Question
How can I compute the grid impedance in HIFREQ?

The HIFREQ program does not report directly the impedance of the modelled structure. You can obtain this value by dividing the potential of the energized segment by the current injected in it.

The injected current is the value you specified on input. It is listed in the standard report generated when clicking Report in the main screen of Output Toolbox for HIFREQ.

The potential of the energized segment can be obtained by plotting the "GPR of Conductor Metal" (under Data-Type) in the Configuration part of Output Toolbox for HIFREQ. This will plot the conductor network and label every conductor segment with the value of the potential of that segment. Another approach to obtain the potential of the energized segment is to look in the program's output file (hi_JobID.f09) and search for the string "GPR". The second time you find this string will be at the beginning of a table listing the potential for all segments. In this case, you will have to know the segment number of the energized conductor segment. This you can obtain from Input Toolbox: the segment number is simply the line number of the grid line (in the Main-Grid Conductors data grid in the 'System' screen) containing the conductor information for the energized segment.