Changing magnetic field creates current.
The change is tiny, on the order of 100s of nanoteslas.
But it is on a planetary scale - [ame="http://en.wikipedia.org/wiki/Geomagnetically_induced_current"]Geomagnetically induced current - Wikipedia, the free encyclopedia[/ame]
The current is induced in conductors on the planetary surface, proportional to the size of one.
At this scale, for things like power wires we are talking several volts per km.
But power lines run for 100s of km.
As a result, there is a quasi-DC current in the power grid during the field-change events.
This current can saturate or bias power transformers, designed to work at pure AC.
Saturation means loss of core magnetisation, bias means a drop in efficiency.
(Imagine an eardrum. AC means pressure going back and forth, the sound you hear, DC means constant pressure on one side. With no DC you can hear AC clearly, with some DC, like in an airplane, dive or other rapid change of pressure, eardrum flexes one side (bias) and you can hear worse, with sharp change like a bomb going off the eardrum pops (saturation) and you cease to hear anything. Similar thing with transformers and current.)
When we are talking about a multi-megawatt power station transformer, that means fire and molten metal, as it ceases to transform and starts to dissipate instead.
Lead time on replacement transformers of such scale can be years, and that with a working power grid.
Therefore, the doomsday scenario.
On the good side, such currents arise from natural changes in magnetic field, on smaller scale, so the power companies are aware of them, and have to take them into account.
So, getting prepared for the big CME is not technologically challenging, but only takes a certain amount of money and engineering to fit countermeasures of necessary scale.
ADHD version: In a solar storm large scale infrastructure burns, small scale things like phones, cars, etc don't notice anything unless plugged into the grid.