The onset of important power system problems can be assessed
in part by experience from contemporary geomagnetic storms. At geomagnetic
field disturbance levels as low as 60–100 nT=min (a measure of the rate of
change in the magnetic field flux density over the Earth’s surface), power
system operators have noted system upset events such as relay misoperation, the
offline tripping of key assets, and even high levels of transformer internal
heating due to stray flux in the transformer from GIC-caused half-cycle
saturation of the transformer magnetic core.
Reports of equipment damage have also included large
electric generators and capacitor banks. Power networks are operated using what
is termed as ‘‘N– 1’’ operation criterion. That is, the system must always be
operated to withstand the next credible disturbance contingency without causing
a cascading collapse of the system as a whole.
This criterion normally works very well for the
well-understood terrestrial environment challenges, which usually propagate
more slowly and are more geographically confined. When a routine
weather-related single-point failure occurs, the system needs to be rapidly
adjusted (requirements typically allow a 10–30 min response time after the
first incident) and positioned to survive the next possible contingency.
Geomagnetic field disturbances during a severe storm can
have a sudden onset and cover large geographic regions. Geomagnetic field
disturbances can therefore cause near-simultaneous, correlated, multipoint
failures in power system infrastructures, allowing little or no time for
meaningful human interventions that are intended within the framework of the N–
1 criterion.
This is the situation that triggered the collapse of the
Hydro Quebec power grid on March 13, 1989, when their system went from normal
conditions to a situation where they sustained seven contingencies (i.e., N– 7)
in an elapsed time of 57 s; the province-wide blackout rapidly followed with a
total elapsed time of 92 s from normal conditions to a complete collapse of the
grid.
For perspective, this occurred at a disturbance intensity of
approximately 480 nT=min over the region. A recent examination by Metatech of
historically large disturbance intensities indicated that disturbance levels
greater than 2000 nT=min have been observed even in contemporary storms on at
least three occasions over the last 30 years at geomagnetic latitudes of
concern for the North American power grid infrastructure and most other similar
world locations: August 1972, July 1982, and March 1989.
Anecdotal information from older storms suggests that
disturbance levels may have reached nearly 5000 nT=min, a level #10 times
greater than the environment which triggered the Hydro Quebec collapse
(Kappenman, 2005). Both observations and simulations indicate that as the
intensity of the disturbance increases, the relative levels of GICs and related
power system impacts will also proportionately increase.
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