Power Transmission

Performance of Overhead Transmission Lines – Their Types, Advantages, Disadvantages and Applications

Performance of Overhead Transmission Lines – Their Different Types, Materials, Advantages and Disadvantages and Applications

Introduction

Overhead transmission lines form the most common and economical choice for electric power transmission covering distances above ground. The conductors are kept at sufficient heights using towers and poles to avoid ground potential. Let’s explore the different types, configurations, materials, performance factors, benefits, limitations, and applications of overhead transmission line systems.

What are Overhead Transmission Lines?

Overhead transmission lines refer to above-ground electrical power transmission systems erected on steel tower lattices or wood and concrete poles using conductors like aluminum kept at safe clearances from the ground. They are used for bulk power transmission over long distances and low to medium-voltage distribution feeders.

Importance of Transmission Lines

Overhead lines allow:

  • Transmission of large amounts of electrical power across long distances
  • Interconnection between generating stations, substations, and load centers
  • Expansion of grid access to rural areas
  • Bulk power transfer between utility areas and regions

Overhead Line Basics

Overhead conductors are kept at sufficient heights on towers or poles, allowing adequate insulation and avoiding the risk of short circuits to the ground. Higher voltages require larger conductor separations.

Performance of Overhead Transmission Lines - Their Types, Advantages, Disadvantages and Applications
Performance of Overhead Transmission Lines – Their Types, Advantages, Disadvantages and Applications

Various types of overhead line designs exist:

Types of Overhead Transmission Lines

Conventional Overhead Lines

Conventional overhead transmission lines are the standard configuration in most power transmission networks. They consist of bare conductors suspended between insulators on sturdy towers or poles. While they are cost-effective and reliable, they have limitations when handling high temperatures and minimizing power losses.

High-Temperature Low-Sag (HTLS) Lines

High-Temperature Low-Sag (HTLS) lines have emerged as a remarkable innovation in the quest for enhanced performance. These lines incorporate advanced conductor materials that can withstand higher temperatures without excessive sagging. This reduces the risk of line failures, especially during extreme weather conditions.

Compact Overhead Lines

Compact overhead lines are designed to overcome space constraints in densely populated areas. They feature innovative conductor designs and compact towers, enabling deployment in urban environments without significant land acquisition.

Single Circuit Lines

Have one set of three conductors for three phases. It is economical and straightforward but less reliable.

Double Circuit Lines

Carry two three-phase circuits on the same towers for higher capacities and redundancy.

Multi-Circuit Lines

Combines multiple lower voltage single or double circuit lines on typical towers for very high capacities.

Overhead conductors can be configured in different arrangements:

Transmission Line Configurations

Flat Configuration

Phase conductors are in the same horizontal plane attached using horizontal cross arms of towers/poles.

Vertical Configuration

Phases are arranged vertically using H-frame or delta towers, requiring less right of way.

Triangular Configuration

Phases are at the vertices of equilateral triangles for more compact tower designs.

Transposition

Reversing positions of phases along the line controls uneven conductor heating from currents.

Various materials are utilized in overhead line design:

Overhead Line Materials

Conductors

Aluminum steel reinforced (ACSR) cables or all aluminum alloy conductors are commonly used, offering high strength and conductivity. Conductors are the heart of overhead transmission lines. They are typically made of aluminum or aluminum alloy, offering a balance between electrical conductivity and weight. Recent advancements have led to the development of composite conductors that exhibit exceptional strength and conductivity.

Insulators

Made of porcelain, glass, or polymer materials to provide high dielectric insulation between lines and support structures. Insulators are crucial components that prevent electrical current from flowing into supporting structures. They are commonly crafted from porcelain, glass, or polymer materials, each with its unique advantages in terms of performance and durability.

Towers and Poles

Towers use steel lattices for heights up to 60m. Concrete and wood poles suffice for lower heights under 60m. The support structures for overhead transmission lines vary widely. Traditional lattice towers, monopoles, and H-frame structures are commonly used, each selected based on terrain, environmental conditions, and transmission voltage.

Accessories

Include insulated cross arms, vibration dampers, spacers, clamps, etc., for conductor support and electrical clearance needs.

Several factors govern overhead line performance:

Overhead Line Performance Factors

Electrical Considerations

Choice of voltage level, power handling needs, corona, radio interference, etc. based on length, terrain, etc.

Mechanical Aspects

Conductor tension, tower and pole design, wind and ice loading, vibration, sag-tension analysis, etc., are considered.

