A high altitude electromagnetic pulse (HEMP) or solar-geomagnetic disturbance (GMD) can introduce geomagnetically-induced currents (GICs) flowing through grounded power transformers that have the potential to severely impact the operation of large-scale electric power grids. A top-down mitigation design strategy considers grid-wide dynamic behavior during an HEMP/GMD event and uses optimal control theory to determine the compensation signals required to protect critical grid assets. As examples, the approach is applied to both a standalone transformer system, small power 3- and 4-bus systems, and large-power 20-bus system.