Fault Characteristics Analysis And Isolation Protection Research Of HVDC System Based On The Coordination Of A Superconducting Fault Current Limiter And A DC Circuit Breaker

The flexible DC transmission technology has been rapidly applied in large-scale new energy access and flexible interconnection of large power grids.However,the DC side of the flexible DC system has small damping,which leads to the rapid rise and high amplitude of the short-circuit current on the DC side,which seriously threatens the safety of various equipment in the system.At the same time,facing the DC circuit breaker with limited breaking capacity,it is also difficult to isolate the DC fault.Therefore,it is necessary to effectively limit the DC short-circuit current.In different current limiting methods,the superconducting current limiter using high-temperature superconducting technology has become one of the research focuses.The core theme of this paper is the application of resistive superconducting current limiters in multi-terminal flexible DC transmission systems.The main research work of this paper is as follows:Firstly,the DC fault characteristics of the MMC-HVDC are analyzed.The transient response process of the fault current when bipolar short circuit and unipolar grounding fault occur on the DC side are theoretically analyzed.And based on the Nan’ao project,a simulation analysis was carried out;The influence of the resistive superconducting current limiter at the outlet of the converter station on the system fault transient and the selection requirements of the current limiter resistance are analyzed.The technical requirements that the resistive superconducting current limiter needs to meet in five aspects: response speed,suppression of fault current amplitude,suppression of fault curren t rise rate,response duration and self-recovery time are summarized.Secondly,in view of the technical problem that it is difficult to evaluate the actual current limiting effect of the resistive superconducting current limiter prototype used in Nan’ao project.In this paper,according to the theoretical analysis of the thermal balance of superconducting materials after quenching,the resistance of the current limiter at any time after quenching corresponds to its own heat accumulation.Therefore,the impact test was carried out on the current limiter prototype,and the corresponding relationship between its resistance value and the heat accumulation was obtained,and the mathematical expression of the current limiter resistance-heat accumulation was obtained by fitting the relationship curve.Furthermore,based on this mathematical expression,the electrical simulation model of the current limiter prototype is established,and the simulation verifies the correctness of the model and the validity of the cur rent limiting effect.Finally,a method for fault isolation based on the cooperation of current limiter and DC circuit breaker is proposed.Firstly,the structure and working process of the circuit breaker are analyzed;The principle of the cooperation bet ween the current limiter and the circuit breaker is analyzed,and the time sequence coordination scheme is designed.Considering the transient process stages of the DC fault when the current limiter and circuit breaker act,the equivalent circuits of the D C side fault at each stage are established,and the mathematical relationship between the peak current and time,the energy dissipated by the circuit breaker and the fault breaking time is theoretically deduced;Simulation analysis of inter-pole short-circuit fault and single-pole grounding fault under different fault depths,the results verify the positive effect of the proposed method in limiting short-circuit current peaks and accelerating fault isolation,and affirm its adaptability to multi-terminal flexible DC systems.