Development of a self protected light triggered thyristor

The next generation of HVDC valves are likely to use Light Triggered Thyristors (LTTs) to eliminate firing electronics close to the thyristor. Maximum cost and system benefits from the use of these LTTs will be achieved by also making them self protecting thereby removing the remaining protection electronics from the high voltage valve. This paper describes the development of a light triggered thyristor which self protects when subjected to voltages greater than its design capability (Vbo protection) and against forward dV/dt, particularly during the critical turn-off period (Forward Recovery or FRP Protection). Modern converter thyristors employ amplifying gate structures to achieve fast and efficient turn-on. The cross sectional structure of a light triggered thyristor is described and the mechanism by which the amplifying gate turns the device on when exposed to a pulse of laser light is explained. The amplifying gate is also the route by which safe turn-on is achieved in the new Vbo and FRP selfprotection structures to be described