In order to reduce the influence of electric energy quality problems

such as voltage harmonic

voltage sag or swell

voltage unbalance

on the doubly-fed induction generator(DFIG)wind power system

a scheme of a nine-switch unified power quality conditioner(UPQC)applied to a DFIG wind turbine system is proposed after a contrastive study on the topological structures of various UPQCs and modeling analysis of nine-switch converter. On the basis of modeling the operation circuit of voltage and current compensation unit in UPQC

the corresponding control algorithm is designed. According to the power grid condition

3 types of working scenario are set up to optimize the operation scheme of UPQC

including harmonic circuit compensation

mild voltage compensation

and heavy voltage compensation. Simulation and experiments are made to verify the validity of the scheme. The results show that

the nine-switch UPQC can suppress harmonic voltage and current

eliminate voltage deviation and phase jump to significantly improve power quality of DFIG

thereby realizing flexible fault ride-through(FFRT)in a wide range of fault voltage conditions.