Research On Coordinated Control And Stability Analysis Of DC Microgrid

DC microgrids are becoming the main power supply architectures and future residential buildings based on its higher reliability and system efficiency.A typical configuration of DC microgrid is consisted of grid connection converters(G-VSC),distributed generations(DGs),dc load with POL(point of load)converter and energy storage system(ESS).The coordination control and stability analysis of them is key issues in the operation of DC microgrids.The power control strategy and system-level stability analysis of DC microgrids were studied in this paper.Conventional hierarchical control strategy use volotage-power droop control to share power in DC' microgrid,it unable to achieve the priority of renewable energy generations.This paper present an improved control strategy based on the analysis of the microsource structure and power generations output characteristics.This control strategy is divided into three levels:the third level provides the superior power grid with scheduler interface;the second level is to determine the microsource priority by design of the DC bus voltage grade,without communication link,schedule the microsource,and achieves the priority generation and automatic current sharing with the renewable energy power,only by taking local information;The first level accomplishes the implementation of converter control.The simulation and experiment result shows that the control strategy can achieve independent control of micro source and realize the smooth-switch between every operation mode.Conventional voltage-power droop control using resistance virtual impedance couldn't reduce power oscillation in DC microgrid.Based on the common small signal model of microsource interfaced converter under droop control,the influence factors of output voltage was theoretically analyzed.An improved droop control strategy based on compensate virtual impedance was presented.This droop control strategy consists of a resistance droop coefficient to achieve the power sharing and compensate virtual impedance to improve the dynamic characteristics of DC microgrid.A small signal model of a simple DC microgrid was used to analyze the design process of virtual impedance parameters.Simulation and experimental results demonstrate that the proposed compensated virtual impedance can improve the dynamic characteristics of the DC microgrid,and enhance the damping property.DC microgrid small signal stability analysis is linear and based on the impedance matching theory.This criterion can not distinguish between the source and load characteristics of the bidirectional converter,and unable to determine the impact of power distuibance on the stability of DC microgrid at the same time.In order to solve this problem,this paper propose a parallel determination method based on the load input impedance and source output impedance,it equivalent to a one port network.Based on the small signal model of DC microgrid,the influence law of filter parameters on the small signal stability was studied in DC microgrid.The result will guide the design of DC microgrid.DC microgrid based on DBS(DC Bus Signaling,DBS)control strategies,realize the conversion of DC microgrid between different operating modes according to the changing of bus voltage,which is characterized by a non-linear system,the switching system stability analysis will need nonlinear analysis theory.In this paper,the common mathematic model of DC microgrid under distributed control strategy was built,and depend on this model,large signal stability was studied based on Lyapunov's direct method,the parameter boundary conditions of DC microgrid was estimated.The influence of switching mode to the stability of DC microgrid was analysis based on switching stability theory.It provided an important reference for the DC microgrid system-level design under DBS control.