Research On The Stability Of Single Sampling LCL-type Grid-connected Inverter

With the massive consumption of fossil fuels,the energy crisis and environmental pollution are becoming more and more serious,which stimulates a growing interest in the renewable energy-based distributed power generation system(RE-DPGS).In the RE-DPGS,LCL-type grid-connected inverters have been widely used due to the higher power density and the superior power quality.However,the LCL filter suffers from a severe resonance problem,which challenges the inverter stability.This stability issue becomes more critical under the weak grid condition,where the LCL resonance frequency varies in a wide range due to the variation of grid impedance.Therefore,it is crucial to develop a proper active damping solution,which can guarantee the system stability and save the system cost.In this thesis,the LCL-type grid-connected inverter with the inverter current feedback is studied.First,the mathematical model of the inverter is established by considering the one and half sampling period time delay,and the delay effect on the capacitor-current-feedback active damping is interpreted.Then,based on the Nyquist stability criterion,the system stability conditions are deduced with the various grid impedance.These stability conditions can be categorized into three cases.Particularly,in Case 1 and Case 2,the gain margin requirements for system stability are opposite.An optimal capacitor-current-feedback coefficient is then derived for the three cases.With this feedback coefficient,stable operations will be retained for all resonance frequencies except one sixth of the sampling frequency,and a critical stable operation is performed for the resonance frequency of one sixth of the sampling frequency.Finally,by introducing a phase-lead compensator into the current regulator,the system stability is guaranteed even for the resonance frequency of one sixth of the sampling frequency.Furthermore,instead of directly sampling the capacitor current,a method to extract the capacitor current from the known inverter-side inductor current is proposed,so that a closedloop control can be realized with a single current sampling.A notch filter is needed to achieve this goal,but on the other hand,it introduces an extra open-loop right-half-plane pole and reduces the system low-frequency gain.To address these two issues,an integrator is added into the current regulator,which improves the system stability together with the optimal capacitorcurrent-feedback coefficient and the phase-lead compensator.A 6-kW prototype of the single-phase grid-connected inverter is built,and the experiments are performed to verify the theoretical analysis.