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

Abstract

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.

Investigation of Three-Phase Thyristor Converters under Generalized Impedance Unbalance

“…This possibility with unsymmetrical firing angels has already been shown for forced-commutated converters both with an unbalanced mains voltage and with unbalanced AC-line impedances [5,6] and for line-commutated power converters with an unbalanced mains voltage [7,8]. A first approach has been shown for line-commutated converters with a simplified unbalance of the AC-lines short-circuit impedance [9]. In this work, a generalized approach is presented, wherein the simplifications of [9] do not have to be complied with.…”

“…A first approach has been shown for line-commutated converters with a simplified unbalance of the AC-lines short-circuit impedance [9]. In this work, a generalized approach is presented, wherein the simplifications of [9] do not have to be complied with. Hence the unbalance of the AC-lines short-circuit impedance can be of any kind and practical problems can be investigated more realistically.…”