| With the large-scale access of distributed power sources and DC loads to the distribution network,the study of flexible DC distribution networks has become a hot research topic today.One of the key technologies is its protection strategy research.At present,the research on the protection strategy of flexible DC distribution network is still in its infancy,in particular,the development speed of protection research is limited by the development of DC circuit breakers.Therefore,the medium-voltage flexible DC distribution network was taken in this paper as the research object,and two protection strategies was proposed for the DC distribution network line in view of the line fault of the DC distribution network.The research on the protection method helped to improve the stability of the flexible DC distribution network system,and corresponding theoretical support was provided for its development.Firstly,the basic topology and basic control methods of the flexible DC distribution network were analyzed,and on this basis,the model of the medium-voltage double-terminal flexible DC distribution network was built.According to the built model,the fault characteristic analysis of DC distribution network was carried out,and the correctness of the fault characteristic analysis was verified by simulation.Secondly,an on-site quantity protection strategy suitable for flexible DC distribution network lines was proposed.The established DC distribution network model was divided into regions with the DC circuit breaker as the boundary;the characteristics of transient voltage changes and fault transient current changes during DC distribution network faults were analyzed,and the results of internal faults and external fault were analyzed accordingly.The instantaneous power characteristics of the fault in the area were identified.Based on the above theory,a line protection strategy for flexible DC distribution network based on instantaneous power characteristics was proposed.The protection strategy consists of startup criteria,fault identification criteria and fault location criteria,and the influence of the size of the transition resistance on the protection strategy was analyzed.Simulations are carried out to verify the selectivity and effectiveness of the strategy.The proposed strategy was simulated on PSCAD using the constructed flexible DC distribution network model,and the strategy was verified to be selective and effective.Finally,in view of the problems of intolerance to high transition resistance and abnormal data in the above protection methods,a pilot protection strategy for flexible DC distribution network lines based on Spearman correlation coefficient was proposed.The correlation characteristics of fault transient current waveforms were analyzed when the DC distribution network has internal faults or external faults,and the characteristics were used as protection criteria to determine internal and external faults,fault section location and fault type judgment.The strategy was simulated on PSCAD with the built flexible DC distribution network model,and it was verified that the protection method has better reliability and effectiveness by using a higher transition resistance value and changing different distributed capacitances. |
