Research On Three-level Grid-Connected Inverter With APF Function

With the increasing global energy problems and increasing power demand and power quality requirements,the application of distributed power grid-connected inverters has gradually matured.The three-level grid-connected inverter has become the focus of scholars because of its high efficiency,simple control and the same structure as the parallel active filter.In the case of grid balance and three-phase load symmetry,the DC-side capacitor voltage imbalance for the three-level midpoint clamp type inverter,the LCL filter resonance suppression strategy,and the three-level realization of power quality control are proposed.The inverter's quasi-PR controller superimposes the usage strategy.The main contents of this article are as follows:Analyze the relationship between the voltage and current of the inverter system,establish the mathematical model of the three-level inverter in abc coordinates,analyze its working principle and the current magnitude and direction of the space vector action,and determine the DC side capacitor voltage imbalance.The reason is to use SVPWM control based on basic space vector recombination to suppress the imbalance of the upper and lower capacitor voltages on the DC side to avoid the quality problem of the output voltage and current waveform of the inverter.In order to reduce the higher harmonics generated by the high frequency SVPWM control in the inverter side inductor current of the inverter,the inverter is connected to the grid through a third-order LCL filter.The range of values of the LCL filter parameters is determined according to the power flow direction,the current ripple generated by the SVPWM,and the resonant frequency range.The grid connection of the inverter is realized by using two current control loops on the basis of the LCL filter.One current loop control forms a closed loop by using the current of the filter capacitor as a feedback variable,thereby reducing the instability of the LCL filter,thereby improve the stability of the system;use another current control loop based on quasi-PR control to control the output current of the inverter.For improving the power quality and reducing the cost,a grid-connected control strategy that takes into account the power quality control function of the inverter is proposed to filter the harmonic current and reactive current in the grid current.In the traditional i_p-i_q method,the delay compensation link is added to avoid the inverter output current caused by the delay and the harmonics and reactive current to be compensated in the grid current are seriously inconsistent,and the compensation of harmonics and reactive current is improved.At the same time,based on the double closed-loop current control,the quasi-PR controller superimposition and the harmonic and reactive reference current are superimposed on the grid-connected current reference value to realize the grid connection of the inverter and the power quality.In order to verify the correctness of all the theoretical analysis in this paper,relevant validations were carried out on Matlab/Simulink and experimental platform respectively.The SVPWM control strategy for verifying the basic space vector recombination under off-grid conditions suppresses the DC-side capacitor voltage fluctuation of the inverter.Under the grid-connected condition,verifying the current loop with the current of the filter capacitor as the feedback variable can reduce the instability of the LCL filter,and verify that the inverter is connected to the grid by performing the quasi-PR control and the superposition of the reference current on the current loop.Filter out the effects of harmonics and reactive currents.