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Fire safety is always one of the first considerations when you’re specifying a transformer, especially if the unit is close to populated areas or high-value structures. Engineers at NASA’s Johnson Space Center (NASA/JSC) in Houston used fire properties as one of their primary criteria in the fluid selection process when they replaced an aging network of 10 dry-type and liquid-filled transformers. All of the new liquid-cooled units are filled with silicone dielectric fluid.
These transformers were part of a renovation project at NASA’s Project Management facility. The units supply power to offices, R&D facilities, laboratories, and storage areas. Because the new equipment is located just 25 ft from the nine-story structure, personnel safety was the first priority in specifying the design.
A spokesman from the silicone dielectric fluid supplier explained distinctions between the most popular fluid technologies, which carry identical flammability ratings per the NEC:
“Even though hydrocarbon-based fluid and silicone are both classified as less flammable, there are dramatic differences in the burn characteristics of each material. The contrast is especially apparent when considering the rates of heat release, smoke protection, and generation of other combustion products.”
He also pointed out the radiative and convective heat release rates (HRRs) for silicone are about 1/10th the rate for hydrocarbon fluid. The amount of heat released by a burning material is a critical factor in assessing potential fire hazard, because this contributes to a fire spreading and hampers firefighting efforts. So the higher the heat release rates in a large-scale fire, the greater the hazard.
These two fluid types have demonstrated substantial differences between their flash points (temperature at which vapors will ignite in the presence of a flame) and fire points (temperature at which a fluid sample will sustain combustion after being exposed to an ignition source.)
With 25 years of service behind the old units, NASA engineers knew that technical advancements in transformer design made it possible to replace the existing equipment with more efficient designs. Furthermore, the original transformers were housed in a remote rooftop location, making even routine maintenance a challenge.
To increase accessibility and simplify service, engineers established an outdoor transformer yard on the Project Management building’s east side, where two transformers are also installed. Both are 1500kVA units filled with transformer fluid. Classified according to ANSI/IEEE 141-1986, each weighs approximately 11,500 lb and has a liquid volume of about 430 gal. Eight other silicone-filled power transformers have also been installed at NASA/JSC, ranging from 500kVA to 1500kVA. Each of the new units is located next to the building it serves.
For extra safety, the transformers incorporate external cooling fans, which allow them to run above the base nameplate kVA rating for short periods under emergency conditions. Engineers have also installed heaters in the high- and low-voltage cable termination compartments, to keep them dry when they’re not energized.
The decision to install silicone-filled transformers at NASA was based on performance, safety, and economic considerations. Silicone transformer fluids generally provide excellent long-term performance and are essentially maintenance-free. As with most liquid transformers, diagnostic tests can provide valuable predictive information about the condition of the unit. This benefit is not available with dry-type transformers.
Estimates indicate silicone dielectric fluids are currently in service in more than 100,000 power transformers worldwide. Their growing use can be attributed primarily to superior fire safety characteristics, including high molecular weight hydrocarbons. With silicone’s greater ignition resistance, lower HRR, and tendency to self-extinguish, it significantly reduces the potential fire hazard. In addition, silicone fluid has a favorable health and environmental profile, and can be reprocessed or recycled to extend its service life. Silicone is not classified as a hazardous waste, and degrades naturally by contact with the soil.