Research On Key Technologies Of Short Circuit Fault Protection For DC Microgrid

The DC microgrid is a medium or low voltage DC grid,which contains a large number of power electronic devices,and it is also an important part of intelligent power system.Introduction of the DC microgrid has improved power system in economy,safety,reliability and its ability to absorb renewable energy.Short-circuit fault protection is one of the key technologies in power systems.Since the impedance of the transmission line in DC grid is much smaller than that in AC grid,the short-circuit fault current of the DC grid increases faster than that of the AC grid.Moreover,since the fault current is a DC current in DC grid,it takes a long time for the fault current to reach the zero-crossing point,and the fault current cannot be interrupted by the method of breaking the line at the zero-crossing point in AC grid.Based on the above two factors,the fault protection of DC grid is facing severe challenges.Compared with other types of DC grids,there are fewer studies on fault protection of DC microgrid.Since the DC microgrid is different from other types of DC grids in terms of level of voltage,type of connected units,topology of network,length of transmission line,etc.,it is of great significance to analyze the fault characteristics and fault protection of DC microgrid.Based on the needs of research in fault protection of DC microgrid,three key issues in DC microgrid are studied in this paper.And the three key issues are modeling and fault characteristic analysis,fault detection and fault location,and optimimal configuration of fault isolation devices respectively.A series of work has been done step by step,and the results obtained by research are summarized as follows:Aiming at the problem that the transient model of converters in DC microgrid ignore the influence of control system,a fault analysis and modeling method for the converter in the faulty DC microgrid are proposed.And the transient models of voltage source converter(VSC)and three commonly DC/DC converters are modeled.Moreover,the accuracy of the established models is verified with the help of control hardware-in-the-loop(CHIL)experimental platform.On the other hand,aiming at the problem of the complex topology modeling process of the DC microgrid with more connected units,a discrete modeling method,which considers intermediate nodes,for mesh-type topology is proposed.The discrete modeling method has a small calculation burden and can be implemented automatically.The accuracy of the established DC transmission line model is verified with the help of Matlab/Simulink simulation.Finally,a fault DC microgrid model is established by integrating the faulty converter models and faulty transmission line model.Furthermore,the accuracy of the established faulty DC microgrid model is verified with the help of a CHIL platform.Aiming at the problem that the slow detection speed and the prone to misjudgment and missed judgment in fault detection,as well as the low fault location accuracy and high sampling frequency requirements in fault location,a fault detection and location method based on the improved Pearson correlation coefficient(PCC)method is proposed.On the one hand,the PCC method is improved to realize the fault detection function by monitoring the difference between the sampled current and the steady-state reference current.And the adjustment factor introduced in the Pearson method can effectively avoid the misjudgment that occurs when the current curve is different but the current rate of change is the same.On the other hand,the PCC method is used to compare the sampled current and the transient current to achieve fault location.The sampled signal used for location is selected as the current signal with a fixed period before the fault is detected.Finally,based on the four-terminal DC microgrid,a sufficient numerical analysis is completed for the proposed fault detection and location method.Aiming at the multi-dimensional,strong coupling,and non-linear optimal configuration problem of fault parameters in DC circuit breaker(DCCB)and DC fault current limiter(DCFCL),an optimal configuration method of DCCB and DCFCL in DC microgrid based on the elitist nondominated sorting genetic algorithm(NSGA-II)is proposed.Firstly,the fault isolation mechanism of DCCB and DCFCL is analyzed,and the relationship between fault isolation and the parameters such as breaking current of DCCB,breaking time of DCCB,current limiting inductance of DCFCL is also analyzed.Secondly,on this basis,a fault parameters evaluation model and an optimal configuration method based on the improved NSGA-II algorithm for fault isolation devices are proposed.Finally,the proposed method is implemented with the help of the code compilation function in Matlab,and the improved effectiveness and convergence speed of the NSGA-II algorithm is verified.Also,the process of the set of optimal solutions and the final optimal solution are given.