Electrical power generated at power stations needs to be transmitted to distant load centers. For this purpose, two main systems are used – transmission systems and distribution systems.
Based on the installation method, there are two major types of transmission lines:
- Overhead Transmission Line
- Underground Transmission Line
1. Overhead Transmission Line
An overhead transmission line is a system of conductors suspended by poles or towers. These lines carry high-voltage electricity and are generally more economical due to natural air insulation.
Classification of Overhead Transmission Lines
- Resistance (R) and Inductance (L): Act as series impedance.
- Capacitance (C) and Conductance (G = 1/R): Act as shunt admittance.
Materials used in towers include wood, steel, concrete, aluminum, and reinforced plastics. Conductors are mainly made of aluminum (sometimes steel-reinforced) or copper.
Types Based on Length & Voltage:
| Transmission Line | Voltage | Length | Effect of R, L, C |
|---|---|---|---|
| Short Transmission Line | Below 20 kV | Up to 50 km | R & L |
| Medium Transmission Line | 20 kV to 100 kV | 50 km to 150 km | R, L, C |
| Long Transmission Line | Above 100 kV | More than 150 km | R, L, C |
2. Underground Transmission Line
Underground transmission lines are buried cables used for transmitting electricity. These are ideal for densely populated or environmentally sensitive areas.
Key Features:
- Less visual impact and aesthetically pleasing
- Higher safety due to no exposed wires
- More expensive than overhead lines
- Used mainly up to 132 kV
Difference Between Overhead and Underground Transmission Lines
| Characteristic | Overhead Lines | Underground Cables |
|---|---|---|
| Initial Cost | Low | High |
| Public Safety | Less | More |
| Maintenance Cost | High | Low |
| Occurrence of Faults | High | Low |
| Visual Appearance | Poor | Good |
| Fault Identification | Easy | Difficult |
| Communication Line Interference | Yes | No |
| Voltage Drop | More | Less |
| Lightning Damage | Yes | Immune |
| Wire/Cable Jointing | Easy | Difficult |
| Operating Voltage | Up to 765 kV or more | Up to 132 kV |
| Charging Current | Less | More |
| Surge Effect | More | Less |
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Frequently Asked Questions (FAQs)
What are the types of transmission lines?
The two main types of transmission lines are Overhead Transmission Lines and Underground Transmission Lines.
Which is better: overhead or underground transmission?
Overhead lines are more cost-effective for long distances and high voltages, while underground cables offer better safety, aesthetics, and are suitable for urban areas, but are more expensive.
What is the voltage range for underground transmission lines?
Underground transmission lines are typically used for voltages up to 132 kV.