In high voltage direct current (HVDC) applications, the AC/DC thyristor power converter is subjected to very high voltages and each valve in the converter bridge must be able to withstand several hundred kV. As the voltage capability of a single thyristor is limited to less than 10 kV. Necessary firing and protection facilities at the thyristor level must therefore be energized by current coming from the valve itself at the same level. Furthermore, triggering signals between these facilities and the control system at ground potential must be transmitted optically. Though a rather sophisticated but reliable solution based on a compact thyristor control unit (TCU) has been evolved for conventional electrically triggered thyristors (ETTs) direct optical triggering of the thyristor would be advantageous to simplify the valve electronics and possibly further improve converter reliability. The object of this presentation is to compare different circuit configurations based on light-triggered thyristors and to summarize laboratory and field test experiences of the chose solution. Some comments will also be made on a new design utilizing recent improvements in optical components.
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