Recovery system for short circuits through switching devices in power circuits

Claim 1

In a power circuit comprising at least one series circuit including at least two solid-state thyristor devices connected in series circuit relationship with inductor means between a pair of power supply terminals that in turn are connected across an electric potential power supply, a load circUit coupled to a point on said series circuit between said thyristor devices, means for gating said thyristor devices alternately into conduction, and commutating means including said inductor means for turning off said thyristor devices after desired periods of conduction, the improvement that comprises the addition of: first automatic short circuit recovery means operative when both of said thyristor devices erroneously turn on simultaneously comprising impedance means coupled in series between said power supply and series between said impedance means and series circuit, said capacitor and the inductor means of said series circuit being tuned to series resonance at a frequency selected to supply reverse current for a period of time sufficient to render said short-circuited thyristor devices nonconductive; and second automatic short circuit recovery means for turning off said series connected thyristor devices following a repeated short circuit that occurs after the first recovery means has operated and before the capacitor comprising a portion of said first recovery means has recharged, said second automatic short circuit recovery means comprising means for sensing zero voltage between the supply terminals, means for sensing the voltage across said impedance means, and means responsive to the reduction to zero of the voltage between the supply terminals and to the impedance voltage for at least temporarily disabling said power supply to reduce the power circuit current below the holding value of said thyristor devices.