Transmission lines are used to carry electrical power from generating stations to substations and load centres. Classifications of transmission lines are used depending on voltage level, current type, construction method, and line length. This classification helps engineers choose the right system for safe, efficient, and economical power transfer.
Different Types of Transmission Lines
1. Based on Voltage Levels
- Ultra High Voltage (UHV): Above 765 kV
Used for very long-distance bulk power transfer. Common in countries with very large and interconnected power grids. - Extra High Voltage (EHV): 220 kV to 765 kV
Used for long-distance transmission of large amounts of power between major grid substations. - High Voltage (HV): 110 kV to 220 kV
Used for regional transmission and interconnection between large substations. - Medium Voltage (MV): 33 kV to 110 kV
Mainly used for sub-transmission and linking major substations to distribution substations. - Low Voltage (LV): Below 33 kV
Used for local distribution and supply to end users through distribution networks.
2. Based on Current Type
- AC Transmission Lines (Alternating Current)
Most commonly used system for power transmission. Suitable for both short and long distances.
Typical voltage levels include 66 kV, 132 kV, 220 kV, and higher. - DC Transmission Lines (Direct Current / HVDC)
Used for very long distances and submarine or underground cables because losses are lower.
Common HVDC levels are 100 kV, 500 kV, 800 kV or more.
3. Based on Line Configuration
- Overhead Transmission Lines
Conductors are supported on towers or poles above the ground.
This is the most common and economical method, especially for high and extra high voltage lines. - Underground Transmission Lines
Cables are laid below the ground.
Used in cities and congested areas where overhead lines are not practical or safe.
4. Based on Length of Line
- Short Transmission Lines (Up to 80 km)
Have small voltage drop and negligible capacitance effect.
Commonly used to connect nearby substations. - Medium Transmission Lines (80 km to 250 km)
Show moderate voltage drop.
Capacitance and inductance effects become noticeable.
Used for regional power transfer. - Long Transmission Lines (Above 250 km)
Have significant voltage drop and strong capacitance and inductance effects.
Often need reactive power compensation.
Used for long-distance bulk power transmission.