Experimental Findings and Theoretical Explanations on Time-Varying Immunity of LCC-HVdc Inverters to Commutation Failure Caused by Sending End AC Faults

Our preliminary work has recently reported that even sending end ac faults may cause commutation failure (CF) in line-commutated converter-based high-voltage direct current ( LCC -HVdc) inverters. However, the time-varying immunity to this new CF phenomenon is unclear yet. Thus, in this article, the immunity to the CF versus varying fault initiation time is first revealed by systematical hardware-in-the-loop experiments with different fault types and properties considered. The experimental findings are as follows: first, for both inductive and resistive faults, the time-varying immunity for the line-to-line and single-phase line-to-ground faults is much more prominent than for the three-phase and double-phase line-to-ground faults. Second, for the inductive faults, the immunity increases in the order of the three-phase, double-phase line-to-ground, line-to-line, single-phase line-to-ground faults. For the resistive faults, the immunity decreases in the order of the three-phase, double-phase, single-phase line-to-ground, line-to-line faults. Third, the immunity for the inductive faults is much lower than for the resistive faults regardless of the three-phase, double-phase, single-phase line-to-ground, or line-to-line faults. Second, these findings are theoretically explained by developing equivalent dc circuit and inverter reactive power consumption analysis. Third, some practical considerations are clarified as to the impacts of various complex factors on the time-varying immunity. Finally, the time-domain waveforms sufficiently validate the time-varying immunity.