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Very-large-current composite superconductors are used in SMES coils and fusion applications. These superconductors have large cross-sectional areas of high purity aluminum to improve their stability. Once a normal zone is initiated in such superconductors the current transfers from the superconducting strands to the aluminum stabilizer according to the Maxwell's equations and the temperature distribution. However, the time constant of current diffusion in the aluminum stabilizer is very long as electrical resistivity of aluminum is very low. Therefore, excess Joule heating is generated in a small region of aluminum stabilizer near superconducting strands, and the temperature increases locally. Some 2D numerical analyses have been carried out in order to investigate the transient stability of the superconductor applied to the helical coil of LHD in National Institute for Fusion Science. But, as the performance of computers have improved, huge numerical simulations are new feasible. So we wrote a 3D finite element analysis code ourselves to carry out some now analyses that we compared with 2D analysis.