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.
Thyristor modules are housed in air insulated racks suspended inside a building that is called a valve hall:
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]
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.
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).
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.
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 5.12 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.
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)