Is there any possible low-frequency electrical resonance can be caused in a power grid?
Long story short:

Grid frequency is what is used by the power companies to adjust the energy production. A generator under heavy load turns slower, and you can counter that by applying more mechanical power to it.

This primary control scheme really isn't instantaneous and perfectly easy to apply. That means that within the European grid, you'll often see sudden load changes (or sudden production changes, e.g. from a nuclear plant suddenly dropping out of the net) "ripple" through the grid, get overcompensated, then oscillate back.

If you understand German or feel OK with an English synchro track also available there: there was a rather entertaining more than technically detailed talk on that topic at 32c3. The idea was that it's not that hard (not that easy, either) to fabricate cases where elegantly placed sudden changes to the grid constructively overlay their effects in a manner that makes power grids fail. And what I took away from it: an emergency shutdown of a large power plant is less bad than what happens every 15 minutes synchronously due to the fact that electrical energy production of brokered in quarter hour packets