A classic HVDC converter station has an AC switch yard and transformers on one side of the valve halls, and a DC switching and filtering yard on the other side of the valve halls.
The AC yard includes switching and extensive filtering. The DC yard is usually smaller. Following is the valve halls portion of a typical HVDC converter station layout:
Figure 6.1 Example layout of valve halls portion of a bipolar HVDC converter station, one valve hall (yellow) per pole. Transformers are highlighted light red. Two smoothing reactors in series for each pole are shown in purple areas. Notice the long high voltage wall bushing extending from each valve hall supplying DC electricity to the smoothing reactors. DC yard would be above, and larger AC yard below this drawing. [Siemens/SARI]
Figure 6.2 Isometric drawing of a similar HVDC station layout. Spare transformer is to the left of valve halls in this drawing. AC filtering prevents the converter station from causing interference of the AC grid (including capacitor towers, right of Spare Parts Area). DC switching and filtering is indoors in this design (in DC Hall buildings). Underlaying the grounds are copper wires and ground stakes, which equipment, buildings, transformers, etc. are grounded to. [Siemens/SARI]
A simple HVDC line would have a grounded return conductor, with schematic as follows:
Figure 6.3 Simple HVDC line. [Alberta Energy 2009]
The AC filters, referred to as Switched Filter Banks, connect the AC lines to ground through capacitors, resistors and reactors. These filters can be connected to (and disconnected from) the AC lines as needed (for smoothing, reactive power, etc.).
In that simple HVDC line, a positive pole sends high voltage electricity and a grounded return provides low voltage back. The low voltage return could actually be through the ground instead of by wire, as will be explained shortly.
A more common HVDC line is bipolar, with multiple pairs of valve bridges, illustrated in the following schematic:
Figure 6.4 Bipolar HVDC line, 3 GW. [Alberta Energy 2009]
In this system, if one of the poles goes down (becomes inoperative), the other pole can supply half the electricity of normal operation with ground return until the inoperative pole is repaired.
The ground return line is low voltage when both poles are in operation. Theoretically, the ground return line would have no electricity flow if both poles are perfectly balanced, but that does not quite happen in actual practice, so that some electricity does flow in the ground return under normal operation.
The ground return line can be replaced with ground electrodes, as illustrated in the following schematic:
Figure 6.5 Bipolar HVDC with ground electrodes. [Alberta Energy 2009]
Ground (earth) return can be a ring of electrodes (Figure 6.6), or wide shallow fill, or a deep bore hole one meter (1 m) wide, or ocean electrodes, etc.
Figure 6.6 Ring of ground electrodes, 300 m radius, for ±500 kV HVDC in Manitoba, Canada.
Indoor DC Yard
Indoor DC switching and filtering yards may be used for higher voltage stations, to help protect higher rated components from weather and pollution. Higher voltage systems will be discussed in the next page of this report.
The following photograph, of a new indoor DC yard, shows a disconnect switch that is in the closed position across the top of the photograph:
Figure 6.7 Indoor DC yard under construction, Ningdong-Shandong (China), 4 GW ±660 kV EHVDC. Smoothing reactor is in background. Disconnect switch in closed position is in foreground. [Retzmann]
Figure 6.8 DC disconnect switch of the previous photograph, in open position. See short video of a disconnect switch in operation at the end of this page.
Transformers are becoming larger, to handle more power capacity. That causes difficulties transporting a transformer from where it is manufactured to where it will be installed. Larger transformer sizes also create long-term security risks by not being able to be manufactured and replaced as easily.
To address these problems, single phase transformers have been developed, each transforming a single phase of three-phase AC (using three single-phase transformers to transform three-phase AC). This requires using more transformers, but each can be less large and more easily replaced than a larger three-phase transformer, allowing stations to store spare transformers on site.
The size and type of transformer depends on what size transformer can be shipped to the station site. The maximum size possible for shipping is determined before designing and manufacturing a transformer for a project.
Figure 6.9 Transporting a transformer in Germany. [Retzmann]
Electrical schematic drawings show traditional three-phase transformers, to illustrate electrical characteristics of an HVDC project. For higher voltages, however, multiple single-phase transformers take the place of each three-phase transformer.
If realized, a single three-phase transformer for a 12-pulse bridge could have windings as follows:
Figure 6.10 Windings diagram of a single transformer for a 12-pulse bridge. The transformer is three-phase three-winding. [KTH]
That would be the largest possible transformer for a 12-pulse bridge. An alternative could be to use two transformers:
Figure 6.11 Two transformers for a 12-pulse bridge. Each transformer is three-phase two-winding. [KTH]
To further reduce the size of each transformer, each three-phase transformer in the diagram above can be replaced with three single-phase transformers, as follows:
Figure 6.12 Six transformers for a 12-pulse bridge. Each transformer is single-phase two-winding (one valve winding). [KTH]
For a valve bridge with lower power rating, a possible compromise could be to use three transformers per bridge:
Figure 6.13 Three transformers for a 12-pulse bridge. Each transformer is single-phase three-winding (two valve windings). [KTH]
Figure 6.14 Single-phase three-winding tranformer at Siemens manufacturing test facility, before shipment to the Tian Guang HVDC project in China.
Figure 6.15 Single-phase two-winding tranformer, for the Three Gorges HVDC project in China. [Siemens]
References for this page:
1. Dietmar Retzmann, HVDC Station Layout, Equipment LCC & VSC and Integration of Renewables using HVDC, Siemens 2011 / CIGRE 2012. pdf (22 MB)
2. D. Wu, U. Astrom, L. Arevalo, R. Montaño, B. Jacobson, Selection between indoor and outdoor DC yards, CIGRE 2014. pdf
3. General Guidelines for HVDC Electrode Design, CIGRE 2017. pdf
4. Donghui Zhang, Marcus Haeusler, Hong Rao, Chun Shang, Tao Shang, Converter Station Design of the ±800 kV UHVDC Project Yunnan-Guangdong, Siemens 2008. pdf