HTS TRANSFORMER DEVELOPMENT IN JAPAN

After Alsthom designed an LTS 220 kVA transformer and operated it successfully under a 70 kW load, many small-scale LTS units ranging from 10 kVA to 100 kVA were manufactured in Japan in order to confirm various aspects of basic operating behavior. Subsequently, larger units were built and tested by Nagoya University in conjunction with Takaoka (100 kVA), Kansai Electric in conjunction with Mitsubishi (2,000 kVA using Nb₃Sn), Osaka University in conjunction with Toshiba (40 kVA), and Kyushu University in conjunction with Toshiba (1,000 kVA). In addition to acquiring better understanding of the sensitivity of performance to changes in the critical design parameters, one of the more important results from these trials was acquiring better understanding of the stability of the ac conductor and the quench protection afforded by the particular design. Low ac losses were achieved in the LTS conductor through the use of twisted and transposed conductors having fine filaments and high resistivity matrices. The HTS conductors available for use in transformers at the time of this study neither have fine (submicron) filaments nor are readily twisted and transposed, due to the somewhat brittle nature of the ceramic filaments. Moreover, the stabilizing material surrounding the filaments is composed of silver or silver alloy, which do not exhibit high resistivities. However, ability to operate at elevated temperatures makes it possible for the HTS conductors to tolerate higher ac losses than are tolerable in LTS systems, due to the large reduction in refrigeration costs. When LTS design parameters were used to estimate bounds of performance for HTS wires, Iwakuma et al. (1996b) concluded that an Ag-10%Au alloy sheath suffices for resistivity reduction, and that the filament diameter/thickness must be smaller than 25 microns and 10 microns for the case of round wires and flat tapes, respectively. They also determined that the ac losses of transposed Bi-2223 tapes are less than those of nontransposed tapes and about equivalent to the losses measured in single wires (Fig. 3.5).

Fig. 3.5. (left) Total ac loss vs. B_m in the Nb-Ti single-wire and 2-strand parallel conductors; (right) The differences between the ac losses of a single wire and those of parallel conductors.

It is interesting to note that the Japanese industrial corporations that were involved in the development of LTS transformers have not reported activities in the area of HTS transformer development. Moreover, while the electric utilities are playing a pivotal role in transformer programs in the United States and Europe, the same is not true in Japan, where the driving force for development is in industrial and academic circles, with no apparent support from the utilities. At the CEC/ICMC meeting held in May 1996 in Kitakyushu, Japan, Kazuo Funaki (1996) of Kyushu University presented details of a 500 kVA HTS transformer program that was supported by the collaborative efforts of Fuji Electric and Sumitomo Electric Corporation (SEC). This author assumes that Sumitomo was principally responsible for supplying the HTS tapes, and that the majority of the transformer design and construction tasks were assumed by Fuji Electric and Kyushu University. Table 3.6 shows the characteristics of the HTS strands and the winding sequence. Note that the matrix is pure silver, not the silver-gold alloy recommended by Iwakuma, and that the filaments are not twisted, although there have been reports from other HTS wire manufacturers that it is possible to twist multifilamentary HTS conductors.

Table 3.6
Characteristics of the HTS Strands and the Winding Sequence, Fuji/SEC/Kyushu University HTS Transformer

Design parameters of the 500 kVA unit (Fig. 3.6) are highlighted in Table 3.7, where the two values for the winding diameters refer to sandwiched layers that are constructed so as to reduce the effective self-field.

Results of the testing of this unit (Fig. 3.7) are summarized in Table 3.8.

Losses, estimated calorimetrically, amounted to 115 W, taking into account ac losses in the windings and heat leakage from the cryostat and current leads. Future plans for the Fuji/SEC/Kyushu University group's activities include changing the cooling method from pool boiling in liquid nitrogen to continuous flow of supercooled nitrogen with a refrigerator, with the intent of improving the current-carrying capacity of the winding and dielectric breakdown strength.

Fig. 3.6. Fuji/SEC/Kyushu University HTS transformer unit.

Table 3.7
Transformer Design Parameters (Fuji)

Fig. 3.7. View of transformer test setup.

Table 3.8
Transformer Characteristics (Fuji)