Analysis And Suppression Of Sub/Super-Synchronous Oscillation Of Grid Connected Doubly-Fed Induction Generator Based On Impedance Model

At present,with the installed capacity of wind power generations in China increasing,wind power generations have become the third main power source after thermal power and hydropower in China,and the proportion of annual wind power generation in the total amount is also increasing year by year.Series compensation capacitors are usually installed on transmission lines to reduce the loss through the wind power transmission from the Three North Region to the load center,which the Three North Region is the region with abundant wind resources and far away from the load center.However,this approach will take the risk of sub/super-synchronous oscillations in wind power grid-connected systems.In severe cases,the wind turbine and thermal power generator will run off-grid,which seriously endangers the safe operation of the power system.Existing methods for analysis sub/super-synchronous oscillations mainly include frequency scaning,etc,but these methods are complex and no longer applicable when large-scale power electronic devices are involved in the power system,and impedance analysis can clearly demonstrate the stability of the system.Therefore,this paper uses the impedance model and the time domain simulation to analyze the sub/super-synchronous oscillations of grid-connected Doubly-Fed Induction Generator(DFIG)systems,and effectively suppress the sub/super-synchronous oscillation in the system,the main work is as follows:First,the model of the whole gird connected DFIG system is derived,which contains the model of the DFIG motor,main circuits models,duel-looped control systems and their phase-locked loop at the both side converter.Then,because stator of DFIG and AC side of grid-side converter are directly connected with power grid,impedance model of DFIG in dq frame can be obtained by using the established small signal transfer functions of grid-side and rotor-side of DFIG.Second,the open-loop transfer function of DFIG grid-connected system is obtained by using the equivalent impedance model of the power grid and DFIG derived above.Then,using the Generalized Nyquist Criterion on the obtained model,the influence of power grid parameters,motor parameters of DFIG and controller parameters on the system stability is analyzed,and thus the main factors that cause sub/super-synchronous oscillations can be given.In addition,the simulated model of a single 5MW DFIG grid-connected system is built in PSCAD/EMTDC simulation software to analyzes the system parameter stability and finally verify the impedance model.At last,try to add a damping controller to suppress the sub/super-synchronous oscillation caused by the grid-connected DFIG through series compensations.This controller composes of a second-order low-reject filter in the outer power controlling loop or the inner current controlling loop of the rotor-side DFIG.A conclusion that the stability margin of the system with the damping in the out power controlling loop is lower than that the damping in the inner current controlling loop,which may fail to suppress the sub/super-synchronous oscillation due to the influence of the inner current controlling loop.The effectiveness of the proposed method is further verified by adding dampers in the inner current controlling loop to suppress the sub/super-synchronous oscillation of DFIG caused by mismatched control parameters.Finally,the simulated model of the DFIG in PSCAD is established to verify the correctness of the above analysis results.Through this paper,it can be found that the additional damping controller in the rotor-side DFIG is used to reshape the system impedance model,and effectively suppress the sub/super-synchronous oscillation for the safe and reliable operation of the power grid.