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Saturday, May 8, 2010

Despite the Best Made Plans, Part II

So what does a power plant do for power when it trips?

In the case of steam plants that have enormous amounts of auxiliary systems, just dropping dead would damage and gunk up a variety of systems that I won't spend much time explaining on here. Suffice to say, it is Bad.

Therefore, power plants tend to draw their power, known as "station service", from the substation that the plant is attached to. You see, if the unit trips, the substation generally stays online. One or two breakers is all it takes to separate the unit from the grid, and there is no reason at all to dump the whole station. Alternatively, the plant may have a station service connection to the closest distribution feeder circuit, which is not necessarily even attached to the plant's substation. Or, just as likely, the plant will be capable of working off of either source.

But what happens in a regional outage, when everything trips, a bona fide area blackout?

Well, power plants that need it use pretty much the same kind of system that hospitals and 911 centers tend to use; an on-site, automatic Diesel generator, albeit on a larger scale. Sometimes more than one if station service demand is high. In the event of the loss of all station service sources, the generator kicks in within a few seconds, hopefully before the conveyors and pumps and belts and blowers and what have you get clogged up or damaged.

Now, a typical large steam plant may require around 10% of its rated output for typical station service, which is substantial. That would require equally substantial diesel generators. But this happens so rarely, it is not cost effective to plan on running the power plant's office lighting, vending machines, coffee pots, really anything considered non-essential during the rare area blackout that happens every decade or so.

These days, offices and computer data centers have large uninterruptible power supply systems on dedicated circuits that exclusively serve essential applications, but let everything else drop in a power outage. This is not a new practice, as power plants have done this for ages. But instead of running essential servers, etc, the backup generators run all those auxiliary systems and keep the turbine shaft slowly rotating so it does not warp out of balance, until full power is restored and the plant comes back online.

So, there's the setup for Despite the Best Made Plans, Part II.

When a large power facility, with three turbines totaling a combined 430MW, completed a years-long project and added a fourth turbine rated for 550MW by itself, the on-site backup generation got a very necessary upgrade as well. Every station circuit was scrutinized to determine which were critical, so the others could be left off the emergency system and help keep the demand on the emergency system manageable.

Well, everything went peachy, everything tested good. The new turbine got its kinks worked out over a few months and then was released for service.

The plant personnel faithfully tested the backup generator every week, flopping the critical systems over to it for a few hours at a time, and it worked great.

And then, the Day of Big Fun finally arrived.

What caused the regional blackout isn't important. It was notable, but not quite worth big national news attention.

This particular power station got whacked, all four turbines tripped, and all station service sources went out. The new backup generation system was ready to show its stuff, and true to plan it fired right up. Yay!

And then, about ten minutes later, it died.

I type too much, let's cut to the end.

There was a very small diesel tank located by the generator, fed from a very large diesel tank located a short distance from the building for safety purposes. When the small tank got low, a float sensor activated a pump to transfer fuel from the big tank, refilling the little one as needed.

Guess which critical system wasn't on a critical circuit. Yup, the transfer pump's dedicated circuit was not attached to the generator it served.


The fix wasn't very expensive, really, but the same can't be said for how the problem was discovered.

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Also see: Despite the Best Made Plans (Part I)

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