Research On Current Limiting And Protection Of DC Microgrids Cable Fault

DC microgrids has simple structure,low loss,high quality of power supply,green development,and can solve the problem of distributed power supply connected to the grid,has wide development prospects.However,the outlet of DC microgrids converter usually has a large voltage regulator capacitor.After the cable fault occurs,the rapid discharge of the capacitor generates a rapid increase in fault current,which threatens the safety of the fragile power electronic devices in the system.Due to the small coverage area,short line length,small capacity and low voltage grade of DC microgrids,the traditional protection technology cannot be applied well.The extension and development of DC microgrids are limited by the lag of protection technology.Therefore,it is of great significance to study the protection scheme of DC microgrids.First of all,this thesis introduces the research status of DC microgrids protection technology,the protection is divided into single terminal protection and double terminal protection.The problem of threshold setting is difficult for single terminal protection,and the protection based on communication may have delay time,which cannot meet the requirements of power electronic devices for quickness.On this basis,the problems that need to be solved for DC microgrids protection are summarized,and the future development direction is prospectedSecondly,the mathematical models of grid-connected interface,photovoltaic power supply and battery modules in DC micro-grid are established.Appropriate converter models and control methods were selected for different modules,and a typical four-terminal radial DC microgrids system was built based on the PSCAD/EMTDC simulation platform,which provided a verification tool for the subsequent cable fault analysis and protection method researchLater,considering the existing detailed fault analysis of the VSC grid-connected interface and photovoltaic power module,there is little analysis of the battery energy storage module.Therefore,taking the battery energy storage module as an example,the cable fault analysis of the converter outlet is carried out.In order to ensure the comprehensiveness of the fault analysis,the different faults of the battery under different working conditions are expounded one by one.The fault transient process is divided into different stages according to the change of fault current loop,and the expression of the related fault quantity at each stage is given.The influence of fault on uncontrollable power electronic components is emphasized.Based on the PSCAD/EMTDC simulation platform,the aforementioned faults of the battery module are simulated and verified,and the fault characteristics are summarized.In addition,the effects of battery charging and discharging state,fault distance and transition resistance on the fault transient process are compared to provide a theoretical basis for the study of protection methods.Finally,current limiting protection integration scheme is proposed.In this method,the current limiting measures are combined with the protection action.On the premise of limiting the amplitude and growth rate of the fault current,the current zero crossing is made,and the fault current is cut off by using the semi-controlled power electronic devices to achieve the purpose of protection.This protection method limits the amplitude of fault current flowing through the fragile power electronic devices,prolongs the peak time of fault current,reduces the strict requirements on the speed of protection scheme,and ensures the safety of the converter from threats.On the other hand,the integrated scheme of current limiting protection is simple in principle,and can be widely applied to all modules of DC micro-grid.It can ensure reliable operation in the case of single-pole grounding fault,inter-pole fault,and different distance fault and transition resistance grounding fault.Based on PSCAD/EMTDC simulation platform,the failure of VSC grid-connected interface,photovoltaic power supply and battery module is simulated respectively to verify the protection performance of the integrated scheme of current limiting protection.