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

• Published in: IEEE transactions on industry applications Vol. 58; no. 4; pp. 5005 - 5014
• Main Authors: Zhu, Jianhang, Hu, Jiabing, Li, Yingbiao, Ma, Shicong, Wang, Tiezhu
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.07.2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Capacitors; Converters; Grounding; Linear time-periodic (LTP); Mathematical models; modular multilevel converter (MMC); Parameters; Power lines; Power system dynamics; Power system stability; single-phase grounding faults; small-signal stability; Stability analysis; Stability criteria; Systems stability; unbalanced grid conditions (UGCs); Voltage control
• ISSN: 0093-9994; 1939-9367
• DOI: 10.1109/TIA.2022.3176596

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.