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World's First Successful Test of the Light-Triggered Thyristor in an Actual Transmission Line System

The light-triggered thyristor, which does not require a power supply for its insulated gate, is a key device in the field of power electronics.

World's First Successful Test of the Light-Triggered Thyristor in an Actual Transmission Line System

Power semiconductor devices are semiconductor devices such as diodes and transistors that are used in power supply and inverter applications, where they perform various functions including AC-DC conversion, voltage step-up/step-down, frequency conversion, and so on. In 1972, upon receiving a strong request from what was then the Systems Department of the Heavy Electronics Division, the Electron Devices Laboratory of the Research and Development Center (currently the Corporate Research & Development Center) began the full-fledged development of a high-power gate turn-off thyristor (GTO) (a thyristor that can be turned off by means of gate control). Scientific papers describing this type of device had already been published in the 1960s, and General Electric Company, Westinghouse Electric Corporation, and RCA Corporation in the U.S. developed GTOs in the early 1970s. However, they were unable to obtain the expected performance and either scaled down their activities or withdrew from this field. A number of Japanese companies also worked on GTOs, but all of their development efforts failed.

Toshiba's Semiconductor Division had also been engaged in the development of a small-sized GTO, but had decided to abandon this work due to insufficient yield rates and therefore had a cautious view of this field. Even so, the Systems Department made their strong request for renewed efforts with its sights set on power converters for use in industrial applications and electric railway systems. The president of the company agreed with this view and gave instructions for a new clean room to be established for this purpose, with the Systems Department to support the development work in cooperation with the Semiconductor Division.

The greatest challenge in this project was to find a way of increasing the turn-off current, with the aim of expanding the range of applications. In 1976, the company broke the world record for withstand voltage and current rating by a large margin and, in 1978, announced the development of a 2,500 V-600 A GTO with double the previous withstand voltage. This was followed by a 4,500 V-3,000 A GTO, which was adopted for the Nozomi Shinkansen ("Bullet Train") cars in Japan as well as for the locomotives of a rapid rail transit system in Europe and greatly contributed to the development of Toshiba’s power electronics-related business.

At around the same time, the light-triggered thyristor, which can be triggered by a light-emitting diode (LED), was under development. Compared with conventional electrically triggered thyristors, it realized greater miniaturization of equipment and the use of fewer components, leading to improved reliability. Upon receiving another strong request from the Systems Department, which wanted the extra-highvoltage thyristor valves used in frequency converter stations and DC power transmission systems to be exclusively of the light-triggered type, development efforts in this field were initiated in 1978. Already a 4,000 V-1,500 A electrically triggered thyristor had been commercialized, and the development of a high-power light-triggered thyristor using general-purpose LEDs as the trigger with nearly 100 times the light gate sensitivity while maintaining the same electrical characteristics was a risky venture.

From December 1983 to February 1985, a test was conducted on an actual transmission line system at the Sakuma Frequency Converter Station of Electric Power Development Co., Ltd. with as many as ninety 4,000 V-1,500 A light-triggered thyristors connected in series. The success of this project, the first time in the world that the light-triggered thyristor had been tested in an actual transmission line system, was a deeply moving experience for the people involved, and demonstrated the feasibility of DC power transmission using this device.

On the other hand, from around 1981, when the development of high-power light-triggered thyristors of 4,000 V capacity had reached a certain point, research with the objectives of realizing higher-output LEDs and higher withstand voltages also began. After trial manufacturing of more than 10 types of light gate structure, the company succeeded in the development of a new multistage amplification light gate structure that fully met these research objectives and, in 1982, a device with the world's largest capacity of 8,000 V-1,200 A was exhibited at an exhibition of Toshiba’s technologies. Then, in 1984, an 8,000 V device with an integrated overvoltage protection function was released. Toshiba's 4,000 V-1,500 A devices were widely used for reactive power compensators installed in transformer substations, while our 6,000 V-2,500 A devices were mainly adopted for frequency converter stations and DC power transmission systems, as well as for industrial applications such as converters for motor control in steel works. In 1990, the Semiconductor Division developed a 6,000 V-2,500 A light- triggered thyristor with an overvoltage protection function.

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