We have been developing thyristor valves for high-voltage dc transmission systems for more than 20 years. During this period, the size, power loss and reliability are dramatically improved and one of the technical advancements which support the improvements is to increase the voltage and current rating of thyristors.
However when thyristor voltage rating becomes higher than 6kV, increasing the voltage rating does not lead size and power loss reduction of the valve because of turn-off characteristic deteriorations.
This paper describes the method which can optimize the thyristor characteristics in such a way that the size and power loss of valves are minimized. Two different approaches for HVDC and back-to-back systems are presented.