Re: Attack which layer?
If you are really determined, hack the power company for distribution layout drawings,which are constantly changing and therefore widely distributed within the engineering arm after the organisation, and then coordinate physical attacks on the infrastructure exploiting redundancy info gleaned from the distribution drawings.
It doesn't take much to take out a transformer, you could do it noisily ( explosive) or quietly ( smash a low drain valve off with a sledge) massive loss of cooling oil would knock the unit out. The vast majority of distribution sites are unmanned so it's quite feasible to be in and out before the organisation can react.
The organisations involved are fully aware of this weakness which is far harder to defend against than locking up your control network.