You can do the test with low voltage, you need a three-phase low voltage source to aplly excitation voltage for possitive sequence test and one-phase for cero sequense test.
You can find the procedure for this test in the IEEE Std. C57.12.90-1999.
You can read the IEEE Std C37.110-1996: IEEE Guide for the aplycation of current transformer used for protective relaying purposes.
Chapter 4.5 : Dynamic characteristic.
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...
The 7UT513 relay have a tool for comisiioning, if you are sure about the transformer is OK, try this.
1. Change the setting 1601 to "Trip bloked".
2. Put the transformer on load.
3. Whit the transformer in service, verify Idiff and Ibias.
Many problems in this relay in my company...