LVRT During-fault-stage Equilibrium Point is Unnecessary for Renewable Grid-tied Systems
Yayao Zhang, Min Cheng, Meng Zhan, Jiawei Yu, Yihua Zhu, Mingkang Wu · 2024
Transient synchronous stability of renewable grid- tied systems is of paramount importance. Most of previous studies focused on the grid-tied converter in the low voltage ride through (LVRT) during-fault stage and performed stability analysis based on the belief that if the system has (or has no) a stable equilibrium point in the during-fault stage, the system will be stable (or unstable). Therefore, it has been broadly accepted that the existence and/or stability of equilibrium point in the during-fault stage is necessary. However, the whole LVRT contains four different stages and the during-fault stage is only one among them. The transient synchronous stability of whole system should be determined by the initial state in the late post-fault stage and the attraction basin of its equilibrium point. The late post-fault stage is essential for stability and all four stages are important. Clearly treating the during-fault stage as the whole LVRT stages makes the nonlinear switched dynamics problem be greatly simplified and, certainly, is incomplete. In order to clarify this point, this paper analyzes the influence of existence (or nonexistence) of equilibrium point in the during-fault stage on the transient synchronous stability of permanent magnet synchronous generator grid-tied system. We can find that the existence of an equilibrium point in the during-fault stage is only a sufficient condition, rather than a necessary condition. The theoretical analysis is also verified in the hardware-in-the-loop experiment Therefore, a large number of previous studies and results on the transient synchronous stability of renewable grid-tied systems are incomplete and questionable.