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Journal Journal: Mars, Ho! Chapter Thirty Five

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Destiny woke me up about seven thirty; I'd been the one up early the day before because of that engine. "Wake up, sleepyhead, or you won't have time for breakfast." She'd already made coffee had the robots make chicken cheese omelets. God but I love that woman, meeting her was the best thing that ever happened to me in my life. Of course, were it not for the monsters I'd never have met her. You take the wonderful with the insanely horrible, I gues

Comment Re:The failure mode is transformer core saturation (Score 4, Interesting) 91

... the induced DC from a solar storm isn't as instantaneous as a lightning strike. It takes minutes to develop, which leaves time to disconnect the lines and affected transformers if they are properly monitored.

But ARE they monitored for DC? It's not a usual problem.

Warnings on the order of minutes might be useful if the transmission line were the only one invoved. Unfortunately, the power grid is a GRID. Lots of multiple, parallel, transmission lines, and many, many, more going elsewhere and often creating loops.

Redundancy is a good thing in most situations. But when you have to drop a high line, and don't drop all the others simultaneously, you shift the load onto those that are still connected. When you're cutting off because you're near the limit - either due to heavy load at the time or because of the DC issue - you can drive the others beyond their limits (or throw things out of sync and add a bunch of "reactive current" to the load) and create a cascading failure. (Indeed, this is how the first Great Northeast Blackout occurred: Three of a set of four high-lines crossing the St. Lawrence Seaway near Niagra tripped out, and the redistributed load put one after another generator above its limits, blowing its protective breakers and making it progressively harder on those remaining.)

Gracefully shutting down the grid is not something you do on a couple minutes' notice, even if you have a plan in place.

As I understand, the induced DC is something on the order of hundreds of volts, which is much less than the tens of thousands of volts transmitted across ordinary high voltage transmission lines; disconnecting them should not result in arcing problems across the switches.

First, the problem with the induced near-DC is not the voltage, but the current. Transformers and transmission lines have as little resistance as possible, because it's pure loss of valuable energy. The magnetizing alternating current (i.e. the part of the AC that's there all the time, not just when there's a load) is also limited by the inductance of the transformers, but that doesn't impede the direct current at all. A couple hundred "DC" (very low frequency - fractional cycle per minute) volts, induced for minutes around the loop, can drive a hysterical amount of current.

Once the transformer is saturated, most of the damage comes, not from the direct current, but from the line power, which ends up dissipating lots of energy in the transformer. Meanwhile, at these voltages and currents, the switches that interrupt the AC are largely dependent on the momentary off time as the cycle reverses to quench the arc. If, say, the event happened when the line was running at about half its rated load, the direct current will be higher than the alternating current, so there will be no off time. This can keep the current flowing even through an open breaker (while dissipating megawats IN the breaker). Interrupting DC is MUCH harder than interrupting AC.

Heck, at these voltages even interrupting AC is hard. (The video is of an interrupter where the jet of arc-suppressing gas failed for one leg.)

Comment The failure mode is transformer core saturation. (Score 5, Informative) 91

High induced votlages in open wires are a problem, but they're not the big one.

The biggie is common-mode currents in long high-voltage transmission lines adding a strong DC component to the current in the substation transformer windings - high enough that when the same-direction peak of the AC's cycle adds to it, the core saturates. Then the inductance of the transformer drops to the air-core value and no longer substantially impeeds the current.

The current skyrockets. The resistive heating of the windings (and the force on the wires from the magnetic fields) goes up with the SQUARE of the current. The windings quickly soften, distort, form shorted turns, melt, open, short out to the frame, etc. The transformer is destroyed, or committed to a self-destructive progressive failure, in just a handful of such cycles - too fast for the circuit breakers to save them (even if they DO manage to extinguish the arcs with the substantial DC component to the current.) Even if the transformer doesn't explode and throw molten metal, gigawatt sustained arcs, and burning oil (or burning-hot oil replacement) all over the substation area, it's still dead.

