By the end of September 2004, Siemens Power Transmission and Distribution (PTD; Erlangen, Germany) commissioned the 940-km (584-mile) long Gui-Guang high-voltage direct-current (HVDC) transmission link between Guizhou and Guangdong in southeast China, six months ahead of the contractually agreed date.

After a short trial period, the HVDC system is now transmitting 3000 MW of electricity from the west of China to the up-and-coming economic zone centering on Guangzhou and Shenzhen in the southeast of the country.

Siemens PTD received the 350 million euros order from the State Power Corporation of China in Guangzhou in October 2001. The contract covered construction and equipping of the two converter stations for the HVDC link.

The HVDC transmission link features two poles, each rated for 1500 MW at +/- 500 kV DC, and consists of one converter station in Anshun in the province of Guizhou and one in Zhaoqing in the province of Guangdong, about 200 km (124 miles) from Hong Kong. The two stations are connected via a 940-km long overhead line. Siemens delivered and installed the complete HVDC transmission system including converter valves, converter transformers, smoothing reactors, high-voltage switchgear and the associated control and protection systems.

Siemens' light-fired thyristors with integrated overvoltage protection reduced the number of additional electronic components for the HVDC converter valves by 80%. A further benefit is that the firing and monitoring electronics are entirely at ground potential and therefore accessible during system operation. This recently commissioned HVDC transmission system is the first in China to work with such thyristor technology.

Gui-Guang is Siemens PTD's third HVDC transmission project in China. The first was the Ge-Sha link between Gezhouba and Shanghai (1200 MW over 1040 km [646 miles]). This system entered operation in 1989. The second project was the Tian-Guang line between Tianshengqiao in the southeast of the country and Guangzhou (1800 MW over 960 km [597 miles]), which has been in operation since June 2001.

China is more than just a rapidly growing market for power transmission projects. In the next 20 to 30 years it will be the world's largest single market for HVDC technology.

HVDC transmission comes into its own when conventional methods using alternating current encounter their technical and economic limits. HVDC transmission makes sense when it comes to connecting networks that are operated with different frequencies for long submarine cable connections or for moving power over extensive distances. Significantly lower transmission losses and the simplified design of the overhead lines make HVDC the most efficient way of transporting high quantities of energy over distances of 600 km (373 miles) and more.

Furthermore, HVDC transmission systems allow quick regulation of the load flow, thus contributing to the stability of the connected networks and limiting the effects of disturbances such as blackouts.

With energy demand increasing worldwide HVDC transmission will continue to gain in significance, especially when more and more energy reserves are tapped whose sources are far away from where the electricity is consumed, as is often the case with hydropower stations for example.