Stability Analysis of MMC under Grid Voltage Phase Change

Grid voltage faults may cause grid voltage phase changes in grid-connected modular multilevel converter (MMC) systems, leading to power factor degradation and excessive reactive power in grid-connected MMC systems. To see the effect of reactive power on the stability of an MMC system, in this paper, the effects of the grid voltage phase on the control loop and hence stability of the system are analyzed. The Lyapunov method is adopted to identify the critical grid phases in grid-connected MMCs. By comparing the effects of the various parts of the control loop, it is concluded that the unstable response of MMCs under phase change is caused by the instability of the active power loop. The analytical results are verified by simulations of a three-phase 21-level MMC.