You can find the procedure of how to measure the Zo for Dry type transformer in the IEEE C57.12.91-2001 standard.
If you hahe a transformer whit only une neutral point externaly:
Basically you need a AC monophasic source to excit the windings, cables and another hardware to connect and of course measurement equipment.
You have to short circuit the terminals of the widings that you want to measure and then you apply AC power between the short circuits terminals and the neutral terminal.
Take the value of the current and voltage fron the source and calculate Zo=3E/I.
The procedure is diferent for others types of transformers (if you have delta windings or if you have one or two neutrals externals terminals.
If you want i can send you a test procedure via e-mail but this document was made in spanish.
