Transmission Lines: Theory, Parameter, Function and Everything You need to Know

A transmission line is an essential component of the electrical power system, designed to carry electricity from power plants to substations and, ultimately, to end-users. These lines are made of conductive materials such as aluminum or copper and are operated at high voltages to reduce energy loss during long-distance transmission. Typically supported by towers or placed underground, transmission lines ensure a steady and reliable supply of electricity across various regions. They are classified by voltage levels, from low to ultra-high voltage, and play an important role in connecting renewable energy sources like wind and solar to the grid. While overhead transmission lines are cost-effective and commonly used, underground lines are preferred in urban areas due to their safety and visual appeal. Advances in technologies like high-voltage direct current (HVDC) and smart grids are further improving the efficiency and dependability of transmission systems.

COMPONENTS OF A TRANSMISSION LINE

A transmission line consists of several crucial components that work together to efficiently and safely deliver electrical power. Below is a breakdown of each key part:

1. Conductors: Conductors are the wires that carry electricity across the transmission line. These are generally made from conductive materials such as aluminum or copper. Conductors often have a steel core for added strength, with an outer aluminum layer for better conductivity. The choice of conductor material and size depends on factors such as voltage and distance.

• Examples: Common conductor types include ACSR (Aluminum Conductor Steel Reinforced) and AAC (All Aluminum Conductor).

2. Insulators: Insulators are used to support the conductors and prevent electrical leakage. They provide insulation, ensuring that the current stays within the conductors and does not leak to the ground or other structures. Insulators are typically made from porcelain, glass, or composite polymers, each offering different benefits in terms of durability and weather resistance.

• Varieties: Insulator types include pin-type, post-type, and suspension-type, which are chosen based on the voltage level and design of the transmission system.

3. Towers and Poles: Towers or poles provide the necessary support for the transmission line, keeping the conductors at the correct height and distance from the ground. They must be able to withstand environmental forces like wind and ice. Towers are typically made of steel, with lattice or monopole designs depending on the terrain and transmission requirements. For shorter lines, wooden or concrete poles may be used. Lattice towers are often used for higher-voltage lines because they offer more strength and stability over longer distances.

4. Ground Wires: Ground wires, positioned at the top of towers, protect the line by diverting lightningstrikes safely to the ground. They help avoid damage to the transmission line and its components.  Typically made from steel or aluminum, these wires are often coated with a layer of galvanization to prevent rusting.In addition to protecting against lightning, ground wires also help reduce electromagnetic interference (EMI) that could affect other electrical equipment.

5. Substations: While not directly part of the transmission line, substations play an essential role in managing electrical power as it moves through the grid. They contain equipment to regulate voltage and direct the flow of electricity.Substations house transformers (for voltage adjustment), circuit breakers (for safety), and switchgear (to control power flow).They are usually located near major population centers or industrial areas to efficiently distribute electricity.

6. Line Accessories: These accessories help maintain the integrity and stability of the transmission line. They include components that address issues like mechanical stress and weather-related impacts. Spacers: Maintain proper spacing between conductors to prevent them from touching and causing short circuits, Reduce corona discharge, which occurs when electrical energy ionizes the air around high-voltage equipment, causing energy loss.

7. Protection and Control Equipment: Protection and control devices ensure safe operation by detecting faults and disconnecting sections of the transmission system when necessary.

• Types:
• Relays: Detect faults or irregular conditions like short circuits.
• Fuses and Circuit Breakers: Automatically disconnect parts of the system in case of a fault.
• Surge Arresters: Protect equipment from voltage spikes caused by lightning or switching.

CLASSIFICATION OF TRANSMISSION LINES

Transmission lines can be categorized in various ways, such as by their voltage level, distance of transmission, type of current they carry, conductor configuration, and location. These classifications influence the design and operational parameters of the lines. Below are the primary classifications:

1. Classification Based on Voltage 

Transmission lines are divided based on the voltage they handle. The voltage level impacts the line’s design, distance it covers, and transmission efficiency.

• Low Voltage (LV): Up to 1 kV. Used for short-distance power distribution in local networks or residential areas.
• Medium Voltage (MV): 1 kV to 69 kV. Typically used for regional power distribution in small cities and industrial zones.
• High Voltage (HV): 69 kV to 230 kV. Commonly used for medium to long-distance transmission from power stations to substations, minimizing power loss.
• Extra High Voltage (EHV): 230 kV to 765 kV. Designed for long-distance transmission with minimal power loss, often used in large-scale national or regional grids.
• Ultra High Voltage (UHV): Above 765 kV. Used in extremely long-distance transmission, often for inter-country connections or vast regions.

