What is Transmission Line? types, parameter, voltage level, advantages

What is Transmission Line?

A transmission line is like a big electric road. Just like trucks carry goods from one city to another, a transmission line carries electricity from power stations to our homes, schools, offices, and factories.

Simple Example:

Imagine a water pipe. Water flows through the pipe and reaches your tap. In the same way, electricity flows through a transmission line and reaches every place where it is needed.

Why Do We Need a Transmission Line?

When electricity is made at power stations, it needs to travel long distances to reach cities and villages. A transmission line helps electricity move safely and quickly without much loss.

Introduction:

To carry electricity from where it’s made to where it’s used, we need a strong and smart system. This system has two main parts:

1. Transmission System – This carries electricity over long distances from power plants to big substations.
2. Distribution System – This brings electricity from substations to homes and businesses.

There are different types of transmission and distribution systems. Not all of them look the same. Some areas may only need a simple setup, while others might need a full system with more steps.

The electricity is usually generated by three-phase alternators. The starting voltage is often 11,000 volts (11kV), but it can be more, like 33kV. To send it over long distances, we increase this voltage using step-up transformers. This helps save energy and reduces the size of wires needed.

But we can’t keep increasing the voltage too much. Higher voltage means more insulation, more expensive equipment, and more safety risks. That’s why engineers carefully choose the right voltage, balancing cost and safety.

In India, we use different high-voltage levels for AC (Alternating Current) transmission:
66kV, 132kV, 220kV, 400kV, and even 765kV for the biggest transmission lines.

Type of Transmission line

Primary Transmission –

When electricity is made at the power station, it needs to travel a long way to reach our homes, schools, and offices. The very first step of this journey is called primary transmission.

In primary transmission, electricity is sent in bulk (in large amounts) from the generating station to a big substation near cities or towns. This is done using three-phase (3-ф), three-wire overhead lines.

The voltage of electricity here is very high, so it can travel long distances without losing much energy. These strong lines carry power from point A to point B, which is the primary transmission line.

⚡ Why Use High Voltage?

High voltage helps electricity travel far without wasting energy. It’s like using a big water pipe to move more water quickly and easily.

🏙️ Secondary Transmission –

After the electricity reaches the first substation near a city, it enters the secondary transmission stage.

Now, power is again sent through three-phase, three-wire lines, but this time it goes from the receiving substation to smaller substations inside the city or near places where people live and work.

This part is shown as point C to point D in the diagram.

Here, the voltage is reduced (stepped down) to 33,000 volts (33 kV) or 11,000 volts (11 kV) so it’s safer and easier to use for homes, shops, hospitals, and factories.

1. Type of voltage:
   a. A.C. Transmission line
   b. D.C Transmission line

2. Type of system:
   a. Overhead transmission
   b. Underground transmission
   
3. Length of power transmission line system:
   a. Long power transmission line
   b. Medium power transmission line
   c. Short power transmission line
   
   

Classification of Transmission and Distribution Lines by Voltage Level

Electricity travels through wires before it reaches our homes, schools, factories, and farms. But not all wires carry the same amount of electricity. Some carry very high voltage, while others carry low voltage, depending on how far and how much electricity they need to deliver.

To keep everything safe and working properly, we follow standard voltage levels. These levels are clearly fixed in the Indian Grid Code, and electricity must always stay within these limits.

⚡ Standard Voltage Levels for Transmission Lines

Below is a simple table that shows the standard voltage range for transmission lines – these are the big wires that carry electricity over long distances:

These high voltages help carry a large amount of power over long distances without much loss.

🏠 Standard Voltage Levels for Distribution Lines

Once the electricity comes close to our homes and businesses, it needs to be reduced to safer levels. That’s where distribution lines come in. These lines carry electricity at lower voltages so we can use it safely.

