Stability Analysis Of Three-phase Grid-connected Inverter Based On Positive-sequence And Negative-sequence Impedance

The system stability is reduced in weak grid,and the interaction between grid impedance and inverter output impedance even result in the system instability.This thesis shows that a balanced three-phase AC system can be treated as a positive sequence subsystem and a negative sequence subsystem based on the symmetric component method,and the stability of the three-phase AC system is finally determined by the stability of each subsystem.The content of this thesis are summarized as follows:In this thesis,the positive-sequence and negative-sequence output impedance models of the inverter are obtained by harmonic linearization method.The effects of phase-locked loop and current loop on the positive and negative sequence impedance model of inverter are considered under the sequence impedance framework.And the positive-sequence and negative-sequence output impedance model is expressed as the positive-sequence and negative-sequence equivalent PLL impedance in parallel with positive-sequence and negative-sequence equivalent current loop impedance.From the perspective of the circuit structure,physical meaning and the dominant factor of the positive-sequence and negative-sequence PLL impedance and current loop impedance are revealed respectively.Aiming at the problem that the existing inverter controller usually has poor adaptability to weak grid conditions,the impedance optimization method to improve the grid-connected stability of the system is explored from the perspective of sequence impedance models.The effects of current loop bandwidth and SRF-PLL bandwidth on the system stability are discussed.An improved current control strategy based on the sequence impedance phase correction is proposed,thus improves the stability of three-phase ac system.Finally,the proposed control strategy is applied to the grid-connected system of the three-phase inverter in simulation.The results verify the effectiveness of the proposed control strategy.In this thesis,the positive-sequence and negative-sequence impedance model of three-phase inverter with impedance reshaping strategy is derived.Based on this model,the effects of feedforward function and phase-locked loop on grid-connected stability is analyzed.Using the impedance reshaping control strategy,the stability margin of the system is expanded by optimizing the feedforward parameters and improving the phase-locked loop structure.In addition,the proposed method can extend the impedance range of the three-phase grid-connected system in stable opration and enhance the system adaptability to weak power grid.Finally,the effectiveness of the impedance reshaping control strategy is verified by simulation.