The forced commutation of thyristors connected in series

Widespread high voltage application of forced commutation was then, and still is, some years ahead. The thesis puts foward methods which have been devised for ensuring satisfactory thyristor turn-off in this context and suggests Some possible future trends in application. Individual tbyristor turn-off characteristics are first studied together with the equivalent diode characteristics. The difficulties of using thyristors in series for high voltage working are described and the limitations of the conventional resistive-capacitive voltage sharing network discussed in relation to forced commutation. Three forms of improved voltage sharing network are presented together with developments which use these in suitable combination. All have been tested with series-connected thyristors operating in a high voltage force-commutated chopper circuit. The advantages and disadvantages, and a design procedure, are given for each form of network. The two most appropriate voltage sharing arrangements are applied to the series thyristors and diodes in a high voltage, variable frequency d.c. chopper, and their overall influence on chopper performance is considered in detail. Owing to the possibility of cascade failure when many semiconductor devices are connected in series, extensive component damage can result from circuit malfunction or incorrect component connection. Great care has therefore. been necessary in the design, construction and checking of the high voltage experimental equipment. It is concluded that the voltage sharing methods devised make possible forced commutation of thyristors at high voltages and do not, in themselves, provide the practical constraints on system performance. The methods are compared on a technical and economic basis. Possible future applications are reviewed, with particular emphasis placed on the transformation of d.c. at high voltages using chopping techniques. Other problems which must be solved before such applications become realisable outside the laboratory are outlined and suggestions made for future work.