Research On Fault Characteristic Analysis And Protection Schemes Of Medium-voltage DC Distribution Line

With the rapid development of distributed generation resources,the continuous increase of various kinds of DC loads and the increasingly wide application of various types of energy storage,the problems of high loss of AC and DC energy conversion,slightly poor flexibility and more converter links are becoming increasingly prominent in AC distribution system.As a powerful supplement to AC distribution network,flexible DC distribution network has the advantages of easy and flexible connection with distributed generation resources,excellent power quality and low line loss.It has a strong development momentum in recent years.However,as the core technology of flexible DC distribution network,the immature and incomplete development of relay protection is the direct factor restricting the rapid development and efficient application of flexible DC distribution network.This paper takes the medium-voltage DC distribution system as the research object,using the method of combining theoretical analysis and simulation verification,the fault characteristics of mediumvoltage DC distribution lines are systematically analyzed,on this basis,the single-pole breakage protection and short-circuit protection schemes of medium-voltage DC distribution lines are proposed.Firstly,a typical ring medium-voltage DC distribution network model is established,and the coordinated control strategy of key equipment and each port is defined.On this basis,by establishing the equivalent model of the system before and after the fault,the voltage and current characteristics of the single-pole breakage fault on the DC line,the link line between the sending-end converter and the DC bus,and the link line between the receiving-end converter and the DC bus are analyzed.And the influence of system grounding mode on the characteristics of the single-pole breakage fault is discussed.It shows that,the flow path of the fault current is related to the location of the single-pole breakage faults;the voltage characteristics of singlepole breakage fault on DC line are related to the power flow direction of converter station;the voltage characteristics of single-pole breakage fault on the link line between the sending-end converter and the DC bus are related to the control mode of converter;the system grounding mode has little effect on the fault characteristics.It provides a reference for formulating the single-pole breakage protection scheme of medium-voltage DC distribution line.Secondly,in view of the problems existing in the single-pole breakage protection of medium-voltage DC distribution line at present,a single-pole breakage protection scheme of medium-voltage DC distribution line based on single-terminal transient current is proposed.The current characteristics of the single-pole breakage fault on the DC line are analyzed.On this basis,based on the variation difference of line current between fault pole and non-fault pole after single-pole breakage fault,the protection starting criterion,fault line selection criterion and fault pole selection criterion are constructed.Simulations in PSCAD/EMTDC show that the proposed protection scheme only uses single-terminal current information,has small amount of calculation,can quickly and reliably identify single-pole breakage fault,and is not affected by the length of protection data window,and can avoid the problem of difficult setting of single-pole breakage protection in complex flexible DC distribution networks such as multi-terminal and ring form.Thirdly,aiming at the problems existing in the short-circuit protection of mediumvoltage DC distribution network at present,a short-circuit protection scheme of medium-voltage DC distribution line based on clustering center distance measurement is proposed.The scheme only uses one-terminal current as the characteristic quantity.Different types of short-circuit faults at dif-ferent locations of the line are simulated in advance,and combined with the actual faults,the post-fault currents are collected and processed to form historical data samples.The K-means clustering algorithm is then used to find the best clustering centers under different types of faults,and fault identification and pole selection are realized by comparing the distance between realtime data and each clustering center.The method doesn’t need complex feature extraction and calculation process,and the complex setting of conventional current protection can be avoided.Finally,the simu-lation is carried out in PSCAD / EMTDC,and the results show that the proposed protection has good selectivity and rapidity,and the resistance to fault resistance is improved compared with the conventional current protections based on local information.Finally,the main research contents and results are summarized.