there is no way that you could cut off the power supply. well, not easily anyway. there is a certain room at the bottom of the damn that is guarded by
armed men. not to mention, if you did kill the guards, and you did cut the power, they have backup generators that would kick on. and they would have
a crew down there ASAP to fix the problem. so it wouldnt really matter if you cut there power supply.
Considering their threats would be either a terrorist or actual military, chances are none of that would stand up to a MISSILE smacking the
dam...
However, of course there are backup generators to ANY facility like this, even hospitals...

There's also a good chance that any subterranean
facilities are likely using geothermal energy (so good luck killing that, hehe...)