A global power grid is a utility network which distributes power throughout a planet.
A conspiracy led by Admiral Leyton in 2372 involved sending Red Squad to sabotage Earth's global power grid with a computer virus. The planetary blackout that followed was blamed on Changeling infiltrators. Leyton had hoped that the public's fear of Changelings would allow Starfleet to take political control of Earth in a military coup d'etat. (DS9: "Homefront", "Paradise Lost")