Research On Control System For Single-phase Grid-connected Inverter With LCL Filter

Due to the exhaustion of traditional energy,the development and utilization of environmentally friendly new energy is in line with the development trend of the world today.Inverters play an important role in the grid-connected system of new energy generation.LCL-type grid-connected inverters have attracted wide attention from researchers for their excellent high-frequency harmonic suppression capability.Since the LCL filter is a third-order system with resonance,this paper focus on the resonance suppression method of LCL-type grid-connected inverter.Firstly,the introduction and analysis of bipolar SPWM modulation and single-pole frequency-doubled SPWM modulation technology is given.By analyzing the harmonic distribution characteristics of the inverter output voltage through the bilateral Fourier transform,the similarities and differences between the two modulation methods are pointed out.The LCL filter is designed according to the grid-connected current standard and its design steps are given.Secondly,six common passive damping strategies are studied.On the basis of the detailed analysis of the damping suppression mechanism,the feedback control of inverter side inductor current,split capacitor method,and capacitive current and grid-connected current double closed-loop control are introduced,analyzed and compared in detail,and their advantages and disadvantages are pointed out.Then,considering that the active damping suppression strategy under PI control is difficult to achieve zero steady-state error control of the fundamental frequency,that the introduction of the fundamental wave resonance can realize the tracking control of the fundamental wave component without static difference is indicated on the basis of the analysis of PI controller,PR controller and quasi-PR controller.A new single-current closed-loop control strategy with simple structure,low cost and strong robustness is proposed.This method only needs to sample the grid-connected current and track the fundamental frequency without static difference through the configuration of the current loop.Finally,the simulation model built on MATLAB/Simulink proves that this strategy can achieve better control effect under both steady state and dynamic conditions.The hardware circuit of the single-phase grid-connected inverter is designed and the experimental model is built.The experimental results verify the correctness and effectiveness of the proposed single-current closed-loop control strategy.