Impedance is indeed given in Per Unit, % of nominal nameplate ratings.
Transformers for our internal distribution were a few percent, 2 to 5 -ish.
Our main transformer (the one connecting generator to grid) was 17% .
The impedance will be mostly inductive so as to not waste kilowatts as heat.
Electrical system designers spend a lot of time optimizing voltage drops so everything stays within ~10% over operating range of plant.
The main transformer connecting generator to grid was 894MVA so you can see what 0.1% loss would be in kw - almost a megawatt . It's worth making the big ones highly efficient.
Sorry i just dont remember the exact numbers but the guidelines in that reference should give you a feel.