HVDC Valves with Light-Triggered Thyristors

Abstract

Conventional series-connected High Voltage Direct Current (HVDC) thyristors are triggered electrically (referred to as ETTs). Using Light-Triggered Thyristors (LTTs) instead, valve electronics can be considerably simplified. Extensive comparative tests have shown that for a cost-effective solution the LTT should be self-protected, and full-sized rather than of the smaller, auxiliary type. The obvious light sources, Light-Emitting Diodes (LEDs) or Laser Diodes (LDs), are relatively weak and require a very sensitive LTT gate. A principal concern was that the LTT presented unexpected turn-on difficulties when triggering from AC voltage at low firing angle. Both LED and LD triggering of 45 cm2, 7 kV self-protected LTTs are currently being tested in a 125 kV, 1050 A test valve in the Konti-Skan HVDC transmission link (Danish side). The light sources are in both cases exceedingly powerful and specially developed for this project. So far, the LED solution has evinced unacceptable degradation. most probably due to the high LED current. The LD system exhibits tolerable degradation margins but appears to be too expensive at present to pose a competitive threat to ETTs. A new, promising, cost-efficient solution, based on commercial 50 mW continuous-wave (cw) LDs, is under test on a laboratory scale.