Understanding HVDC and EHVAC Transmission Lines: Why Overhead Lines for HVDC?

The transmission of electricity is a fundamental challenge in modern societies, and the efficiency of this process is critical for both economic and environmental reasons. Two prominent technologies in this domain are High-Voltage Direct Current (HVDC) lines and Extra High-Voltage Alternating Current (EHVAC) lines. Each has its unique applications and characteristics.
The main difference between these two transmission methods lies in the nature of the current they use. HVDC lines transmit power as direct current (DC), which carries a constant direction (positive and negative). On the other hand, EHVAC lines transmit power as alternating current (AC), which changes direction periodically. Both systems can transmit power at very high voltage levels, typically ranging from 500kV to 1000kV or more, to minimize line losses and increase the efficiency of power transmission.

Key Differences Between HVDC and EHVAC Lines

The choice between HVDC and EHVAC lines depends on several factors, including distance, terrain, and the nature of the power grid. HVDC lines are often used for long-distance teleconveyance, while EHVAC lines are more common for shorter to medium distances within a power grid. This choice is critical as it significantly affects the overall efficiency and cost-effectiveness of power transmission.

The Role of Overhead Lines

Both HVDC and EHVAC lines predominantly use overhead lines with bare conductor wires, which are installed openly in the air. This design choice is not arbitrary but is a strategic decision aimed at maximizing efficiency and cost-effectiveness. The overhead installation eliminates the need for more expensive insulating materials used in cable systems. Most overhead lines are supported by metal structures fitted with insulator strings, such as ceramic or glass insulators, which help maintain the required voltage levels and electrical isolation between different phases.

Why Overhead Lines for HVDC?

Despite the potential advantages of using cables, HVDC lines typically do not use cables for overhead transmission. There are several compelling reasons for this:

Lower Losses due to DC Transmission

Direct current (DC) transmission in HVDC lines has inherent advantages. In the DC system, the power loss is primarily due to the resistance of the conductors and is independent of the distance over which the DC is transmitted. This implies a significant reduction in transmission losses compared to AC systems, where the inductance and capacitance of the transmission lines also contribute to the loss. In contrast, AC transmission involves additional reactive power losses due to the changing current direction, which are not present in DC systems.

Economic Considerations of Overhead Lines

Cables, especially high-voltage cables, are complex and expensive to manufacture and install. Overhead lines, using bare conductor wires, are simpler and cheaper. This simplicity in design translates to lower initial and maintenance costs, which is a significant factor in the overall cost of power transmission projects.

The Role of Air as a Natural Insulator

One of the primary advantages of using overhead lines in both HVDC and EHVAC systems is the application of air as a natural insulator. There is no need for additional insulating materials, which simplifies the installation process and reduces costs. In both scenarios, insulating ceramics or glasses are used at intervals to support the conductors and maintain the necessary electrical clearance, but the basic elements are freely exposed to the air, a readily available and cost-effective insulator.

Conclusion

In conclusion, while both HVDC and EHVAC lines have their merits and applications, HVDC lines predominantly use overhead lines with bare conductor wires due to the inherent benefits of direct current transmission and the overall cost-effectiveness of such a system. The choice of HVDC or EHVAC lines is influenced by a complex interplay of factors, but the advantages of fewer losses, lower costs, and the availability of air as a natural insulator make it a highly efficient choice for long-distance power transmission.

Keywords: HVDC lines, EHVAC lines, power transmission efficiency