Research On The Key Technologies Of Resistive DC HTS Fault Current Limiter

With the continuous breakthrough of DC engineering in voltage level and transmission capacity,DC short-circuit fault is continuously upgraded.In order to protect the electrical equipment in the system,it is generally necessary to effectively isolate the fault within 5ms.However,the level of DC short-circuit current can reach tens of thousands of amperes in milliseconds,and there is no natural zero crossing,which challenges the existing DC circuit breaker to quickly clear the DC short-circuit fault and cut off the DC short-circuit current.Therefore,to effectively limit the DC short-circuit current has become an urgent problem in the field of DC protection.In recent years,with the rapid development of high temperature superconducting technology,the second generation of high temperature superconducting materials have achieved large-scale production and production cost reduction.The resistance type DC SFCL(Superconducting Fault Current Limiter,SFCL)composed of it has the advantages of simple principle,integration of detection,triggering and current limiting,small on-state loss during stable operation,which can achieve ideal current limiting effect,in order to suppress the rising speed and amplitude of DC fault current It provides a new solution.In this dissertation,aiming at the problem of two pole short-circuit fault current suppression in DC power grid,combined with the research status at home and abroad,starting from the research on the characteristics of the second generation HTS(High Temperature Superconductivity,HTS)tape,the transient characteristics and design methods of the resistive HTS current limiter and the DC power grid protection strategy based on the resistive HTS current limiter are deeply analyzed and studied.The theoretical analysis and experimental research on the critical current anisotropy and quench recovery characteristics of YBCO superconducting tape used in the resistive HTS current limiter are carried out.The critical current of superconducting tape under the influence of external magnetic field is calculated by finite element simulation,and a mathematical model is established to describe the dynamic characteristics of quench and recovery of superconducting tape,so as to obtain the calculation of quench recovery time of superconducting tape Methods:the effects of polyimide film and PTFE film on the recovery time were investigated by experiments.A winding method of insulating film is proposed to accelerate the recovery of superconducting tape.Based on the finite element simulation model,the critical current of the current limiter is calculated.It is concluded that the critical current of the current limiter is not affected by the number of turns and the number of permutations of the coils.The current limiter is made and tested by critical current test,DC rated current test and so on.Through the analysis of the test data,the current limiting performance parameters of the current limiting unit are obtained,which provides the basis for the design of the current limiter.Based on the deep analysis of the mechanism of the bipolar short circuit fault in the DC side of MMC(Modular Multilevel Converter,MMC)converter,the short circuit current calculation model and the transient model of the current limiting process of superconducting current limiter are established to realize the synchronous calculation of the DC system circuit parameters and the physical quantities of the current limiter.Based on this,the dissertation proposes to adjust the two situations of the bipolar short circuit fault in the DC side of MMC converter,namely,the blocking and non blocking of the converter.The number of current limiting units and series parallel mode meet the requirements of short-circuit rejection rate and reclosing action time of MMC DC system.The multi terminal flexible DC power grid protection strategy and DC protection device action sequence coordinated by the resistive htdc superconducting current limiter and other protection devices are formulated.The MMC converter+resistive superconducting current limiter+DC circuit breaker coordination scheme is proposed.Taking a three terminal flexible DC project in China as an example,the effectiveness of the proposed protection strategy is verified on PSCAD/EMTDC simulation platform,realizing selective isolation of DC fault and effectively improving the reliability of multi terminal flexible DC transmission system.