Material Insights · Understanding the Material World

HVDC links: How it works

How converters let operators steer exact megawatts

What this tab covers How the energy actually moves, step by step: the science that makes it work and the engineering that makes it practical.

An HVDC link turns alternating current into direct current at one end, sends it along an overhead line or cable, and turns it back into AC at the other. It loses less power over long distances, works under the sea, and can join grids that are not in step with each other.

Step by step

  1. Collect AC powerPower arrives from the AC grid or an offshore wind farm at a converter station, where a converter transformer sets the voltage.
  2. Convert to DCPower-electronic valves (thyristors in line-commutated converters, or fast-switching IGBT transistors in voltage-source converters) turn AC into DC.
  3. TransmitDC flows on two poles (one positive, one negative, hence voltages written as ±800 kV) on overhead towers or through insulated land and subsea cables.
  4. Convert back to ACA second converter station produces AC synchronised to the receiving grid.
  5. Control the flowOperators set exactly how many megawatts flow and in which direction, which an ordinary AC line cannot do.

The science

Resistive loss rises with the square of the current (P = I^2 R), so very high voltage keeps losses down. DC adds two advantages. An AC cable must keep charging and discharging its own capacitance; over a long cable this charging current takes up much of the conductor's capacity, so beyond a few tens of kilometres HVDC becomes cheaper. And because DC has no frequency, it can link grids that run at different frequencies or out of phase. The price is the converter stations, each losing roughly 0.7 to 1% of the power passing through.

The engineering

A converter station is a large hall of valves, filters, transformers, cooling and controls. Line-commutated converters (LCC) are cheaper per MW and carry the largest powers but need strong AC grids at both ends; voltage-source converters (VSC) are more compact, can restart a dead grid and support weak grids, and have become the dominant type. The record holder is China's Changji to Guquan link: ±1,100 kV, 3,293 km and 12,000 MW.

Key numbers

Highest voltage in service ±1,100 kV (Changji to Guquan, China)1 Longest and largest link 3,293 km, 12,000 MW1 Converter losses per station About 0.7 to 0.8% (LCC); about 0.8 to 1% (modern VSC)45 Longest land and subsea interconnector Viking Link, 475 miles (765 km), 1.4 GW8

Sources

  1. The world's most powerful transmission system facilitated by Hitachi ABB Power Grids Hitachi Energy, 2020
  2. Overview of HVDC Technologies and EPRI's HVDC Research (R. Adapa, presentation to ERCOT) EPRI / ERCOT, 2023
  3. Market Integration of HVDC Lines: Internalizing HVDC Losses in Market Clearing (Tosatto, Weckesser, Chatzivasileiadis) arXiv (Technical University of Denmark), 2018
  4. National Grid announces commercial operations of Viking Link, the world's longest land and subsea interconnector National Grid, 2023