Accurate nonlinear time-periodic model and stability analysis of LCC-HVDC considering commutation failure and switching control

• A nonlinear time-periodic model for LCC-HVDC with commutation failure and control mode switching is proposed. • Local stability boundaries are obtained via Poincaré map and numerical continuation. • An efficient algorithm is proposed for attractor identification and global stability assessment. • Case Studies based on the CIGRE Benchmark model and a real-world project reveal a discrepancy between small- and large-signal stability. Line-commutated-converter based high-voltage direct current (LCC-HVDC) systems are essential for long-distance bulk power transmission, yet accurately modeling their dynamics under complex operating conditions remains challenging due to non-smoothness, nonlinearity, time-periodicity, and control mode switching. This paper develops a unified smooth nonlinear time-periodic ordinary differential equation (ODE) model for LCC-HVDC by approximating the switching dynamics. The proposed model accommodates both symmetrical conditions (SCs) and asymmetrical conditions (ASCs), as well as abnormal transients caused by commutation failure (CF) and controller switching. On this basis, the small-signal (SS) stability of steady-state periodic orbits (PO) under off-nominal conditions is analyzed using Poincaré map and numerical continuation, and the stability boundaries of controllers under the switching control scheme and external grid are obtained. In addition, a large-signal (LS) stability analysis method based on the Poincaré recurrence theorem is proposed to identify coexisting attractors and quantify stability under large perturbations. Results show that off-nominal operating conditions may trigger Neimark–Sacker and fold bifurcations, while some controller parameters have little effect on SS stability but significantly reduce LS stability The effectiveness of the proposed modeling and analysis framework is verified using both the CIGRE benchmark model and a real-world project in South China.