Photo by Egor Kamelev from Pexels.

Transmission lines sag. The magnitude of the sag depends on several factors like the distance between the towers or poles, the weight of the conductor and temperature, both the ambient temperature and the heat of the line, which is the function of the load on the line. Sags should not be a problem, since there are defined formulas to calculate the sags as well as rules on required clearance to ground or vegetation. Nevertheless, losing transmission lines due to short circuits caused by tree-to-(power) line contact can be a major reason why blackouts cascade. If the system is already stressed, losing a line results in overloads on remaining lines. The overloaded lines heat up and sag more, maybe beyond emergency ratings or tree trimming has not been kept up, increasing the risk for tree-to-line contact and more lines will be lost. It happens fast, like a cascade, as was the case in the 1996 Western North America blackout, the 2003 Northeast blackout, the 2003 Italy blackout, and many other blackouts.


“Resilience and Stability of Ecological Systems” was a paper by the Canadian ecologist C.J. Hollings published in the Annual Review of Ecology and Systematics 1973. The concept of resilience was used to describe the persistence of natural systems in face of changes in the ecosystem. The paper has had a major impact within ecology and the concept of resilience has later been expanded to many other areas, including the electric grid.

The 2003 Northeast blackout and extreme weather events like hurricane Katrina (2005), superstorm Sandy (2012), polar vortex (2014), hurricanes Harvey, Irma and Maria (2017), as well as awareness of new risks, such as physical and cyber-attacks, etc., have contributed to the increased attention to the resilience of the electric grid and ways to strengthen it.