| High capacity HTS power equipment requires high current carrying HTS cables.As far as the first generation of high-temperature superconductors wires/tapes and the second generation of high-temperature superconducting tapes are widely used at present,due to their unique geometry and tape structure,different methods and strategies will be adopted in the winding of high current cables.Compared with traditional superconducting wires,REBCO coated conductors have a larger width-to-thickness ratio and obvious anisotropy.Therefore,at present,they are often made into high current cables at home and abroad in the form of multiple stacks or space transposition Roebel cable,spiral wound CORC cable,etc.When these high-capacity cables work in changing transmission currents or external fields,there have AC loss,which will increase the workload of the cooling system.So,the research on the AC loss of HTS cables has always been one of the hot topics in the research of applied superconducting at home and abroad.In addition,HTS belongs to ceramic structure,so it is difficult to avoid the existence of local defects in the preparation,and it is also easy to produce local defects under the action of various forms of stress in use.Therefore,the influence of local defects on the AC loss of cable is also an unavoidable problem.Moreover,the arrangement of the tapes in the REBCO superconducting cable also directly affects the overall AC loss of the cable.How to optimize the arrangement of the tape in the cable to reduce the overall AC loss is also worthy of high attention.Although the first generation of high temperature superconducting wire Bi-2212 belongs to the round wire structure,which is the same as the traditional wire,it can still be made into high current cable by multi-stage twisting.However,due to the multifilaments structure of Bi-2212 round wire,it needs a lot of calculation time and computer memory to evaluate the AC loss of Bi-2212 wire after it is wound into superconducting cable.In order to improve the calculation efficiency,homogenization method is widely used in practical calculation.With the development of application,superconducting power equipment will be widely used not only in low frequency field,but also in high frequency field.Therefore,the homogenization method to evaluate whether the AC loss of the Bi-2212 round wire at high frequency is effective needs related inspection or verification.In addition,after heat treatment,the filaments of Bi-2212 wire may have local defects and irregular structure,which will affect the AC loss of the wire.Finally,the local performance of Bi-2212 cable may be degraded during the process of twisting Bi-2212 round wire into cable.Therefore,it is necessary to find a fast and efficient method to detect the local performance of twisted Bi-2212 cable.In this paper,firstly,the related research contents of HTS cable are briefly reviewed,and the numerical analysis method for the related problems to be solved is established,and the transmission AC loss platform is built independently(see Chapter 1 and Chapter 2).On this basis,the AC loss characteristics of REBCO tape with local defects are studied in this paper,and calculates the AC loss of REBCO tapes with multiple strands in parallel and multiple strands in series under different arrangements by establishing a 3-D finite element model(see Chapter 3).In Chapter 4,a new structure of 5-groove REBCO superconducting cable(cable 2)is constructed for the 5-groove REBCO superconducting cable(cable 1)proposed by the European Nuclear Energy Agency(ENEA),and the critical current and AC loss behaviors of two REBCO superconducting cables under self-field and different external magnetic fields were studied by the numerical methods.Then,the AC losses of Bi-2212 round wires and cables under wide frequency(50 Hz-500 k Hz)are calculated by homogenization method.Finally,a non-contact method is proposed to measure the local characteristics of Bi-2212 cable,and the influence of the local defects of the filaments and the irregularity of the filaments on the AC loss are further discussed(see Chapter 5 and Chapter 6).Through experiments and numerical calculations on the influence of local cracks in REBCO tape on AC loss,it is found that transverse cracks in REBCO tape have a greater influence on AC loss,but longitudinal cracks along the length of the tape have a slight impact on AC loss,and the loss of cracks near the edge of the tape is greater than the AC loss near the center.When the double-stranded REBCO tapes arranged in parallel,the transmission loss in the tape anti-series connection mode is greater than the loss in the tape parallel connection mode and series connection mode,especially when the defect near the other tape,it will increase significantly.For double-stranded REBCO tapes arranged vertically,the transmission loss in the tape anti-series connection mode is smaller than the tape parallel connection mode and the series connection mode.For multi-strand REBCO tapes arranged in parallel,whether they are arranged horizontally or vertically,when the defect is located in the middle tape of the multi-stranded REBCO tape,the overall AC loss of the cable is less affected.For the REBCO tapes arranged vertically in series,the defect in any tape has the same effect on the overall AC loss.In the comparative numerical calculation of the critical current and AC loss of the two different structures of REBCO cables,the critical current of the new structure cable is lower than the original structure under the self-field,but the critical current of the new structure cable is higher under the higher external field.In addition,the transmission loss under the self-field of the new structure cable is larger than that of the original structure cable.However,when the external magnetic field is large enough,the AC loss of the new structure cable is smaller than the original structure cable.In specific practical applications,the performance of the cable can be effectively improved by changing the arrangement of the superconducting tape in the cable.By the study of AC loss under Bi-2212 round wire and cable under wide frequency,the introduction of homogenization method can effectively reduce the calculation time while satisfying higher calculation accuracy.In addition,with the increase of the research system,the calculation accuracy of the homogenization method will be higher and higher.In the non-destructive testing of the local characteristics of the Bi-2212 sub-cable,it is proposed to use four mutually perpendicular Hall arrays to detect the residual magnetic field of the Bi-2212 sub-cable magnetized by the pulse field and the slope field.When the local defect along the length of the sub cable,the sum of the residual magnetic fields by the four arrays will decrease,while the sum of the residual magnetic fields at other positions hardly change with the change of position.The results show that the multi array combination method is effective in detecting the local characteristics of sub cables.Through the calculation of the influence of the local defects of the filament in the multifilaments round wire on the AC loss,it is found that the existence of the defect will increase the AC loss,and the transverse defect will lead the AC loss to increase significantly.The more irregular the filament is,the greater AC loss is,but the AC loss decreases with the increase of the pitch of the irregularly distributed filament. |
PDF Full Text Request