Research On Line Protection Strategy And Fault Ride-Through Method Of DC Distribution System

In recent years,with the development of power electronics technology,the massive access of distributed generation and the continuous increase of DC load on low voltage side,flexible DC distribution system has gradually become a research hotspot.Based on the double-ended medium and low voltage DC distribution system,this thesis focuses on the DC side line fault mechanism,the formulation of protection scheme and the DC medium and low voltage fault ride-through method.The main work is as follows:(1)The topologies of three existing DC distribution systems are compared and analyzed,and the topology used in this thesis is optimized.The medium and low voltage side voltage levels,the key equipments,the system wiring mode and the grounding mode in the DC distribution system are established.The simulation model is built in PSCAD/EMTDC platform.Aiming at the most common single pole grounding short circuit fault and the most serious inter pole short circuit fault in DC distribution system,the fault mechanism is studied,the analytical expression of fault characteristic quantity in each stage after fault is deduced,and the correctness of theoretical analysis is verified by simulation.(2)Aiming at the problem that the fault current is not obvious after the single pole grounding short circuit fault,a DC distribution system protection scheme based on the coordination of control and protection is proposed.According to the current differential change on the AC grounding branch,the parallel small resistance will be switched quickly to improve the fault characteristic quantity,which saves the communication control time compared with the traditional protection scheme.The differential protection based on zero mode current is used for fault location,which improves the accuracy of fault location;At the same time,for inter pole short circuit fault,low-voltage over-current protection is used as the starting criterion,and the high setting section of current differential protection is used for fault location.Simulation results show that the proposed protection scheme can quickly and effectively locate the fault section,and has good sensitivity and reliability.(3)Aiming at the problem that the DAB is locked quickly after the inter pole short circuit fault on the medium voltage side,the voltage on the low voltage side drops sharply and the fault recovery speed is slow,a new type of fault ride-through method without locking the DAB during the fault is proposed.By adding a fault current blocking module and a compensation capacitor branch in the DAB,it can quickly identify,cut off the fault current path and input the compensation capacitor after the fault occurs.During the duration of the fault,the DAB does not need to be locked,and relies on the remaining power of the module capacitor and the compensation capacitor to supply power to the load at the low voltage side.The simulation results show that the proposed method can effectively reduce the voltage drop at the low voltage side,and the capacitor can be quickly charged to normal operation after the fault is cleared.