Research On Stability Of A Two-stage Power Conversion System Based On Sequence Impedance

With the rapid development of the renewable energy industry,distributed energy storage technology has been steadily implemented,which brings power conversion system(PCS)—the critical energy interface between storage unit and power grid—much attention.However,the power distribution network connected with PCS might be weak grid,making the stability issue of the interconnection system increasingly prominent.Therefore,research on the stability of PCS has its theoretical and practical worth.In this thesis,the two-stage PCS is selected as research object for its flexible control characteristics,and research is conducted with respect to sequence impedance modeling,stability analysis,and improving stability interacting with weak power grids.The specific research content is organized as follows:The stability analysis based on sequence impedance is analysed for its decoupling considering dq impedance is coupled between d axis and q axis.The preceding stage of the twostage PCS discussed in this thesis is a Buck/Boost converter regulating the current flow of the battery,and the posterior stage is a three-phase full bridge converter regulating DC bus voltage and the reactive current.The harmonic linearization method is ultilized to model the sequence impedance of the proposed PCS at PCC,and the output impedance of the preceding DC/DC,the DC voltage regulator,PLL controller and the mirror frequency coupling effect are taken into account when modeling so as to guarantee accuracy.The impedance measurement results from simulation verify the validity of the sequence impedance,and comparison between the proposed impedance and other impedance modeled via simplified approach such as neglecting PLL or output impedance of the preceding stage is drawn to further verify its superior accuracy.The effects of a set of factors on stability is analysed in detail based on the impedance obtained,including grid impedance,the power of the stable state point,and the parameters of various controllers;where certain conclusions concerning stability are drawn and some attention points of parameter design proposed,and all the conclusions and attention points are verified through simulation.Then the influence of the grid voltage feedforward strategy on the impedance of the PCS is analyzed,and how feedforward control improves the weak grid adaptability of the PCS is explained.The practical engineering applications where PIR controller is preferred out of harmonic suppression are taken into account,and analysis suggests that the ordinary PCC voltage feedforward contol could worsen the stability of PCS when the resonant control exists.To solve this problem a kind of bandstop PCC voltage feedforward strategy is proposed,the effectiveness of which is verified through analysis,simulation and experiment.What’s more,a kind of asymmetric feedforward strategy is proposed as the improvement of the ordinary one.Simulation and experiment results prove that asymmetric strategy has superior grid impedance adaptability over symmetric one.Sliding mode control is introduced into the prosterior DC/AC control system for its desirable robustness.The zero dynamic characteristics of DC/AC converter are analyzed at first,based on which,control strategy composed of PI control for outer loop and sliding mode control for inner loop is designed.The criterion for parameter design is concluded and the robustness of sliding mode control is analyzed.Then the sequence impedance of PCS under the sliding mode control is modeled based on the equivalent control.The analysis shows that the expression of sliding mode equivalent control is similar to P control,so the sliding mode control has a wider grid impedance adaptive range than PI+ feedforward control,which is also verified by simulation and experiment.Finally,a disturbance compensation strategy based on linear extended state observer is proposed to deal with the problem that small coefficient of the sliding mode switching term might lead to steady-state error.Simulations and experiments suggests that the proposed strategy ensures the sliding mode control to be zero-steady-state-error even with a small switching coefficient.