Research On DC High Temperature Superconducting Fault Current Limiting Technologies And Applications

Flexible and efficient dc technology,especially flexible dc technology,can realize the flexible interconnection of power grid and the smooth access of large-scale renewable energy.It is considered to be an important revolution in the development of power system in the future.Unlike ac system,dc system is a "low inertia" system whose dc fault transient process is fast.Once a dc fault occurs,a single fault may quickly affect the entire dc system and cause a power outage.In addition,the short-circuit current will rise sharply when a fault occurs.The overcurrent level that reaches tens of times the rated current within a few milliseconds has had severely demand on the breaking capacity and operating speed of dc circuit breaker,and seriously threatened the safe operation of important primary equipment such as converters.Therefore,effectively limiting the dc fault current is the key of the development of dc technology.Among variaties of current limiting methods,the dc current limiter implemented by traditional technology has certain limitations.For example,the resistive type fault current limiter will generate power loss during normal operation.The system stability and response will be affected when the inductance of inductive type current limiter is too large.The current limiting device manufactured by high temperature superconducting technology can break the dilemmas faced by the traditional current limiter.The unique characteristics of superconducting materials,such as zero resistance and high current density,make them have great research value and development potential in dealing with the scientific and technical problems of dc system stability.The application of high temperature superconducting fault current limiter in dc system will provide a strong support for the development of a more timely and effective dc current limiting technology.This Ph D dissertation takes the application research of high temperature superconducting fault current limiting technology in dc system as the main work.First,starting from the fault characteristics and current limiting requirements of dc system,the effect on the dc fault transient is analyzed under the circumstances of different types of fault current limiters installed in different locations of dc system.Based on the device level,the current limiting technical requirements in four aspects,namely,response speed,current rising rate limitation,current amplitude limitation and recovery time,are proposed.The current limiting technology requirements of different types of dc superconducting fault current limiters are analyzed.For the resistive type limiter,regarding the limitation of the maximum short-circuit current as the main current limiting target,this thesis aims at the difficulties in evaluating the actual current limiting effect of the resisitive type dc superconducting fault current limiter engineering prototype,a testing method of current limiting performance is studied based on the assosicated relationships between quench resistance and heat accumulation.By using the method of ac short-circuit test equivalent to dc short-circuit test and the method of "factory test + system simulation",the current limiting performance of a real prototype of a ±160k V dc resistive superconducting fault current limiter in the application scenario of Nan'ao Modular Multilevel Converter High Voltage Direct Current(MMC-HVDC)system is evaluated.For the inductive dc superconducting fault current limiters,the impact of conventional reactors on dc system operation is first analyzed from the perspective of impact on operating loss,dynamic response,and system stability.Furthermore,a novel inductive dc superconducting fault current limiter based on the principle of magnetic saturation is proposed,targeting in limiting the rising rate of short-circuit current.Its current limiting performance and reliability are discussed through the analysis of the key parameters,such as the turns of current limiting coil,the magnetomotive force ratio on both sides and the normal operation current.In the modeling stage of this fault current limiter,a finite element model and a Matlab model are built respectively,and the practical significance of the two models in device and system performance analyses is analyzed.Finally,in the testing stage of this fault current limiter prototype,through the measurement of the flow capacity of the 1G and 2G superconducting tapes in the measurement system,the critical current variation of superconducting tapes,with the change of magnetic field angle(0°,90°)and the change of magnetic field strength,are analyzed to complete the selection of superconducting tape and the prototype construction.The current limiting performance of the inductive dc superconducting fault current limiter is tested and verified.The improvment schemes based on this prototype are later proposed repectively,which are conducive to realizing both the resistive and inductive current limiting properties and to realizing the acceleration of fault curret declining rate.