Here are the common voltage levels used in the distribution network:

• 66 kV
• 33 kV
• 22 kV
• 11 kV
• 400 volts / 230 volts
• Sometimes also: 6.6 kV, 3.3 kV, 2.2 kV
  
  
  
  

AC and DC Transmission: Simple Explanation for Everyone

Electricity can travel in two main ways — through AC (Alternating Current) or DC (Direct Current) systems. Both have their own good points and bad points. We use them based on what we need, how far the electricity has to go, and what’s more cost-effective.

Let’s understand them both in a very easy and clear way.

🔌 What is AC Transmission?

AC means Alternating Current. This is the type of electricity we usually get at home, in schools, and in shops.

✅ Advantages of AC Transmission

• Easy to increase or decrease voltage: With the help of transformers, we can easily step up or step down the voltage. This is super helpful!
• Good for long distances: High voltage AC can be sent over long distances more economically.
• Cheaper equipment: The tools and machines used in AC systems are easy to maintain and cost less.
• Most power stations use AC: That’s why it’s very common everywhere.

❌ Disadvantages of AC Transmission

• More wires needed: AC systems usually need 3 wires instead of one.
• Higher cost of poles and wires: More wires mean more money spent.
• Power loss due to inductance and capacitance: These are invisible effects in the wires that cause some power to be wasted.
• Slightly more complex structure: Setting up an AC transmission line needs more care and planning.

⚡ What is DC Transmission?

DC stands for Direct Current. This is the type of electricity you find in batteries and some modern power systems.

✅ Advantages of DC Transmission

• Less power loss: DC does not have issues like inductance or capacitance, so it’s better for very long distances.
• Only two wires needed: This saves a lot of money on conductors.
• Simple structure: DC transmission lines are easier to build and handle.
• Stable and smooth power flow: It is constant and does not change direction.

❌ Disadvantages of DC Transmission

• Voltage can’t be changed easily: It is hard to step up or step down voltage in DC without special converters.
• More expensive devices needed: The equipment used to convert AC to DC and back again can be costly.
• Not common for local power: Most of our daily-use devices and systems work on AC.

⚖️ Which One is Better – AC or DC?

There is no one perfect choice. It depends on what we are trying to do.

• For short distances and everyday use, AC is better.
• For very long distances or special projects, DC is more efficient.

Thanks to modern technology, both AC and DC can be used smartly in different situations.

Classification of Voltage Levels in Power System – Explained Simply

Electricity travels a long way before it reaches our homes, schools, and workplaces. On this journey, the voltage of electricity keeps changing depending on where it is and what it needs to do. Let’s understand this step by step in a very easy and simple way.

🔌 What Is Voltage?

Voltage is like the pressure that pushes electricity through wires. Just like water flows faster through a pipe if you increase the pressure, electricity flows better when the voltage is right. Different parts of the power system use different voltages.

⚙️ Different Voltage Levels in the Power System

The electric power system has different stages — from where electricity is made to where it is finally used. At each stage, the voltage is different. Here’s a simple breakdown:

🏭 1. Generation Voltage

This is where electricity is produced, like in power stations.

• 💡 Common voltages: 11 kV and 33 kV
• 🏭 Where used: Power plants and generators

🌐 2. Primary Transmission Voltage

Here, electricity is sent over long distances through big towers and thick wires.

• ⚡ Common voltages: 132 kV, 220 kV, 400 kV, and even up to 765 kV
• 🏔️ Where used: Between cities, regions, and states

🏙️ 3. Secondary Transmission Voltage

Now the voltage is reduced a bit and sent closer to towns and cities.

• 🔋 Common voltages: 33 kV and 66 kV
• 🏢 Where used: From big substations to smaller ones near cities

🏘️ 4. Primary Distribution Voltage

This voltage brings electricity near your neighborhood.

• 🔌 Common voltages: 6.6 kV and 11 kV
• 🌇 Where used: Local areas and small substations

🏠 5. Secondary Distribution Voltage

This is the final voltage that reaches your home or shop. It is safe to use in everyday appliances.

