High Voltage Direct Current (HVDC)
Page 7: Higher Voltage
To recap earlier discussion of thyristor bridges: a six-pulse thyristor bridge performs rectification (and inversion) of three-phase AC power to/from DC; using two six-pulse bridges (as a 12-pulse bridge) smooths out the DC power curve compared to using only one six-pulse bridge; each thyristor (valve) in a bridge is actually many thyristors connected in series to achieve higher rating.
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:
Figure 7.1 Schematic diagrams of possible HVDC valve bridge circuits for bipolar DC. Left shows a single 12-pulse bridge per pole. The other two examples illustrate two 12-pulse groups per pole: in series (center), or in parallel (right). [Astrom/Lescale] (see links in References below)
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:
Figure 7.2 Schematic diagram of Xiangjiaba-Shanghai ±800 kV UHVDC Project. [Lescale et al]
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.
Figure 7.2 Aerial photograph of Xiangjiaba station under construction, viewed from the DC side. Building in center of the site is two LV valve halls, with transformers on both sides. Two taller buildings are HV valve halls, with transformers on the sides facing the LV halls. More recent satellite images
show spare transformers on site.
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:
Figure 6.7 Schematic diagram of Yunnan-Guangdong ±800 kV UHVDC Project. [Zhang et al.]
In these systems, valve groups (bridges) are connected in series, with valve groups that are closer to ground/neutral at lower voltage:
Figure 6.8 Comparison of series valve group voltage levels for HVDC (left) and Ultra HVDC (right). [Schenk/Fritsche]
Figure 6.9 Series bridge voltage levels for ±800 kV Ultra HVDC (UHVDC). [Freyhult et al]
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.
Figure 6.10 Single phase two winding transformer for 1100 kV (1.1 Million Volts), in manufacturing test facility. [Siemens]
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 includes six of the transformers shown in the photograph above (per pole of a bipolar line).
References for this page:
1. U. Astrom, V.F. Lescale, Converter Stations for 800 kV HVDC, ABB CESPI 2006. pdf
2. V.F. Lescale, U. Astrom, W. Ma, Z. Liu, The Xiangjiaba-Shanghai 800kV UHVDC project status and special aspects, ABB CIGRE 2010. pdf
3. Donghui Zhang, Marcus Haeusler, Hong Rao, Chun Shang, Tao Shang, Converter Station Design of the ±800 kV UHVDC Project Yunnan-Guangdong, Siemens 2008. pdf
4. Mario Schenk, Ronny Fritsche, 800 kV Ultra HVDC Transformer, TrafoTech Mumbai 2010. pdf
5. Thomas Freyhult, Mats Bergland, Ake Carlsson, UHVDC Meeting the needs of the most demanding power transmission applications, ABB 2012. pdf
6. Xu Deng, Anwen Xu and Yuting Qiu, UHVDC Electrical Equipment, Chapter 21 in Ultra-high Voltage AC/DC Power Transmission, Springer 2017.
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