Environmental Influences

Altitude, temperature, winds, storms, icing, lightning, pollution, etc. affect material choice and clearances.

Construction and Maintenance

Terrain impacts construction ease, access, and maintenance needs. Live line maintenance methods are used.

Overhead line advantages include:

Advantages of Overhead Lines

Overhead transmission lines offer several advantages, making them a preferred choice for long-distance power transmission. They are cost-effective to install, have a relatively low maintenance requirement, and facilitate easy monitoring and maintenance. Moreover, their high voltage-carrying capacity enables efficient power transfer over extended distances.

Lower Costs

Overhead lines need lower material and installation costs per km than buried cables.

Easier Maintenance

Issues can be identified more readily by visual inspection. Maintenance access is more convenient.

Lower Inductance

Exposed conductors have lower series inductance than insulated underground cables, allowing reactive power flow.

Better Heat Dissipation

Open air circulation enables better cooling of conductors compared to buried cables.

However, some disadvantages also exist:

Disadvantages of Overhead Lines

Despite their advantages, overhead transmission lines also have drawbacks. They are susceptible to weather-related damage, such as ice and wind loadings, resulting in outages. Additionally, they may have aesthetic and environmental concerns, especially in residential areas.

Right-of-Way Needs

Require suitable space for adequate conductor clearances, unlike buried cables.

Weather Vulnerability

Unlike protected underground lines, exposed to stresses from winds, storms, lightning, icing, contamination, etc..

Visual and Ecological Impacts

Tower/pole routes need vegetation clearance, affecting natural habitats and landscapes.

Theft and Damage Risks

Compared to underground lines, more prone to physical damage risks, pilferage, etc..

Major applications of overhead transmission lines include:

Applications of Overhead Transmission

Overhead transmission lines are extensively used in various applications, including:

  • Interconnecting Power Grids: Overhead lines connect different regions, ensuring a stable and continuous power supply.
  • Powering Remote Areas: They extend electricity access to remote and rural locations.
  • Supporting Renewable Energy: Overhead lines facilitate the integration of renewable energy sources into the grid.
  • Industrial Power Supply: They provide the necessary power for industries and factories.

Long Distance Bulk Power Transmission

Used for backbone transmission of large power blocks across hundreds of miles between regions at very high voltages like 400kV and above.

Distribution Feeders

Low voltage overhead conductors below 50kV distribute power from grid substations to rural areas, communities, industries, etc..

Rural Electrification

Economical option for expanding grid connectivity to remote villages using single and double circuit medium voltage lines.

Temporary and Emergency Supply

Quickly rigged temporary overhead lines restore emergency power after disasters when regular networks are damaged.

Comparison with Underground Cables

  • Overhead lines are more economical but suffer weather-related disturbances, unlike more protected buried cable routes.
  • Overhead routes need right-of-way space and have higher visual impacts versus underground options across populated areas.
  • Access is easier for overhead lines, aiding visual inspection and live-line maintenance.
  • Inductance is lower for overhead conductors than insulated underground cables, allowing reactive power flow.

Enhancing Performance and Efficiency

To meet the growing electricity demand and improve grid reliability, continuous advancements are being made to enhance the performance of overhead transmission lines. Smart grid integration and transmission line upgrades are pivotal in achieving these goals.

Smart Grid Integration

Integrating overhead transmission lines into smart grids enables real-time monitoring and control, minimizing downtime and optimizing power flow. This improves overall system efficiency and reliability.

Transmission Line Upgrades

Upgrading overhead transmission lines with advanced conductors, insulators, and monitoring systems can significantly enhance their performance. These upgrades reduce power losses and improve the line’s capacity to transmit electricity efficiently.

Conclusion

Understanding overhead transmission lines’ performance, types, materials, advantages, and disadvantages is essential in maintaining a reliable and efficient electrical grid. As technology advances, these lines will continue to play a vital role in powering our communities and industries.

Summary

Overhead transmission lines provide the most widely adopted and cost-effective option for electric power transmission and distribution services. The selection of voltage levels, configurations, designs, materials, and construction methods depends on electrical load, lengths, and terrain factors. While economical compared to underground cables, overhead lines require right-of-way space and remain more exposed to weather events and physical risks.

FAQs

  1. Why are overhead lines most common for power transmission?

Overhead lines offer the most economical option for bulk power transmission over long distances, owing to lower installation and maintenance costs.

  1. What are the different overhead transmission line configurations?

Common configurations are single circuit, double circuit, multi-circuit, flat phase arrangement, and vertical and delta configurations.