• 🏡 Common voltages:
  • 415 volts for three-phase (used in factories, big machines)
  • 220 volts for single-phase (used in homes and small shops)

✅ Summary Table – Voltage Levels at a Glance

Advantages and Disadvantages of High Voltage Transmission

Electricity is like water flowing through a pipe. If we want to send a large amount of electricity from one place to another, we need a smart way to do it. That’s where high voltage transmission comes in.

Let’s understand it in a simple and natural way.

✅ Why Do We Use High Voltage for Power Transmission?

When electricity travels over long distances — like from power stations to our homes — it is sent at high voltage. Why?

Because high voltage means low current, and when current is low, less energy is wasted. It’s like sending a big water tank through a narrow pipe — high pressure helps move more water with less effort.

✨ Main Advantages of High Voltage Transmission

Here are the simple benefits:

1. 🔌 Smaller Wires Are Needed

When we increase the voltage, we can reduce the size (or thickness) of the wires. This saves metal like copper or aluminum, which makes the wires lighter and cheaper.

2. ⚡ Less Energy Is Lost

Low current means there is less heat in the wires. That means less energy is wasted during the journey. This makes the system more efficient.

3. 📉 Better Voltage Control

With high voltage, the voltage at the end of the line doesn’t drop too much. This gives better voltage regulation, so our devices work properly without damage.

4. 📈 Higher Efficiency

When we lose less power, the overall efficiency improves. This means more power reaches the homes and factories that need it.

5. 💰 Cost Saving in the Long Run

Even though high voltage needs stronger equipment, the cost of wires and support towers becomes less because of lighter materials.

⚠️ Disadvantages of High Voltage Transmission

But everything has two sides. Let’s look at the challenges of high voltage transmission:

1. 💵 High Installation Cost

The setup cost is high. We need bigger transformers, stronger switches, and special protective devices, which are expensive.

2. 🗼 Taller Towers Needed

To carry high voltage wires safely, we need tall transmission towers. These towers keep the wires high above the ground for safety, but they cost more and need more space.

3. 🧯 More Insulation Required

High voltage can jump across gaps. To prevent accidents and short circuits, more insulation is used. This adds to the cost and maintenance.

Different Systems of Transmission of Electrical Power

(Simple Guide for Everyone)

Electric power is generated in power stations and needs to travel long distances to reach our homes, schools, factories, and farms. This journey is made possible through power transmission systems.

In most cases, we use a three-phase (3-φ), three-wire AC transmission system to send power over long distances. This system is strong, reliable, and efficient. When the electricity finally reaches towns and cities, a three-phase, four-wire AC distribution system is often used to deliver electricity to homes and buildings.

But did you know? There are many other systems that can also be used to transmit or distribute electricity, depending on the situation and need. Let’s understand them one by one.

🔹 A. DC (Direct Current) Systems

These are older but still useful in some cases, like in special industrial applications or railways. Here are the types:

• 2-wire DC system – Two wires carry current to and from the load.
• 2-wire DC system with earthed midpoint – The center point of the power supply is connected to the earth (ground) for safety.
• 3-wire DC system – This gives two voltage levels and saves conductor material.

🔹 B. Single-Phase AC (Alternating Current) Systems

Used mostly for small loads, like in homes or small shops.

• 1-phase, 2-wire system – Simple and common for homes.
• 1-phase, 2-wire system with grounded midpoint – Adds more safety.
• 1-phase, 3-wire system – Offers two voltage levels and can reduce wiring cost.

🔹 C. Two-Phase AC Systems

These are rarely used today, but they still exist in a few old installations.

• 2-phase, 3-wire system
• 2-phase, 4-wire system

🔹 D. Three-Phase AC Systems

These are the most widely used systems for transmitting power over long distances.

• 3-phase, 3-wire system – Simple, low-cost, and commonly used in transmission.
• 3-phase, 4-wire system – Often used for distribution to consumers, as it includes a neutral wire.
  
  
  

Comparison of Conductor Material Required for Transmission Lines

The conductor material requirement depends on:

• Type of system (DC or AC)
• Type of transmission (Overhead or Underground)
• Voltage levels (either between conductor and earth, or between conductors)

A. DC Systems

✅ Best Saving: DC two-wire with earthed midpoint (only 25% of material for overhead)

B. Single-Phase AC Systems

🚫 High Material Usage: Single-phase AC uses more conductor material compared to DC.

