Hitoshi Kanetaka

Abnormal quench process with very fast elongation of normal zone in multi-strand superconducting cables

Abstract An abnormal quench process with apparent high propagation velocity of the normal zone, ·fast quench’, was studied in relation to electromagnetic and thermal couplings among strands for multi-strand superconducting cables. The quench process was numerically evaluated by simultaneous equations of heat balance and equivalent electrical circuit of the multi-strand cables. Experimental results are well explained by the numerical simulation concerning threshold current of the fast quench for a two-strand cable. The quench process of a six-strand cable was also calculated numerically and the relation between the fast quench and current commutation among strands is discussed.

Additional AC losses due to alternating magnetic field component longitudinal to strand axis in the armature winding of fully superconducting generators

The armature winding of fully superconducting generators will be constructed by multiply stacked cables of multifilamentary superconducting strands. In the superconducting cable exposed to an alternating transverse magnetic field, additional AC losses called longitudinal losses are generated by a field component longitudinal to the strand, because the angle between the axes of the strand and the final cable changes spatially with periods of cabling pitches. The longitudinal losses depend upon the structure of the cable, the direction and pitch in each level of cabling, etc. The additional losses are evaluated in electromagnetic environments of the armature winding. The structure of the cable is discussed to minimize the total AC loss under a stability condition.<<ETX>>

Quench analysis in a 72 kVA superconducting four-winding power transformer

Abstract Quench processes in a 72 kVA superconducting four-winding power transformer were studied experimentally by sudden short-circuiting of the secondary load. An instantaneous decrease in the transport current was observed in both of the primary and secondary main windings during the first half cycle immediately after the short circuit. The equivalent propagation velocity of normal regions along the wire during the period of instantaneous decrease in current was about two orders of magnitude larger than existing theoretical predictions. Two-dimensional propagation of the normal region was also observed after the very fast propagation. In the primary main winding, the equivalent velocity of the very fast propagation was linearly increased with the voltage applied to the normal region of the winding. This type of very fast progagation was explained by a model in which a broad part of the winding is changed into the normal state via a flux-flow state with very low resistivity, due to the excess current. In the secondary main winding, similar fast propagation was also generated by local heating of the winding above a threshold level in the region of the transport current. The experimental results show that the instantaneous transition to the normal state is attributed to a quench process peculiar to superconducting cables, due to the redistribution of the transport current among strands. It is proposed that the abnormal transitions to the normal state can be useful for self-protection of a.c. superconducting windings from burning.

Experimental evaluation of AC losses in superconducting multifilamentary wires for 50/60 Hz use exposed to a magnetic field with arbitrary angle with the wire axis

In multiple twisted superconducting cables exposed to an external AC transverse magnetic field, a component of magnetic field parallel to the strand axis due to the twisting structure of the cables causes an additional AC loss (longitudinal AC loss) in the strands. The authors experimentally evaluate the longitudinal AC loss by an equivalent measurement with short samples of the strands exposed to an oblique magnetic field to the strand axis. The total loss obtained at 60 Hz was equivalent to a direct sum of the hysteresis loss, the coupling-current loss for the transverse component and the additional loss for the longitudinal one. In triple twisted superconducting cables composed of insulated strands, measured AC losses could be also quantitatively explained in the same manner as for the short samples. These results suggest that the longitudinal loss may be dominant in an AC magnetic field with relatively large amplitude along with the coupling-current losses inside and among strands. The authors also discuss an optimum condition for the twist pitch of the strand to reduce the total AC loss of the strands in the cable.<<ETX>>

Experimental evaluation of longitudinal field loss in multiply-twisted superconducting cables for a.c. use

Abstract In multiply-twisted superconducting cables exposed to an external a.c. transverse magnetic field, a component of the magnetic field parallel to the strand axis brings about an additional a.c. loss (longitudinal field loss) in the strands. We have experimentally evaluated the longitudinal field loss in two types of triply-twisted superconducting cables composed of insulated strands with different diameters, by eliminating the hysteresis loss and the transverse coupling-current loss from the total loss in the strands. The evaluated longitudinal field losses were quantitatively explained by a theoretical expression obtained, to date, from consideration of the effects of the longitudinal component of the applied field on the strand. The present results suggest that the longitudinal field loss can be dominant in an a.c. magnetic field with relatively large amplitude. To minimize the total loss of the strand in the multiply-twisted cables, an optimum condition for the twist pitch of the strand was discussed.