I'll take a stab at it. I'm not an electrical engineer but I do have some experience with industrial control systems.
From Wikipedia: "Generally substations are unattended, relying on SCADA for remote supervision and control." These SCADA systems may or may not be connected to Internet, which could allow an attacker to remotely access and modify the code that controls transformers and other electrical equipment.
In another comment, someone mentioned the power company was, in this case, pumping C02 into the substation in order to contain smoldering electrical insulation. This means that, most likely, the copper conduit heat up beyond defined tolerances. This could be due to more current being carried than those conduits are rated for. Normally, the SCADA system would be responsible for keeping these currents within tolerances. What I am saying is that they could deliver a payload to the PLCs via the SCADA that could trick the transformers into taking more load than they could handle.
I believe the idea is, you make the monitoring values read too low (lower than the actual voltage in the wires), so the automatic control system adjusts by making the voltage higher. If you can get it to make the real voltage higher than the equipment can safely handle, you may get a fire.
From Wikipedia: "Generally substations are unattended, relying on SCADA for remote supervision and control." These SCADA systems may or may not be connected to Internet, which could allow an attacker to remotely access and modify the code that controls transformers and other electrical equipment.
In another comment, someone mentioned the power company was, in this case, pumping C02 into the substation in order to contain smoldering electrical insulation. This means that, most likely, the copper conduit heat up beyond defined tolerances. This could be due to more current being carried than those conduits are rated for. Normally, the SCADA system would be responsible for keeping these currents within tolerances. What I am saying is that they could deliver a payload to the PLCs via the SCADA that could trick the transformers into taking more load than they could handle.