As I understand it... normally, There's a direct connection to ground. (no resistors)

The storm induces current along the length of the overhead wires. Click on the diagram

This complete loop of wires and ground, would be "mitigated", with resistors as shown.

Geo-magnetic storms can shut down the power grid.

"But to just shut the grid off and restart it is a $10 billion proposition"

http://www.wired.com/wiredscience/2009/04/2012storms/