Commutation dv/dt Effects in Thyristor Three-Phase Bridge Converters

A significant factor to be considered in properly designed thyristor converter circuits is dv/dt. The intent of this paper is to analyze a three-phase bridge circuit and show the following. 1) dv/dt faults due to turn-on transients are a threat only for operation in the inverter mode. 2) dv/dt faults due to the thyristor recovery transient are a threat for rectifier operation from about 50 to 0 percent of maximum dc output voltage (a between 60 and 90 degrees) as well as in the inverter mode. 3) dv/dt due to recovery is generally less severe and more easily suppressed than that due to turn-on. 4) Turn-on induced dv/dt can be calculated approximately by the use of simple equivalent circuits. 5) Recovery induced dv/dt can also be calculated if peak reverse current is known. 6) Thyristor critical dv/dt rating at 50 to 60 percent of rated anode voltage may be more useful than present standard ratings.