HVDC

HVDC, or high-voltage direct current, is a method of transmitting electrical power over long distances using direct current at high voltage. It is commonly used for submarine cables, underground lines, and to interconnect asynchronous AC networks because it can carry large amounts of power with relatively low losses and allows precise control of power flows.

Historically, the first commercial HVDC link was the Gotland–mainland Sweden line, commissioned in 1954, which used mercury-arc valves. Since the 1970s, line-commutated converters based on thyristors became standard for long-distance, high-power links, while voltage-source converters based on IGBTs have become widespread since the 2000s, enabling multiterminal networks and offshore connections.

A typical HVDC installation comprises converter stations at each end, including converters, transformers, filters, and protection and control systems; transmission media such as cables or overhead lines; and supporting equipment. Back-to-back HVDC links connect two adjacent grids without a long DC line.

Two main families exist: line-commutated converters (LCC) and voltage-sourced converters (VSC). LCC systems rely on the AC system to provide reactive power and require robust AC networks, whereas VSC systems can supply their own reactive power, operate with weaker grids, and easily form multiterminal configurations. VSCs are more expensive but are favored for offshore wind connections and urban or modular deployments.

Applications include long submarine cables and cross-border interconnections, integration of offshore wind, and bulk power transfer between distant or asynchronous grids. HVDC remains a key technology for efficient, controllable, and resilient power transmission over challenging routes.