Analytical expression for LVRT of BDFIG with enhanced current control to CW and reactive power support from GSC

During the low-voltage ride-through (LVRT) of the brushless doubly-fed induction generator (BDFIG), the oscillation of the power winding (PW) flux will induce high current in the control winding (CW), causing possible violation to the current constraint and weakening the power support capability of the machine-side converter (MSC). The existing PI control is designed based on the steady-state operation thus sensitive to the intense oscillation of the PW flux. To isolate the CW current control from the disturbances of the oscillating PW flux, the enhanced CW current control is newly proposed by introducing the change rates of the PW flux and the rotor flux to the inner-loop current control of the MSC as the feed-forward compensation terms. The analytical expressions of the PW current, the CW current, the rotor current, and the CW voltage are newly derived to quantify the LVRT effect of the enhanced control. With the DC-voltage constraint considered, the capacity of the grid-side converter (GSC) is newly utilized to improve the reactive power support capability. Dynamic simulation results show that the enhanced control reduces the oscillation of the CW current, the PW current and the rotor current. The accuracy of the analytical expression is validated by comparing it with the step-by-step simulation results. The selected references of the GSC current provide the additional reactive power support without violation to the constraint of the DC-voltage.