Research On Resonance Suppression Strategy Of Grid-connected Inverter With LLCL Filter In Weak Grid

With the continuous expansion of new energy grid-connected scales,the research of new energy grid-connected generation technology in weak grid has received extensive attention.The micro-grid composed of distributed generators is generally located in remote areas.Due to the long transmission line,it needs to be multi-stage voltage conversion to access the distribution network,so that the impedance of the grid cannot be ignored,and the grid is characterized by weak grid.The grid impedance in weak grid cannot be neglected,which makes the grid impedance coupled with the high-order filter,and easily leads to the resonance phenomenon of the system,which affects the stable operation of the system.Therefore,this dissertation studies the resonance suppression of grid-connected inverters with LLCL filter in weak grid,analyses the resonance mechanism of single and multiple grid-connected inverters in weak grid,studies the causes of resonance and proposes effective resonance suppression strategies.The main research works are as follows:(1)The structure and working principle of LLCL filter are studied.The performance of LLCL filter is analyzed by frequency response characteristics.The performance of LLCL filter is further optimized.The parameters limitation conditions and design method are introduced in detail.LLCL filter consists of four energy storage elements and has inherent resonance peaks,which will affect the stability of the system.In order to solve the influence of LLCL filter resonance peaks on system stability,the passive damping method and active damping method to suppress the filter resonance peaks are compared and analyzed,and the virtual resistor method is selected for grid-connected inverters with LLCL filter,which lays a foundation for the research of the resonance suppression strategy of the subsequent chapters.(2)In order to the resonance phenomenon of single grid-connected inverter with LLCL filter in weak grid and the influence of grid impedance on system stability,a coordinated control strategy combining indirect sliding mode power control with virtual resistor is proposed.By introducing virtual resistor control into indirect sliding mode power control,the resonance phenomenon in grid-connected inverters can be effectively suppressed,the additional power loss in the system can be reduced,and the stability of the system can be improved.Compared with traditional PI control,the control strategy is robust to external disturbances and parameter changes,and can improve the harmonic distortion rate of grid-connected current.(3)To solve the resonance problem caused by parallel operation of multiple grid-connected inverters with LLCL filter in weak grid,a hybrid damping control strategy combining passive damping and active damping is proposed.Firstly,the mathematical models of multiple grid-connected inverters with LLCL filter are established.On this basis,the resonance mechanism of the multiple grid-connected system is analyzed.It is concluded that the key factors of resonance are the grid impedance and the variation of the parameters of LLCL filter.Therefore,without changing the topology of grid-connected inverters with LLCL filter,the proposed control strategy is applied to multiple grid-connected system in weak grid.Finally,the simulation and experimental results show that the proposed control strategy can effectively reduce the harmonic distortion rate of grid-connected current and grid-connected voltage,and improve the stability of the system in weak power grid.