I read yet another article about this issue in the paper today which, like all the others, does not explain why a solar storm should damage the grid. After some searching, I found a rather technical slideshow that seems to explain at least some of it. From what I could understand of this, the problem is that the neutral line in a 3-phase system is grounded, and solar storms induce something called "geomagnetically induced currents" along the surface of the Earth. These cause what is effectively a DC current flow, up one grounding point and down another one, hundreds of km away - clearly not something the transformers and grid wires are designed for. What I did not understand is there was some further talk of excess induced magnetic fields, saturating transformer cores and thereby causing magnetisation of the bodyshells of transformers, creating heating from eddy currents and....damage. But if these currents are either DC or changing very slowly I don't understand where the eddy currents come from. Can anyone confirm or correct the above and explain about the eddy current issue?