“Tension” and voltage are equivalent terms, with voltage more quantitative and tension more qualitative in connotation. Electricity as delivered by the utility to your house is not distributed at the same voltage level at which it will ultimately arrive. It goes out at levels of tens or hundreds of thousands of volts for cross-country distribution, which is stepped down to a few thousand volts for neighborhood distribution, then to a few hundred volts for buildings.
Certainly, good shielding practices can harden systems to flare-induced electrical surges. The shield gets the surge, but whatever is within the shield does not.
Wouldn't buried lines be protected from any kind of solar flare? All the lines in my neighborhood are buried.
No. High Tension lines are literally installed under high tension, mechanical strain, to reduce sag between supports.
They tend to increase together as the higher voltage insulator supports get more and more expensive as the voltage goes up. So the economics of longer spans with corresponding tensions tends to go up with voltage. But water, road and other physicals constraints also justify higher tension and longer span construction.