Research On Control Strategy Of Single-Phase Grid-Connected Inverter Based On LLCL Filter

With the rapid development of human society and economy,human beings urgently need to develop renewable new clean energy.Grid-connected inverters is the core component of distributed generation system,and the filter is the core component of the inverters.Choosing the appropriate filter control method can improve the performance of the whole grid-connected inverters system.Therefore,based on the LLCL grid-connected inverters,this paper focuses on the control technology of LLCL filter.By analyzing the basic structure and working principle of grid-connected inverters,the performance of LLCL filter is deeply studied.Combining with the restriction requirements of grid-connected inverters,the design process and principles of the main parameters of the filter are briefly described.Finally,according to the proposed parameter design method,the parameters of the simulation model and the experimental prototype are calculated.Through detailed analysis of the working principle and transfer function of LLCL filter,it is found that as a high-order underdamped system,LLCL grid-connected inverters also have resonance peaks.Aiming at the underdamped characteristic of the system,the damp strategy of increasing damp is usually adopted to restrain the negative influence of resonance on the system.At present,passive damping method,active damping method and mixed damping method are commonly used to suppress resonance problems.By using a weighted current method based on active damping,the output current of the inverter side and the grid-connected side current are weighted in a reasonable proportion and used as feedback.In theory,the system is reduced to one order to compensate for the problem of under-damping of the system.In view of the shortcomings of the weighted current method,the active damping strategy is introduced to better suppress the resonance peak of the system.Finally,the feasibility of the weighted current method is verified by simulation experiments.In addition to the resonance problem caused by the structure of the filter itself,grid voltage fluctuation and DC flow into the inverter system will also interfere with the performance of LLCL grid-connected inverters.Based on the principle that feedforward grid voltage can effectively weaken the adverse effect of grid voltage on output current of grid-connected side,combined with the characteristic that QPR current controller can track fundamental voltage without static error,it makes the output current of grid-connected side track the reference current better.The experiment on the simulation platform proves the correctness of the method.Aiming at a series of problems caused by DC injection,taking full advantage of the merits of PI controller which can accurately track DC component,a new control strategy of PI and QPR compound control is formed by combining PI controller with basic QPR current controller.The DC component of instruction current is successfully filtered out from the output current of grid-connected side.Simulation and experiment confirm the validity of theoretical analysis.