At the core of a UHVDC transmission system is the converter. It consists of a number of thyristor modules equipped with Siemens Energy thyristors of up to 6-inch. Often, the valves of one pole are arranged in two series-connected 12-pulse groups to overcome possible transport limitations. A major benefit of this layout is the relatively small size of the converter transformers. It also increases the redundancy of the bulk power transmission system, as each of the four converters can be bypassed with the assigned DC line still operating at a reduced voltage level, e.g. 400 kV for an rated 800 kV UHVDC system.
The next level of HVDC technology was ushered in when Siemens Energy developed its 6-inch thyristor valve module and all other components required for UHVDC converter stations. Thanks to the ability to produce the entire range of components required for UHVDC voltage up to 1100 kV DC power transmission, Siemens Energy can supply turnkey solutions for complete UHVDC transmission systems.
Yunnan-Guangdong, the world's first UHVDC transmission project, which had been contracted to Siemens Energy by China Southern Power Grid in 2007, started commercial operation of the first pole in December, 2009. The first bulk power transmission system equipped with 6-inch thyristors from Siemens Energy, the Xiangjiaba-Shanghai UHVDC system in China, commenced operation in 2010. Another huge Siemens Energy ±800 kV UHVDC project was implemented in Brazil.
