고온 초전도 케이블의 전류 통전 및 절연설계 연구
(A) Study on the insulation design and transport current of high-T_(c) superconducting cable
고온초전도케이블 전류통전 절연설계 전기공학;
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Since the superconducting cable used with Ag sheathed Bi-2223 high-T_(c) superconductor(HTS) tape is suitable for the power cable in the metro city, the cable has been strongly and continuously developed all over the world. The application of HTS to power cable has been studied because the cable has large current capacity, compact size and low AC losses. Some HTS cable systems have already been developed and evaluated. However, for optimum design of HTS cable, the most important key element is low AC loss and insulation of cable. The essential conditions for low AC loss of cable is a high critical current and a homogeneous layer current of cable. And it is important to establish of insulation thickness of cable because the cable has experiences of short circuit. From these points of view, it is studied that the AC loss of HTS tape in chapter Ⅱ, the mechanical properties of HTS tape in chapter Ⅲ, the transport current properties of mini-model cable in chapter Ⅳ, the current distribution of HTS cable in chapter Ⅴand Ⅵ. Finally, in chapter Ⅶ deals with an insulation of cable. The results are the followings 1) The AC losses under self field are investigated at 77K on the HTS tape. The transport method is used to measure of AC loss and numerical calculation is carried out based on Norris theory to compare with experimental results. The AC loss of HTS tape mainly ascribed to the hysteresis loss and loss density was concentrated on the edge of tape. 2) For the characterization of the mechanical aging, I investigated the critical current and AC loss of HTS tapes stressed by bending and mechanical vibration. The influence of bending strain was compared between cyclic single and cyclic double bend test. After that, the bending fatigue properties are discussed when the specific strain was applied repeatedly. The most degradation by repeated bending strain was observed when the initial stress was applied. In case of mechanical vibration, the critical current of HTS tapes dependence on vibration frequency, time. And the degradation was much severer in 1-filament than 19-filament tape. The degradation of critical current may be caused both by the smaller Ag ratio of filament and by transverse cracking developed by vibration fatigue when the frequency increases. 3) The result shows that the total transport current of HTS cable in LN2 was 475[A], and transport current passed through almost the outer layer. Also, AC losses in outer layer of HTS cable was proportion to I^(2) and higher than losses of inner layer. A calculated AC loss density was concentrated on the edge of tape and most of loss density in cable was distributed outer layer more than inner layer. 4) It is important to control layer current distributions of a coaxial multi-layer HTS cables, because a homogeneous layer current distribution decreases AC loss and can supply the largest operational current. Thus, I have proposed the theory that can control current distribution based on a concept of flux conservation between two adjacent layers. The theory was effective for the operational current less than the critical current of the cable. It is important to investigate the current distribution under the condition of operational current more than the critical current of cable, because the cable has experiences of fault current. I have extended the theory that treat the operational current more than the critical current by considering V-I nonlinear characteristics of HTS tapes including flux flow resistance and contacting resistance between the cable and terminals. In order to verify the extended theory, I have fabricated a two layers cable with the same twisting layer pitch, and hence cause inhomogeneous current distribution. It was observed that almost all the operational current less than the critical current flowed on the outer layer because of its lower inductance. In case that the operational current increased above the critical current of second layer, the flux flow resistance appeared and distorted the current waveform with phase deviations. Finally, in case of operational current more than critical currents of both layers, the flux flow resistances affect strongly current waveform and thereby the currents of both layers were determined by the flux flow resistances. 5) It is necessary to investigate the ac, impulse breakdown stress of a liquid nitrogen/LPP composite insulation system for insulation design of HTS cable. Based on the experimental results of LPP, the insulation thickness of a 22.9kV class mini-model HTS cable is established at 2.7mm to prevent insulation degradation associated with discharges inside the butt gap of cable.