Transmission Towers vs Underground Systems: Cost, Maintenance, and Efficiency Compared

Have you ever wondered why underground cable systems are necessary when transmission towers are already in place? Is it a matter of power transmission efficiency or other factors, such as construction costs? This article will provide a detailed comparison and analysis of transmission towers and underground systems, focusing on three dimensions: cost, maintenance, and efficiency. This article aims to provide a sound basis for decision-making by power companies, engineering contractors, and project planners.

What are Transmission Towers and Underground Cable Systems?

Transmission Tower Basic Definition

Transmission towers, also known as overhead transmission towers, are steel or concrete structures used to support and secure high-voltage transmission lines. They are typically constructed from angle steel or steel pipes, assembled by bolts or welding, and are capable of withstanding the vertical loads and lateral tension of the lines. Common types include suspension towers, tension towers, corner towers, and terminal towers, each chosen based on the line's alignment, span, and load. The primary function of a transmission tower is to maintain a safe height for conductors, ensuring a safe distance between lines and from the ground, and preventing phase-to-phase short circuits and potential hazards to personnel and facilities.

Introduction to Underground Cable Systems

Underground cable systems transmit electrical energy by laying insulated cables underground. They utilize multi-layer insulation and shielding structures, burying high-voltage cables within underground pipes, tunnels, or trenches to protect against external interference and environmental damage. Underground cables typically consist of a conductor, insulation, shielding, and protective layers, making them suitable for use in urban environments where space is limited or aesthetically pleasing.

Comparison of Applicable Environments and Constraints

Transmission towers are more suitable for long-distance, high-voltage transmission, especially in open or rural areas. They are relatively economical to construct and easy to maintain. Underground cable systems are often used in urban centers, environmentally sensitive areas, or areas with high visual acuity requirements. However, they have a long construction period, high costs, and complex maintenance. Overall, the suitability of each system depends on the project's terrain, environment, budget, and technical requirements.

Cost Comparison of Transmission Towers and Underground Cables

Construction Cost Analysis

Material, Design, and Installation Costs of Transmission Towers: Overhead line construction typically requires steel, concrete foundations, conductors, and insulators. The design phase requires load calculations, wind-induced vibration analysis, and line optimization. Installation includes tower assembly, foundation pouring, and conductor installation. In comparison, the per-kilometer cost of a transmission tower is relatively low, especially in areas with flat terrain and favorable construction conditions.

Expenses for excavation, laying, and isolation and protection of underground cables: Underground systems require trenching, construction of pipes or tunnels, and the use of high-performance insulation materials and waterproofing. During construction, issues such as underground obstructions, traffic diversion, and environmental protection must be addressed. These factors significantly increase construction costs, especially in densely populated urban areas, where the per-kilometer cost is often several times that of overhead lines.

Cost differences at different voltage levels: As voltage levels increase, the insulation thickness and shielding requirements for underground cables increase, leading to exponential increases in costs. Transmission towers, while also requiring higher-strength materials, experience a smaller overall cost increase.

Therefore, overhead lines are generally more economical for long-distance high-voltage or ultra-high-voltage transmission.

Long-Term Operating Costs

Maintenance and frequency: Transmission tower lines are above ground, making fault points easily visible and maintenance relatively simple and quick. Underground cable faults are difficult to locate, requiring specialized testing equipment and excavation, resulting in higher maintenance costs.

Lifecycle costs: Return on investment analysis: Overhead lines typically have a lifespan of 40-60 years, while underground cables, due to insulation aging and moisture, have a shorter lifespan and higher replacement costs. Over the entire lifecycle, transmission towers offer a superior return on investment.

External factors affecting costs: Complex terrain, poor geological conditions, urban planning requirements, and extreme weather conditions can all impact costs. For example, overhead line construction is more difficult in mountainous areas or earthquake-prone areas, while underground cables face challenges such as groundwater or soil corrosion. These factors should be comprehensively assessed during the initial planning stages.

Maintenance And Reliability Of Transmission Towers And Underground Systems

Ease of Maintenance:

• Because transmission tower lines are exposed, inspection and maintenance are convenient, and technologies like drone inspections and infrared thermal imaging can improve efficiency.
• Underground cables, on the other hand, require cable testing and precise positioning before excavation and repair, which takes a long time and affects a large area.

Power Outage Risk:

• Overhead lines are more susceptible to weather disruptions (such as lightning strikes and typhoons), but faults can be quickly addressed and outages are shorter.
• Underground cables are less susceptible to environmental interference, but if a fault occurs, repairs take longer, leading to a higher risk of power outages.

Maintenance Technical Requirements:

• Overhead line maintenance primarily involves mechanical and structural overhauls, with a moderate level of technical difficulty.
• Underground cables, on the other hand, require more stringent requirements, including electrical testing, insulation restoration, and environmental control. Labor and equipment costs are significantly higher than for transmission towers.

Efficiency and Performance Comparison of Transmission Towers and Underground Cables

Transmission Efficiency

Differences in Conductive Performance Between Overhead and Underground Lines:

• Underground cables have poor heat dissipation, resulting in higher conductor temperature rise and limited current-carrying capacity.
• Conductors installed in transmission towers, on the other hand, have excellent heat dissipation, allowing them to carry higher currents and improve transmission efficiency.

Analysis of Transmission Distance and Power Loss:

• Underground cables have a significant capacitance effect, resulting in significant reactive power losses over long transmission distances, making them unsuitable for ultra-long-distance transmission.
• Overhead transmission lines on transmission towers, however, can achieve low energy losses if properly designed, making them suitable for long-distance, high-power transmission.

Safety and Environmental Factors

Transmission Towers' Impact on Land Use and the Environment: Overhead lines require a wider corridor, occupying a relatively large area and impacting the landscape. However, the construction process is shorter and environmental disturbances are manageable.

Underground Cable Heat Dissipation and Construction Risks: While underground systems offer minimal visual impact, they create significant disturbances during construction, potentially impacting the underground pipeline network and the ecological environment. Heat dissipation is also difficult, requiring cooling measures, increasing energy consumption and making maintenance more difficult.

Applicable Scenarios and Selection Recommendations

When to Choose a Transmission Tower: Suitable for long-distance, high-power, and budget-sensitive transmission projects, especially in rural or suburban areas, where it offers a more cost-effective solution.

When to Choose an Underground System: Suitable for projects in densely populated urban areas, with high landscaping requirements, or limited terrain. While costly, it can meet spatial and aesthetic requirements.

Application Examples of Hybrid Solutions: Some projects utilize a combination of overhead and underground solutions, such as using a transmission tower in the city's periphery and transitioning to underground cables within the city center to balance economic and environmental considerations.

Conclusion

Transmission towers and underground cable systems each have their advantages and disadvantages, and the choice depends on the project's scale, budget, environmental, and technical requirements. From a cost perspective, transmission towers offer advantages; while underground cable systems are more suitable for aesthetics and space constraints. Considering both ease of maintenance and long-term efficiency, transmission towers remain the preferred option for most long-distance transmission projects.

Project decision-makers are advised to thoroughly assess terrain, load demands, environmental requirements, and long-term maintenance strategies during early planning. When necessary, they can combine the advantages of both options to achieve the optimal solution. As a professional transmission tower manufacturer, JOISYNC offers comprehensive support from design, manufacturing, and installation to ensure project safety and maximized economic efficiency.