If you give 50 engineers the task to come up with ideas for breaking a countries infrastructure on a budget I'm pretty sure 45 of them will go for the electricity plants and will look for the most vulnerable place to cause a dead short in a high voltage circuit. There aren't that many of them, they're our Achilles heel. I'm not at all surprised that the military has taken this idea into their arsenal.
I remember reading somewhere that in Total War scenarios you should poison the water source if you are not able to capture and control it. I think it was Art of War by Sun Tzu.
Basically the idea is to short the two sides of the electrical wires so that you end up breaking the transformers. This means that lots of things at the substation will have to be replaced before it can be put back into action. A few well placed attacks like that and you can easily take out an entire city's power or more.
transformers, generators, motors and power lines have protection relays which sense conditions such as short circuits and isolate the short circuited part of the ... circuit before equipment can be damaged. see http://www.selinc.com
As you say this would cause a power outage, but not necessarily break any equipment as the protection is quite robust and moreso the more critical the facility.
The US military got this: http://www.fas.org/man/dod-101/sys/dumb/blu-114.htm. Carbon fiber instead of the chain, but same principle.