V/sub BO/-protection of power thyristors using an external trigger thyristor

Abstract

I n s o p h i s t i c a t e d equipments l i k e HVDC valves w i t h many t h y r i s t o r s i n series, p r o t e c t i v e functions e x t e r n a l t o t h e t h y r i s t o r s are employed.One such desired f e a t u r e i s VBo-protection.Conv e n t i o n a l l y , u n t i l r e c e n t l y , t h e VBo-protective f u n c t i o n was handled by a p r o t e c t i v e c i r c u i t containi n g many components.As a r e s u l t o f the t e c h n i c a l success o f t h e Separate L i g h t Trigger T h y r i s t o r (SLTT), launched commercially i n 1984, e f f o r t s were concentrated on making t h i s basic c h i p a l s o capable o f p r o t e c t i n g f o r overvoltages i n t h e o f f -s t a t e d i r e c t i o n . Thus, when t r i g g e r e d by l i g h t o r avalanche c u r r e n t due t o a VBo-voltage, i t would supply a gate s i g n a l t o t h e Power T h y r i s t o r . This paper describes t h e d i f f e r e n t design o p t i o n s and t h e j u s t i f i c a t i o n f o r t h e one chosen. The behavior f o r d i f f e r e n t waveforms, DC t o very h i g h d v / d t range (28 kV/us), i s i n v e s t i g a t e d and n o n -r e p e t i t i v e and r e p e t i t i v e pulse c u r r e n t r a t i n g s are established. The concept o f t h i s separate t r i g g e r device a l s o necessitates a b r i e f review o f t h e advantages and disadvantages, emphasizing i t s use i n s e r i e s connected s t r i n g s o f Power Thyristors. The r e s u l t i s i n f o r m a t i o n s u f f i c i e n t t o t a i l o r t h e design f o r s p e c i f i c p r o t e c t i o n schemes and Power Devices w i t h respect t o safe VBo-triggering.