Improvement of turn-off performance of series-connected thyristors by magnetic components

Conventional voltage sharing networks for series-connected thyristors have serious deficiencies with forced commutation. It is shown that great improvement in turn-off performance over that attainable with the usual shunt resistive and capacitive components can be achieved by using an additional series saturating inductor, transformer(s) connected across the thyristors, or a combination of these. The latter methods pose a severe insulation problem and methods of reducing this are described. Sufficient reasoning is given to form a design basis for the arrangements considered and justifying experimental results are quoted. These methods of voltage control, while applicable generally to series operation of thyristors, are mainly aimed at future applications of long strings of thyristors to high-voltage invertors and choppers.