Operation Analysis And Stability Control For LCL-Type Grid-connected Inverter In Non Ideal Grid Environment

With the increasingly serious problem of the global energy shortage and the proposal of the“double carbon”target,new energy generation technology has been developed rapidly.As the interface between new energy generation and the grid,the control performance of the grid-connected inverter plays a critical role to the output power quality and the stability of the generation system.However,limited by the geographical location of China’s new energy generation system,many grid-connected inverters are located far away from the electric power backbone network,and the grid connection interface is weak and complex,resulting in non-ideal grid environments such as grid background harmonics and weak grid.Among them,the background harmonic in the grid voltage causes the grid-side current distortion,which reduces the output power quality of the inverter,and the resonance caused by the LCL filter adversely affect the system stability;Under the weak grid,the Phase-Locked Loop（PLL）forms a coupling network with the current harmonic controller through the grid impedance,which induces low-frequency oscillation of the grid-side current,affecting safe operation of the equipment;In addition,the frequency coupling effect caused by the inherent asymmetric control of d-and q-axes in the PLL in weak grids makes the grid-connected inverter generate disturbance current and coupling current simultaneously under the harmonics disturbances at the connection point,reducing the output power quality of the inverter.Besides,the frequency coupling effect of PLL changes the equivalent output impedance characteristics of grid-connected inverter and further worsens the system stability,even causes the grid-connected inverter to be cut off from the grid in serious condition.The output power quality and stability of the grid-connected inverter in non-ideal grid has become a key factor restricting the promotion and popularization of new energy power generation.Therefore,it is of great significance to study the operation characteristics and stability control of the grid-connected inverter in non ideal grid environment for expanding the application of the new energy generation in the power market.Taking the typical three-phase LCL-type grid-connected inverter as the research object,this paper focuses on its operation characteristics and stability optimization control scheme in the non-ideal grid environment,and verifies the theoretical analysis through simulations and experiments.The specific research contents of this paper are as follows:（1）Aiming at the grid-side current distortion caused by the grid background harmonics and system instability caused by the resonant of the LCL filter,the current controller to improve the waveform quality of the grid-side current and the active damping control strategy to enhance the system stability are studied.By deriving the expression of the grid-side current,it is clarified that the essential reason for grid-side current distortion is that the grid background harmonic affects the output current through the equivalent impedance of the grid-connected inverter.On this basis,a proportional integral+quasi harmonic controller is designed to suppress the grid-side current harmonic;The stability of the grid-connected inverter with grid-side current feedback under digital control is analyzed through the open loop transfer function of the control system.Aiming at the instability problem caused by the LCL filter,an active damping control strategy of High Pass Filter（HPF）based on delayed positive feedback is proposed,and the stability of the grid-connected inverter is improved by selecting appropriate control parameters.The simulation and experimental results show that the HPF based on delayed positive feedback broadens the active damping range and ensures the stability of the grid-connected inverter when filtering parameters change;The proportional integral+quasi current harmonic controller can effectively suppress the grid-side current distortion caused by the grid background harmonic and improve the waveform quality of the inverter output current.（2）The coupling between PLL and the current harmonic controller under weak grids leads to low frequency oscillation of the grid-side current,and to solve this problem,a robust control strategy for current harmonic suppression system of the grid-connected inverter is studied.Firstly,the coupling relationship between PLL and the current harmonic controller is clarified by constructing the synchronization vector model of the grid-connected inverter under weak grid;The causes and laws of the PLL affecting the stability of the grid-connected inverter under weak grid are revealed by establishing the output impedance model of the inverter considering the influence of the PLL under grid background harmonics;According to the influence laws of the PLL on the stability of the grid-connected inverter,an improved PLL based on the second-order low-pass filter is designed,and the parameters of the improved PLL are designed.The simulation and experimental results show that PLL reduces the phase of the q-axis output impedance Z_qq of the grid-connected inverter.As the grid impedance Z_g increases,the frequency of the intersection point between Z_qq and Z_g shifts to the left,which is close to the compensation frequency of the current harmonic controller.The system is unstable due to the phase step caused by the harmonic poles,and the grid-side current may oscillate;The improved PLL can modify the low frequency characteristics of the output impedance of the inverter,and the problem of the grid-side current harmonic suppression system instability caused by the PLL in weak grid is solved.（3）The frequency coupling effect of the PLL under weak grid further reduces the stability of the grid-connected inverter.Aiming at this problem,the influence of the frequency coupling effect of the PLL on the output impedance characteristics and the stability of the grid-connected inverter under weak grid is studied,and optimal control methods are carried out.The frequency coupling mechanism caused by d-axis and q-axis asymmetric control of the PLL in weak grid is clarified by small signal disturbance analysis;Self admittance and coupling admittance models characterizing coupling characteristics are established in dq coordinate system,which proves that the frequency coupling effect of PLL cannot be ignored;By deducing the equivalent output admittance model of the three-phase LCL grid-connected inverter considering PLL frequency coupling effect,it is found that the output impedance of the grid-connected inverter under the coupling characteristics of the PLL is not only related to self admittance and coupling admittance,but also influenced by the grid impedance.It is pointed out that the frequency coupling effect of the PLL reduces the output impedance phase of the grid-connected inverter and ignoring it may lead to inaccurate system stability judgment results;An improved PLL based on second-order low-pass filter and Second-Order Generalized Integrator Phase Locked Loop（SOGI-PLL）are proposed to suppress the frequency coupling effect of the PLL and improve the stability of the grid-connected inverter.The simulation and experimental results show that the PLL frequency coupling effect in the weak grid makes the grid-connected inverter generate dual-frequency current response under the single-frequency voltage disturbance,which deteriorates the grid-side current quality of the grid-connected inverter under the harmonic disturbance;The frequency coupling effect of the PLL changes the frequency characteristics of the inverter output impedance model during low and medium frequencies and reduces the system stability;Both the improved PLL and the SOGI-PLL can attenuate the influence of the coupling admittance and improve the system instability caused by PLL frequency coupling effect.Based on the above three aspects,this paper realizes the operation optimization control of the photovoltaic grid-connected inverter in the non-ideal grid environment such as distorted grid and weak grid.The simulation and experimental results show that the method proposed in this paper can improve the output power quality of the grid-connected inverter,strengthen the system stability,and enhance its ability to operate stably in the non-ideal grid environment.