Sunspots, solar flares, or other phenomena can produce large clouds of plasma and magnetic fields that erupt from the corona of the sun. These are called coronal mass ejections (CMEs). The shock wave that precedes a CME accelerates solar energetic particles (SEPs), which are high-energy particles consisting of electrons and ions. During low-intensity CMEs, these particles are mostly deflected by the geomagnetic field of the earth. However, when the CME is sufficiently large the magnetic field associated with SEPs perturbs the otherwise constant geomagnetic field around the earth resulting in a geomagnetic disturbance (GMD).
A GMD is of concern to electrical grid operators because the changing geomagnetic field induces a geoelectric field that drives stray currents in transmission lines, their connections to earth, and the earth itself acting as a conductor. This current is referred to as geomagnetically induced current or GIC. GICs can cause:
- Damage to bulk power system components such as transformers, potentially requiring replacement in large numbers
- Harmonic currents that can cause system equipment to trip, potentially causing a system disturbance
- Loss of reactive power support that can lead to voltage instability and power system collapse
While system restoration due to voltage instability may take hours to a few days, replacing a large number of transformers could take months given the lead time to acquire and install replacements. An example of the potential impact is the March 1989 event when a large GMD led to the collapse of the Hydro-Quebec power system. The GMD damaged transformers and caused harmonic currents that resulted in seven static var compensators (devices that deliver reactive power to the grid) disconnecting. The loss of the compensators in turn resulted in a reactive power shortage leading to voltage collapse and a system-wide blackout. More than 6 million customers were without power for up to nine hours.
Balancing authorities typically perform a documented evaluation (called a GMD vulnerability assessment) of potential susceptibility of their system to voltage collapse, cascading, or localized damage of equipment due to GMDs. This is necessary because the potential impacts associated with a severe GMD are not typically included in planning studies performed by balancing authorities, so a separate study is needed to evaluate risk and develop a corrective action plan to mitigate the potential threats.