| In recent years,distributed power generation based on renewable energy has grown rapidly,and DC microgrid systems containing distributed power sources have attracted widespread attention at home and abroad.When the system encounters a short-circuit fault,the distributed power sources and loads in the system may be affected,which will pose a certain threat to the stable operation of the DC microgrid.At present,few studies have been considered on the transient stability of the system after a short-circuit fault,and the effects of relay protection actions on the transient stability of the system have been less considered.Therefore,It is necessary to consider the transient stability analysis of the relay protection operating characteristics.First,the DC microgrid is modeled based on MATLAB/Simulink,including the modeling of photovoltaic power generation systems,energy storage systems,and converter interface circuits.Among them,MPPT control is adopted by photovoltaic power generation system to provide maximum power generation.;lead-acid batteries are selected as the energy storage system to maintain the stability of the DC bus voltage;The converter is modeled by an average model based on the state space,and its control strategy is analyzed,and its feasibility is verified by simulation.Line load shedding is used as a case to simulate and verify the correctness of the overall model of the DC microgrid.Secondly,the possible faults in the DC microgrid system are classified,and the short-circuit faults between the poles and the ground short-circuit faults are analyzed.Transient processes in different stages are analyzed using equivalent fault circuits,and the correctness of theoretical analysis is verified by simulation;According to the characteristics of the fault,a protection scheme with current differential protection as the main protection and undervoltage protection as the backup protection is given.Taking the short-circuit fault between poles as an example,the correctness of the scheme is verified by simulation.Finally,using the method based on the theory of mixed potential function to analyze the transient stability,the asymptotic stability regions of the system before the failure and the system after the relay protection action are made respectively.By comparison,the gradual stability range of the system after the relay protection action is reduced;The influencing factors of transient stability are analyzed from two aspects:fault removal time and capacitance.The critical fault clearing time under the two types of faults is estimated by using the time when the maximum fault current occurs,to ensure that the relay protection operates correctly within the critical fault clearing time,so that the system restores stability,the use of different sizes of capacitors is verified by simulation. |
