1. College of Electrical Engineering, Zhejiang University, Hangzhou 310027, China;2. China Electric Power Research Institute, Beijing 100192, China

To meet the need of rapid accurate simulation of the large-scale AC/DC hybrid power system, dynamic phasor method is applied to a model of line commutated converter under unbalanced conditions. Firstly, the principle of switching instants and commutation under unbalanced conditions is analyzed. A calculation method for triggering angle and commutation angle considering the third harmonic at AC as well as the second harmonic at DC is established. The traditional switching function model is improved for unbalanced conditions. Also, calculation process is simplified by ignoring non-essential harmonics, and an advanced dynamic phasor model for the line commutated converter, which is suitable for normal operating and unbalanced conditions, is proposed. Based on the Cigre-Benchmark DC system, the model has been compared with the simulation results from PSCAD, and the result shows the effectiveness of the model. Based on the improved switching function model, the failure of the commutation is further determined. The result shows that the failure of the commutation can be determined efficiently by considering the second harmonic at DC and the third harmonic at AC.