Multi-terminal VSC-HVDC

In December 2013, the first multi-terminal VSC-HVDC system went into operation. Today, the ground-breaking system’s enduring success has made it a reference project for electric utilities and renewables organisations around the world. VSC-HVDC is set to become an important enabling technology for the grid of the future. 

What is VSC-HVDC?

High voltage direct current (HVDC) is widely acknowledged as the most effective means to transmit bulk power over long distances when using AC becomes less efficient or economic. However, its use has traditionally been limited by problems surrounding power flow and protection when connecting to weak AC systems. 

This is changing, thanks to developments in Voltage Source Converters (VSCs). By incorporating a voltage source on the DC side when delivering to AC systems, VSC power conversion can maintain a prescribed voltage, irrespective of the magnitude or polarity of the current flowing through the source. What’s more, bidirectional power flow can be achieved without DC voltage power reversal or complicated mechanical switching. 

The world's first multi-terminal VSC-HVDC link

A core differentiator of VSC-HVDC is its ability to overcome the limiting two-terminal point-to-point set-up of conventional HVDC connections. A multi-terminal HVDC system can connect more converters via a single DC system, paving the way for DC grids.

A landmark multi-terminal VSC-HVDC system developed for China’s Nan’ao Island grid reinforcement and wind power plant went online in December 2013. It consists of three converter stations at different locations, connected by a combination of subsea DC cable, land DC cable and overhead DC line. The goal was to reduce transmission losses, increase network security and eliminate the need for reactive power compensation when incorporating wind power generated on the island into the onshore grid. 

RXPE was commissioned by China Southern Grid to supply the 200MW VSC-based HVDC valve and control package. We also provided station controls for all three terminals as well as the overall multi-terminal control and protection system.   

The completion and ongoing success of this project is a significant milestone in the development of power transmission technology. RXPE has overcome one of the industry’s most complex challenges, the intricacies of control and protection for multi-terminal HVDC.

Looking ahead

The Nan’ao Island VSC-HVDC system broke new ground in transmission management to deliver multiple benefits. Many organisations have been following its progress with interest.

Going forward the project will provide a blueprint for safe and efficient long-distance, large-capacity DC transmission. As well as offering much potential for transmission between remote locations, it can facilitate cross-border transmission ties where a power deficit in one country is balanced by surplus power in another.

Two years ago, VSC-HVDC was a pioneering technology. Today, it is a proven technology set to become an important enabler for the long-term improvement of power transmission.

Lalit Tejwani explored the potential of VSC-HVDC to overcome power challenges in India and the wider South Asian region in the October 2015 issue of Power Line magazine. Sign up to our Premium Content library to access the full article.