Impact of PLL and VSC control parameters on the AC/MTDC systems stability

The impact of PLL and VSC control parameters on the dynamic interactions between the AC and MTDC systems are discussed. A modularized method to incorporate the VSC-MTDC into transient stability programs (TSPs) is given. The proposed programs are validated via PSCAD simulation. Three results are obtained: (1) PLL has an insignificant effect on system stability. (2) The damping of MTDC grids is greatly affected by VSC control parameters. The inappropriate parameters would cause adverse effect on the damping performance of MTDC grids, resulting in large oscillation among DC voltages. However, these parameters have a negligible effect on small signal stability of AC systems. (3) Two forms of instability phenomena are found. One is caused by insufficient synchronizing torque; the other is caused by poorly damp of MTDC grids. As for the latter, it is the adverse interactions among the VSC controllers that cause the MTDC grids instability, thereby making the overall system easier to collapse.