Procedure and device for detecting the non-conducting state of turn-off thyristors

A procedure to operate a turn-off thyristor which can be set to a current conducting state by a logic command ON and set to a current blocking state by a logic command OFF, comprising the steps of:

• detecting a gate current flowing into a gate terminal of the gate turn-off thyristor;
• generating an auxiliary signal after an occurrence of the logic command OFF corresponding to a time change of the gate current;
• and
• generating a status signal indicating the transition of the gate turn-off thyristor into a current blocking state, upon a polarity change of said auxiliary signal.

A procedure to operate a turn-off thyristor settable to a current conducting state by a logic command ON and settable to a current block state by a logic command OFF, comprising the steps of:

• detecting a gate current flowing into a gate terminal of the turn-off thyristor;
• generating an auxiliary signal upon the occurrence of the logic command OFF corresponding to the time change of the gate current;
• monitoring the detected gate current to determine whether it falls below a negative threshold current value;
• and
• generating a status signal upon the occurrence of one of the following conditions:
• a changing of the auxiliary signal polarity and a finding of the gate current below the negative threshold current value indicating a transition of the gate turn-off thyristor into a current blocking state;
• and
• a continuing of the gate current at a level equal to or greater than the negative theshold current value and an expiration of a preset time duration after an appearance of the logic command OFF.

A device for operating a turn-off thyristor having a gate terminal and a cathode terminal connected to a switch-off current circuit, comprising:

• a current detecting unit as a part of the switch-off current circuit;
• a current sensitive input connected to the current detecting unit;
• an output of the current detecting unit connected to an input of a differentiating circuit;
• an output of the differentiating circuit connected to an input of a polarity detector;
• and
• connecting means for connecting an output of the polarity detector to a status signal output.

A procedure for operating a converter having a push-pull bridge circuit with a first turn off thyristor connected in a first part of the bridge and a second turn off thyristor connected in a second part of the bridge, comprising the steps of:

• detecting a first gate current of the first turn-off thyristor and generating a first auxiliary signal, after the occurrence of a first logic command OFF, corresponding to the time of change of the first gate current;
• generating a first status signal, indicating the transition of the first turn-off thyristor to a current-blocking state on the occurrence of a change in polarity of the first auxiliary signal;
• providing a logic command ON to the second turn-off thyristor enabled by the first status signal;
• detecting a second gate current at the second turn-off thyristor and generating a second auxiliary signal, after the occurrence of a second logic command OFF, corresponding to the time change of the first gate current;
• generating a second status signal, indicating the transition of the second turn-off thyristor to a current blocking state on the occurrence of a change in polarity of the second auxiliary signal;
• and
• providing a logic command ON to the first turn-off thyristor enabled by the second status signal.