The electric power grid, and
therefore the power to your home and
business, can be disrupted by space
weather. One of the great discoveries of
the 19th century was the realization
that a time-varying magnetic field is
able to produce an electrical current in
a conducting wire. The basic idea is
that the time rate of change of the
magnetic flux (i.e. lines of magnetic
force) passing through a current loop is
proportional to the current that is
generated around the loop. A slightly
earlier but equally important discovery
was that a current-carrying wire
produces a magnetic field. The
application of these principles is
widely prevalent in modern society, for
example in electrical power generators,
electrical power transformers, and
electrical motors.
Time-varying magnetic fields and
electrical current systems, however, are
not just artificial phenomena, but in
fact are quite common in nature. Natural
electrical current systems which vary in
time can be found inside the Earth, in
the oceans, and in the upper atmosphere
of the earth (above ~100 km) where the
constituents of the atmosphere include
positively charged ions and negatively
charged electrons which move about in a
myriad of complicated ways. Many of
these upper atmospheric current systems
are constantly present and modulate in a
regular way in response to the rotation
of the earth, the gravitational pull of
the moon, and the slow variation of
solar radiation over the course of the
solar cycle. At times, however, these
current systems can be greatly enhanced
and exhibit rapid changes with time and
space, a phenomenon typically referred
to as a geomagnetic storm. Geomagnetic
storms in turn are caused by
disturbances that propagate away from
the Sun, travel through interplanetary
space and interact with Earth’s space
environment.
We might expect that the early inventers of the telegraph systems did not realize that the electrical circuit they were constructing was threaded by lines of naturally produced magnetic flux, and even more surprising that this flux could vary with time and induce a natural current in their system. However, it was not too long after their deployment that reports of anomalous currents were observed which could at times prohibit communication or could enable a system to be run without an electrical power source, or in more dramatic instances cause the recording paper to catch fire (see review by Boteler 2003 and references therein). Similar effects continued to be noticed from time to time with the next generation of communication lines (coaxial cables).
Another system of artificial electrical circuits began to grow with the advent of electrical power systems. Just like the telegraphs, this complicated collection of circuits is threaded by naturally produced magnetic flux and just like the telegraphs, rapid variations of this magnetic flux during geomagnetic storms causes an unexpected, naturally produced current to flow through the system. This effect was first reported after the 24 March 1940 geomagnetic storm (Davidson, 1940; see also Germaine, 1940 for reports of effects on long-line communication cables). Numerous large and moderate impacts to the grid have been reported over the years including a power blackout in 1958 (Lanzerotti & Gregori, 1986), equipment tripping and voltage stability issues (4 August 1972), a nine-hour blackout in Canada and a transformer loss (13 March 1989 - see photo), a blackout in Sweden during the October 2003 storm. (See Boteler, 2001, for a comprehensive compilation of effects).
Assessing the impact of geomagnetic storms on the electrical power grid involves a number of considerations. The path for current flow that responds to the varying external currents in the upper atmosphere follows artificial current paths on the ground (the power lines) as well as various natural current paths (e.g. ground conducting structure below the surface and in nearby bodies of water). Once the natural current paths are adequately accounted for the net geoelectric field that is imposed on the artificial current paths results in a quasi D.C. (periods of 10 seconds to 10’s of minutes) current in the power lines. These geomagnetically induced currents cause the ‘exciting current’ in power transformers to operate out of their designed range, resulting in saturation of the magnetic core material inside the transformer. Once the core saturates, the transformer no longer provides any back ‘electromotive force’ (a kind of electrical inertia) and the currents and voltages in the windings become abnormally large. Depending on the transformer design, this can lead in some cases to heating of the surrounding structures due to induced ‘Eddy Currents’ which has the potential to damage parts of the transformer. An additional impact of transformer saturation is that the voltages and currents no longer have a simple sinusoidal (60 cycle) form and this can cause protective equipment elsewhere in the grid to operate when it shouldn’t. These equipment ‘trips’ can take needed equipment off line and cause voltage stability problems. An additional issue for the system is that all of the transformers that are saturating show up as a significant inductive load on the grid as a whole. This means that a system that is near peak levels of demand prior to the geomagnetic storm event may not be able to meet the total power demand when the geomagnetic storm occurs, leading to partial or system wide blackouts.
References
Boteler, D.H., Geomagnetic Hazards to
Conducting Networks, Natural Hazards,
28: 537-561, 2003
Boteler, D.H, Geomagnetic Hazards, GSC
Bulletin 548, 2001
Davidson, W.F., The magnetic storm of
March24, 1940 – effects in the power
system, Edison Electric Institute
Bulletin, 1940
Germaine, L.W., The magnetic storm of
March 24, 1940 – effects in the
communication system, Edison Electric
Institute Bulletin, 1940
Lanzerotti, L.J. and G.P. Gregori,
Telluric currents: the natural
environment and interactions with
man-made systems; in The Earth’s
Electrical Environment, (ed) R. Roble
and E.P. Krider; National Academy Press,
Washington D.C., pp 232-257, 1986