A novel solution to restore a three-phase thyristor inverter from commutation failure due to voltage dip

From industrial experience, it is known that three-phase line commutated thyristor inverters encounter commutation failure during voltage dips. Following commutation failure, both upper and lower thyristors belonging to the same leg of the inverter conduct and a surge current results. This paper gives an analysis of this phenomenon in the event of a single-phase dip. A novel and simple solution for turning off the thyristors in the shorted leg is presented. The proposed scheme detects the short circuit and switches off the thyristors to avoid damage due to excessive current build up. When the three-phase source recovers from the dip, the inverter resumes its normal operation. The proposed scheme has quick response and requires no separate voltage source for control circuits. Experimental verification of the thyristor's turn off mechanism is presented to validate the operation principle.