Stability and Multiconstraint Operating Region of Grid-Connected Modular Multilevel Converter Under Grid Phase Disturbance

Grid-connected converters may operate under grid fault conditions, such as the short-/open-circuit and the transmission line switching, which may be accompanied by a phase angle difference between the grid and converter. This variation will affect the normal operation of the converter since its phase-locked loop is not able to track the grid voltage phase angle immediately. In this article, an instability phenomenon with unregulated ac line current is identified in a modular multilevel converter (MMC) based high-voltage direct-current transmission system when there is a sudden phase angle change on the grid side. A set of analytical constraints is developed for the MMC in terms of the Lyapunov stability, the modulation index, and the maximum power transfer capability. Then, a multiconstraint operating region of the MMC under various phase disturbances is derived. Moreover, the parameter influence on the operating region is analyzed and two stability-enhancing methods for the MMC system are proposed. Finally, the boundaries of the operating region and the stability-enhancing methods are verified by simulations and laboratory measurements of three-phase grid-connected MMCs.