Design-Oriented transient stability analysis of LCC-HVDC inverter with DC voltage control under grid fault

• The transient process of LCC-HVDC is analyzed from a dynamical point of view. • First application of phase diagrams to the analysis of the subsequent commutation failures. • A control method for stability enhancement based on the dynamic equation of the extinction angle is proposed. • Provides a new entry point for the study of commutation failures. Subsequent commutation failure (SCF) is an inherent issue for the line-commutated-converter based high voltage direct current (LCC-HVDC) in the receiving end. Nonlinear natures of LCC, such as the DC-AC conversion, commutation overlaps, relation between the transmitted power and grid voltage, etc. complicate the modeling and theoretical analysis for SCF under grid faults, which also poses challenges to the suppression of SCF. To address the above issues, the phase portraits analysis method is developed to depict the dynamics of SCF during grid faults. In the proposed analysis method, the large-signal model of the inverter is derived, hereafter, the dominant inducements of SCF, including control parameters, grid sag depth, and grid impedance are elaborated quantitatively. Moreover, an adaptive PI controller is designed for DC voltage control in both steady state and grid fault conditions, which can ensure the safe operation of the inverter under various degrees of grid fault. Then parametric boundaries for maintaining the highest DC voltage during transients are also obtained. Experimental results verify the effectiveness of the analysis and the proposed strategy.