This happens to MANY of the giant transformers in the power grid. Each set of three transformers that has one or more failed members means a high-voltage transmission line that is shut down until the transformer is replaced.

There are essentially no spares - these are built to order. Building one takes weeks, and there are few "production lines" so little parallelism is available. What is destroyed overnight will take years to replace, while each intercity power transmission line is not functioning until the transformers at its end ARE replaced.

The current occurs because the transformers are organized in a "Y" arrangement, and the center of the Y is grounded at each end (to prevent OTHER problems). The transformers have enough extra current handling capacity to avoid saturation from the DC through that center connection to/from ground from ordinary electrical and solar storms - just not a giant one like we get every couple centuries.

The solution is to put a resistor in that ground connection, to limit the DC in the lines (and dissipate the energy it represents). Indeed, a few lines have such resistors already.

But a suitable resistor is a box about the size of one of the transformers. It's very expensive. And it only makes a substantial difference to the operation of the lines in such a once-in-centuries event. So most executives don't spend the money (and get dinged for costing the company millions) to put them in, to prevent a failure mode that hasn't happened in the generations since Tesla and Westinghouse invented the three-phase long-line power grid.

Or at least they don't until the regulators or their stockholders require it. Which means said decision-makers need a little educational push to decide it's worth the cost and get it done.

Thus articles like this. B-)

Comment Re:Avoiding Amazon Web Services? (Score 2) 168

AWS started as a way to gain revenue from the spare capacity they had for cyber monday, but it's now ~200x the size of Amazon's actual needs and is its own revenue and profit center. If a new CEO wanted to at this point he could spin it off into a separate company with contracts to host services for Amazon. I'm honestly not sure what it would gain you other than access to a pile of capital to use elsewhere, but for the time being Amazon doesn't seem to be hurting for access to capital.

Comment Good (Score 4, Interesting) 225

That's probably a good thing since students shouldn't be static consumers of information and tablets are really subpar for most kinds of content creation. Add in the fact that a Chromebook costs half as much as even an ipad mini and overall the schools are probably making the rational choice.

Comment Presbyopia (Score 1) 550

I'm up around retirement age. My eyes don't chage focus much at all. So I have to swap lenses to go from distance to close-up vision. (Yes I could use some kind of bi/tri/progressive-focal lenses. But at the moment swapping is adequate for me.)

Until they find a way to correct presbyopia (and they don't see to be even researching it), I'd still have to don/remove glasses anyhow. With my extreme astigmatism, extreme nearsightedness, and substantial age, I'm not a good candidate for lasic and stand a substantial chance of visual artifacts from it. I'm also a target shooter, so my glasses double as eye protection.

Given all this, the potential benefits for me would be small and the risks and cost oughtweigh them.

But if they ever find a way to fix presbyopia the equation could change substantially.

Comment Re:EVD (Score 1) 170

It's exactly as many syllables as "ebola" but carries more information, what's not to like?

Indeed, it carries MUCH more precision than just "Ebola", which can mean any of the following:

"Ebola River" is a tributary to the Congo River.

"Ebola Hemorrhagic Fever" was the name of a disease first discovered in people living in the remote Ebola River watershed.

"Ebola Virus" (abbrev. "EBOV") is the infectious agent that causes "Ebola Hemorrhagic Fever"

"Ebolavirus" is the taxonomic genus to which the "Ebola virus" belongs.

"Ebola Virus Disease (abbrev. "EVD") is now the more common name for Ebola Hemorrhagic Fever. We can call it that because we have definitively identified the infectious agent that causes the disease (EBOV). Changing the name pre-emptively differentiates EVD from other hemorrhagic diseases that might arise from the same area.

Laymen simply say "Ebola" and let their audience sort out what they mean -- if indeed they mean anything precisely. I once had this conversation with an elderly relative.

Relative: 90% of bats have rabies.

Me: That's hard to believe.

Relative: It's true! I read it in the paper.

So I went to the paper and found out that she had it hopelessly garbled. TEN percent of bats SUBMITTED FOR TESTING had positive SCREENING tests.

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