“The electrical power grid that powers Northern America is not a single grid, but is instead divided into multiple wide area synchronous grids.”
“In 2009, The Wall Street Journal reported that spies from China and Russia, as well as from other countries, ‘penetrated the U.S. electrical grid and left behind software programs that could be used to disrupt the system. According to U.S. officials, the spies did not seek to damage the power grid; however they might be able to do so during a war or crisis. Most of the intrusions were detected by American intelligence agencies.”
https://en.wikipedia.org/wiki/North_American_power_transmission_grid
“Most”