Investigation of Three-Phase Thyristor Converters under Generalized Impedance Unbalance

Abstract

Line-commutated current converters operated in electrical power systems will work asymmetrically, if the system is not balanced.This asymmetrical operation causes non-characteristic harmonics in the currents and therefore a higher distortion level. As a high distortion level is always disadvantageous, it is worthwhile to find appropriate methods to lower the distortion level by reducing the amplitudes of these non-characteristic harmonics. In this paper, three-phase thyristor converters operated in systems with unbalanced AC-line short-circuit impedances are investigated. To achieve a competent and well-grounded understanding of the problem a suitable method of calculation has been developed. This method firstly offers the opportunity to calculate the operation of line-commutated current converters in an unbalanced three-phase system. Furthermore it offers the opportunity to optimize the operation of the unbalanced system. This optimization is done by balancing the systems operation and reducing the amplitudes of the non-characteristic harmonics using asymmetrical firing angles.