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US/Canada Power Outage Task Force Event Timeline
bofus writes "The U.S./Canada Power Outage Task Force issued the Aug. 14, 2003 Sequence of Events at noon today. While no conclusions are drawn at this point, it does paint a pretty good picture of what happened and when it happened."
12:05:44 - 1:31:34 PM - Four Generator trips
2:02:00 - 2:02:00 PM - Transmission line disconnects in southwestern Ohio
3:05:41 - 3:41:33 PM - Transmission lines disconnect between eastern Ohio and northern Ohio
3:45:33 - 4:08:58 PM - Remaining transmission lines disconnect from eastern into northern Ohio
4:08:58 - 4:10:27 PM - Transmission lines into northwestern Ohio disconnect, and generation trips in central Michigan
4:10:00 - 4:10:38 PM - Transmission lines disconnect across Michigan and northern Ohio, generation trips off line in northern Michigan and northern Ohio, and northern Ohio separates from Pennsylvania
4:10:40 - 4:10:44 PM - Four transmission lines disconnect between Pennsylvania and New York
4:10:41 - 4:10:41 PM - Transmission line disconnects and generation trips in northern Ohio
4:10:42 - 4:10:45 PM - Transmission paths disconnect in northern Ontario and New Jersey, isolating the northeast portion of the Eastern Interconnection
4:10:46 - 4:10:55 PM - New York splits east-to-west. New England (except Southwestern Connecticut) and the Maritimes separate from New York and remain intact.
4:10:50 - 4:11:57 PM - Ontario separates from New York west of Niagara Falls and west of St. Lawrence. Southwestern Connecticut separates from New York and blacks out.
If they're using AC generators (which I suspect provides most of the power - they're used in coal, nuclear, and hydro plants, but not solar, and not newer high-voltage DC transmission lines), then the frequency output is related to the spin of the shaft. Two things control the speed of the shaft - power in (water pressure) and power out (load demand from the grid), and a control system tries to keep the frequency a constant 60 Hz.
The trick is that the control system can only react so fast - suddenly disconnect an entire town, and the load drops, causing the power in to spin the generator too fast. If the control system overcorrects, then you'll get too low of a frequency. If a far-away generator drops out and you've got to supply more current to your local region, then the demand has gone up, slowing the frequency.
If you've been around generators, you can hear this exact phenonemoa - if the load changes suddenly, the motor will hunker down a little and then catch back up to normal speed. Usually a flywheel can damp out extremely short transients, but it would be prohibitively big if it were sized to handle transients as large as the control system (throttle) will allow.
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It's not that bizarre if you think about it... the bulk power frequency is actually one big juggling act between all of the generators that are synchronized on the system...
The Eastern Interconnection (everything in North America east of the Rockies and north of Texas) is tied together at many stations, such that there are many parallel paths to deliver energy to a customer load, providing an excellent level of stability. Simply put, the frequency is the prime measure of the balance between energy production and consumption. Energy generation is not a smooth process, it spikes as fuel is delivered and burned. If enough generators are synchronized with one another, they can automatically cover for each other's dips, and thus the frequency stays balanced.
Now, when the system split, imagine you had all of the generation on the west side, and all the load on the east side. For those of us in PA, we saw a huge loss of load, and the frequency shoots up. For those on the wrong side of the blackout, you suddenly lost your generation source, and your frequency drops.
Transmission equipment is easily damaged at low frequencies, so many are equipped with underfrequency relays that open breakers to protect themselves. What happened is that lines tripped and load sheds, forming smaller and smaller zones, until there were only small pockets of load and generators remaining (see the notes on western NY). Without the rest of the interconnection to syncronize with, your local generator was trying to maintain the frequency by itself as best it could, and was probably all over the map due to uneven fuel burn. Then, a few minutes later, you might have auto-reclosing of breakers (try-backs). If a line trips, some are programmed to auto-reclose, which, in an event like this, can suddenly add thousands of MW of load to an already stressed system, pulling the frequency down even more until everything is black.
There are two things you need to keep in mind here. The first is that phase in AC systems performs much the same function as voltage in DC systems; just as power flows from higher voltage to lower voltage across a DC connection, power flows from leading phase to lagging phase along an AC connection. (This has to do with reactance; all power lines are inductive.) Counterintuitively, voltage helps move power but it mostly balances VARs (volt-amperes reactive); if you have a local low-voltage situation, you can connect a capacitor to add some VARs and the voltage will come up. This is part of why big inductive loads cause line voltage to dip.
The second thing is that frequency variation is just a phase change over time. If the local frequency falls for a bit, it means that the local phase is moving behind the rest of the grid. This is what you would expect if some large load was added (or a generator lost) and more power had to come from elsewhere on the grid; the delta-phase across the interconnecting lines has to shift to allow more power to flow. What little energy buffering there is is mostly the rotational energy of generators and motors, so phase changes don't quite happen instantaneously.
If you had a serious local power shortage leading to shutdown, under-frequency is exactly what you would expect. Generators trip off-line, and the phase of the local grid backs off to pull more power from outside. It would take a full second at 59 Hz to shift one cycle, so this can go on for a fair fraction of a second. If the phase change over a transmission line increases past 90 degrees it will have to trip off-line, and once the local grid is an island you can have just about any frequency that the system will try to operate at. It's my understanding that most generators trip off-line at more than a fractional Hz off 60, if for no other reason than that they aren't designed or certified to operate on a grid that's obviously malfunctioning and such a condition means trouble. Mechanical resonances at off-operating rotational speeds are another reason to shut down.
Last, I suspect your conclusion is correct.
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