Research On High-power Three-phase Grid-Connected Inverters With LCL Filter

The renewable energy power generation systems such as wind power and photovoltaicpower have led to a rapid development in the last decade. The main function of gridconnected inverter is DC to AC conversion，it is the key module of the renewable energypower generation system.The traditional inverter is connected to the grid through an inverter bridge and an Lfilter, the L filter can prevent the switching harmonics caused by the inverter from enteringto the grid. In a high-power inverter,a bulky and expensive inductor will be needed toreduce the current harmonics.The LCL filter can achieve more harmonics attenuation withthe same inductance.Therefore, the LCL filter is more suitable for high-power applications.But the control strategy of LCL filter is more complicated, the system may becomeunstable because of the resonance problem brought by the LCL filter, appropriate dampingcan solve this problem.In this paper,The relationship between the LCL filter parameter values and it’sperformance in reducing the switching harmonics is analyzed first,then the mathematicalrelationship equations are deduced.The impedances of the inductors and capacitor aretransformed into the p.u. values,hence equations deduced before are simplified.At last,theconstraints of the parameter values are obtained based on the requirements of LCL filterperformance.The simulation and experimental results verify the effectiveness of thismethod,the switching harmonics have been effectively reduced.The mathematical model of the system is deduced both in stationary-frame andsynchronous-frame, and the closed loop control system in stationary-frame is implemented,the closed loop control system has a capacitor current inner loop and a grid current outerloop, the P controller is adopted in the inner loop and PR controller is adopted in the outerloop.The space-vector modulation strategy is adopted in the PWM modulation module.The controllers are designed after the characteristics of PR controller are analyzed. Theexperimental results are presented to verify the stability of the system and effective damping of the capacitor current inner loop.Based on the control system designed in theprevious section, the harmonic compensators are added in the control system, thesimulation results show that the harmonic compensators are effective in reducing theselected current harmonics.The relationship between the harmonic impedance and the controller parameter isanalyzed, by changing the parameter values of the controller, the harmonic components ofgrid current in different conditions are compared, the experimental results are presented toverify the relationship between the harmonic impedance and the controller parameters.