Secular change in characteristics of thyristors used in HVDC valve

The Hokkaido-Honshu HVDC link entered service in 1979 for the first phase (150 MW, 125 kV-1200 A) and in 1980 for the second phase (300 MW, 250 kV-1200 A) and has since been operating in good condition. In the Kamikita converter station situated on the Honshu side, the thyristor valves for the first and second phases use 1344 thyristors rated 4 kV-1.5 kA. To examine secular changes in the characteristics of thyristors and estimate their life expectancy, we picked up eight thyristors from the thyristor valves, measured the main characteristics and compared measurements with the corresponding values measured before they entered service. Further, two thyristors were subjected to temperature and voltage acceleration tests to check for a change over time of leakage current. From the results of these investigations it was verified that the thyristors that have been in service for 16 years have the characteristics and service life equivalent to those of the new ones. But further investigation is required as to the change of turn-off characteristics