Study On Fault Characterization And Protection Technologies In Low Voltage DC Microgrid

With the ever-rapid development and progress of society,it is increasingly difficult for traditional AC power grids to meet the requirement of renewable energy access and users for better power quality.Low voltage DC microgrids have attracted more and more attention of researchers for its better compatibility with distributed energy sources,better power quality and lower transmission loss.For the reason that lack of practical operating experience and cost-effective DC circuit breakers,the protection technologies of low voltage DC microgrids has become a major obstacle to its development.Fault characteristic analysis is an important prerequisite and theoretical basis for the effective design of protection scheme.The existing DC protection technologies are mainly aimed at high voltage applications such as HVDC transmission.Meanwhile,low voltage DC microgrid lacks corresponding fault characteristic research,protection equipment and solutions.Based on these problems,this thesis takes low voltage DC microgrid as the research object,analysis the fault process and characteristics of two major faults,designs a new solid state DC circuit breaker based on SiC JFET to protect line-to-line fault,and proposes a method for ground fault location based on wavelet transformation to solve the location problem with single-pole to ground fault.The main research contents of this thesis are shown as follow:(1)To solve the problem of lack of fault characteristics in low voltage DC microgrids,the fault characteristics are explored through theoretical analysis and fault simulations.First,combining the features of DC microgrid structure,the process and characteristics of line-to-line short circuit fault and single-pole ground fault are investigated.Different stages during fault process has been proposed and simulation model is built in Matlab/Simulink.Based on theoretical analysis and simulation results,the fault characteristics and protection features of low voltage DC microgrid are proposed and lay the foundation for the design of protection solutions.(2)Based on the characteristics of line-to-line short circuit fault in low voltage DC microgrid and status quo of DC circuit breakers,a new SiC JFET-based solid state circuit breaker(SSCB)has been proposed.This new DC circuit breaker has the advantages of rapid isolation of fault and simple control system.First,the topology and working principle of the proposed DC circuit breaker are discussed and the optimization of design is introduced.The coordination between upstream and downstream devices is designed.Then detailed simulation model of new DC circuit breaker is built in PLECS and laboratory prototype is designed and built.Both of the simulation and experiment results verify the effectiveness of the proposed solid state DC circuit breaker design.(3)A new wavelet-based single-pole to ground fault location method is proposed in this part to solve the problem of difficulty in fault location in low voltage DC microgrid.First,based on the discharge phenomenon of shunt capacitance during ground fault occurs,the common mode current is chosen as the characteristic for ground fault location.Then common mode current in transient is decomposed with suitable wavelet function and appropriate criteria are set to distinguish different ground fault points.Finally,an algorithm for locating ground faults is proposed and the method is verified through changing value of ground resistance.