During a transformer short-circuit, the electrical arc vaporizes oil and creates a Dynamic Pressure Peak which travels at the speed of 1,200 meters per second (4,000 feet per second). This phenomenon occurs within a few milliseconds. Because of reflections in the tank the pressure peak will generate pressure waves. The integration of all of the waves pressure peaks creates static pressure. Then, the pressure becomes equal throughout the entire transformer tank within 50 to 100 milliseconds after the electrical arc, and causes the transformer tank to rupture.
LACK OF INTERNATIONAL TRANSFORMER STANDARDS AND REGULATIONS
Transformers can only withstand a small overpressure and are not designed as pressure vessels according to ASME Codes and Controls. Consequently, transformers have proven to be very dangerous. Because transformer standards describe electrical requirements but do not cover mechanical design.
TRANSFORMER ELECTRICAL AND MECHANICAL PROTECTION LIMITS
- Pressure Relief Valve inadequacy: Pressure Relief Valves are suitable for slow pressure rise whereas pressure gradients developed during low impedance faults are extremely fast. Transformers that have exploded were usually equipped with Pressure Relief Valves.
- Buchholz and Rapid Pressure Relay inefficiency: transformer electrical protections are not designed to react to sharp pressure gradients. During the 62 TRANSFORMER PROTECTOR tests, the Buchholz always failed to detect any gas and oil movement or pressure variation.
- Electrical Breaker opening time: the best breaker technology trips in 50 milliseconds, far too late to prevent the explosion because most of the gases are generated within milliseconds after short-circuit. Consequently, the tank pressure keeps increasing even after the breaker has opened.