Optimal Control Of Full-scale Wind Power Converters Under Nonideal Grid Conditions

With the extensive exploitation and utilization of wind power in China,its proportion in the regional grid continues to increase,and the interactions between wind power and power grid become more and more remarkable.Non-ideal grid environment is one of the major challenges that restrict the large-scale development of wind energy resource,affecting the effective grid connection,safe and stable operation of wind turbines.Wind power converter is a strong and controllable link of the wind turbine,and its optimal control could achieve wind turbine’s good operation under non-ideal grid conditions.In this paper,wind turbine with fully-rated converter is taken as the research object,and several key control technologies under non-ideal grid conditions are systematically studied.The specific contents are as follows:The current control technology of full-scale wind power converter under unbalanced grid conditions is studied.A multi-target current control strategy based on sliding mode variable structure for grid-side converter(GSC)is proposed.The sliding mode control(SMC)parameters are designed based on exponential approach law,which improves the dynamic characteristics of the controller and system chattering.The current reference command is calculated by using the extended instantaneous power theory,which improves the sinusoidality of GSC’s current,and eliminates the double-frequency pulsations in active power and extended reactive power.Based on the analysis of the impacts of input harmonics and dc offset,a positive and negative sequence separation method based on the improved second-order generalized integrator(SOGI)is presented,which realizes the independent control of positive and negative sequence current loops in an unbalanced grid.Based on the RTLAB hardware-in-loop(HIL)experimental platform,the feasibility of proposed control strategy is verified.A proportional multi-resonant(PR_M)regulator is applied for current optimization control of the GSC under harmonic distortion grid.To deal with the troubles that the order of LCL filter and PR_M regulator is higher and the control parameters design of current loop is harder,a parameter design method of PR_M regulator based on discrete root locus is proposed,which ensures the system is robust against grid frequency fluctuations and changes in filter parameters.The influence of harmonic distortion grid on phase-locked loop(PLL)is analyzed.A least squared-error filter(LSF)with frequency adaptive function is presented to improve the PLL’s synchronization performance in the unbalanced and harmonic distortion grid.In addition,the computational capacity of digital controller is reduced as well.Based on the RTLAB HIL experimental platform,the feasibility of proposed control strategy is verified.Aiming at the resonant interaction problem of LCL-type wind power converter in the weak grids,a small-signal impedance model of converter considering multi-control loop is established.The impacts of PLL,inner current control loop,outer dc-bus voltage control loop and filter parameters on output impedance’s frequency characteristics are compared and analyzed,and the resonant interaction mechanism between the weak grid and LCL-type wind power converter is revealed.On this basis,the stabilization control strategy of converter under weak grid conditions based on reducing PLL’s bandwidth is studied.It is pointed out that this method improves the phase of q-q axis impedance.From the perspective of increasing the amplitude of q-q axis impedance,a weak grid stabilization control method based on the filter capacitor current feedback is proposed.Based on the RTLAB HIL experimental platform,the feasibility of proposed control strategy is verified.The stability control issue of fully-rated wind power converters in the fault grids is studied.The transient current stress of GSC during grid faults is analyzed.The uncontrolled mechanism of dc-bus voltage under sudden voltage changes is revealed.A control method to automaticly transfer imbalanced power between grid side and machine side is proposed.This method adopts MSC and GSC to control dc-bus voltage and torque respectively,and transfers the imbalanced power to generator’s rotor during the grid fault,which solves the problem of dc-side voltage transient surges under low grid voltage conditions.Meanwhile,the over-modulation and dc-link voltage oscillation under high grid voltage conditions are avoided through GSC’s automatic excitation control,and the purpose of supporting the grid under low grid voltage conditions is realized.Aiming at PLL’s frequency instability problem in the faulty grid,a reduced-order model for the grid-connected fully-rated wind turbine is established.A control method by reducing output active current and delaying the injection of inductive reactive current is proposed to enhance the wind turbine’s transient frequency stability.The simulation model based on PSCAD is built to verify the correctness of mechanism analysis and the feasibility of proposed control method.