2. Classification Based on Transmission Distance

Transmission lines are also classified based on how far they carry electricity. This affects the design and the equipment used.

• Short-Line Transmission: Up to 50 km (31 miles). Generally, uses lower voltage and simpler designs with minimal voltage drop.
• Medium-Line Transmission: 50 km to 250 km (31 to 155 miles). Requires more advanced line designs and voltage regulation to manage losses over medium distances.
• Long-Line Transmission: Over 250 km (155 miles). Uses high-voltage systems and more complex technologies to minimize losses and ensure stable power delivery.

3. Classification Based on Current Type

Transmission lines are also classified based on the type of electrical current they carry: Alternating Current (AC) or Direct Current (DC).

• Alternating Current (AC): AC transmission is the most widely used, efficient over medium to long distances, and easy to step up or down using transformers. AC is more flexible, with cheaper infrastructure and easier maintenance. However, it can suffer from reactive power losses and inductive impedance over long distances.
• Direct Current (DC):  HVDC transmission is ideal for very long distances or for underwater power cables. HVDC systems have lower energy losses over long distances and offer greater stability and efficiency, especially for interconnecting setparate grids.

4. Classification Based on Conductor Configuration

Transmission lines can also be classified based on the arrangement of the conductors, which affects their electrical and mechanical properties.

• Single Circuit Transmission Line: A single circuit carries power via one set of conductors. It is used for shorter distances or lower-capacity lines.
• Double Circuit Transmission Line: A double circuit uses two sets of conductors on the same tower, transmitting more power and improving reliability, as one circuit can continue operating if the other fails.
• Multi-Circuit Transmission Line: Multi-circuit lines use several sets of conductors, allowing for high-capacity transmission across large distances, typically in high-demand areas or complex grids.

5. Classification Based on Location

Transmission lines are classified by their installation method: either overhead or underground.

• Overhead Transmission Lines: The most common and cost-effective option, overhead lines are mounted on towers or poles, exposed to environmental conditions. While more prone to damage from weather, they are easier to maintain.
• Underground Transmission Lines: Used in urban or densely populated areas where overhead lines are impractical, underground lines are more costly to install and maintain but are less vulnerable to environmental damage.

Benefits and Drawbacks of Transmission Lines

Benefits:

1. Effective Power Transfer: Transmission lines efficiently carry electricity over long distances with minimal power loss, especially when high-voltage lines are used.
2. Broad Coverage: They connect power stations to far-reaching areas, ensuring electricity is available in cities, industries, and remote locations.
3. Affordable to Build: Overhead transmission lines are relatively inexpensive to install, making them a cost-effective solution for large-scale power distribution.
4. Facilitates Renewable Integration: Transmission lines enable the inclusion of renewable energy sources, such as wind and solar, into the grid, enhancing energy diversity.
5. Dependable Operation: Advanced transmission lines with smart grid features can quickly detect and isolate faults, reducing the duration and impact of power outages.
6. Adaptability: Transmission networks can be expanded as the demand for electricity rises, offering room for growth and future needs.

Drawbacks:

1. High Costs for Underground Lines: While overhead lines are less expensive, underground transmission lines incur high installation and maintenance costs, especially over long distances.
2. Environmental Impact: Overhead lines can create visual pollution and disrupt wildlife habitats, affecting the surrounding environment.
3. Susceptible to Weather Damage: Overhead lines are prone to damage from harsh weather conditions, such as storms, heavy winds, and lightning.
4. Energy Losses: Despite being efficient, transmission lines still experience energy losses, particularly over long distances due to resistance and impedance.
5. Maintenance Demands: Transmission lines require regular maintenance to remain operational, especially in remote or difficult-to-access locations.
6. Land Use Challenges: Building transmission lines requires significant land, which can lead to disputes over land use and obtaining the necessary rights-of-way.

