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Thursday, November 18, 2010

Classic Kaboom!

This clip has been around for a long time, so you may have seen it elsewhere.

This is a 138/23kV substation, and a capacitor bank on the low side (used to boost voltage) is for whatever reason experiencing a relatively low-current ground fault. For some reason, the station's protective equipment is not detecting and then clearing the fault.

The sustained fault arc is cooking and welding and destroying everything it can reach, like a death ray blaster stuck on maximum. How hot does an electrical arc get, you ask? Oh you didn't ask? Well I'll tell you anyway. Depending on the amperage and voltage of the arc, the temperature can vary widely between 9,000°F and 36,000°F. Hot. For reference, the outer visible surface of the sun, the photosphere, is only about 10,000°F. Hot.

So, where were we? Oh yes, this hotter-than-the-sun arc is blazing away next to this unlucky transformer. It doesn't take long for the transformer's flammable mineral oil to overheat, boil and expand, and finally cause the transformer's overpressure safety valves to release the oil as a high-pressure spray.

Incidentally, transformer banks are typically equipped with pressure relays to detect and de-energize the bank within a second or less when something like this goes down. Since there are two independent show-stopping problems going on (arc fault current and transformer overpressure) that are not causing any kind of shutdown, I have to conclude that the station's entire protective relaying system was inoperative.

Back to the long boring story. The high-pressure flammable oil spray meets the hotter-then-the-sun arc, and the result is a foregone conclusion. The spray and associated fireball with its conductive smoke particulate byproducts also seems to somehow finally cause a good hard high-side fault to occur (watch and listen for the flash/bang right as the fireball goes up), and this fault is at last detected, by whatever station is feeding this one. The other station says "Whoa, something's going down out there!" and opens up the feed. Alas, a tad late to save the day.

The transformer alone will cost perhaps about $750,000 to replace, the other destroyed station components at least that much again, and don't forget to add the cost of the environmental cleanup, and other peripherals.

Repeat message from an earlier post. Stay away from the pretty sparkly show:




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