C. Two-Phase AC Systems

🔸 Not commonly used due to complexity and higher material use.

D. Three-Phase AC Systems

✅ Most Efficient AC System:
Three-phase three-wire system requires less material for both overhead and underground transmission.

• DC systems are more material-efficient, especially for overhead lines.
• However, DC systems are technically more complex and harder to manage.
• Three-phase AC systems are the most practical and efficient for large-scale transmission:
  • Reliable
  • Less conductor material required
  • Widely adopted worldwide
    
    
    
    

Transmission Line Construction

The construction of overhead transmission lines involves several key steps:

1. Site Preparation
2. Foundation Construction for Line Supports
3. Erection of Line Supports (Steel Towers)
4. Wire-Stringing Operations

1.10.1 Site Preparation

Before setting up transmission lines, the site must be prepared. Here’s what happens:

• Land Acquisition:
  A strip of land is taken for the construction. This land is called the Right of Way (RoW).
• Right of Way (RoW):
  It’s the clear strip of land along which the transmission line is built. The line runs through the center of this strip, and towers are placed at fixed intervals.
• Clearing the Site:
  Before tower construction, the locations are cleared of trees and vegetation.
• Width of RoW:
  The required width depends on:
  • Voltage of the line
  • Design and height of towers
  • Amount of sag (wire droop)
  • Wind conditions
  • Safety standards

RoW Requirements by Voltage (India)

Access Roads

• Construction Vehicles Need Access:
  Roads may be upgraded or new ones built to allow vehicles and equipment to reach each tower location.
  
  

Construction of Foundations for Line Supports

Most electrical towers are built on concrete foundations. The type and size of the foundation depend on:

• The soil condition (hard, soft, rocky, etc.)
• The type of steel tower (lattice or tubular)
• The terrain (plain, hilly, forest, etc.)

Steps in Foundation Construction:

1. Digging Holes:
   • Lattice towers need four holes (for four legs).
   • Tubular steel poles need one central hole.
2. Steel Reinforcement and Concrete:
   • Steel bars are placed in the holes for reinforcement.
   • Concrete is poured into these holes.
   • The foundation is made slightly above ground level.
3. Curing Time:
   • Construction of the tower begins only after the concrete is fully set and hardened.

Construction of Line Supports (Steel Towers)

Line supports are structures that hold overhead conductors (wires) above the ground, maintaining:

• Proper ground clearance
• Spacing between conductors
• Mechanical strength for wind, weight, etc.

Types of Line Supports:

1. Pole Structures:
   • Made of concrete, steel, or aluminium.
   • Used for low to medium voltage lines.
2. Tower Structures:
   • Usually steel lattice towers.
   • Used for high voltage lines (110 kV and above).

Tower Construction Process:

• Towers are fabricated in parts at factories.
• Parts are transported to the site and assembled on the ground.
• Cranes are used to lift and install the parts.
• In difficult terrain, helicopters may be used for construction.
• Electrical poles may be assembled in sections and then erected.

Wire Stringing Process

Wire stringing is the process of installing conductors and earth wires onto the transmission towers.

Main Steps in Wire Stringing:

a. Installing Insulator Strings:

• Attached to the arms of towers.
• Number of discs in each string depends on voltage level.

b. Stringing Pilot Wire:

• A light pulling rope is passed from tower to tower using machines or helicopters.
• Ropes pass through rollers (stringing sheaves) at each tower.

c. Conductor Wire Pulling:

• The conductor is attached to the pilot rope.
• A puller machine pulls the wire; a tensioner machine maintains even tension and sag.

d. Sagging and Dead Ending:

• Proper sag and tension are applied using tensioners.
• Sag depends on temperature:
  • High temperature = more sag (wire expands)
  • Low temperature = less sag, more tension
• Proper installation ensures safe ground clearance.

e. Splicing:

• Temporary joints are replaced with permanent splices/connectors.
• After splicing, the wire is fixed at dead-end towers.

