Masashi Yagi

A simulation code that analyzes the temperature and pressure in a high-temperature superconducting (HTS) power cable cooled by a forced flow of sub-cooled liquid nitrogen (LN) has been developed. Analysis of the HTS cable is useful for comprehending the normal operating condition and the generation of heat during the transient state. In Japan, an excessive current of 63 kA may flow in an HTS cable for 0.6 s in the worst case when a short-circuit current accident occurs in a 275 kV class cable. To assess the safety under this condition, a simulation of a 275 kV class 20 m model cable was performed, and the simulation results of the temperature profiles qualitatively agreed with the experimental results. In this study, finite-difference method was used to solve the non-linear partial differential equations of the heat-transfer phenomenon through heat conduction. By solving these equations, the temperature profiles of the LN coolant and the cable cores were analyzed. The GASPAK’ software package (Cryodata) was used to evaluate the fluid properties when the LN temperature in the cable was calculated. According to this result, the simulations focused on the analysis of a real-scale HTS cable in this work.

The REBCO high-temperature superconducting cable, rated at 275 kV and 3 kA, was developed in the Materials and Power Applications of Coated Conductors project in 2008. The cable is expected to be put to practical use as a backbone power line in the future because the transmission capacity is about the same as the UHV overhead transmission line. Transmission loss is significantly reduced by using superconducting wires instead of Cu or Al conductors. Furukawa Electric plans a demonstration of the 275 kV high-temperature superconducting cable. The cable system consists of a 30-m cable, terminations, a cable joint, and a cooling system. The 30-m cable has already been manufactured and installed on a test site. In this demonstration, various electrical evaluations will be conducted, such as a long-term test under a current of 3 kA, and the test voltage will be determined assuming 30 years of insulation degradation.

Coated conductors are subjected to short-circuit currents 10 to 30 times greater than the operating current in the use of superconducting power cables. The Ic and thickness of the stabilizer are considered to be nonuniformly distributed in the real manufacturing process. Thus, the coated conductor may be damaged locally by the hot spot due to the overcurrent. Therefore, it is important to clarify the local degradation characteristics of the coated conductor and determine its tolerance against the fault current in the actual operation. In a previous study, overcurrent experiments and numerical simulations on thermal stress-strain were carried out on a coated conductor with a nonuniformly distributed Ic. In this study, two samples with nonuniformly distributed copper plating thickness were tested. Numerical simulations based on the finite element method were performed to simulate the temperature and thermal stress-strain distribution caused by the overcurrent. The nonuniform thickness of the copper plating and the longitudinal length of the nonuniform area were considered as the simulation parameters.

The current margin of a jointed model cable of the 275 kV class high-temperature superconductor (HTS) power cable was measured under fault condition. The cable-to-cable joint is necessary to connect long continuous sections of the power cable. At the joint the superconductor is prone to degradation due to an excessive temperature rise caused by the current passing through normal conducting zones of the joint. The jointed, and for the comparison the joint-less cables were subjected to a short-circuited fault current of 63 kA rms for 0.6 s. In the results, the allowable current margins were about the same for both model cables. The jointed cable degraded at the joint area from the result of the magneto-optical (MO) image. The numerical simulations were performed to clarify current distribution and temperature rise at the cable joint area. The temperature rise as a function of the joint gap was also calculated. The experimental and simulation results were showed in this paper.

YBCO tapes are expected to be used in future high temperature superconductor (HTS) applications as they have better J_c characteristics at high temperatures and in high magnetic fields. For power applications such as transmission cables, YBCO tapes and a copper former are connected in parallel and they might be subjected to a short-circuit fault current that is 10 to 30 times the normal operating current. Therefore, the over-current behavior, including the hot-spot and the distribution of transport current between the copper-laminated YBCO tapes and the copper former, is very important to examine the stability and feasibility of the cable. This paper describes the experimental results for over-current pulses of a 1-m single-layer cable fabricated with five copper-laminated YBCO tapes and a copper former. Further, we performed numerical simulations by using a newly developed computer program based on the 3D finite element method.