TRANSMISSION LINE EFFICIENCY: KEY FACTORS AND ENHANCEMENTS

1. Factors Affecting Transmission Line Efficiency Transmission line efficiency is influenced by conductor resistance, voltage levels, line length, and environmental conditions. Resistance in conductors leads to power losses, which can be minimized by using low-resistivity materials like aluminum or composite-core cables. Higher voltage reduces current flow, significantly lowering resistive losses as per Joule’s Law:

where Ploss​ is power loss, I is current, and R is resistance. Since power transmission follows P=VI, increasing voltage (V) reduces current (I), thereby minimizing losses. Longer transmission lines experience greater resistance and inductance, contributing to efficiency loss. Additionally, environmental factors such as high temperatures, humidity, and the corona effect at elevated voltages can further impact energy dissipation.

2. Transmission Line Efficiency Calculation

Transmission efficiency is determined by comparing received power (PR​) to sent power (PS​), calculated as:

Power losses primarily result from resistive heating (I²R), dielectric losses in insulators, and corona discharge at high voltages. Utilities use this efficiency equation to assess power transmission performance, helping identify potential improvements through design optimization and operational adjustments.

3. Methods to Improve Efficiency

Several strategies can enhance transmission efficiency. Increasing voltage reduces current, significantly lowering resistive losses, making high-voltage and high-voltage direct current (HVDC) systems ideal for long-distance power transmission. Using conductors with lower resistance, such as aluminum or composite-core cables, helps reduce energy dissipation. Power factor correction, achieved through capacitor banks, minimizes reactive power losses by optimizing the power factor (cosθ) in the equation:

where θ represents the phase angle between voltage and current. Bundled conductors reduce inductance and corona effects, further improving efficiency. Regular maintenance, such as insulator cleaning and ensuring proper conductor tension, also contributes to reducing energy losses.

4. Typical Efficiency Values

Transmission line efficiency varies depending on design and operating conditions. Short-distance transmission lines typically achieve efficiencies between 95–98%, while long-distance alternating current (AC) lines generally operate within the 90–95% range. HVDC transmission, preferred for extended distances due to its lower energy losses, often surpasses 95% efficiency. These values highlight the need for optimizing transmission infrastructure to maximize energy delivery while minimizing losses.

5. Challenges in Maintaining Efficiency

Ensuring high efficiency in transmission systems presents several challenges. Aging infrastructure can increase conductor resistance, leading to higher energy losses over time. Load variations cause inefficiencies when power demand fluctuates, making it crucial to maintain system balance. Environmental conditions, such as extreme weather, heat, and humidity, impact conductor performance, increasing losses. Addressing these challenges requires regular upgrades, advanced materials, and optimized grid management to maintain consistent efficiency and reliability.

6. Importance of Transmission Efficiency

Efficient power transmission is essential for economic, environmental, and operational benefits. Reducing energy losses lowers operational costs for utilities and consumers. Improved efficiency reduces the need for additional power generation, lowering carbon emissions and supporting environmental sustainability. Reliable power transmission enhances grid stability and ensures uninterrupted electricity supply. By prioritizing efficiency improvements, utilities can achieve cost savings, support sustainable energy goals, and provide better service to consumers.

HOW POWER LOSSES ARE OCCURRED IN A TRANSMISSION LINE?

Power losses in transmission lines arise mainly due to the resistance and reactance of the conductors, which result in energy being converted to heat. These losses can be categorized into different types:

1. I²R Losses (Resistive Losses): Power loss occurs as a result of the conductor’s resistance. When electricity flows through the transmission line, heat is generated due to resistance, leading to energy dissipation.The loss (P) in the conductor is proportional to the square of the current (I) and the conductor’s resistance.
   • Impact: Higher current leads to greater losses. To minimize these losses, transmission lines are typically run at high voltages, which reduces the current and thus the losses.

2. Corona Losses: Corona losses happen when the electric field around the transmission line becomes strong enough to ionize the surrounding air. This causes energy to be lost as ionized particles, often visible as corona discharge. These losses become more significant at high voltages and in humid conditions. The design of the line, such as the size of the conductor and the spacing between them, also plays a role in corona discharge.

3. Inductive and Capacitive Losses (Reactive Power Losses): Transmission lines possess inherent inductance and capacitance. Inductive losses result from magnetic fields created by current flow, while capacitive losses arise from the electric fields between the conductors and the ground.These losses affect the voltage-current phase relationship and lead to reactive power, which does not contribute to real power but causes additional losses, especially in long-distance transmission.

4. Leakage Losses: Leakage losses occur when energy escapes from the transmission line due to poor insulation or the presence of conductive materials.Although small compared to resistive losses, leakage losses still add up and contribute to the overall power loss, especially in cases of inadequate insulation or contamination on the line.

