Research On The Stability Of Digitally Controlled LCL-type Grid-connected Inverter System

As fossil energy reserves continue to decrease and environmental issues become increasingly prominent,countries around the world are paying more and more attention to the development and use of renewable energy.Among them,distributed power generation is one of the most effective methods for the large-scale development and utilization of renewable energy.As the core of the distributed power generation system,the stability of the grid-connected inverter will affect the normal operation of the entire system.However,because of the influence of geographical conditions,energy distribution and environmental factors,the actual power grid shows the characteristics of weak grid.At the same time,the control system of the grid-connected inverter widely adopts digital control technology,which will lead to the risk of the inverter instability in the weak grid.To this end,this article will conduct in-depth research on the stability of digitally controlled LCL-type grid-connected inverter in the weak grid,and propose corresponding measures to improve the control performance of grid-connected inverter,thereby ensuring the safe and efficient operation of the whole distributed power generation system and the grid.First,three common methods for determining stability of grid-connected inverter system in the frequency domain are summarized,which lays a theoretical foundation for subsequent research.Based on the argument principle,the Nyquist stability criterion is introduced to judge the stability of the closed-loop system through the open-loop amplitude phase curve.This criterion is the basic basis for analyzing and designing the system.Based on this,the logarithmic frequency stability criterion is derived by the corresponding relationship between the logarithmic frequency curve and the amplitude phase curve.Furthermore,the impedance stability criterion of modeling the grid-connected inverter system and the grid system separately is introduced.Using this criterion,the stability of the grid-connected inverter in a weak grid can be easily judged.Secondly,the frequency characteristics of the output impedance of grid connected inverter system under digital control are analyzed.The imaginary and real parts of the output impedance expression are analyzed in the frequency domain,and it is found that the different system parameters under digital control may make the output impedance appear negative damping at one-sixth of the sampling frequency.It can be known from the impedance stability criterion that a system with negative damping in output impedance will face the risk of resonance or even instability in the weak grid.In order to improve the robustness and stability of the system,this paper proposes a method of grid voltage proportional feedforward and regulator parameters optimization.It is found that this method can not only improve the phase of the output impedance in the middle and high frequency,but also improve the system’s ability to suppress low frequency harmonics.A 3k W single-phase LCL-type grid-connected inverter prototype is built for experimental test,and the validity of the theoretical analysis is verified by the experiments.Finally,for the LCL-type grid-connected inverter using capacitive current feedback active damping under digital control,the effect of digital control delay on system performance is analyzed in detail.Based on this,a delay compensation method that can take into account both active damping characteristics and loop bandwidth is proposed,then the specific parameter design process is given.This method reduces the control delay without introducing switching noise,thereby improving the stability,robustness and dynamic performance of the system.The stability analysis and prototype experimental tests verify the effectiveness of the proposed delay compensation method.