Research On Fault Location Of Two Terminal Flexible DC Distribution Network Based On MMC

Flexible DC distribution network is an indispensable part of smart grid in the future.Because it is easy to connect distributed power and DC load,reduce the generation of grid connected harmonics,and has no reactive power and stability problems of AC system,it has become the research focus of relevant scholars at home and abroad.The short-circuit fault of DC distribution network is mainly single pole ground fault and double pole short-circuit fault.Compared with single pole ground fault,the double pole short-circuit fault seriously affects the safe operation of the whole distribution network.Therefore,after the short-circuit fault occurs in DC distribution network,it is necessary to quickly identify the fault type and fault measurement distance,and recover the power supply of the system as soon as possible.In order to solve the problem of fault type identification and bipolar short-circuit fault location in DC distribution network,in the case of building a topology structure of dual terminal flexible DC distribution network based on MMC,first analyze the short-circuit fault of DC line,summarize the fault characteristics,then put forward the appropriate fault type identification method according to the fault characteristics,and finally carry out fault location for bipolar short-circuit fault.The main research contents are as follows:(1)Based on MMC,the short circuit fault analysis of two terminal flexible DC distribution network.For the single pole grounding fault of DC bus,the transient voltage and current change process are analyzed,and the DC bus current change caused by branch line fault is compared to summarize the fault characteristics;for the double pole short circuit fault,the fault voltage and current at the converter end and the fault voltage change characteristics at each terminal of DC line are analyzed,and the fault characteristics are summarized Analyze and carry out simulation verification.(2)Research on fault type identification of two terminal flexible DC distribution network based on MMC.Based on the analysis of single pole ground short circuit and double pole short circuit fault,considering that fault current may be affected by branch fault,this paper uses transient voltage with obvious fault characteristics as the basis of fault type identification.For bipolar short-circuit fault,the transient voltage deviation coefficient at the end of the section is defined,and the fault section is identified according to the different transient voltage deviation coefficient at the end of the fault section and the no fault section.The accuracy and reliability of fault type identification and fault section judgment are verified by simulation considering various fault situations.(3)Research on bipolar short circuit fault location of flexible DC distribution network based on MMC.Two different methods are used for fault location.One method is to use the equivalent capacitance of the converter to locate the fault and write down the fault loop equation.The fault distance is calculated from the transient voltage and current information of the converters on both sides.In the other method,the fault distance is calculated from the discharge voltage and current information of the fault capacitance at both ends of the fault section.This method requires that the data at both ends of the section be synchronized.Under the condition of under damping,the influence of the transition resistance can be eliminated theoretically.Both methods have high ranging accuracy.PSCAD / EMTDC simulation verifies the effectiveness of the two ranging methods.