Geomagnetically Induced Currents (GICs) are often measured in amps rather than milliamps, and their values can vary significantly depending on the specifics of the geomagnetic storm and the electrical grid. During the 1989 geomagnetic storm that led to the collapse of the Hydro-Québec power system, GICs of hundreds of amperes were measured. While transformers are designed to handle thousands of amperes of alternating current, they are generally not designed to handle direct current like GICs, which can lead to transformer saturation and increased reactive power demand.
Now, let’s talk about resonances. The electric grid can have resonant frequencies due to the combination of inductive, capacitive, and resistive elements. Resonances can cause the system to magnify the effects of incoming disturbances, much like how a tuning fork resonates at its natural frequency. When a geomagnetic storm induces currents in the Earth, those currents generate a magnetic field that interacts with the magnetic fields of transformers and transmission lines. If the frequency of these geomagnetic disturbances happens to match or come close to a resonant frequency of the electrical grid, the amplitude of the induced currents can be significantly magnified.
To be specific, resonances can occur in multiple parts of the system:
1. Transformer windings have their own resonant frequencies, at which the impedance becomes high, leading to larger voltage across the windings for the same amount of GICs.
2. Long transmission lines can have characteristic impedances that interact with the impedances of transformers and other elements to create resonant circuits, thus amplifying the GICs in localized regions.
3. Harmonic resonances can occur when nonlinear elements like transformers generate harmonics that coincide with resonant frequencies in the grid, thereby magnifying the effective GICs.
So, even though individual transformers may be able to handle small DC currents without immediate failure, the presence of resonances and the cumulative effects across a large, interconnected grid make GICs a non-trivial concern.