Research On Resonance Mechanism And Control Strategy Of LCL-type Grid-connected Inverter

Traditional fossil energy has gradually become exhausted with the development and utilization of human beings.In order to meet the needs of human society,renewable energy based on wind and solar energy has come into people’s eyes.As the main way of using new energy,distributed generation technology has the characteristics of less transmission loss,friendly environment and safety,and has been greatly developed in recent years.As the key interface equipment between new energy generation and power grid,grid connected inverter plays a key role in power conversion,so its performance has an important impact on the stable operation of grid connected system and the safety and reliability of power grid.In recent years,LCL grid connected inverter has received extensive attention and research because of its better switching frequency sub harmonic attenuation ability.However,LCL filter,as the inherent resonance summit of the third-order system,causes the resonance phenomenon of the system output current,even leads to the system disconnection in serious cases.At the same time,due to its large number of non-linear equipment connected to the common coupling point,it appears non ideal grid characteristics,on the one hand,will produce a large number of background harmonics,which make the output current of the system appear harmonic resonance through the power loop,on the other hand,make the grid show the characteristics of weak grid,there is a wide range of changes in grid impedance seriously threatening the stability of the system.In addition,the grid-connected inverter generally adopts digital control,and the digital control delay due to calculation and modulation under digital control will change the characteristics of the system in the frequency domain,thus reducing the phase margin and dynamic response ability of the system,and significantly weakening the stability range of the system.In this paper,the single-phase LCL grid connected inverter is taken as the research object,and its resonance suppression technology under the condition of digital control and non ideal grid is studied in depth,in order to effectively improve the adaptability and stability of the system under complex conditions.The inverter side current feedback control is widely used in the distributed generation system because of its simple implementation and more practical conditions.However,the digital control delay and the wide range variation of grid impedance under the non ideal grid conditions limit the stable range of the control strategy based on single current feedback,weaken the robustness of the system,and the resonance peak of LCL filter is not suppressed,so the quality of incoming current may be affected.At the same time,when there are a large number of grid background harmonics in the power grid,capacitor-voltage proportional feedforward is usually added to the inverter-side current feedback control to suppress it.However,the analysis of this paper shows that there is a reverse resonance peak in the open-loop gain under digital control,which changes with the grid impedance.At the reverse resonance peak,the phase of the system is greatly reduced,resulting in a significant reduction in the stability range of the system,even leading to system instability.Therefore,this paper proposes a method based on the feedforward phase compensation method based on all-pass filters in weak grid can improve the phase of the system in the range of reverse resonance peak by adding a phase compensation link in the capacitor voltage feedforward channel,which improves the stability of the system.At the same time,this paper gives the corresponding design method of compensator parameters.The simulation results obtained on the 3k W single-phase LCL inverter grid-connected platform fully verify the effectiveness of the proposed control strategy.Grid current feedback control strategy is another basic inverter control structure,which is a direct control of grid current with smaller control error.Similar to single inverter side current feedback control,digital control delay and wide range variation of grid impedance under non ideal grid conditions limit its stable range and control performance.Meanwhile,the uninhibited resonance peak of LCL filter may affect the output current quality of the system.The capacitor current feedback active damping strategy with PCC voltage feedforward is based on this kind of control structure and is widely used,but its more state quantity feedback makes the control complexity and parameter design complexity of the system further increase,at the same time,more sensors are needed and the economy is poor.Therefore,based on the above control strategy,this paper proposes a control strategy based on capacitor-voltage proportional feedforward and grid-current proportional-differential feedback through block diagram equivalent transformation and reasonable design,which simultaneously suppresses the influence of LCL filter resonance and PCC voltage harmonic,in addition,it has certain adaptability under the condition of wide range of grid impedance changes.The 3KW single-phase LCL inverter grid connected platform fully verifies the effectiveness of the proposed control strategy.