Research On Control Strategy For Photovoltaic Grid-Connected Inverter Based On LCL Output Filter

With the rapid development of solar photovoltaic power generation technologies,grid-connected control of photovoltaic has become a research hotspot.The grid-connected inverter,which is regarded as the interface device between the photovoltaic array and the power grid,can affect the power quality directly.LCL-type grid-connected inverter is widely used in the field of photovoltaic grid-connected generation due to its superior harmonic suppression capability.But LCL filter is a third-order undamped system and can easily lead to resonate,and causes instability.According to the characteristics of LCL-type grid-connected inverter,this thesis researches the filter resonance damping methods,current accurate tracking control and power quality improvement.The main research contents are as follows:(1)The topology and mathematical model of L-type,LC-type and LCL-type grid-connected inverter are analyzed,and the LCL-type grid-connected inverter with high frequency harmonic suppression capability and cost advantages is adopted.(2)Through analyzing the resonant reasons and the damping method of LCL-type grid-connected inverter,the capacitor current feedback active damping method with good damping effect and practical value is adopted to prevent the loss of the passive damping.(3)In view of the traditional proportional integral control,proportional resonant control and repetitive control method cannot filter effectively the harmonics and decrease the steady-state error,a cascade current double closed-loop control strategy of the repetitive control and quasi proportional resonant control is proposed.The inner-loop uses capacitor current feedback to suppress high frequency resonance caused by LCL filter.The outer-loop adopts cascade dual current control strategy to inhibit the effect of voltage fluctuations and non-linear load on the grid current and achieve non-static error control and high power factor network synchronization.The simulation verifies the correctness and effectiveness of the proposed control strategy.(4)The state feedback pole assignment method based on the repetitive-quasi proportional resonant cascade current control strategy is proposed,considering the grid impedance,in order to inhibit the effect on the system power quality and stability.Finally,the study is validated through simulation.Studies have shown that: the state feedback pole assignment method can inhibit effectively the grid impedance impact on system stability,and improve the grid current waveform quality by the combination of the repetitive-quasi proportional resonant cascade current control strategy.