Short Answer:

High-voltage DC (HVDC) transmission is used to transfer electrical power over very long distances using direct current instead of alternating current. Its main function is to reduce power losses during long-distance transmission and to improve efficiency. HVDC is especially useful when electricity needs to be sent across seas, through underground cables, or between regions with different power systems.

HVDC transmission provides stable and controlled power flow, requires fewer conductors, and allows connection of asynchronous grids. It is commonly used in modern power systems to connect remote power plants, integrate renewable energy sources, and transfer electricity between countries.

Detailed Explanation:

Function of high-voltage DC transmission

Introduction to HVDC Transmission

High-voltage DC transmission is a system where electricity is transmitted in the form of direct current at very high voltages, typically above 100 kV. Unlike AC systems, where current changes direction regularly, HVDC carries current in a single direction. This unique feature offers several advantages, especially for long-distance and high-power transmission.

HVDC transmission is not new but has become more important due to increasing power demands, expansion of renewable energy, and the need to connect power grids over large geographical areas.

Main Functions of HVDC Transmission

1. Long-Distance Power Transfer with Low Losses
   HVDC is more efficient than AC for very long distances (typically over 600 km). AC systems suffer from more losses due to reactance and continuous change in current direction. HVDC avoids these losses and delivers more usable power over the same line.
2. Interconnection of Asynchronous Grids
   Different regions or countries may operate their power grids at different frequencies or timings. HVDC allows these grids to be connected without needing to match frequency, thus improving reliability and enabling power sharing across borders.
3. Stability and Controlled Power Flow
   HVDC systems allow precise control of the amount and direction of power flow. This helps maintain grid stability, avoid overloads, and respond quickly to changes in demand or supply.
4. Efficient Underground and Underwater Transmission
   AC transmission through cables over long distances suffers from capacitive losses. HVDC works better with underground or submarine cables, such as those used between islands or across countries separated by water.
5. Integration of Renewable Energy
   Wind farms, solar plants, or hydroelectric stations are often located far from cities. HVDC lines connect these remote sources to the main grid efficiently, supporting clean energy distribution and reducing dependency on fossil fuels.
6. Reduced Right-of-Way and Infrastructure
   HVDC systems require fewer conductors and simpler tower designs. This reduces land usage and is especially useful in crowded or environmentally sensitive areas.

How HVDC Transmission Works

• Conversion Process: At the sending end, AC is converted to DC using a converter station.
• Transmission: DC travels through HVDC lines over long distances.
• Reconversion: At the receiving end, DC is converted back to AC using another converter station, so it can be used in normal electrical systems.

Modern HVDC uses two main technologies:

• Line Commutated Converters (LCC) – Used for bulk power transmission
• Voltage Source Converters (VSC) – Used for flexible, compact systems and connecting renewable sources

Common Applications of HVDC

• Connecting offshore wind farms to mainland grids
• Cross-border electricity trading (e.g., between India and Bhutan)
• Linking remote hydro power stations to distant cities
• Providing backup power links between national grids
• Urban power supply where overhead lines are difficult

Conclusion

The function of high-voltage DC (HVDC) transmission is to enable efficient, stable, and flexible transfer of power over long distances or across different power systems. It reduces energy losses, supports the use of underground and submarine cables, and helps in integrating renewable energy sources. HVDC has become a critical part of modern electricity networks, especially for connecting regions with different power demands and structures.