Reverse voltage surge protection for high-voltage thyristors

An improved scheme for protecting a thyristor having a pair of main electrodes adapted to be connected in an electric power circuit where they are periodically subjected to a reverse bias voltage (anode potential negative with respect to cathode), said thyristor being adapted to switch from a reverse blocking state to a reverse current conducting state if the instantaneous magnitude of the reverse bias voltage increases to a level sufficiently in excess of a predetermined repetitive peak reverse voltage to cause an avalanche breakdown therein, wherein the improvement comprises:

• a. non-linear resistance means comprising at least one metal oxide varistor;
• b. switching means having a normal high-resistance state and being operative to switch abruptly to a low-resistance, current conducting state if subjected to a voltage which attains a predetermined breakover value;
• c. means for connecting said resistance means and said switching means in series with each other and in parallel with said thyristor;
• d. the switching characteristic of said switching means being coordinated with the voltage-current characteristic of said resistance means so that said switching means will switch to its current conducting state in high-speed response to a surge of reverse bias voltage exceeding said reverse voltage peak and nearly equaling the reverse breakdown level of said thyristor;
• e. said voltage-current characteristic of said non-linear resistance means being coordinated with the reverse breakdown characteristic of said thyristor so that, when said switching means switches to its current conducting state, surge current will divide between said resistance means and said thyristor in such proportions that the maximum reverse current flowing through said thyristor is lower than a predetermined critical magnitude which can damage said thyristor.