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High Voltage Direct Current (HVDC)

Page 5:  Converter Stations
Valve Hall

A thyristor can only handle part of the total voltage of rectification and inversion. Thyristors must be connected in series in modules to handle more voltage.

Figure 5.1  Each thyristor in a bridge (yellow) is multiple thyristors in series (light blue). [Alberta 2009]

Figure 5.2  A module containing thyristors in series for the Nelson River Bipole. Thyristors are the white disks at the back of the module. Transparent plastic tubes carry cooling water through the heat sinks. Capacitors in the module assist with commutation and protect against transient voltages. [Wtshymanski 2004]

Figure 5.3  Newer generation module containing more thyristors. Aluminum framing holds all components, and serves as a corona shield. [Siemens 2008]

Thyristor modules are housed in air insulated racks suspended inside a building that is called a valve hall:

Figure 5.4  Thyristor modules in a valve hall for a 500 kV HVDC pole. In this example, six suspended module towers (three pairs) implement a 12-pulse bridge. Each pair of suspended towers implements a quadruple valve (column of the bridge valves). The high voltage end is at the bottom and includes separate corona shields. Person in lower right corner of photograph shows scale. [Siemens 2008]

Figure 5.5  Quadruple valve diagram.


Thyristor module towers suspended from valve hall ceilings provide air insulation underneath the suspended tower, and better seismic event survivability.

Figure 5.6  Simulated dynamic behaviour during earthquake. [ABB]


Transformers

AC transformers provide electricity to the thyristor modules. The transformers are designed for HVDC stations, providing galvanic separation of the AC and DC systems, and large range of voltage regulation.

Historically, transformers were separate from the valve hall of a station, with conductors connected from transformer bushings to through-the-wall bushings. Newer transformers are attached to the valve hall, with transformer bushings directly entering the building.


Figure 5.7  Line drawing of a valve hall. Transformer (left) is positioned outside, with bushings entering valve hall wall to supply AC power to suspended thyristor valve module towers.

Transfomers are on the AC side of the thyristor modules. Thyristors convert electricity from AC to DC, and can quickly adjust the voltage by adjusting firing angles, however voltage adjustment should be done with the AC transformers when possible.

As voltages reach new highs, a three-phase AC transformer becomes too large to transport from a manufacturing facility to the converter station. New transformers are developed for single phase operartion, instead of three phase, requiring multiple smaller transformers to replace a larger transformer. At such high voltages, the relatively smaller transformers are still quite large.


Figure 5.8  Valve hall transformers. [Siemens 2006]


Using more transformers, that are each smaller, facilitates replacement of transformers later, allowing storage of spare transformers in the station yard for possible future replacement needs.

Transformers have high voltage bushings, usually a vertical bushing for AC input (see photo above), and diagonal bushings to supply AC electricity into the valve hall (next photo).


Figure 5.9  Valve hall at Three Gorges dam, China. Large transformer bushings (lower right) supply AC phase electricity to suspended valve towers (left). Transformers are behind the wall on right. [ABB]

Figure 5.10  Valve hall under construction in Porto Velho, Brazil. Transformers will be installed against the valve hall wall, and high voltage bushings will be attached to the portion of each transformer that is within the valve hall. Valve halls are shielded and grounded. [ABB]

A high voltage bushing is an insulated conductor terminal that prevents arcing (“flashover”). For transformers, the bushings insulate the conductor from the transformer exterior (e.g., prevents arcing from the high voltage conductor to the outside of the transformer). The transformer bushings that supply AC to the valve hall also insulate the AC conductors from the wall of the valve hall (prevent arcing from the conductors to the valve hall wall that the bushings penetrate).

Transformers are on the AC side of thyristor valves. Thyristor firing angles can be adjusted to change voltage, although most voltage adjustment should be done by the AC transformers (tap changing). Multiple wires and hollow tubes are used as high voltage AC conductors to maximize surface area since AC skin effect does not use the geometric interior of conductors for current flow.


DC Filtering

The DC line coming out of the valve hall is smoothed and protected from short circuit surges on the DC line with smoothing reactors in series. Reactors were traditionally oil filled and looked like transformers. Smoothing reactors may now be air core (dry-type).


Figure 5.11  Air core smoothing reactor. High voltage wall bushing delivers DC electricity from the valve hall (right). [Siemens]

Figure 5.12  Pair of air core smoothing reactors in series. [SARI]


Additional filters are used in the DC switch yard to further smooth the DC current on overhead lines, preventing interference on telephone lines, etc.


Figure 5.13  Towers of capacitors for line filtering, in DC yard of Henday Converter Station, Canada. [Manitoba Hydro]

Line filtering uses capacitors, resistors, and reactors (air core inductors).


References for this page:

 1.  Hartmut Huang, Markus Uder, “Application of High Power Thyristors in HVDC and FACTS Systems”, Siemens 2008. (see Figure 5.3 above for link)


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