Just think, for a few hundred dollars of balloons, you could disrupt a few major cities for a while!

Then there is the expensive military method:
https://cyberarms.wordpress.com/2010/04/28/the-weapon-that-disabled-iraqs-power-grid/

 

What may not be obvious is that flame is a fair conductor of electricity. I learned that back in fifth grade. Smoke is not nearly such a good conductor, but soot deposits on insulation tends to lead to flash-over incidents. This was especially a problem on larger screen TV sets in smoky spots bars, back in the era of tube-type television receivers.