If you sink an oil tanker and a pollute large area for years or decades, that's terrorism because holy fuck that's scary. But what if you do it in a way that nobody knows it was intentional and don't tell anyone about it?
You're absolutely right: blowing up substations can be terrorism. But you need someone claiming the attack and making demands.
Damaging property if the motives and outcome align with what we consider terrorism is terrorism.
But the meaning of terrorism is rather vague, and broad.
If I blow up a substation and it causes blackouts and chaos, that can be terrorism.