Multiple Switching Functions Based HSS Model of LCC Considering Variable Commutation Angle and Harmonic Couplings

The line commutated converter (LCC) based high voltage direct current (HVDC) transmission technology facilitates efficient power transmission over long distances, but also arouses system stability concerns. Although a considerable number of switching function based models and state average models of the LCC have been proposed for system stability analysis, none of them simultaneously took variation of the commutation angle and couplings of characteristic harmonics into consideration. This paper demonstrates that the two factors can be critical to system stability analysis, and on this basis, a novel multiple switching function based harmonic state space (HSS) model of the LCC is proposed focusing on impedance based stability analysis. The commutation angle's variation is depicted with switching functions that correspond to different commutation processes and the LCC model is built based on HSS to reflect harmonic couplings. Influences of commutation angles' variation as well as harmonic couplings, and their necessity for impedance modeling, are discussed through comparing the proposed model with other models. Hardware-in-the-loop (HIL) experiments validate the ability of the proposed model on capturing the two factors and demonstrate its satisfactory accuracy and efficiency in impedance based harmonic stability analysis of the LCC.