Research On Fault Identification Of Direct Current Line In Modular Multi-level Converter Based Multi-terminal Direct Current System

Modular multi-level converter based multi-terminal direct current system(MMC-MTDC)is an ideal way of energy interconnection.However,MMC-MTDC belongs to a "low damping" system.The prominent overcurrent phenomenon of the fault line because of the current fed to the fault point by multiple converter stations.The converter stations will be locked and the widespread power outages will be caused.Identifying the fault correctly and effectively is the premise of reliable operation of line protection.Therefore,fault identification is one of the technical difficulties that need to be solved at present.The following research is done from fault characteristic analysis and fault identification model design of MMC-MTDC DC line.The single-ended and double-ended characteristics of internal and external faults are analyzed.The single-ended faults with different high-frequency transient components and the double-ended faults with different current mutation trends at both ends are obtained.The coupling characteristics of inter-pole lines are analyzed.It is concluded that the coupling effect of the bipolar current is small in the low frequency band and the offset degree of the bipolar voltage is different in the short time window.According to the above fault characteristics,the main identification model based on single-ended transient energy and the pilot backup identification model based on current mutation trend and voltage standard deviation are proposed.The main identification model can identify the fault quickly through the single-ended electrical volume which meets the requirements of the main identification model on the quickness.The pilot backup identification model realizes fault identification by using the mutation trend of double-end current which has high reliability and strong ability to resist transition resistance.It is suitable as a backup identification model for DC line.In addition,both identification models can identify the fault pole.A four-terminal flexible DC model based on MMC was built according to the project parameters of Zhang Bei flexible DC by using the PSCAD/EMTDC simulation platform.The preset faults and the factors affecting the property of the identification model are simulated and verified.The simulation results show that the identification models can identify DC line faults correctly and effectively.In addition,both identification models are not affected by fault location,fault type,data synchronization and line distributed capacitance.The main identification model does not need communication and the anti-transition resistance can reach 250 ohm.The pilot backup identification model has communication delay,but the anti-transition resistance can reach 500 ohm.The paper has 46 pictures,10 tables and 85 references.