Progress in the development of an 8-kV light-triggered thyristor with integrated protection functions

Light-triggered 8-kV thyristors with an integrated breakover diode were fabricated. The adjustment of the breakdown voltage of the breakover diode (BOD) is realized by a well-defined curvature of the junction between the p-base and the n-base. A multiple amplifying gate structure, together with an integrated turn-on current limiting resistor, guarantees a safe turn-on behavior in the case of overvoltage triggering as well as during light triggering. These thyristors have successfully been put into service at the Celilo Converter Station of the Pacific Northwest-Southwest HVDC Intertie by Bonneville Power Administration in Portland/Oregon/USA. There, a mercury arc valve was replaced by a valve containing these light triggered thyristors with integrated overvoltage protection. In a further development, the amplifying gate structure was optimized in order to simplify the fabrication process and to maintain the high light sensitivity while obtaining a higher dV/dt and a higher dI/dt capability. This was realized by shrinking the optical gate, by carefully adjusting the triggering sensitivity of the amplifying gates and by widening of the turn-on current limiting thyristor. With these measures, initial investigations regarding the integration of a forward recovery protection were also performed.