A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Solar storms. was: New inovation in astro navigation?
From: George Huxtable
Date: 2010 Aug 6, 15:47 +0100
From: George Huxtable
Date: 2010 Aug 6, 15:47 +0100
My previous posting on this topic was sent to Navlist on 4 August. It is copied, in full, below. On 5 Augusr, Frank Reed responded- "Here's a brief discussion of the induced currents: http://en.wikipedia.org/wiki/Geomagnetically_induced_current One thing to note here is that there are big currents in the ground and that may help explain what was going on in those old telegraph systems in 1859." ======================= I thank Frank for that link, which together with other information has caused a major rethink on my part. I'm now aware that I missed an important factor, when assuming that the body of the Earth would be everywhere at more or less the same voltage. That is not the case, and completely undermines my argument. When a solar storm event occurs, the time-varying magnetic field above the Earth's surface, caused by circulating currents above the atmosphere, induces corresponding voltages in the Earth's crust, associated with circulating electric currents in the crust which try to oppose those changes. That crust is by no means a perfect conductor, but has a finite resistivity. The voltages which result, between pairs of ground-points on the Earth's surface which happen to be connected together with conducting wires, can be very significant. I now accept that those voltage differences are the cause of disruption, both to early telegraphic signals, and to modern ac power transmission networks. The calculation of small induced voltage in the loop between a telegraph wire and ground was, I think, correct, but it is overwhelmed by the voltages induced, within the Earth, between those ground-points at its ends. Sorry if I've misled anyone. George. contact George Huxtable, at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. =========================== The following is a copy of a message I sent to Navlist on 4 August- 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. (end of copied message)