Recent technical advancement of thyristor valve for HVDC transmission system

Thyristor valves have progressed with an increasing thyristor capacity. Over the past 15 years the volume of a thyristor valve has reduced to about one-third and power loss to about one-half. The advance of thyristor valves, however, seems to have reached a saturation level if simply an increase in the thyristor capacity continues to be sought. To increase the dielectric strength of thyristors requires increasing the silicon wafer thickness and resistivity. Increased thickness and resistivity of silicon wafer, however, increase conduction loss of a thyristor valve. The conduction loss can be reduced by prolonging the carrier life-time of silicon wafer, which in turn increases the loss of a damping circuit. For improving such a tradeoff relationship, an ion irradiation technology was developed. It is shown that optimization of thyristor characteristics is important in optimizing the thyristor valve.