Bang‐bang‐type optimal constant current control of parallel AC‐DC power transmission systems

Abstract

The transient stability of an ac-dc hybrid system can be improved greatly by controlling the power flow of the dc transmission line. In this study the optimal changeover curve of dc current setting value on the phase plane with a quadratic curve are approximated. The coefficients of this curve are calculated in advance taking into account the loading condition of the power system and hence the proposed control scheme can be used for emergency control of the power system. The main conclusions from this study were that: to facilitate transient stability analysis, the dc transmission line is represented by equivalent load admittances, which vary every second with the operating condition of the transmission line; the action of the speed governor increases the damping coefficient of the power system and reduces the control time which means that the admissible range of the dc current setting value can be reduced by increasing the governor gain; and the optimal change-over curve of the dc current setting value is approximated by a quadratic curve, whose coefficients can be calculated in advance taking into account the loading condition of the power system. The suboptimal changeover curve thus obtained makes it possible to change the settingmore » value of dc current without time delay in emergency condition. While the case of three-phase short-circuit fault was specifically discussed, it is possible to extend the results obtained to the case of successful or unsuccessful reclosing. The proposed method could also be applied to multimachine systems by using equivalent network reduction methods.« less