Modern weapons of war include those specifically to knock out power grids for both short and long periods of time. In 1999 the US knocked out Serbia’s national grid with such a weapon:
Source: CNN at http://edition.cnn.com/US/9905/03/secret.weapon/
“”May 3, 1999
Web posted at: 11:09 p.m. EDT (0309 GMT)
WASHINGTON (CNN) — U.S. Air Force F-117 stealth fighters dropped special bombs containing carbon fibers designed to short out electrical transformers, blacking out much of Serbia Sunday night, Pentagon sources said.
The Pentagon is refusing to officially answer questions about the munitions, which are classified, but sources said similar weapons were used against power grids in Iraq during the 1991 Persian Gulf War.
“I have nothing to say about it,” said Pentagon spokesman Ken Bacon.
NATO spokesman Jamie Shea said Monday morning that aircraft Sunday evening struck the five main electric yards that distribute power to the Serb armed forces.
Pentagon sources said the special bombs explode over targets, then shower the electric transformers and lines with tiny carbon fibers, shorting out the systems.
The bombs can effectively shut down power for hours or days, depending on their use, the sources said.
NATO officials said the intention in Sunday night’s attack was to cut power that supplies “the military machine of President Milosevic,” not to inconvenience the people of Serbia.
The use of the secret weapons was also designed to limit damage to power plants which take a lot of time and money to rebuild, Pentagon officials said.
NATO aircraft on Sunday night struck the transformer yards of Opranovac station in western Serbia, at Nis in southern Serbia, and in Bajinabast, Dermo and Novi Sad.
The alliance began its bombing campaign on March 24, after Yugoslavia rejected an international proposal to end a civil conflict between Serbs and ethnic Albanians in the Serbian province of Kosovo.” end quote. source CNN
It goes without saying that conventional weapons can cause great damage to the gird.