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In case this thread develops further, I've rechristened it with a more
appropriate name.

Frank Reed wrote, on 4 August-

"...  The solar storms impacted the earth's magnetic field causing fairly
abrupt changes in direction of the field lines over hundreds of miles. And
changing magnetic fields do their Faraday duty inducing currents in those
long electrical lines of early telegraph systems. That steady current,
above normal levels, then lit the paper on fire."
======================

To me, that seems implausible, which was why I wrote, on 3 August-

"I've considered the effect of voltages induced in the current loop created
by the wire and its ground return, by changes in the magnetic field passing
through that loop, but at first sight the order-of-magnitiude seems quite
insufficient to give rise to sparking."

We need to put in a few numbers to back up that judgment. The Earth's
static magnetic field at the surface ranges up to 50 microTeslas or so (or
0.5 Gauss in old-money). The biggest recorded solar-flare event, which
occurred in 1859, gave rise to changes in the Earth's horizontal magnetic
field of 1.6 microTeslas. In such events, that field change typically
builds up, then dies away, over a period of a few hours.

Next, we need to estimate the size of the loop involved; that of the
telegraph wire as one conductor, and the earth return as the other. We
might estimate the height of the wire, strung between its poles, to be 4
metres. I don't know what the maximum span of early telegraph wires,
between repeater stations, would be, but let me guess at 1000 kilometres.
In which case the area embraced by the loop would be 4,000,000 square
metres.

If the magnetic field through a coil of 1 square metre changes by I Tesla
per second, that induces a voltage between its ends of 1 volt.

So if the magnetic field, passing through that loop of 4,000,000 square
metres changed by the amount of the 1859 event, 1.6 microTeslas, in 1
second, the voltage induced would be just 6.4 volts; hardly enough to give
rise to serious sparking. But in a solar event, it doesn't change by that
much in a second; the changes take place over a good fraction of an hour.
In which case, the induced voltage would be correspondingly less, of the
order of millivolts rather than volts.

I'd be grateful if someone would check over those numbers and point out any
errors, if they're found.

So we need to seek another explanation, and I think we can attribute it to
the raining down of ionised particles, some of which are collected by the
exposed length of wire. It seems quite plausible for that 100 km of wire to
intercept the few microamps of current that are needed to overwhelm the
sensitive galvanometer at the receiving end, and a few milliamps might well
be sufficient to set things on fire in such high-impedance circuitry.

====================

The Quebec event of 1989 is another matter altogether. I thank Richard
Langley for providing (as he does so often) a link to a relevant and
well-informed article, at http://www.breadandbutterscience.com/SSTA.pdf ,
"Solar storm threat analysis", by James A Marusek. My only caveat about
that article is that it's a bit alarmist.

That article has an explanation of the Quebec event, in terms of
interconnections of the electrical power grid. Let my try to explain, and
you can refer to his paper to check whether I have it right. I'm not an
electrical engineer, and have no such specialist knowledge.

Power stations are linked to each other by a network of 3-phase
high-voltage cables, connected to the generators via step-up high-power
transformers. These have a "neutral" terminal, at the mid point of the Y
configuration of the transformer, which is connected to local ground.
Normally little or no current passes through that connection. My own guess
is that it's there, mainly, to provide some protection to the insulation of
the transformer in the event of a local lightning strike.

Large areas of that part of Canada are occupied by the Laurentian Shield, a
bedrock of solid granite which has very low electrical conductivity. I
presume that a meshwork of buried conductors surrounds each power station
to provide as good a connection as possible to the local ground.

Now imagine a solar flare, that interacts with the atmosphere and provides
an uneven rain of charged particles at the Earth's surface. Where these
fall on a conductor, such as the sea surface or conducting geology, then
that surface will remain at the potential of the whole Earth-body. But
where they fall on an insulating layer, such as that granite, a local
voltage may be developed (which may perhaps be no more than a few volts) as
the surface current finds its way to the body of the Earth, or to another,
conducting area of the surface (perhaps hundreds of miles away). But now,
what happens if we link two distant places on that granite shield with a
copper path? That could provide an easier route for that local surface
current to flow and equalise the potentials at its ends. That is what the
grid-link can do, via the ground-connection of the transformers at its
ends, and the buried meshwork.

So this is a mechanism for the area around the station to collect the local
ground-current and channel it to another station, via the ground
connections of the transformer, which provides no barrier to such dc
(steady)current, as opposed to the normal 60 Hz ac of the supply. That
current flow could amount to many amps, which might well be well within the
current-carrying capacity of the wiring, but it can have an insidious
effect on the iron core of the transformer.

The dimensions of a transformer core are chosen such that the magnetisation
of its iron swings, over a cycle, lies between just less than its
saturation value in one direction, and just less than its saturation value
in the other direction, when the rated ac voltage is applied. Any less, and
the iron would saturate, with disastrous consequences, as the coil around
it effectively loses all its inductance, and becomes like a short-circuit
across the power-station's output. Any more iron than that critical value
would be wasteful. It will always be designed to run close to the
saturation limit.

But now, if we impose a dc ground-current, through those same coils, that
current imposes a steady magnetisation in one direction. In one half-cycle
of the ac swing, it adds to the magnetisation of the core, taking it closer
to, or even over, the saturation limit (in the other half-cycle, it
opposes, so then presents no problem). The resulting asymmetric pulses of
ac current, at 60 Hz frequency, can be enormously damaging to a
transformer, giving rise to overheating and mechanical damage due to
vibration. That's what lay behind the failure of one of Quebec's major
power transformers, which within a short time escalated into a runaway
breakdown of the whole grid system.

To an armchair commentator such as me, there seems an obvious solution.
When such dc ground-currents are detected, above a certain danger level,
simply break the ground link (at one end, not both) or replace it by a
capacitive impedance. Would any dangerous voltages develop? Are these
auto-transformers, in which case the low-voltage windings, from the
alternators, are not isolated? If so, would it be safer if they were fully
isolating? Do we have any electrical power engineers on Navlist who can
explain what problems would ensue?

That Quebec event has certainly demonstrated our vulnerability to solar
events, which might well be much more intense than that of 1989.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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