Subsynchronous Interaction Between Direct-Drive PMSG Based Wind Farms and Weak AC Networks

TLDR

Sub‑synchronous oscillations have been observed in direct‑drive PMSG wind farms in Xinjiang, China, a phenomenon previously unreported and whose mechanism remains unclear. The study aims to model a representative multi‑PMSG system and introduce a subsynchronous damping control loop to mitigate the instability. Using the manufacturer’s PMSG model, the authors performed small‑signal eigenanalysis, time‑domain simulation, and impedance analysis to characterize the interaction and evaluate the damping strategy. The analyses and field data show that weak AC grids with low short‑circuit ratio induce a negative‑resistance effect that destabilizes the SSI mode, saturates the PMSG controller, sustains power oscillations, and can excite torsional vibrations when the frequency aligns with generator modes.

Abstract

Recently, sustained power oscillation at subsynchronous frequency was captured in direct-drive permanent magnetic synchronous generator (PMSG) based wind farms in Xinjiang Uygur Autonomous Region, China. This new type of subsynchronous interaction (SSI) detected in practical systems has never been reported and analyzed before. Therefore, its mechanism and characteristics are not yet clearly clarified. In this paper, a simplified but representative system model with multiple PMSGs interfaced with AC networks is established first based on the actual system and the PMSG model provided by the manufacturer. Then, small-signal eigenanalysis, time-domain simulation, and impedance model analysis are carried out to investigate the interactive dynamics between them. The results show that such interaction between direct-drive PMSG wind farms and weak ac grids characterized by low short-circuit ratio would cause negative-resistance effect for the SSI mode, leading to unstable oscillation. In such cases, the controller of PMSG would saturate soon, resulting in sustained power oscillation in the system. If unfortunately the oscillation frequency matches the torsional mode of any nearby turbogenerator, severe torsional vibration would be excited on the shaft of the latter. The analysis results are finally validated with field measurements of an actual SSI event. To address the problem, a supplementary subsynchronous damping control loop is attached to the controllers of PMSGs to reshape the impedance and thus to stabilize the SSI.