5. Skin Effect: The skin effect is a phenomenon where alternating current (AC) tends to flow mostly on the surface of the conductor, rather than evenly through the entire conductor. As frequency increases, current density becomes concentrated at the surface, raising resistance and causing more losses.The skin effect increases resistance, especially at higher frequencies, leading to greater power losses.

6. Eddy Current Losses: Eddy currents are circulating currents induced in the conductor or nearby metal components due to alternating magnetic fields. These currents cause heat, resulting in energy losses.Eddy current losses are more prominent in materials with high magnetic permeability, and are generally reduced by using materials that produce lower losses.

7. Dynamic Losses (Sags and Tension Losses): Changes in the physical properties of the transmission line, such as sagging or variations in tension due to temperature shifts or mechanical stress, can lead to additional losses. These factors can reduce the efficiency of the power flow.

TRANSMISSION LINE PROTECTION TECHNIQUES

1. Overcurrent Protection – Detects excessive current due to faults like short circuits or overloads. Relays and fuses isolate faulty sections to prevent damage.
2. Distance Protection – Measures line impedance to locate faults, segmenting lines into zones for precise disconnection, ensuring minimal disruption.
3. Differential Protection – Compares currents at both ends of a transmission line to detect imbalances, allowing fast and accurate fault isolation.
4. Pilot Protection – Uses communication between line terminals for quick fault detection and isolation, enhancing protection for long-distance lines.
5. Directional Protection – Determines fault current direction to disconnect only the affected area, preventing unnecessary outages and ensuring grid stability.
6. Reclosing Schemes – Automatically restores power after transient faults (e.g., lightning strikes) by reclosing circuit breakers, reducing service interruptions.
7. Lightning Protection – Prevents voltage surges using lightning arresters and shielding wires, reducing insulation damage and power failures in storm-prone areas.
8. Earth Fault Protection – Identifies phase-to-ground faults caused by insulation breakdown, preventing equipment damage and safety hazards.
9. Backup Protection – Provides a secondary safeguard, ensuring fault clearance if the primary protection system fails, improving reliability.
10. Automation & Control – Uses SCADA and remote systems for real-time monitoring, fault detection, and automated responses, enhancing efficiency.
11. Fault Locators – Determines fault locations using impedance analysis or traveling waves, enabling quicker repairs and reducing downtime.
12. Thermal Monitoring – Tracks conductor temperatures to prevent overheating and possible failures, with sensors triggering alarms if limits are exceeded.
13. Cybersecurity Measures – Safeguards digital protection systems from cyber threats using encryption, firewalls, and intrusion detection.
14. Relay Coordination – Ensures relays operate in a sequence to isolate only faulty sections, preventing unnecessary power cuts and system failures.
15. Surge Protection – Shields equipment from voltage spikes caused by switching or lightning using surge arresters and capacitors, preventing insulation damage.

Frequently asked question

1. What causes power loss in transmission lines?

Power loss in transmission lines is mainly due to the resistance of the conductors (I²R losses), inductive and capacitive reactance, corona effects, and leakage currents.

• What is the purpose of transmission towers?

Transmission towers hold the overhead wires in place, maintaining safe clearance from the ground and other objects to ensure the line operates properly.

• What is a transmission line?

A transmission line is a network of conductors designed to carry electrical power from power plants to substations and then to consumers over long distances.

• What is the difference between AC and DC transmission lines?

AC transmission lines carry alternating current, which reverses direction, while DC transmission lines carry direct current, which flows in one constant direction, often more efficiently over long distances.

• How is the capacity of a transmission line measured?

The capacity of a transmission line is measured by the amount of power (in megawatts) it can safely carry, taking into account its voltage, current-carrying capacity, and impedance.

• Why is high voltage used in transmission lines?

High voltage allows for lower current, reducing energy losses due to resistance in the conductors and improving efficiency when transmitting power over long distances.

• What are the different types of transmission lines?

Transmission lines are classified as short, medium, or long lines, based on their length and electrical properties.

• What is corona in transmission lines?

Corona occurs when the electrical field around a high-voltage line ionizes the surrounding air, causing energy loss, noise, and sometimes visible discharge.

• How are transmission lines protected from faults?

Transmission lines are protected by devices like circuit breakers, fuses, and relays that detect faults and isolate the affected sections to prevent further damage.

1. What is meant by “transmission line impedance”?

Transmission line impedance is the total opposition to the flow of electrical current in the line, which is a combination of resistance and reactance.