素線間に高抵抗層を有する撚線導体内の循環電流

Abstract

Nonuniform current distribution is generated in a conductor consisting of strands coated by a high resistive layer, such as chromium plating, as a result of superimposition of transport and induced circulation currents. The characteristics of the induced circulation current are analytically studied by using a distributed model circuit. The parameters mostly used in this calculation are those of US-DPC coil, which at first exhibited instability and so-called ramp rate limitation (RRL) because of current imbalance in the conductor consisting of chrome-plated strands. Thus the conductance among strands and the inductance of unit length loop and length of the conductor are mostly assumed to be 10kS/m, 0.5μH/m and 150m, respectively. The analysis results indicate that the induced circulation current can be classified into the boundary and interstrand-induced circulation currents, hereafter referred to as BICC and IICC. BICC is induced only across the joint at the ends of the conductor, resulting in a constant along the conductor axis, when the total leakage magnetic flux of the loop is not zero. Its decay time constant is quite long, more than a few hours. In contrast, when the leakage magnetic flux distributes along the conductor axis, IICC is induced among strands in the conductor to eliminate this flux. Since the leakage magnetic flux normally becomes largest where the magnetic field is highest, it becomes larger where the time variation of the magnetic field is larger. Its decay time constant is much less than that of BICC. If the leakage magnetic flux linearly changes along the US-DPC conductor, it is evaluated to be about 10s. This IICC therefore becomes dominant in a pulse charge, whose ramping time is less than 10s. Moreover, it is found that the variation of the leakage magnetic flux with the relatively long cycle, such as more than a few 10-meter lengths, causes IICC with a decay-time constant of more than several hundred milliseconds. Such an IICC can greatly enhance the total IICC in a short charge. The nonuniformity of the current distribution therefore becomes larger in the shorter charge where the magnetic field is high. Consequently, RRL can be explained qualitatively by these effects from IICC. Note that the decay time constant of IICC generated from magnetic flux as a result of cabling is too short to explain RRL of the US-DPC coil.