What is a Geomagnetic Disturbance (GMD)? A GMD is a disruption of the earth’s magnetic field caused by sunspots. Sunspots are places on the sun where a magnetic dipole field pokes through the sun’s ambient magnetic field (the sun has a magnetic field just like the earth, but about 100 times stronger). These magnetic dipole fields, or intense magnetic loops, which are hundreds of times stronger than the ambient magnetic field, cause visible effects known as “sun spots or solar flares.” They also cause a temporary change in the sun’s magnetic field, which affects our earth’s magnetic field, at least temporarily.
This temporary change in the earth’s magnetic field induces a direct current (DC) or ground induced current (GIC) in long transmission lines. This GIC has caused several electrical transmission system problems including transformer saturation, current harmonics, increased reactive power demand, and distorted AC currents. These GIC effects are greater at northern latitudes, but can affect any location depending on GMD direction.
The most famous GIC “storm,” which occurred in 1989, caused a major Canadian blackout, as well as problems along the Mississippi gulf coast. A GIC storm in 1921 disrupted New York City’s train service and was 10 times stronger than the 1989 event. A “super storm” that occurred in 1859 was twice as strong as the 1921 storm and the strongest GMD event ever recorded.
Which electric utilities should be concerned? The farther north, the greater the risk – most experts consider those below the 35th parallel to be safe. Utilities operating extremely long transmission lines at extra high voltages (500 kV and above) are most susceptible. The Federal Energy Regulatory Commission (FERC) has been directed by Congress to mandate preventive actions, so NERC (National Electric Reliability Corporation) has developed an advisory system, and along with FERC, developed prevention and mitigation standards.
Much of this material came from a presentation by Warren Whitson, Southern Company Transmission, presented at the Atlanta IEEE-IAS on April 16, 2012. His slides are available at http://ewh.ieee.org/r3/atlanta/ias/2011-2012_Presentations/Differences%20Between%20Bonding%20and%20Grounding.ppt.