Geomagnetic storms damage power systems through a specific mechanism involving very slow currents. The vulnerability depends on geology and grid layout as much as on storm strength.
A changing magnetic field drives current through the ground
When the magnetic field around Earth is disturbed, that change induces electric fields in conducting material beneath the surface.
The ground and the power grid together form a circuit, since transmission lines connect distant points and are grounded at substations along the way.
Current flows along the line and returns through the earth, producing what engineers call geomagnetically induced current in a network designed for something else.
Transformers are the vulnerable component
Large transformers are built for alternating current, and their magnetic cores are sized for the way that current reverses many times each second.
The induced current is effectively direct, and it biases the core so that it saturates during part of each cycle rather than operating in its intended range.
A saturated core draws distorted current and dissipates heat in places not designed to shed it, which is the pathway to internal damage.
Line length and geology set the exposure
A longer transmission line spans a greater difference in induced electric field, so more voltage is available to drive current through it.
Ground conductivity matters just as much. Where bedrock resists current, more of it prefers the grid, concentrating flow into the transmission network above.
This is why exposure is uneven across the country and why utilities in some regions carry meaningfully higher risk than storm intensity alone would suggest.
Effects propagate beyond the affected transformer
Distorted current draws additional reactive power from the system, which can depress voltage across a wide area rather than at one substation.
Protective relays may interpret the distortion as a fault and disconnect equipment, removing capacity precisely when the system is already stressed.
The cascading potential comes from these secondary effects, since a single overheating transformer would otherwise be a contained maintenance problem.
Operators have procedures rather than a switch
Advance warning allows utilities to reduce loading on vulnerable paths, postpone maintenance that would take redundant capacity offline, and stage crews.
Monitoring devices installed at substations measure the current directly, giving operators evidence rather than inference during an event.
Blocking devices exist that prevent direct current from entering transformer neutrals, though installation is expensive and targeted at the most exposed equipment.
Vulnerability assessments now model the grid and the underlying geology together, so the engineering response is aimed at the specific transformers a storm would actually stress.