GMD – Geomagnetic Disturbance

Many times each year, the sun ejects a portion of its corona into space. If the highly energetic, electrically, and magnetically charged matter characteristic of these Coronal Mass Ejections (CMEs) encounters the Earth it distorts the Earth’s geomagnetic field. When the trajectory of a substantial CME happens to intersect the earth it can induce a Severe Space Weather event, creating damaging Geomagnetically Induced Currents (GICs) in long transmission lines and transformers, the “ligaments” of a national-scale power grid.

Severe space weather-induced Geomagnetic Disturbances (GMDs) can last anywhere from minutes to days. Depending on the magnitude and duration of the GIC and the design and condition of transformers, especially the critical Extra High Voltage (EHV) transformers, voltage instabilities and transformer damage or degradation can occur. Extra High Voltage (EHV) transformers can be especially vulnerable to damage or degradation from such unplanned, transient currents.

In modern times, satellite-based sensors record powerful Coronal Mass Ejections relatively frequently, usually on trajectories that do not impact the earth in its orbit. Historically, the frequency at which powerful CMEs have affected the earth is estimated as once per 100 – 200 years. The largest CME event in relatively recent history occurred in 1859 and is known as the “Carrington Event.” It was followed 62 years later by another event of similar magnitude, the 1921 “Railroad Storm.” Although both events caused serious damage to the global telegraph network and related systems that existed at those times, the impact of similar storms on modern power grids could be far more serious.

A similar Coronal Mass Ejection occurred quite recently, in July 2012.  According to NASA researchers, if the eruption had occurred one week earlier, a massive cloud of magnetized plasma would have intersected the earth in its orbit. NASA scientist Daniel Baker said that the resulting geomagnetic storm would have been at least as powerful as the Carrington event.