The internet is the fastest and cheapest way to transfer data to pretty much anywhere so power companies use it to monitor and control some systems. Many substations and very small plants are actually remote controlled. It is very secure but nothing is 100% foolproof.
Even if they ran their own cables completely separate from the internet, the cables could still be cut at some point and intercepted. There is just no 100% secure way to transfer data in real time that fast.
“It is very secure”
No such thing if billions of people have access to the network.