HTS Power Transformers

Utilities rely on transformers in electric transmission and distribution systems to change the relationship between voltage (a measure of electric “pressure”) and amperage (the current’s strength). By increasing the voltage, they make it more cost-effective to transmit power over long distances. However, they later must restore it to the much lower voltages needed by homes and businesses.

Conventional transformers boost power to long-haul levels and then (closer to end users) step it back down. However, heat generated in constant voltage changes reduce a transformer’s useful life. In addition, the systems must be cooled using chemicals that carry a potential for hazardous leaks, combustion or explosion.

Replacing the copper coils in a conventional transformer with HTS wire can provide significant benefits:

Smaller and Lighter: Due to the high power density of HTS wire, an HTS transformer is about 40-60 percent smaller in size and weight than conventional transformers of the same power rating. This feature can help reduce manufacturing, shipping and installation costs. In addition, it provides flexibility in siting by reducing space requirements -- a critical factor in overcrowded urban substations.

Longer Life: Heat generated by electrical resistance and constant changes in temperature (thermal cycling) during operation are major factors in the breakdown of the electrical insulation on the copper wire used in conventional transformer coils. This effect is eliminated with HTS coils which operate at constant temperatures. The absence of thermal expansion and contraction in the coils removes shear stresses the electrical insulation normally experiences, and prevents premature failure of the insulation due to mechanical fretting. In addition, conventional transformers can be overloaded for only short periods (200 percent for 30 minutes, according to IEEE/ANSI standards). Thermally independent HTS transformers can carry overloads when needed, with no decrease in equipment lifespan and with manageable additional load losses.

Greater Efficiency: Energy losses are reduced by eliminating resistive losses in the windings and by a reduction in the size of the transformer core. Total efficiency improvements will vary by design.

Environmentally Benign: Inexpensive and environmentally benign liquid nitrogen replaces the conventional oil as the electrical insulation (dielectric) and provides the necessary cooling media.  Liquid nitrogen is safe, non-flammable, and environmentally friendly.  Using it as a dielectric and coolant instead of oil eliminates a potential source of explosions and the potential for soil contamination from oil leaks.  HTS transformers can be designed to meet indoor specifications.

Fault Current Limiting:  Material properties of HTS wire provide fault current limiting capabilities. A development effort to integrate fault current limiting capabilities directly into the transformer design is currently underway at AMSC.

Transformer development projects to date based on the use of first generation HTS wire have achieved technical success.  The commercial introduction of HTS transformers is expected to occur by the end of the decade based on the commercial availability of second generation (2G) HTS wire, which will have both the electrical properties and price point needed for this important application.

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