| Fault current limiting and fault breaking,as the two most important parts of fault protection technology,are the cornerstones of the flexible DC transmission systems to large capacity and high voltage.At present,the development of fault protection technology is lagging behind the increasing rate of voltage le vel,so the flexible DC transmission system under transient operation has the problem of poor operation reliability.In this regard,on the basis of classifying and su mmarizing the existing fault current limiting and breaking methods,referring to differen t evaluation criteria,this article selected the fault current limiting and breaking methods suitable for current HVDC flexible transmission projects : resistance superconducting fault current limiter(RSFCL)for limiting and DC circuit breaker(RSFCL)for opening,and has been expanded in depth,in order to better meet the development needs of flexible DC transmission projects.First of all,starting from the characteristics of superconducting materials,the working principle of resistive superconducting current limiter was explained,and its circuit topology and three modeling methods were given.Using the "exponential" modeling method,relying on the on-site impact test waveform data of the Nan’ao RSFCL prototype,a single current limiting resistor structure current limiter simulation model was built on simulation platform,and its feasibility verification was completed.Based on the structure of the prototype,a highly flexible fault current limiting operation combination structure is proposed.Due to this theoretical foundation,its corresponding simulation model was built,and by simulating its effect on the fault current limitation when the system was grounded through different transition resistances,the conversion threshold calculation method for the three operating states was given,which further demonstrated the feasibility of the theory.Secondly,comparing the advantages and disadvantages of different DCCB,it is pointed out that CM-DCCB has the advantages of high reliability,low loss and excellent economy by analyzing the topological structure of the coupled mechanical DC circuit breaker(CM-DCCB)and its fault breaking process.Combining actual engineering development and considering the limitations of switchgear manufacturing,a design scheme of CM-DCCB suitable for the Nan’ao project was given,and with reference to the mathematical theory relationship between its breaking capacity and the main circuit parameters,the calculation of CM-DCCB parameters that meet the breaking requirements of the Nan’ao system the was obtained.At the same time,aiming at the "rejection" fault of the CM-DCCB in the artificial grounding short circuit test in Nan’ao system,based on the traditional self-protection criterion,an improved self-protection strategy was proposed,and it was effectively verified in the simulation system and the on-site manual short-circuit test.Finally,based on the RSFCL with the above-mentioned fault current-limiting operation combination structure and the CM-DCCB with improved self-protection strategy,a simulation model of the multi-terminal flexible DC system for the actual project in Nan’ao was built in PSCAD,and the change trend of the fault current under the installation of current limiting and breaking measures before and after was simulated when different faults occur in the Nao’ao system.Through analysis and comparison,the current limiting and breaking scheme described in this article is better than conventional schemes,which can better meet the demands of the current development of flexible HVDC systems,an d has more advantages and practical engineering application significance. |
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