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This paper provides details of a computational technique used to calculate currents induced by geomagnetic storms on transmission lines and in transformers of a large power grid. In practice, this technique has been driven by three sets of input data - magnetometer data on an irregular grid, magnetic fields from storm models, and predictions of magnetic fields from satellite data. The technique employs a ground conductivity that varies in both the horizontal and vertical directions and a computationally efficient technique for performing the convolutions needed to obtain the horizontal electric field from the horizontal magnetic field. A quasistatic technique is used for circuit modeling of lines and transformers, and various transformer and core types may be included for modeling of the current flow and of the generation of reactive power demand. This technique has been implemented in the POWERCAST™ code.