Research On Harmonic Interaction And Resonance Of Multi-Grid-Connected Inverters

With the rapid development of power electronization in power system,micro-grid can effectively connect new energy distributed generation system to power grid and improve energy utilization.However,there are interactions among inverters with different characteristics in the microgrid system,which affect the quality of current and stable operation of the paralleled inverters.Especially when new energy is connected to weak power grid through LCL grid-connected inverters,which is with complex background harmonic voltage or low voltage distribution network,the interaction between inverters and power grid becomes more prominent,and even leads to system oscillation,which affects the stable operation of the whole system.This paper takes grid-connected inverters connected to weak grid system as the research background.Based on the control principle of grid-connected inverters,the single-phase grid-connected inverters and three-phase grid-connected inverters are modeled and analyzed in the dq coordinate system and the ?? coordinate system respectively.On this basis,the equivalent circuit model of multi-grid-connected inverters is established,and the interactive coupling between inverters and weak grid is analyzed detailedly.For the problem of LCL resonance suppression,the resonance damped suppression methods of LCL grid-connected inverters are analyzed,including passive damping,active damping and hybrid damping resonance characteristics analysis,and the damped characteristics of three methods are compared and analyzed when the grid impedance is fluctuating.In view of the possible harmonic interaction problems in the system,RC impedance restraining method exists the power loss.To avoid power loss,full feedforward control of grid voltage is adopted to restrain the influence of complex background harmonic voltage on grid-connected current of grid-connected inverters.In view of the possible harmonic interaction,RC impedance restraining method exists the power loss.To avoid power loss,full feed-forward control of grid voltage is used to restrain the influence of complex background harmonic voltage on gridconnected current of grid-connected inverters.For the resonance problem of multi-grid-connected inverters,the influence of number and grid impedance on the resonance characteristics of multi-grid-connected inverters is analyzed,and the law of resonance frequency offset is studied.Aiming at the problem of resonance frequency offset,RC passive damping is used to achieve global resonance suppression.Based on this analysis,more effective suppression methods are studied.According to the open-loop transfer function satisfying Nyquist stability criterion of small loop gain,virtual admittance active damping method is used to suppress resonance.Finally,an experimental platform is built to complete the experimental study of the cascaded system of grid-connected inverters and weak power grids,which verifies the correctness and effectiveness of virtual admittance to suppress resonance and full feed-forward control of grid voltage to suppress harmonic interaction.