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Saturday, October 17, 2009

Tutorial 8: Introduction to System Protection

What have we learned so far? We know the difference between AC and DC power, how energy is scheduled and accounted for, how it is decided which generators will run and how high, how interconnected power companies help each other out when they lose a unit, and quite a bit about power lines from big transmission to little distribution.
Has this been helpful so far?

Today's topic is in regard to why the system protects itself when bad things happen.

The system protects itself for two reasons: To prevent damage, and to maintain stability (ie. not collapse the whole house of cards). Although the correlation is not precise between fire hydraulics and electrical flows, nonetheless major system disturbances can have far-reaching and disruptive ripple effects that could be compared to the water hammer effect.

Extremely high or low amp flows and voltages pose a real threat of causing permanent damage to everything exposed to the swings, from multi-million dollar substation transformers all the way down to your DVD player. It is far better to instantly disconnect the world and drop the whole potato, because if you save the equipment, you have something to put back together. If you disconnect late, there isn't much point in trying to restore a system made up of fried parts.

When voltages and flows go to pre-programmed extreme limits (by line/equipment faults or simple overflows), protection systems activate to remove elements from the system in order to prevent damage to the transmission infrastructure, and also to prevent damage to customer equipment. It isn't unusual for a 'ripple' to occur when elements are removed, resulting in momentary spikes in flows and voltages, sometimes causing more elements to be removed, causing more ripples, causing.... yeah. There are well-paid transmission protection engineers who spend their careers trying to balance instant protection of the assets with not crashing things worse. Frankly, I find that stuff fascinating, but don't want their job.

It is a testament to those protection engineers that hundreds of major transmission lines and thousands of distribution circuits trip out as a matter of routine every day all across North America for all reasons imaginable, yet system protection isolates the problem so quickly that the vast majority of problems are cleared in seconds or a fraction of a second. Unless it is a distribution circuit that tripped and you're on it with several hundred other people, you don't generally notice that anything happened anywhere else.

The August 2003 Northeast blackout in the United States was a major comedy of errors and things going exactly wrong. How is it described by those dissecting airline disasters? An unfortunate sequential confluence of unforeseen events, or something like that. Super rare. It will happen again, but the 99.999999999% norm is that it doesn't, which is remarkable when you take in the scope and complexity of everything that goes into this machine.

For what it is worth, system disturbances do cause split-second bumps that go by faster than a blink of the eye in areas regional to the problem. Most dispatchers are attuned to that, and see it in a momentary hiccup in the lights several times a day at home and at work, often knowing a trip alarm is coming one or two seconds before it appears on the console, or wryly smirking when seen at home, knowing that some dispatcher is about to get interrupted. Sometimes we see it and the alarm doesn't come in, because it happened on a neighboring system. That said, these happen so fast that almost no one else notices them at all. Which is how we like it.

System protection is done in layers, in a way that breaks up the chain of transmission from the power plant to your breaker panel into many segments. Each segment has something protecting it, and each segment has at least one higher level of protection in place if the first one fails. And if the backup fails, the backup is backed up at the next level. And so on. Sometimes protection settings are not tight enough or sensitive enough (to wit: Must-See Video of Line Fault and Reclose) and issues happen, but the concept is generally very reliable.

I think that's enough for today. It's Saturday. You get the general idea, but no math or excessive technical jargon, nor homework.

What we learned: There's a ton of mysterious stuff set up out there designed to prevent blackouts, by removing faulted and overloaded parts of the grid faster than Barack Obama earns Nobel Peace Prizes. Oops, I said I wasn't gonna do that again. Sorry. And if you know a dispatcher and you see a minor nervous tic once in a while, don't worry about it, they just detected a line fault hundreds of miles away with their spidey sense, that's all.

Technical stuff next time. Don't worry though. It's pretty cool. Well, to some of us. Take it easy for the rest of the day.

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