Discussion of thyristor characteristic optimization for HVDC valves

We have been developing thyristor valves for high-voltage de transmission systems for more than 20 years. During this period, the size, power loss, and reliability have been dramatically improved and one of the technical advances which support the improvements is to increase the voltage and current rating of thyristors. However, when thyristor voltage rating exceeds 6 kV, increasing the voltage rating does not lead to size and power loss reduction of the valve because of turn-off characteristic deteriorations. This paper describes a 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. (C) 1999 Scripta Technica.