The volts per kilometer produced by natural events is proportional to the area of the loop passing through the earths magnetic field. you need on the order of 100,000 square meters of area to get a dozen volts.
The problem for the grid is the current produced in the line is enough to cause a few hundred amps of dc current to flow, saturating the transformer core which then draws excessive ac amps from the line. The only reason the current can flow is because the neutral grounded at both ends. all they have to do is lift the ground at one end, put a capacitor in series. There could still be net dc show up from one phase to another, but usually the phases are alternated with each other so each has the same inductance to ground, so it would be very minimal dc if any significant current.
a congressionally funded study iirc estimated it would cost 50,000 dollars for each of the 1000 largest substations to open the loop with a capacitor, resistor and spark gap, why the utilities haven't installed this equipment themselves, i don't know. they have known about this problem for nearly 100 years