Impedance-based Analysis of Potential Stability Risk Between Grid-Forming and Grid-Following Wind Turbine Systems

In order to address the stability problems and the lack of inertial, the control strategy based on the concept of virtual synchronous generator (VSG) is applied to wind turbine system (WTS) to realize the grid-forming WTS. Question remains that whether potential stability risk will be caused if the grid-forming WTS accesses to the power grid in parallel with the traditional grid-following WTS. In this paper, a 2*2 admittance matrix model is novelly developed for the grid-forming type-IV WTS, based on which the impedance characteristics between the grid-forming and grid-following modes are compared. Impedance-based analysis reveals that the grid-forming type-IV WTS matches the grid-following type-IV WTS, while potential stability risk can be caused when it operates in parallel with the type-III WTS in grid-following mode. It is also found that this mismatch lies below the fundamental frequency where the connected grid is inductive, the type-IV WTS is capacitive in grid-forming mode and the type-III WTS has negative damp in grid-following mode. The developed impedance models and theoretical analysis are verified by experiments.