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Large-scale aluminum stabilized superconductors are applied to accelerator magnets, SMES, fusion magnets and so on. Authors have investigated transient stability of large helical device (LHD) conductor, which consists of a NbTi/Cu Rutherford cable, a pure aluminum stabilizer, and a copper sheath, cooled by saturated He I, sub-cooled He I and pressurized He II. The peculiar phenomena, such as "traveling normal zone" and "asymmetrical propagation of normal zone" were observed in the original LHD helical coil and also in small test coils wound with the LHD conductor. These phenomena appeared with He I cooling and were emphasized with He II cooling. In order to investigate the transient stability of LHD composite superconductors against a thermal disturbance and the electromagnetic behavior inside the conductor, the tests with numerical simulations were carried out. In this paper, the effects of the polarity of the external field and the transport current on the direction of the traveling normal zone and the propagation velocity were discussed based on the experimental and simulation results. The hall current distributions in the longitudinal area around the normal zone and their effects on the transient stability were discussed. Important factors in the design of a large-scale aluminum stabilized superconductor were discussed.