HVDC

HVDC, or high-voltage direct current transmission, transfers power using direct current between converter stations that connect to AC systems at each end. It is used when direct-current transmission offers technical or economic advantages over conventional HVAC alternatives.

HVDC is especially attractive for long-distance bulk transfer, submarine or underground cable links, and interconnection of asynchronous systems. Because converter stations can control power flow directly, HVDC is also a powerful tool for operational control and system integration.

Key Aspects of HVDC:

• Converter-Based Link: An HVDC scheme converts AC to DC at one terminal and DC back to AC at the other. The converter technology, line-commutated or voltage-source based, strongly affects the system capabilities of the project.
• Best-Use Cases: HVDC is often favored for very long transmission distances, submarine cables, underground cables, or links between systems that are not synchronized in frequency. In these applications, its advantages can outweigh the higher terminal cost.
• Controllable Power Flow: Unlike a standard AC line, an HVDC link can set power transfer directly within its operating range. This makes it valuable for relieving congestion, limiting loop flows, and improving inter-area controllability.
• Stability and Integration Benefit: HVDC can support weak interconnections, connect remote renewable resources, and reduce some of the reactive and charging issues associated with long AC corridors. Modern VSC designs add functions such as black-start support and flexible reactive control.
• System-Planning Tradeoff: HVDC projects require expensive converter stations and detailed integration studies, so they are not automatically the best choice for every corridor. Their value depends on distance, cable requirement, controllability needs, and the characteristics of the connected systems.