HTS TRANSFORMERS -- OVERVIEW

Immediately following the discovery of HTS materials in 1986, several studies looked into the feasibility of HTS transformers. Savings over conventional units were estimated to be greater than 35%, but the unknown ac loss characteristics of the HTS materials made it difficult to assess viable designs. A comprehensive study conducted for the U.S. Department of Energy found the life-cycle costs of an HTS transformer, on average, to be half those of a comparable conventional unit (Dirks 1993). National savings from the insertion of HTS transformers were estimated to be $25 billion through the year 2030. Over the size range of 30-1,500 MVA, Mumford (1994) estimated costs savings of HTS transformers over conventional designs to be as great as 70% and transformer weight to be 40% less.

A further advantage of HTS transformers over conventional units that is particularly relevant to Japan with its high population density is the inherently smaller size and weight of superconducting devices. As in the case of HTS power transmission cables that can provide increased capacity in existing ducts, HTS transformers can provide increased power handling in the same available space as a conventional power transformer. The benefits of smaller weight and size are expected to be major factors in the early introduction of HTS transformers in Japan. In Europe there is growing interest in using compact HTS on-board transformers in high-speed trains.

The potential market for superconducting transformers worldwide is in excess of a billion dollars. A review of the U.S. power transformer market shows that more than 90% of units are in the 10-100 MVA range (Table 3.4), representing 70% of the value of all transformers sold. The world market is estimated to be three to four times as large and growing at double the rate of the U.S. market.

There are three major HTS transformer projects currently ongoing in the United States, Europe, and Japan. Table 3.5 shows the team composition, size of the units under development, and the HTS materials used.

Table 3.4
Power Transformer Market, as of 1995-96

Source: National Electrical Manufacturers Assn., modified to include transformer sales not tracked by NEMA.

Table 3.5
Major HTS Transformer Players

The U.S. effort was launched by IGC as an extension of a cooperative research and development agreement with the Oak Ridge National Laboratory. With Waukesha Electric (a division of General Signal) as a commercialization partner and with partial financial support and applications guidance from Rochester Gas and Electric, the IGC team has designed and is in the process of constructing a 1,000 kVA-class HTS demonstration transformer that utilizes a low-cost HTS-coated silver tape. Use of the BSCCO-2212 system ensures stable operation at temperatures as high as 30 K. Operation at 77 K is possible with the use of BSCCO-2223 conductors, but selection of the overall transformer design must weigh the benefits of reduced refrigeration loads due to the elevated operation temperature against the higher conductor costs due to use of the more expensive BSCCO-2223 material and the lower performance due to the increase in temperature. While the demonstration of the Waukesha-IGC transformer is intended for 1 MVA, the base design is targeted for 30 MVA, 138/13.8 kV, 60 Hz, 10% impedance at 18 MVA, Delta-Wye configuration.

ABB, in collaboration with Electricité de France, has used multifilamentary BSCCO-2223 tape produced by ASC to build a 630 kVA, 13.72/0.42 kV, 50 Hz, 4.6% impedance, Delta-Wye demonstration unit.