Nomex® Paper Plays a Key Role in Traction Transformer Design
First put into service on December 11, 2000, Amtrak’s Acela Express trains are still the fastest in North America. Operating at speeds of up to 150 mph (240 km/h), Acela Express trains use the most powerful traction transformers, weight-for-weight, ever built in the United States. The traction transformer design and manufacturing of these Class H transformers was done by Alstom using DuPont™ Nomex® brand paper and pressboard.
The traction transformer design was quite a challenge. Achieving optimum weight/volume/power ratio for the Acela Express traction transformers was complicated by the fact that the supply voltages along the Boston to Washington route range from 25 kV/60 Hz to 12.5 kV/60 Hz and 12 kV/25 Hz. This required the transformers to have triple-voltage capability with twin primary coils for use in-line or in parallel, depending on the supply voltage, and quadruple secondary traction coils.
To deliver the 5.745 MVA power required and to reduce the size of the transformers so they would fit beneath the two 6,000 hp motor coaches of each train, the mass and cross-section of the copper conductors had to be reduced, raising the continuous operating temperature of the transformer to 160°C, with a peak temperature of 180°C. Because conventional materials are unable to provide the required safety margin of 220°C, Nomex® paper was the obvious choice.
In this traction transformer design, Nomex® paper is used to cover the copper windings on the coils while Nomex® pressboard is used for the spacers that separate the coil layers and act as guides for the silicone cooling oil in which the internal transformer components are immersed.
Optimum weight/volume ratio: The use of Nomex® paper and pressboard in this traction transformer design made it possible to reduce the weight of the equipment transformers by between 15% and 25%, while optimizing the volume.
Long-term reliability: Its proven ability to withstand electrical loads, combined with mechanical strength, thermal stability, chemical compatibility, longevity and flame-resistance enable Nomex® paper and pressboard to significantly improve the long-term reliability of these power transformers.
Long service life: Because they are virtually free from the effects of aging, Nomex® paper and pressboard contribute directly to the transformers’ 25- to 30-year expected service life.
Materials Selected and Why
Nomex® paper and pressboard are critical to this traction transformer design because they are the only electrical insulation materials that provide the required safety margin of 220°C, enabling reduction of the size of the transformers while optimizing volume. The negligible aging, ability to withstand electrical loads and mechanical strength of Nomex® paper and pressboard were also important because these properties help reduce maintenance requirements and improve long-term reliability.