Design And Analysis Of LLCL Filter Three-phase Grid-connected Inverter

With the shortage of conventional energy sources and the deterioration of environment,the development and utilization of new energy sources have become urgent tasks for social development.The use of renewable energy for grid-connected power generation is a focus point today.Grid-connected inverters are the key equipment for DC-AC energy conversion and bridging new energy generation equipment and power grids,and their performance directly affects the stability of the power grid operation and power quality.Therefore,it is of great theoretical and engineering significance to carry out design and analysis of grid-connected inverters.An improved LLCL filter is designed.Based on the analysis of the structure and performance of the new high-order LLCL filter,combined with several passive damping methods of the filter resonance peak,the active damping method of a capacitive current feedback control is introduced into the traditional LLCL filter,and the structure of the LLCL filter is improved.Besides,the improved LLCL filter parameters are designed.The control strategy of LLCL filter three-phase grid-connected inverter is designed.Passive damping control strategy will bring extra loss to the system,and will affect the performance of the filter.Therefore,the current double closed-loop control strategy,which takes the filter capacitor current as inner loop feedback and the power grid current as the outer loop feedback,is applied to the improved LLCL filter SiC MOSFET inverter.The parameters of the control system are designed by the pole assignment method,and the stability of the system is analyzed in detail based on the root locus and bode diagram.The simulation model of LLCL filter three-phase grid-connected inverter is set up.On Matlab/Simulink platform,the filter performance of LCL filter and LLCL filter in three-phase grid-connected inverter are compared and analyzed,and the dynamic response simulation of three-phase grid-connected inverter system is carried out.The simulation verifies the validity of the designed control scheme and system parameters.Based on IGBT and SiC MOSFET,an experimental prototype of 5kW/220 V LLCL filter three-phase grid connected inverter is set up.The three-phase grid-connected inverter with current double closed-loop control is built by using SKM75GB12 V IGBT and C2M0080120 D SiC MOSFET respectively.The main circuit,the driving circuit of the switch tube and other peripheral circuits of the inverter are designed separately.Then the characteristics of the two inverters are analyzed and compared with each other,and the performance of the output filter and the stability of the system is verified.In this paper,the structure of high-order LLCL filter three-phase grid-connected inverter is designed,and a new power device SiC MOSFET suitable for high frequency applications is used to build the prototype.On the premise of satisfying the power quality,compared with the traditional LCL filter IGBT grid-connected inverter,the total inductance of the filter is reduced by 78.3%,increasing the power density.Besides,the current double closed-loop control strategy not only effectively suppresses the resonant peak of the LLCL filter,but also ensures the good stability and dynamic characteristics of the three-phase grid-connected inverter system.Finally,the experimental results verify the validity and feasibility of the design scheme.