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George ;
I have in my hand a Journal by the Institute of Post Office Electrical 
Engineers (uk), on  The Design, Laying and operation of the Trans-Atlantic 
Telephone Cable. Which was a joint effort by UK, Canada and USA sometime in 
the early 1950s. It is a good read for the technical minded btw.
In the Introduction on the chapter on Power feed equipment, (1950 volts 
negative supply from one end and 1950 volts positive supply from the other), 
the objective of the design of the power plant  was, to quote:-
 
“To compensate for earth potential differences of up to 1000 volts, of either 
polarity, that may develop between the earths at Oban and Clarenville 
(Newfoundland) during the magnetic storms accompanying the appearance of 
Sunspots and Aurora Borealis”.

Clive.

PS, I am not sure that this will get to the Nav List directly as I haven’t 
been logged on for a long time.  Maybe you could forward it for me.

Clive.


-----Original Message-----
From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of George Huxtable
Sent: 04 August 2010 21:46
To: NavList@fer3.com
Subject: [NavList] Solar storms. was: New inovation in astro navigation?


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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