Usually, power lines over large distances use alternating current (AC). However, in some cases, direct current (DC) is also used, as high voltage direct current (HVDC), where the high voltage is used in order to minimize power losses.
In order to convert AC from the power plant into DC, converter stations use thyristors in a diode bridge circuit. A diode bridge with regular diodes already performs full-wave rectification, converting AC into DC. On the other hand, a thyristor is like a transistor, but is bistable, meaning it does not require a constant secondary supply to stay on: when on, it acts as a diode.
Why do converter stations use thyristors instead of plain diodes? In other words, why is it desirable to be able to switch diodes on and off, when plain diodes already perform AC to DC conversion?