This was an electrical substation that stepped down high voltage (138,000 volts) from a transmission line down to a lower voltage (23,000 volts) for local distribution. It was conveniently located adjacent to a golf course and residential housing. In this clip, a ground fault on a capacitor bank on the low voltage side of the substation creates an arcing fault that behaves like an uncontrollable welding torch from Hell, chewing up everything in its path. Normally, the abnormal current would be detected and power automatically removed almost immediately by substation protective hardware. Unfortunately, in this case, the protective hardware failed or was unable to sense the presence of the arcing fault. Excessive current eventually causes the windings on the substation's power transformer to overheat, severely cooking its innards and raising the flammable mineral oil within to the boiling point. In a vain attempt to prevent the transformer's tank from exploding, pressure release valves or a failing tank gasket vents steam-like clouds of superheated oil vapor. The foggy mist of hot oil is then ignited by the arc, causing it to explode in a ball of flame. This is quickly followed by a phase-to-phase short on the HV side, perhaps caused by a flashover within the flames or by a heat induced fault within the transformer . The phase-to-phase short causes an upstream circuit breaker (in another, larger substation) to blow, finally killing power to this substation.
However, by this time, the overheated transformer's tank fails, and it dumps hundreds of gallons of flaming mineral oil onto the already devastated substation. Local firefighters can only watch from a distance since there's no way to safely fight this fire. The substation is a total loss. As linemen often say, "Firemen don't mess with their wires, and linemen don't mess with their fires". A very sobering look at the explosive power lurking within that quietly humming substation in your neighborhood...
NOTE: Based on recent inputs from employees of Florida Power and Light, this event occurred at the Ives Dairy Substation located near San Simeon Way and Biscayne Boulevard in Miami, Florida. It is believed to have occurred in 2000 or 2001, and the footage was captured by a local resident from his home which was adjacent to the country club and golf course. The root cause was a defective fuse holder associated with motor-operated high voltage switches. Substation switchgear was disabled when a small fuse blew, opening control power for the substation's protection hardware. Normally, the blown fuse would trigger an alarm to the dispatcher and the problem would be promptly fixed by maintenance personnel. However, in this case, the defective fuse holder also prevented the alarm from being sent, so the power company was unaware that the substation had become completely unprotected.
Some time later, a low voltage side capacitor bank failed, creating an arcing fault that could no longer be cleared, since the substation's protection hardware was inoperable. The arcing fault ultimately led to the total destruction of the substation. Although at least one report indicated that the spray of white mist might have been water from a fire suppression system, it is now known that this particular substation did not employ an active fire suppression system. The spray was, in fact, a "fog" of overheated, vaporized mineral oil, and the explosion was quite likely an example of a dangerous BLEVE (Boiling Liquid Expanding Vapor Explosion).