Islanded Small Signal Stability Analysis And Grid-connected Hierarchical Control Strategy Of DC Microgrid

The global energy crisis and environmental crisis prompted the world to find renewable energy to replace oil,coal and other traditional energy sources,which inspired the wind power,solar power and other technologies.As an effective way to integrate the use of renewable energy,DC microgrid technology has important research value and application prospect.In this paper,the small signal stability of islanded DC microgrid is analyzed,and a hierarchical control strategy of grid-connected DC microgrid is proposed.The main work of this paper is as follows:Firstly,the circuits of various basic DC/DC converters including Buck,Boost and Buck-Boost under six basic control are summarized.Based on the unified circuit model of basic DC/DC converter,the output impedance and the input admittance of the various basic DC/DC converters under single constant voltage control loop are deduced.Then,the output impedance and the input admittance of Buck,Boost and Buck-Boost under single current control loop and voltage-current double loop control are successively deduced,which lays the foundation for the analysis of the small signal stability analysis of the islanded DC microgrid.Secondly,an impedance-based system stability criterion is proposed.The criterion reveals that the stability of the system is only related to the impedance or admittance of each part under the premise of each module's own stability.The criterion can be compatible with the impedance between the system nodes;eliminating the reference to the output of the closed-loop transfer function matrix,which simplifying the calculation;no specific requirements on the system structure,which means that it is not only applicable to the radiation system but also can be applied to the ring system;It can analyzes the stability of complex DC microgrids including voltage controlled power supply,current control type power supply,constant power load,and system node impedance,and it is easy to expand.Then,with the proposed system stability criterion based on impedance,the stability of ring DC microgrid under multi-power master-slave control is analyzed,and the stability criterion of ring-shaped DC microgrid under master-slave control is given.The detailed impedance modelings of master power supply,slave power supply and constant power load with the electromagnetic interference filter are carried out.A series of active damping control method based on double quasi-PR controller is proposed.The results of a three machine four node DC microgrid under master slave control show that the proposed stability criterion can accurately judge the stability and the dynamic behavior of the system under the different constant power load,resistive load,and the proposed double quasi PR controller series active damping control method can effectively improve the system stability which does not affect the system steady flow distribution.Furthermore,with the proposed system stability criterion based on impedance,the stabilityof a ring DC microgrid under peer-to-peer control is analyzed and the stability criterion is given.The output impedance modeling of droop control power supply is carried out.A shunt active damping control method base on notch filter is proposed and the setting method of the corresponding parameter is given.With the proposed stability criterion,the stability of the system with various load power are accurately predicted,and with the root locus of the system under various control parameters and constant power load,the dynamic performance of the system is also accurately analyzed.The effectiveness of the proposed stability criterion and shunt active damping control method is verified by a case study of a three machine four node droop controlled ring DC microgrid.Finally,the hierarchical control structure of DC microgrid is summarized,and the hierarchical control of the grid connected DC microgrid based on parallel bidirectional power converter is proposed.The proposed internal loop dq-0 control method suppresses the circulating current among the parallel bidirectional power converters,and the proposed DC voltage squared method in the voltage control loop makes the nonlinear coupling relationship to linea relationship.With the droop control,the current can be equally divided preliminary,but the quantitative analysis of the current sharing error and the DC voltage deviation reveals that the singal droop control will cause a compromise between the current sharing error and the DC voltage deviation.So this paper proposes a dynamic droop coefficient adjustment and voltage adjustment strategy,while can eliminat the current sharing error and DC voltage deviation simultaneously.And the precise power control between the DC microgrid and the AC utiity is realized with the control.