses quartz sand, it features high stiffness and high strength; for conduits of the same wall thickness, the DBS alkali-free fiberglass quartz cable conduit has strength parameters 1.5 to 2 times those of ordinary composite fiberglass-reinforced plastic pipes. It can be buried directly under the driving lane without the need for a concrete protective layer. •It has excellent resistance to chemical corrosion and water, allowing for long-term use in humid environments, highly saline areas, and underwater. •It has excellent insulation properties, with a breakdown strength of over 3.5 kV/mm, and no eddy current losses; it is suitable for the installation of single-core cables. •It further improves the product’s flame retardancy, cold resistance, and heat resistance, allowing it to be used for extended periods in environments ranging from -30 to +130 °C without any impact on the performance of the catheter. •The inner wall is smooth with a low friction coefficient; the ends of the connectors feature 45° chamfers, allowing the cable to pass through smoothly without being scratched. •Installation is simple and convenient; the product itself is lightweight, allowing it to be moved by one person, and installation can be carried out with two people. This helps to **reduce the construction time and lower installation costs. The flared socket connection method makes it easy and fast to lay the pipes, thereby avoiding prolonged road excavation and the disruption it causes to urban traffic. •It features scientific designability: different fibers and winding angles are selected in various parts and aspects to achieve equal strength, thereby making full use of the excellent tensile strength of alkali-free glass fibers and ensuring an optimal balance between the stiffness and strength of the catheter. Moreover, the use of materials is more rational, reducing the production cost of the product. •In terms of cable performance, DBS alkali-free fiberglass quartz cable conduits have no eddy current losses or electrical corrosion. Its socket-type connection method is resistant to deformation, enhancing the safety and reliability of cable operation. •When laying cables in urban areas, it usually affects the operations of shops on both sides as well as road traffic. Using DBS alkali-free glass fiber quartz cable conduits as protective tubes for the cables can **shorten the construction period, reduce the impact on traffic and the indirect losses caused by cable installation projects, thereby bringing about positive social benefits. Key technical specifications: density 1.80 g/cm3, coefficient of friction 0.4, corrosion resistance (80 °C, 100 h) with weight loss of 0.012 mg, heat resistance up to 135 °C, flame retardancy (oxygen index) of 26%, impact resistance (6.8 kg, 300 mm) with no damage, Brinell hardness of 40, and service life of 50 years. As can be seen from the above analysis, the use of DBS alkali-free fiberglass quartz cable conduits is technically feasible and can yield significant social benefits. In an era of accelerating social development, DBS alkali-free fiberglass quartz cable conduits will gradually be recognized and adopted by a wide range of users and cable engineering professionals thanks to their numerous performance advantages and favorable cost-performance ratio. 4 Analysis and Comparison of Economic Indicators: By comparing the same cable project, it can be seen that the overall cost of using DBS fiberglass quartz cable conduits or RMDP cable protection tubes is lower compared to other types of tubes. Although sepiolite fiber cement pipes have a lower unit price, they require concrete encasement during construction. This not only increases the cost of the project, but also slows down the project timeline due to the need for a curing period for the concrete. The costs of DBS and RMDP pipes are not very different, and their friction coefficients are also roughly the same; however, in practical use it has been found that both require backfilling with fine soil or fine sand. This increases the overall cost of the pipe well. Additionally, RMDP can be slightly bent in practical applications, which allows for flexible handling in the presence of obstacles on site and reduces the need to add transition shafts to some extent. The internal-polished reinforced cement pipe made of sepiolite fibers is a new product. Compared to the original sepiolite tubes, it features a lower coefficient of friction and does not require packaging. Its friction coefficient can reach the same level as that of DBS and RMDP, yet its price is **lower than those of the two. Therefore, from the perspective of technical and economic indicators, the new type of sepiolite fiber-lined reinforced cement pipe possesses significant advantages. This type of pipe is currently used only in the Sanhuan Project as a protective conduit for directly buried cables. Although the results are good, it still requires time to prove its effectiveness and the accumulation of experience. By comparing pipe wells with 2m×2m open cable tunnels, it can be seen that, with similar cable installation capacities, the investment in cable tunnels is more than twice that of buried pipe systems. 5 Conclusion In summary, the one-time investment in constructing pipe wells is much lower than that required for tunnel construction. Once built, it eliminates the need for repeated excavations, and its cable capacity is not significantly different from that of tunnels; thus, its economic and technical indicators are favorable. The installation of 110 kV cables through conduits is theoretically feasible, and there is already considerable operational experience in places such as Shanghai; it is an economical and reasonable method of installation that can be widely adopted. Pipe-driven installation is particularly suitable for areas such as newly developed zones with comprehensive planning. The construction of pipe shafts for laying cables at different voltage levels along the same path can be considered.

Average reactance of 110kV high-voltage cable: X=0.18Ω/km, X*=0.006