Research On Bus Voltage Control Of Dc Microgrig And Delay Compensation Of LCL Grid-connected Inverter

With the application of renewable energy and the rise of distributed generation,DC microgrids have gradually become the focus of research due to the absnce of frequency,phase and reactive power issues.The DC microgrid includes off-grid and grid-connected modes,and the key to its stable operation lies in good control strategies.This paper firstly aims at suppressing the DC bus voltage fluctuations in the DC microgrid in off-grid mode,and proposes an energy storage converter to improve the Virtual DC Machine(VDCM)control strategy;secondly,to further expand the active damping range of the LCL grid-connected inverter,An improved duty cycle predictive control strategy with multiple error compensation is proposed.The details are as follows:In the first part,the small signal model of VDCM and the small signal model of DC bus voltage VDCM control are established first.The influence of VDCM parameters on the dynamic and steady state performance of the control system is analyzed,and the damping coefficient is increased.The steady-state error of the bus voltage and the improvement of the damping characteristics of the control system are limited,so a simplified VDCM control strategy with the damping coefficient removed is proposed to eliminate the influence of the damping coefficient on the steady-state error of the DC bus voltage.Secondly,in order to further reduce the DC bus voltage fluctuations caused by power fluctuations in the DC microgrid,an improved VDCM control strategy that adds a differential link after simplifying the integration link of the VDCM control structure is proposed,and a small signal model and a small signal model for improving the VDCM link are established.The DC bus voltage improves the small-signal model of VDCM control.The analysis shows that the differential coefficient increases its damping characteristics without affecting the steady-state performance of the control system.Compared with the VDCM control strategy,the improved VDCM control strategy eliminates the influence of the damping coefficient on the DC bus voltage steady-state error,and further suppresses the DC bus voltage fluctuation caused by the power fluctuation in the DC microgrid.In the second part,first establish the mathematical model of the LCL grid-connected inverter in the dq coordinate system,analyze the impact of the digital control delay on the active damping control of the LCL grid-connected inverter,and find that the digital control delay is reduced Active damping controls the damping range and reduces the robustness of the grid equivalent impedance.Secondly,the analysis of the compensation effect of the traditional duty cycle predictive control strategy of the LCL grid-connected inverter shows that its prediction error is large and the delay compensation effect is insufficient.To this end,an improved duty cycle predictive control strategy through multiple error compensation is conceived,and the frequency characteristic curve of its equivalent resistance is drawn for analysis.Compared with the traditional duty cycle predictive control and improved first-order filter compensation strategy,the improved duty cycle predictive control has a better grid current sine degree,and the adjustment time of grid current and the grid current offset during step The smaller,the better the robustness of the grid impedance.