| With the massive access of renewable energy and the power supply requirements of AC and DC loads,AC microgrids are difficult to achieve the requirements of power-supply reliability,system stability and diversified adaptability.Hybrid AC/DC microgrid is a main grid structure for various forms of distributed renewable energies and loads,and fully utilize the advantages of AC microgrid and DC microgrid to make them support and backup each other.Thus,hybrid AC/DC microgrid has high adaptability,diversity,and high power supply reliability.Hybrid AC-DC converter,as bridges between DC microgrids in different voltage levels and between DC microgrid and AC microgrid,plays an important role in energy transmission and conversion,and voltage conversion and support.Therefore,the ports characteristics of hybrid AC-DC converter will also become significant features,which are the performance,yardstick,and important pillar of the supplied power quality,long-term operation reliability,and stability of the converter and its interactive system in the hybrid AC-DC microgrid.This paper pays attention to the modular modeling and port characteristics of hybrid AC/DC converter.According to the clues from sub-module modeling method to port characteristics,and finally to the stability analisis of the multi-modular interactive system,the comprehensive and in-depth research is conducted,circulating the modular modeling,port characteristic performance of DC/DC stage and DC/AC stage bidirectional converter,and sub-module aggregation for smallsignal stability analysis of interactive system in AC side.The main researches in this paper are shown as follows:1.In this paper,a small input signal impedance modeling and reshaping method for dualactive-bridge-base(DAB)bi-directional converter in DC/DC stage of hybrid AC/DC converter are proposed.For the extended application of input port in DAB converter,such as the cascaded input LC filter and input-series output-parallel connection,the input port of DAB converter performs a heavy dynamic response,a large steady-state oscillation and a unbalanced voltage.Thus,the full-order small-signal input impedance model of DAB converter is established and considers the dynamic effect of all controllers.But alao,from the perspective of input voltage,the stability analysis and dynamic and steady-state characteristics of DAB converter are taken in the application scenarios of the cascaded input LC filter and the input-series output-parallel connection.Based on aforementioned works,the idea,method and design process for input impedance reshaping of DAB converter are proposed via triple-closed-loop control(input voltage,output current,and output voltage feedback)to achieved active damping.Thus,the stable characteristics in input port and output port,fast dynamic response,and small overshoot are cheved.The applicability,stability,and reliability of multi-level voltage conversion of the hybrid AC/DC converter in DC side are further improved.2.This paper discusses the operation characteristics and symmetrical fault-tolerant modalities of DAB converter from the perspective of high frequency link port under the open circuit switching fault(OCSF).In this letter,considering the parasitic parameters in switch and HFL,the proposed modality analysis presents a fundamental operation principle of parasitic parameters in switches and HFL,and completes the OCSF modality analysis system for DAB.In both fault and normal sides,the HFL currents and voltages are detailed during OCSF,and the potential oscillation is revealed.To achieve fault tolerance for OCSF in DAB,a universal topology transform methodology combining the symmetric topology and duality principle is proposed.The paper provides an effective and practical method in analysis and solution for OCSF in DAB,and it is also has reference value for other dc-dc converters.Moreover,the topology transformation methodology further improves the topology function of bi-directional converter in DC/DC stage,so that the bi-directional converter can be applied in more extensive DC applications,solving more complex fault problems.3.Aiming at the inherent resonance of LCL filter in hybrid AC/DC converter in DC/AC stage,a dual-current feedback active damping control strategy based on inverter-side current and grid-connected current feedback is proposed to achieve the resonance suppression of LCL filter,a larger stability margin and the robustness of DC/AC bidirection converter with LCL filter(DC/AC-LCL)and better grid-connected characteristics in weak-grid.In this paper,the mathematical models with and witout time delay are established for DC/AC-LCL.Then,the active damping characteristic of DC/AC-LCL based on the inverter-side current feedback is analyzed,and the Routh-Hurwitz criterion,stability margin,output power factor and active damping characteristic of system are investigated to achieve the ranges of parameters.Moreover,the systematic parameters design process is discussed and verified by the root locus diagram and Bode diagram in a case study.Furthermore,the proposed active damping strategy for DC/AC-LCL can make the hybrid AC/DC converter to achieve stable and excellent characteristics,supporting the power conversion of hybrid AC/DC microgrid and offering the high-quality power for users.4.In view of the uncertain performances of hybrid AC/DC converter in DC/AC stage and power electronic load in AC side under voltage fault,this paper presents the modeling method in ab frame for small signal output impedance model of hybrid AC/DC converter in DC/AC stage with various control strategy and small signal input impedance model of power electronic load in AC side.In this paper,the small signal output impedance models of hybrid AC/DC converter in DC/AC stage in grid connected mode and droop control mode are proposed respectively.The influence of amplitude and instantaneous angle of operation point on hybrid AC/DC converter in DC/AC stage and power electronic load in AC side is considered synthetically.In this paper,the models are further integrated,and the small signal impedance model of hybrid AC/DC microgrid is proposed in ab frame.The stability of interaction system in AC side is analyzed by generalized Nyquist stability criterion.The proposed mdelling and analysis method reduces the uncertainty of system stability of hybrid AC/DC microgrid,when only amplitude interference is considered in large signal operation point.A series of experiments of voltage sag show the correctness and feasibility of the proposed mdelling and analysis method.Moreover,the modeling method can be further utilized in the other grid fault conditions.Besides,this proposed modelling and analysis method can be applied to design the controller of hybrid AC/DC microgrid.Thus,the proposed modelling method can benefit to hybrid AC/DC microgrid to achieve an accuracy stability analysis and a smooth and stable operation. |