  1. What factors affect overhead transmission line performance?

Major factors are electrical considerations, mechanical and structural design, weather and environmental influences, and construction and maintenance.

  1. What are the advantages of overhead lines over underground cables?

Benefits include lower costs, easier inspection and repairs, lower inductance allowing reactive power transfer, and better heat dissipation.

  1. What are the disadvantages of overhead transmission lines?

Issues include right-of-way needs, vulnerability to weather disturbances, visual impacts, and higher physical damage risks.

  1. Where are overhead transmission lines used?

Major uses include long-distance bulk power transmission, distribution feeders, rural grid extension, and temporary emergency power supply.

  1. What material is commonly used for overhead line conductors?

Aluminum conductor steel reinforced (ACSR) cables offer optimal combinations of conductivity and strength for overhead conductors.

  1. Why is conductor transposition used in lines?

Transposition or phase rearrangement helps balance unequal heating effects from currents, improving thermal loading limits.

  1. How does terrain affect overhead line construction?

Terrain impacts construction access, costs, and maintenance needs. Mountains may require helicopter stringing.

  1. Why are covered conductor lines sometimes used?

Covered conductors help protect against physical damage risks and limit short-circuit currents across dense areas.

 

MCQs about overhead transmission lines:

  1. Are overhead transmission lines safe during thunderstorms?
    • Overhead transmission lines are designed and built with safety in mind, but they can be vulnerable during severe weather conditions like thunderstorms. Lightning can strike transmission lines, potentially causing temporary outages. Utility companies employ various protective measures and grounding systems to minimize these risks.
  2. What is the lifespan of overhead transmission lines?
    • The lifespan of overhead transmission lines can vary depending on the materials used, environmental conditions, and maintenance practices. Typically, these lines are designed to last 40 to 80 years or more. Regular inspections and maintenance help extend their operational life.
  3. Can overhead transmission lines be buried underground?
    • While overhead transmission lines are traditionally installed above ground due to their cost-effectiveness and ease of maintenance, it is possible to bury them underground. However, underground installation is significantly more expensive and may require additional technical considerations.
  4. How do HTLS lines differ from conventional overhead lines?
    • High-Temperature Low-Sag (HTLS) lines differ from conventional overhead lines primarily in their conductor materials. HTLS lines incorporate advanced conductors designed to withstand higher temperatures without sagging, increasing their reliability and capacity.
  5. What are the environmental impacts of overhead transmission lines?
    • Overhead transmission lines can have environmental impacts, including visual aesthetics and potential ecosystem disruption. However, modern design practices aim to mitigate these effects by using compact towers, bird protection measures, and routing lines to minimize environmental impact.
  6. Do overhead transmission lines contribute to electromagnetic fields (EMF)?
    • Yes, overhead transmission lines emit electromagnetic fields (EMF) due to the flow of electricity through the conductors. While studies have not conclusively linked EMF exposure from transmission lines to adverse health effects, it remains a topic of research and public concern.
  7. How is the capacity of overhead transmission lines determined?
    • The capacity of overhead transmission lines is determined by factors such as conductor size, line voltage, temperature, and environmental conditions. Engineers use complex mathematical models to calculate the line’s capacity while ensuring it operates safely within specified parameters.
  8. What measures are taken to protect wildlife near transmission lines?
    • Measures like bird diverters, raptor guards, and wildlife-friendly tower designs are employed to protect wildlife near transmission lines. These measures aim to reduce the risk of bird collisions and disturbances to natural habitats.
  9. Are there alternatives to overhead transmission for power distribution?
    • Yes, there are alternatives to overhead transmission, including underground transmission lines and distributed generation systems like microgrids. These alternatives are used when minimizing visual impact or enhancing reliability is a priority.
  10. How do transmission line upgrades improve efficiency?
    • Transmission line upgrades improve efficiency by reducing power losses, increasing capacity, and enhancing the line’s ability to carry electricity over long distances. Upgrades can include replacing old conductors with more efficient materials and installing advanced monitoring and control systems.

Engr. Muhammad Ali Raza

Hello, I'm Engr. Ali Raza, an Electrical Engineering Professional with a passion for innovation and a commitment to excellence. I completed my electrical engineering degree in 2017 and have since been actively engaged in the field, where I've had the opportunity to apply my knowledge and skills to real-world projects. Over the years, I've gained valuable experience in Engineering field, allowing me to contribute effectively to the development and implementation of electrical systems and solutions. I thrive in dynamic and challenging environments, constantly seeking opportunities to expand my expertise and make a meaningful impact in the world of Electrical Engineering.

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