Sumio Kobayashi

Study on overvoltage protection in HVDC LTT valve

The light-triggered thyristor (LTT), which can be directly fired by light, does not need gate electronics on the high-voltage side and is thus a near-ideal semiconductor device for a HVDC valve. In providing overvoltage protection for an LTT without losing its advantage of simple circuits associated with the thyristor, the overvoltage protection system adopted in the conventional electrically triggered thyristor is not necessarily appropriate. The authors have shown that high-speed protective gate pulses generated from the valve base electronics together with the valve arrester are sufficient for the LTT valve overvoltage protection. The adequacy of this protection strategy is demonstrated by factory type tests and also excellent service experiences.

Study of turn-on test method for high-voltage thyristor valve

Digital simulations have been conducted to analyze the thyristor valve turn-on phenomenon while a valve arrester is conducting, which gives the most severe turn-on stress for the valve. The effects of various other parameters have also been clarified. Based on this analysis, two test methods are proposed that verify a valve turn-on during arrester conduction, and their effectiveness is described. The nonrepetitive turn-on test given in the IEC Std Pb. 700 and IEEE Std 857 is studied, and a new test voltage level based on these analyses is proposed. >

Discussion of thyristor characteristic optimization for HVDC valves

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 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.

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.