The Superconducting—AC Equipment (Super-ACE) project was started in 2000 fiscal year as a national project to research and develop superconducting AC power equipment. The goal of the project is to develop basic technologies for high-temperature superconducting (HTS) cable, HTS fault current limiters (FCL), and HTS magnets for reactors and transformers. This paper describes major results of studies on the HTS superconductors, and outlines some of the results of verification testing completed for 500-m cable, FCL and HTS magnets.
A high-Tc superconducting (HTS) power cable is one of major candidates for next-generation underground power transmission systems having various advantageous features such as low impedance, low losses and compactness. The use of HTS power cables requires cooling stations, which can be located in manholes at intervals of several kilometers. Therefore, HTS power cables need to have a single span length of several kilometers. However, the flow properties of liquid nitrogen, which is used as the coolant for HTS power cables, have not been clarified for cryogenic tubes with a length of several kilometers. Based on this , an HTS power cable with a length of 500 m was constructed at the Yokosuka site of CRIEPI, Japan, and tested for almost one year. Moreover, short-circuit tests were performed using an HTS power cable with a length of 10 m to elucidate the behavior of HTS power cables under the condition of flowing short-circuit current. From these tests, positive results were obtained that will help contribute to the realization of HTS power cables and their introduction into actual power grids.
This paper describes the summarized results obtained from the development of high-temperature (High-Tc) superconducting fault current limiters (FCL) during the Superconductive AC Equipment (Super-ACE) project (2000-2004). The two types of FCL were developed during the project; a resistive type using YBCO thin film and a rectifier type using Bi2223 tape. The R&D of the resistive type aimed to establish large-current/high-voltage technology by connecting thin-film elements in multi-parallel/series configuration. The fundamental technology for a thin-film resistive current limiter of 6.6 kV / 1 kA-class was achieved. The R&D of the rectifier type aimed at high-voltage applications for 66 kV systems. A 66 kV / 1 kA-class reactor magnet for an FCL was designed, fabricated and successfully evaluated during the Super-ACE project. The project was successfully terminated as all the experimental results satisfied the targeted goal.
We have developed the production technology for HTS magnets used in power transformers. Two subjects were mainly studied (i.e., high-voltage technologies and large current and low AC loss technologies) to establish the production method for a 66 kV / 6.9 kV 10 MVA-class HTS power transformer. In order to verify the validity of the elemental technologies, a single-phase 66 kV / 6.9 kV 2 MVA-class model HTS transformer was manufactured and tested. The test results indicate that the elemental technologies established can be used for the development of a 66 kV / 6.9 kV 10 MVA-class HTS power transformer.
Advanced silver sheathed Bi-2223 superconducting tapes have been developed for AC applications such as electric power cables and transformers, which require minimum AC loss. Twist and inter-filament resistive barriers were applied to eliminate the electromagnetic inter-filament coupling, which is the primary mechanism that influences the AC loss of the multi-filamentary superconducting wire. A multi-filamentary wire with a honeycomb-type thin ceramic barrier was designed and manufactured in an attempt to realize low AC loss without degrading the transport properties of the conductor. The AC loss of the manufactured wire was reduced drastically when compared to that of conventional wires without barriers. Using the measured properties of the manufactured wire as a basis, a design study along with projected AC loss for a superconducting power cable is presented.
The use of high Tc superconducting (HTS) cable is expected to realize compact, highly efficient transmission lines. In order to achieve high efficiency, it is important to reduce AC loss. However, accurately estimating the AC loss of a real cable system is very difficult because the cable length is very long and the operation temperature varies according to condition. The measurement of AC loss using a short sample is simple and possible using liquid nitrogen at the temperature of 77 K, but evaluating temperature dependence is very difficult because a refrigeration system is necessary. Moreover, a simple AC loss estimation method has been established for shield-less cable only, and is therefore not applicable to HTS cable with a shield layer. We have developed AC loss measurement equipment that uses calorimetrics and measured the temperature dependence for both critical current and AC loss of HTS cable with a shield layer. From the results, we propose a new simple estimation method for temperature dependence on AC loss by measuring the temperature dependence of critical current and AC loss at one point.