Because of the loopholes in regulations and the growing number of transformer explosions and fires in 2010, the National Fire Protection Association has included in the NFPA 850 “Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations”
The Annex A of NFPA Recommendation 850, defines the 4 criteria for Fast Depressurization Systems. These criteria are highlighted below in bold:
“Annex A, Explanatory Material, Chapter 220.127.116.11. Oil-filled transformer explosions and fires can be prevented in some cases by the installation of a passive mechanical system designed to depressurize the transformer a few milliseconds after the occurrence of an electrical fault. This fast depressurization can be achieved by a quick oil evacuation triggered by the dynamic pressure peak generated by the short circuit. The protection technology activates within milliseconds before static pressure increases, therefore preventing transformer explosion and subsequent fire…”
However, the NFPA 850 Recommendation includes definitions that rely on implicit knowledge regarding a set of complicated and extremely rapid physical processes, including dielectric breakdown in transformer insulating oil, the generation of internal arcs, the vaporization of the oil, the build-up of pressures on the transformer tank, and the resultant dangerously high stresses on the tank wall. Consequently, this portion of the NFPA 850 Recommendation may not be clearly understood by professionals entrusted with generation plant protection.
Therefore, the article “Understanding the NFPA 850 Recommendation for Preventing Transformer Explosions in Electricity Generating Plants” was issued and can be downloaded here
Since transformers explode between 200 and 400 milliseconds after the occurrence of a low impedance fault, the TRANSFORMER PROTECTOR is always designed to fully depressurize transformers within 200 milliseconds after short-circuits.