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

The AC transmission network of Modular Multilevel Converters (MMC) 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 there are two unbalance grid conditions (UGCs), i.e. single-phase grounding (SGUGC) and disconnection (SDUGC) during this process. However, the comparative analysis of small-signal stability under two different 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 the linear time-periodic (LTP) theory are built under SGUGC and SDUGC, respectively. Then, the influence of control parameters on the system stability under the two UGCs is compared in detail, and the results provide theoretical support for the control parameters optimization after protection system action. Finally, the detailed nonlinear MMC grid-connected model verifies the analysis results.