Research On The Control Of LCL-type Grid-connected Inverter Under Harmonic Grid Voltage Condition

As a key equipment of distributed generation,grid-connected inverters convert the electrical energy generated by renewable energy into alternating current.In order to meet the grid connection requirements,LCL-type filters are used between grid and inverters.In contrast to L-type filters,LCL-type filters need smaller inductance and perform better in suppression of high frequency harmonics.However,the structure of LCL-type filter is third order without damping which will cause resonance and make the system unstable.As well,the nonlinear loads such as saturable transformer,arc welding machine,and etc.will distort the grid voltage.The grid voltage will distort the grid current further which won’t meet the grid connection requirements.To solve the problems above,this paper discusses the damping methods of LCL-type filters and suppressing methods of harmonic current under harmonic grid.Firstly,the mathematical model of the LCL-type grid-connected inverter is built in three phase（abc）stationary frame;two phase（αβ）stationary frame and two phase（dq）rotating frame respectively.As well,the dual voltage and current closed-loop control strategy of system under ideal grid is established.Then,in order to damp the resonance of LCL-type filters,this paper analyses the stability of system without damping and introduces several passive damping methods.After that,the stability of system with traditional grid current feedback active damping（GCFAD）method is analyzed.And an improved GCFAD method is proposed to extend the valid damping region from（0,f_R）to（0,f_R′）,where f_s/6<f_R<f_s/3,f_s/4<f_R<f_s/2.The optimal damping parameter is acquired based on the analysis of the stability of the system by the Nyquist theorem and the robustness of the system against the grid impedance variation is improved.After then,how the grid voltage influences grid current is analyzed.As well,two kinds of harmonic suppressing strategies are studied in this paper,including the proportional multi resonance controller（PMR）and the compensation scheme based on the band-pass filter（BPF）.After that,the full feedforward schemes of grid voltage are investigated based on two kinds of active damping methods.Considering the complexity of system and effectiveness of grid harmonic current suppression,an improved proportional feedforward control strategy of grid voltage is proposed and the PLL is improved as well.The parameters are designed to suppress the 5^th and 7^th harmonic current.Finally,the hardware is designed based on parameters of system.Simulation and experiment is realized on the MATLAB/Simulink and 10kW three phase grid-connected inverter experiment platform.The results of simulation and experiment verified the correctness and effectiveness of theory in this paper.