Effects of VSI on the Correlation between System Stability and Output Active Power of Wind Farms in Weak Grid

Recently, subsynchronous interactions (SSIs) between wind farms and weak grids attract significant concerns. To investigate the impact of the wind farms output active power on SSI modes in the weak grid, an impedance-based grid-connected system model is established. The dominant factors that affect the correlation between output active power and system stability are analyzed. Based on the small-signal impedance model, the SSI modes of the system is analyzed by the stability analysis results. From the results, the dominant factors affecting SSI instability of the drive permanent magnet synchronous generator (D-PMSG) based wind farms connected to a weak AC grid is presented, including the inner loop, outer loop and phase-locked loop (PLL) parameters of its voltage source inverter (VSI). Meanwhile, the correlation between outer loop and system stability is various at different wind farms output active power level. The stability analysis results are verified by the time-domain simulation in PSCAD/EMTDC.