This invention relates to a dv/dt protection circuit for a thyristor, particularly for a gate turn-off (GTO) thyristor. 
GTO thyristors of a small capacity are known.  Recently it became possible to manufacture GTO thyristors of large capacity, as large as 200A to 600A. GTO thyristors are used as various power converting devices such as inverters.  GTO thyristors of small capacity have been used in apparatus of small capacity such as control devices.  They could not be and were not used in apparatus of large capacity and no difficult problems arose.  Recently GTO thyristors of large capacity have been invented.  If they are used in a converter circuit, problems arise which cannot be easily solved by any known technique.  One such problem is concerned with dv/dt protection circuits. 
dv/dt is an abrupt positive voltage change between the anode and cathode of a GTO thyristor.  When dv/dt surpasses a value specific to the GTO thyristor, the GTO thyristor is inevitably turned on.  This limit value is called critical rate of rise of off-state voltage.  A GTO thyristor has a lower critical rate of rise of off-state voltage than that of an ordinary thyristor.  Thus it is necessary to take measures to protect against dv/dt of a GTO thyristor. 
It will now be described how dv/dt protection has been effected conventionally in, for example, an inverter constituted by four GTO thyristors.  Between first and second GTO thyristors a first reactor is connected, and between third and fourth GTO thyristors a second reactor is connected.  A load is connected between the neutral terminals of the first and second reactors.  The first and second GTO thyristors constitute a series circuit, and the third and fourth GTO thyristors another series circuit.  Between these series circuits there is connected a DC power source.  Namely, the four GTO thyristors are bridge-connected to one another through the first and second reactors.  To each GTO thyristor there is connected a snubber circuit. 
In this inverter, the snubber circuits and the reactors constitute a dv/dt protection circuit.  The reactors work just as a current back flow circuit which an ordinary thyristor circuit requires.  As a result, the self-arc-extinguishing ability of each GTO thyristor cannot be fully utilized.  Further, when the first GTO thyristor being ignited is turned off, current stops flowing in the first reactor, whereby a high voltage is produced on the first reactor.  The high voltage is then applied between the anode and cathode for the first GTO thyristor.  Consequently, the first GTO thyristor is erroneously turned on again. 
To avoid such a mishap, the capacitance of a capacitor in the snubber circuit connected to the first GTO thyristor should be made sufficiently large, or the second GTO thyristor should be turned on before the first GTO thyristor is turned off so that the current flowing in a first coil portion of the first reactor between the first GTO thyristor and the neutral terminal of the first reactor is made to flow into a second coil portion of the first reactor between the second GTO thyristor and the neutral terminal of the first reactor.  This method, however, inevitably diminishes the electric characteristics of the GTO thyristors, and the GTO thyristors cannot fully achieve their high speed make-and-break operation.