Study Of Current-limiting Characteristics Of Resistive And Saturated Iron Core Superconducting Fault Current Limiter In MMC-HVDC System

With the development of modular multilevel converter based high voltage direct current transmission technology(MMC-HVDC),it is urgent to carry out research on DC short-circuit fault protection.As a fault current limiting device,superconducting fault current limiter(SFCL)has extremely low loss during normal operation,and quickly transforms into high impedance state after a short-circuit fault occurs,which can effectively reduce the fault current amplitude,protect key equipments of MMC-HVDC system,and assist DC circuit breaker to interrupt the fault.At present,although there are many researches on resistive and saturated iron core type SFCL,there is still a lack of systematic analysis of current limiting characteristics of SFCL.The current-limiting characteristics of resistive SFCL are equivalent to its quench characteristics,and currently there is no method that can quickly and accurately calculate the quench resistance due to the complex quench process.The application of saturated iron core type SFCL in DC system is still in its infancy,and the current-limiting mechanism and current-limiting characteristics need to be studied.This paper studies the current limiting characteristics of resistive and saturated iron core type SFCL,and analyzes the effects of the two types of SFCL in MMCC-HVDC system.The main work completed and the main results obtained are as follows:(1)The AC and DC overcurrent experiments of superconducting tapes are completed,and the quench resistance changing characteristics and influencing factors are studied.The results show that there is a clear correspondence between the quench resistance and the accumulated energy.Based on the experimental conclusions,the "R-Q curve" quench resistance calculation method is proposed.This method can effectively couple resistive SFCL with MMC-HVDC system simulation to select the appropriate quench resistance value.(2)The structure and current-limiting mechanism of saturated iron core type SFCL in MMC-HVDC system are studied,the influencing factors of the current limiting impedance are analyzed,and the current-limiting characteristics of the SFCL are clarified based on the fault characteristics of MMC-HVDC system.The influence of the excitation current on the current limiting effect of the saturated iron core type SFCL is evaluated.The results show that the excitation current should be set so that the iron core saturation time is close to converter block time in order to maximize iron core utilization and the current limiting capacity.(3)The DC short circuit simulation model of MMC-HVDC system is built,and the current limiting characteristics of the resistive and saturated iron core type SFCL are compared.The results showed that the resistive type has strong limiting capability to steady state current,and the saturated iron core type has strong limiting capability to transient state current.Both two types of SFCL can cooperate with mechanical DC circuit breakers to interrupt the fault,but the resistive SFCL has a better effect on reducing the voltage and energy stress of DC circuit breaker.