Investigation of line-commutated three-phase converters with impedance unbalance

Current converters operated in unbalanced electrical three-phase systems will generally work asymmetrically, which causes the appearance of non-characteristic harmonics and therefore higher distortion levels. In this paper unbalances are considered, that are caused by asymmetrical AC-line short-circuit impedances, which are usually a result of unbalanced transmission lines. In the case of thyristor converters one can react on a given unbalance with asymmetrical firing angles. This offers the opportunity to make the converters operation more balanced und to lower the distortion caused by unbalance. This paper deals with the calculation of a current converter´s asymmetrical operation and its optimization by use of asymmetrical firing angles.