It's not 100% clear yet and there might be other factors (eg gas leakages or harmonic-induced damages to relays), but it seems the main effect is rapid internal

It's not 100% clear yet and there might be other factors (eg gas leakages or harmonic-induced damages to relays), but it seems the main effect is rapid internal heating caused by stray magnetic flux leakages from the saturated core [1,2]. Hot spots beyond a certain threshold end up severely damaging internal components, rendering the transformer inoperable --or at best sharply reducing its operating lifetime [3]. Repairs in these cases pretty much entail retrofitting the whole damn thing, so the months-long disruption is all the same either way. [1] http://www.ferc.gov/industries/electric/indus-act/reliability/cybersecurity/ferc_meta-r-319.pdf http://www.ferc.gov/industries/electric/indus-act/reliabilit... [2] http://fas.org/irp/agency/dod/jason/spaceweather.pdf http://fas.org/irp/agency/dod/jason/spaceweather.pdf [3] http://www.lloyds.com/~/media/lloyds/reports/emerging%20risk%20reports/solar%20storm%20risk%20to%20the%20north%20american%20electric%20grid.pdf http://www.lloyds.com/~/media/lloyds/reports/emerging%20risk...