| DC microgrid has flexible control and no frequency and power angle problems.It has become one of the important networking methods to access new energy and DC load.The DC microgrid often faces the access of various power sources and loads,so it is necessary to coordinate and control different power sources and loads to realize the stable operation of the DC microgrid.With the increase of constant power load,the negative damping characteristic of constant power load weakens the stability of DC microgrid gradually.Therefore,coordination control of DC microgrid and improvement of transient stability of DC microgrid have become important research contents.In this paper,the coordination control of DC microgrid and the transient stability of the system when switching voltage control mode are mainly accomplished as follows:(1)DC microgrid system modeling.The DC microgrid model including AC main network,battery,photovoltaic power generation unit,constant power load and resistive load five ports was established,the mathematical model of each port was established and the control mode of each converter was detailed,which laid a foundation for the analysis and study of the stability of DC microgrid.(2)Design voltage hierarchical control strategy of DC microgrid.On the basis of analyzing the characteristics and voltage control capacity of each port in the DC microgrid,the order of input voltage control of each port is determined.The system is divided into three voltage control modes.The power balance of each voltage control mode is analyzed,and the voltage hierarchical control strategy based on droop control is designed.The working mode of each port is determined according to the local DC bus voltage,and the decentralized autonomic control of voltage is realized.The control modes of each port in three voltage control modes are introduced respectively.The design principles of droop coefficient and switching threshold are analyzed and reasonably designed to ensure the smooth switching of voltage control mode.A simulation model was built in MATLAB/Simulink to verify the effectiveness of the proposed strategy under voltage drop and voltage rise conditions.(3)The transient stability criterion based on electric quantity is derived.According to the equivalent transformation of DC microgrid,a simplified model is obtained,and the circuit equation of the system is written.Then the volt-ampere characteristic curves of power source and load are obtained.The volt-ampere characteristic curves are used to analyze the moving trajectory of the system operation points,and the necessary conditions of system stability are obtained.According to the mathematical model of DC microgrid,the state equation of the system is established,and the stability criterion based on transient quantity of electricity is obtained from the condition of system stability.(4)The mechanism of transient quantity of electricity change in voltage control mode switching is analyzed and the improved control strategy is proposed.Based on the proposed criterion,the variation mechanism of quantity of electricity in voltage hierarchical mode switching of DC microgrid is analyzed.According to the analysis results,the control strategies of the power source and load are improved,in which the voltage deviation and voltage change rate are introduced into the droop control to provide additional transient quantity of electricity support,to improve the transient stability of the system and ensure the smooth switching of voltage control mode.Finally,the simulation model is used to verify the effectiveness of the proposed control strategy in switching to energy storage voltage control mode and switching to load shedding voltage control mode. |
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