Research On Fault Location And Line Protection Of Multi-terminal Ring Flexible DC Distribution Network

In order to meet the continuous development of distributed power generation,DC energy storage devices,and the wide application of DC loads,multi-terminal DC power grids have become a hot topic and development direction for domestic and foreign research.At present,domestic research on DC distribution networks is still in its infancy,and there are still many technical problems in primary equipment,system operation,control methods,and relay protection.Among them,how to quickly isolate the fault in the case of a DC fault to ensure the normal operation of the non-faulty area,and locate and identify the fault is one of the key issues to be solved urgently.In view of this,this article mainly focuses on the fault identification,location and relay protection technology of DC distribution network.The article first compares and analyzes the advantages and disadvantages of common topological structures of flexible DC distribution networks,and selects multi-terminal ring DC grids as the research object of this article;in order to develop efficient and fast DC distribution network relay protection schemes,the unipolarity that often occurs on the DC side The transient characteristics of ground fault and inter-electrode short-circuit fault are analyzed in detail and verified by simulation.It is concluded that both types of faults have a capacitor discharge stage at the initial stage of the fault,which provides a theoretical basis for subsequent fault location and protection.Then,after analyzing the polarity characteristics of the fault current of the multi-terminal DC distribution network line,a pilot protection scheme based on the DC circuit breaker is proposed.This scheme convert the fault full current information into a logic signal through the data processor based on the magnitude and polarity of the line current change rate.The communication system only needs to transmit the logic signal to realize the fault judgment,which has low requirements for data synchronization and sampling rate.The protection scheme has simple principle and high reliability.Finally,a six-terminal DC distribution network model was built on the MATLAB/Simulink simulation platform to verify the feasibility of the protection principle and the insuscepyibility to transition resistance.Finally,in order to facilitate manual removal of the fault point,it is necessary to use the limited transient information after the fault to measure the location of the fault point.Aiming at the problems that the fault location methods cannot locate the pole-to-ground fault and pole-to-pole fault at the same time,and ignoring the effect of multi-terminal converter stations on the injected current at the fault point.,this paper proposed a fault location methed for multi-terminal flexible DC grid based on the full current component on the line.The influence of the transition resistance on the solution of the fault location is solved algorithmically by combining the voltage equations of the two meshes adjacent to the fault point.A 6-terminal flexible DC distribution network model is built on the Simulink and the simulative results show that,the proposed scheme can fast identify and reliably locate the fault when a line fault occurs,and meet the fault location requirements of DC distribution networks.