| With the sharp increase of the proportion of new energy power generation represented by photovoltaic power generation in the total installed capacity and power generation in China,as the key intermediate node of transmission and grid integration,the stable operation of grid-connected inverter is becoming more and more important.In order to improve the control performance and low voltage ride through ability of photovoltaic power generation system,the research on grid-connected inverter has gradually become a hot spot in recent years.In this paper,the three-phase grid-connected inverter is taken as the research object,and the following work is carried out around the low voltage ride through strategy based on model predictive control:(1)The modeling and control process of three-phase grid-connected inverter is analyzed.Firstly,the topology and working principle of three-phase two-level grid-connected inverter with LCL output filter are analyzed,and its input and output characteristics are analyzed based on the establishment of relevant mathematical models.Secondly,the phase-locked technology related to the design of inverter control algorithm is described,and the characteristics of two grid-connected current model predictive control strategies are analyzed and compared.(2)In order to improve the performance of grid-connected inverter,a continuous control set-model predictive control(CCS-MPC)strategy based on disturbance compensation is proposed.Firstly,CCS-MPC is used as the grid-connected current prediction algorithm.By setting amplitude limiting constraints on the prediction optimization process,the current fluctuation in the initial stage of prediction optimization is solved and the dynamic response speed of the system is improved;Secondly,in order to solve the problem that CCS-MPC can not effectively suppress the influence of dead time on power quality,the distribution frequency of dead time disturbance is determined through mechanism modeling,and an extended Kalman filter(EKF)is designed to estimate the deviation voltage amplitude generated by dead time at some specific frequencies,so as to compensate for this deviation and realize the correction of prediction outputs.Finally,the proposed control strategy is verified by simulation.(3)In order to improve the low voltage ride through ability of grid connected inverter,a strategy of realizing low voltage ride through(LVRT)through reactive power compensation is proposed.Firstly,the types and characteristics of low-voltage faults are described,and the mechanism of voltage positive and negative sequence components under faults is analyzed.Secondly,in order to realize fault identification and crossing state switching,double second-order generalized integrator and phase-locked loop are selected to separate positive and negative sequence voltage and extract phase angle,obtain the real-time proportional change of positive sequence voltage under fault,and compensate reactive current to grid-connected inverter in real time according to proportion.Under the action of control algorithm,the system responds to the step change of reference value caused by reactive power compensation.Finally,the fault environment is simulated to verify the ability of the system to maintain continuous grid connection in the process of low-voltage fault.(4)Using the hardware-in-the-loop(HIL)simulation mode of DSP hardware controller and virtual object,the HIL simulation test environment of three-phase grid-connected inverter is established to further verify the strategy proposed in this paper. |