Note:

• The same basic process is followed for 110 kV, 220 kV, and 400 kV lines.
• Differences occur in:
  • Size of conductors
  • Size/type of towers
  • Insulator string lengths
  • Terrain and power capacity

Introduction to Wire Stringing

Wire stringing is a critical phase in the construction of transmission lines. It involves the installation of conductors (line wires), earth wires (ground wires), insulator strings, and various mechanical assemblies onto transmission towers. The objective is to ensure that conductors are laid out with the correct tension and sag, ensuring safety, reliability, and consistent electrical performance over the entire stretch of the transmission line. This process is typically the same for high-voltage lines such as 110 kV, 220 kV, and 400 kV, though certain elements like conductor type and tower design may vary depending on the voltage level, transmission capacity, and terrain conditions.

Fixing of Insulator Strings

The first step in the wire stringing process is to fix the insulator strings to the arms of the transmission towers. These insulators serve a crucial role by electrically separating the live conductors from the grounded metallic structure of the tower. The insulator strings are typically made up of disc-type insulators connected in series. The number of discs in a string depends on the voltage level of the transmission line — higher voltages require more discs to ensure proper insulation. These insulators are mounted on the cross-arms of the towers at predetermined locations, which are specifically designed to hold the weight of the conductors and maintain electrical clearances.

Stringing the Pilot Wire

After the insulators are in place, the next stage involves stringing the pilot wire — a lightweight pulling rope or cable that acts as a guide for the main conductor wire. This rope is passed from tower to tower, often using rope-pulling machines or helicopters in difficult terrain. The pilot wire is threaded through pulleys or rollers (also known as travellers or stringing sheaves) that are mounted on the lower part of the insulator strings. These pulleys help in guiding the wire smoothly during pulling operations. A camlock device is used to secure the rope to the pulley system, ensuring that it remains safely in position as the pilot wire is pulled through the line.

Pulling the Conductor Wire

Once the pilot wire is in place along the entire stretch of the transmission line, it is connected to the conductor wire. The pulling of the conductor wire is a synchronized process that involves two major machines — the puller and the tensioner. The puller machine draws the conductor wire along the path defined by the pilot wire, while the tensioner machine ensures that the conductor is fed at a controlled rate with proper tension. This collaboration between the two machines ensures that the conductor maintains consistent sag and clearance from the ground, which is critical for both electrical and mechanical safety.

Sagging and Dead Ending

As the conductor is pulled across the transmission line, precise tension and sag must be applied. Sag refers to the downward curve that a conductor naturally forms between two towers due to its weight. If the conductor is too tight (less sag), it may break due to thermal expansion or excessive mechanical stress. On the other hand, if the sag is too much, the conductor might violate minimum ground clearance requirements, posing a safety risk. Temperature plays a key role in this — at high temperatures, the conductor expands and sags more, while at low temperatures, it contracts and increases tension. Therefore, engineers calculate and apply the correct tension during installation to account for seasonal variations.

Splicing of Conductors

After achieving the required sag and tension, the temporary joints or splices used during the pulling process are replaced with permanent splicing connectors. These connectors ensure mechanical strength and electrical continuity between conductor segments. The splicing is done carefully to prevent any weak points in the transmission line. Once splicing is completed and verified, the conductors are securely clamped and fixed to the insulators or suspension clamps at intermediate towers and to dead-end towers at both ends. The entire system is then checked for proper alignment, tension, and electrical clearance.

Conclusion

Wire stringing is a detailed and carefully controlled process that plays a foundational role in the erection of transmission lines. Each step — from installing insulator strings to splicing and dead-ending — must be executed with precision to ensure the safety, durability, and efficiency of the power transmission network. While the general method remains similar across voltage levels like 110 kV, 220 kV, and 400 kV, the specific design of conductors, insulators, and towers varies depending on the line’s voltage, length, power capacity, and geographical factors.

FAQ