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What results are you trying to obtain from running a powerflow with the induced DC voltages added?

In powerflow we have 4 types of busses: PQ (Load), PV (generator), and V/angle (slack). Most busses are load busses where the active and reactive power are specified and we solve for the voltage and angle.

As a result you cannot set the voltage at these busses in powerflow, it is solved for. You can change the nominal voltage of a bus but this will only affect how parameters are converted to per-unit, not the voltage of the bus directly. PSSE is phasor-domain fundamental frequency model so if you're trying to apply a DC voltage offset to your powerflow model this is not the software to use.

In GIC simulations a DC network is created using only DC resistances of equipment. Using faradays law, the electric-field magnitude and angle, and the orientation of the lines the DC voltage induced in each line is calculated. These DC voltages are added to the DC resistive network and the currents (GIC) in each branch and the DC voltages at each node are solved for using KVL/KCL. No powerflow data (besides topology) is used for this.

In the GIC studies I have done we use the DC currents found in the transformers to calculate the reactive power loss in transformers (from half-cycle saturation) using linear k-factors (specified in the GIC data file). These Mvar losses are then added to the powerflow case to study voltage stability.

If you want to study the effects of GIC DC voltage offset for something like insulation coordination or protection and control settings you need to use an EMTP software I think.

In my experience these DC voltage offsets will generally be much smaller than the nominal AC voltage of the equipment (by something like 1000 times).