What would most likely happen if the power lines in a petroleum power plant were broken?

What can happen is that the people will be out of electricity. If the heavy power that depends on the transmission lines break and leaves the plant broken the it is a matter of fixing the transmission lines. As you can see those are the most imprtant lines. If some important things like the main switch gear gest lost then you can lose ventilation, and fuel fumes can accumulate and ignite.