To recap earlier discussion of thyristor bridges:
While there is a smoothing benefit to using additional bridges, another benefit is increased voltage and power rating. Higher voltages are possible, generally up to ±500 kV bipolar using a 12-pulse bridge per pole (double the voltage of six-pulse).
For even higher power transmission, newer systems may double up 12-pulse bridges in pairs. Voltages of ±800 kV and higher are considered Ultra High voltage (UHVDC), and voltages between ±500 kV to ±800 kV may be referred to as Extra High voltage (EHVDC).
Doubling up of 12-pulse bridges in pairs is illustrated as the second and third schematic diagrams of the following figure:
For the second and third options in the figure above, bypass switches allow continued operation while a valve group is disconnected because of failure or maintenance. Another use for bypass switches is to increase current for deicing of overhead lines.
We consider the central option of the diagram above (doubling up valve groups in series). The following diagram shows that option, using the thyristor in a rectangle symbol to denote a 12 instead of 6 pulse bridge:
The two valve groups that are closest to ground/neutral are lower voltage than the two outer valve groups. Each of the four valve groups are in their own valve hall: two lower voltage (LV) valve halls, and two higher voltage (HV) valve halls. The four valve halls are denoted with dashed lines in the diagram above.
That project (Xiangjiaba-Shanghai ±800 kV 6.4 GW) is similar to the Yunnan-Guangdong ±800 kV 5 GW UHVDC Project, which is illustrated with the following schematic diagram:
In these systems, valve groups (bridges) are connected in series, with valve groups that are closer to ground/neutral at lower voltage:
This shows an increase of 200 kV per valve group (bridge) for standard ±800 kV UHVDC. In those systems in China, two types of transformers are used, one type for LV valve halls at 400 kV manufactured by China, the other type for HV valve halls at 800 kV manufactured by ABB and Siemens.
Early ±1100 kV UHVDC systems (in China) use double 12-pulse groups in series per pole, with HV at 1100 kV, and LV at 550 kV.
The higher voltage level requires more air insulation distance to prevent arcing (flashover), including longer bushings on the HV transformers, as shown in the photograph above. Not all station equipment is at higher voltage, allowing use of separate valve halls, for LV valve halls to be smaller (less large) than the HV valve halls (which are really large). Each HV valve hall uses six of the transformers shown in the photograph above (per pole of a bipolar line).
1. U. Astrom, V.F. Lescale, Converter Stations for 800 kV HVDC, ABB CESPI 2006.
2. V.F. Lescale, U. Astrom, W. Ma, Z. Liu, The Xiangjiaba-Shanghai 800kV UHVDC project status and special aspects, ABB CIGRE 2010.
3. Donghui Zhang, Marcus Haeusler, Hong Rao, Chun Shang, Tao Shang, Converter Station Design of the ±800 kV UHVDC Project Yunnan-Guangdong, Siemens 2008.
4. Mario Schenk, Ronny Fritsche, 800 kV Ultra HVDC Transformer, TrafoTech Mumbai 2010.
5. Thomas Freyhult, Mats Bergland, Ake Carlsson, UHVDC Meeting the needs of the most demanding power transmission applications, ABB 2012.
6. Xu Deng, Anwen Xu and Yuting Qiu, UHVDC Electrical Equipment, Chapter 21 in Ultra-high Voltage AC/DC Power Transmission, Springer 2017.