This recent USA Today article claims our grid is attacked every three to four days: https://www.usatoday.com/story/news/2015/03/24/power-grid-physical-and-cybe

This recent USA Today article claims our grid is attacked every three to four days: https://www.usatoday.com/story/news/2015/03/24/power-grid-physical-and-cyber-attacks-concern-security-experts/24892471/ https://www.usatoday.com/story/news/2015/03/24/power-grid-ph... As mentioned in another comment, Ted Koppel's book "Lights Out" is a really fascinating read on the threat.