The mechanism analysis of sub-synchronous oscillation in PMSG wind plants

There is a large risk of sub-synchronous oscillation in the large-scale wind plants connected to the series-compensated transmission system. The sub-synchronous oscillation will cause the wind plants to take off from the grid and damage crowbar circuits, which seriously threatens the safe and stable operation of the power grid. The external impedance characteristic of direct-drive permanent magnet synchronous generator within the range of sub-synchronous frequency is studied in this paper to discuss the direct cause of the sub-synchronous oscillation. Also, in order to investigate the mechanism of sub-synchronous oscillation in PMSG, the equivalent system model of PMSG connected to power grid without series-compensated transmission is established in this paper. By means of transfer function analysis, eigenvalue analysis and electromagnetic transient simulation, the mechanism of sub synchronous oscillation is revealed. The results show that the feedforward voltage delay of the grid side control circuit of the PMSG is the direct cause of the sub-synchronous oscillation. In this paper, the specific composition and exact time of the feedforward voltage delay are analyzed, and the suppression measures of the sub-synchronous oscillation are put forward according to the characteristics of sub-synchronous oscillation and its potential danger.