Abstract— Unbalance in the AC/DC converters produce uncharacteristic current harmonics, in addition to the characteristic harmonics in the AC waveform not expected from the balanced operation. In this paper, a three- phase AC-DC converter is fully analyzed and the performance of the converter under balanced and unbalanced conditions is evaluated. Also, the relationship between the unbalance factor and the resultant harmonics is investigated. The method of reducing uncharacteristic harmonic generation through firing angle modulation is discussed. A digital trigger scheme control circuit for valves with individual control circuit for each device has been designed and built. A digital computer simulation program based on this method is developed; and some experimental results are also given.