POWER GRID DAMAGE AND RESTORATION CONCERNS TUTORIALS

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.

Under these scenarios, the scale and speed of problems that could occur on exposed power grids has the potential to cause widespread and severe disruption of bulk power system operations. Therefore, as storm environments reach higher intensity levels, it becomes more likely that these events will precipitate widespread blackouts to exposed power grid infrastructures.

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