Small-Signal Stability of MMC Grid-Tied System Under Two Typical Unbalanced Grid Conditions

The ac transmission network of modular multilevel converters (MMCs) usually adapts the overhead line, so the single-phase grounding faults of the ac side are rather general. At this time, the protection system will cut off the fault line for the first time, so two unbalanced grid conditions (UGCs) are possible, i.e., single-phase grounding (SGUGC) and disconnection (SDUGC). However, the comparative analysis of small-signal stability under two UGCs has never been involved. Therefore, when ignoring the transient process of faults and protection actions and considering the inherent properties of the MMC system, two small-signal models based on linear time-periodic theory are built under SGUGC and SDUGC. Then, the influence of the control parameters on the system stability under the two UGCs is compared in detail. The analysis results show that the control parameters should be optimized to avoid small-signal instability under different UGCs. Finally, the original nonlinear MMC grid-tied system verifies the analysis results.