IEEE Power & Energy Magazine - November/December 2016 - 62

ieee power & energy magazine

GIC

mittently in what are called, for
historic reasons, substorms. during substorms, charged particles
(electrons and protons) are ejected
from the magnetic tail toward the
earth and follow the magnetic
field lines into the high latitude
regions. here, they are deposited
into the upper atmosphere, causing
an aurora like the one that dazzled
montrealers in march 1989. in
Figure 4, (a) shows a photo of an
auroral display, (b) a special camera shot of a typical aurora, and (c)
(a)
(b)
a photo of an auroral display over
superdarn radar.
unfortunately, there is a "dark"
side to the colorful auroral displays. The particles that cause an
aurora are accompanied by large
amounts of lower-energy particles that do not contribute to the
light show but carry electric current. These currents in near-earth
space and the ionosphere extend
for thousands of kilometers and
E
E
E
E
~100 km
create the magnetic field variaB
B
B
B
tions detected at observatories on
the ground as a magnetic distur(c)
bance. a particularly intense current, the auroral electrojet, flows
figure 3. (a) Earth's normal magnetosphere, (b) Earth's substorm magnetosphere,
along the bottom of the aurora in
and (c) the electrojet current (green arrow) producing a magnetic field (B) and
a predominantly east-west direcinducing a geoelectric field (E) on power network. This generates a flow of geomagtion along the auroral oval region
netically induced current (GIC) in transmission lines. (Courtesy of Hydro-Québec.)
around the magnetic pole in which
auroras are most frequently seen.
The auroral oval (and hence the
auroral
electrojet)
is
normally
located at high geomagnetic
out "sideways" from the sun and missed the earth. however,
starting on 9 march, with the sunspot region now located latitudes (across hudson bay in north america), but during
closer to the center of the solar disk, a series of solar flares intense magnetic disturbances the auroral oval expands,
provided the first warnings that eruptions of plasma from the bringing the threat of the auroral electrojet further south.
sun's corona were headed toward earth. These coronal mass during the morning of 13 march 1989, this expansion
ejections (cmes), superimposed on the regular "solar wind" placed the auroral electrojet directly above the hydroflow of plasma from the sun, would have a considerable effect Québec power system.
The rapidly changing auroral electrojet generated magon the earth's magnetic field.
The solar wind compresses the earth's magnetic field on netic field variations, which in turn induced electric fields in
the side facing the sun (day side) and draws the magnetic field the power transmission lines that sent surges of electric curlines out into a comet-like tail on the night side, producing a rent through the hydro-Québec power system, as illustrated
cavity within the solar wind called the magnetosphere (see in Figure 3(c). These geomagnetically induced currents
Figure 3). The arrival of a cme causes a compression of the (Gics) vary on timescales from seconds to hours and appear
magnetosphere, and, as the cme sweeps by, the magnetic field to the power system as a varying dc when compared with the
carried along by the cme connects with the earth's magnetic normal 60-hz ac for which the power system is designed.
field, allowing an efficient transfer of energy. some of the Gics flowing to ground through power transformers prosolar wind energy is dissipated as it flows around our planet, duce an extra magnetic field in the core of the transformer
but some is stored in the magnetic "tail" and released inter- that interferes with transformer operation. The normally
november/december 2016

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