Another recap - nukes at high altitude give you all three phases. A mid-air burst, almost nothing. A ground burst, you get mostly geomagnetic heave.
Of the three phases, the first two are radio signals with mixed frequency at really amazing amplitudes, and they pop circuits by overvolting devices
on the front ends of radios and radars, and by causing gate punchthrough and SEUs in digital stuff.
The geomagnetic heave phase is where you take out the power lines and telecom cabling.
In the US, long HV distribution lines are typically wired up in a wye configuration, with a grounded center point. So what you end up with is a long
flat loop consisting of the three phase wires for the top part of the loop, and the earth as the bottom part, tied to the middle of the wye at both
This makes for a really nice single turn transformer with a huge aperture, and since the wires are usually pretty high above the ground, you can add
in a capacitive couple with the ionosphere.
After the bomb pops off, the prompt phase will cause those Compton effect electrons to head straight down in a big relativistic Zergling rush. This
capacitively couples a big voltage spike into the line just for openers.
Once you're past that, the magnetic field starts bucking around. You get that first early buck when the electrons spiral around the field lines. This
hits the ground with a big magnetic pulse, which also shoves the field lines from the earth away. Once the pulse is over, the field lines pop back
with a vengeance, and then the whole thing oscillates for a while.
All these field changes induce currents into the loop formed by the wires and the ground below. And the bucking takes seconds to complete each way, so
it's effectively a DC current that's being induced into the long lines. In addition, you get other ground loops set up so that the earth takes on a
different potential at both ends of the wire. This is also bad, and induces its own current in the wire.
The currents in long power lines during a HAND can hit 100A, which is not a good thing.
Now, if you've got copper telecom cables that go for long distances, you'll get the same sort of thing in miniature at the switches or SLAMs. Fiber
optics don't care and won't be affected. But high impedance connections at the ends of long copper wires will have a common mode voltage from hell
induced on them. If not dealt with, the switch/SLAM will fry due to common mode induction.
The situation with power lines is a bit more complex. The first e-field coupling spike can cause dV/dT failures in transformers by just punching
through the insulation between windings, which can ruin your whole day, and will take out a lot of the power line communication system by popping the
Most of the issues, though, will happen with the transformers due to DC currents. Transformers, especially the touchy high voltage ones that drive
long transmission AC lines, don't like DC currents at all. The why is a bit complicated, but essentially it's due to saturation.
A transformer's core steel works by having lots and lots of little magnetic domains. AC power on the primary rotates the little magnetic domain
alignments in the cores, coupling a field into the secondary. That's how it's supposed to work. There are only so many domains in that steel, though.
As you apply more and more current in the primary, you flip more and more domains. And eventually you start running out, and more current in the
primary doesn't increase the field in the core any more. This is what we call saturation. You just can't put any more magnetic field into the core,
because it's taken all the flux lines it's going to. There aren't any more domains to recruit. So more current just creates crappy field lines in the
air around the core, but they don't end up coupling power into the secondary.
Saturation is bad. You never design a transformer to operate saturated in a normal world (swinging chokes are a different thing), or even to approach
saturation. You can't put a HUGE margin in though, because you have to buy margin between your operating point and saturation by putting in more
steel, or better steel, or more windings or whatnot. So you have to pay for that margin, and mostly you can't justify it in a business case for
More, there's a thing called hysteresis, and that's more than I want to explain, but it's related to the core efficiency. The fatter and less
symmetric your hysteresis curve (also called the BH curve), the more of your electrical power goes into warming up the core instead of going down the
As you enter the geomagnetic heave phase, the flux changes caused by the heave enter the wire-Earth loop and induce a big current. And there's another
loop in the ground itself that changes the ground potential between the ends of the line, which also adds to the fun. The quasi-DC current has a lot
of non-optimal effects on the distribution system.
One, it saturates the transformers really well. That 100A+ current recruits all the domains, or most of them, to the DC current flow. As a result, the
energy coming into the primary can't make magnetic fields out of the power, at least not in the core. So what you do produce radiates into the area
around the core, heats up the transformer casing, induces voltages into the connections and surrounding structures and what not. Also, since the
core's not accepting any more flux, the effective AC impedance of the transformer closely approximates to zero, and it turns from a transformer into
an expensive short circuit. What power's not going into heating the case red hot just dumps its energy into the primary in the form of I2R losses, the
wires turns red hot and becomes what we call a molten puddle.
Also over on the secondary side, that 100A current is more than the secondary is able to tote and IT starts heating up to failure.
If you get less of a hit and don't saturate totally, you get some other effects that are more slowly bad but will still cause a failure. The current
raises you on that hysteresis curve, and instead of being thin and tall, it starts becoming fat and distorted. The fatter the curve, the more loss in
the core. A lot more loss. Instead of being 90%+efficient, you're down to the 30% point and most of the power is going into burning up the core. And
since that hysteresis curve isn't symmetric anymore, you get harmonics. Some of the nice 60Hz power going in is now 120Hz, 180Hz, 240Hz etc, and the
currents tend to circulate in the transformer, further heating it, they also cause eddy currents in the core. That asymmetry also starts coupling a
lot of magnetic flux into mechanical stress in the core, so you get this organ chord as the thing starts disassembling itself. Harmonics also cause
hot spots in the windings, so between heating and shaking the core around and heating up spots on the wiring, you will very shortly get a short and
Bob's your uncle.
The good thing, if there is one, is that shorter runs have dramatically less problems, so the local distribution from the power plant next county over
will likely safe itself and shut off, leaving the turbines to deal with an unload, which isn't going to be nice either. But it's not necessarily
curtains for local power. You will lose the AC links between areas, though.
continued in part IV...
edit on 20-5-2015 by Bedlam because: (no reason given)