Is PI test is applicable for a Dry type transformer. Since there is no cellulose insulation in the system I think PI is not applicable for either a VPI or Cast resin transformer. All the solid insulation used in these transformers have solid insulations which are negligabily polarised while using a megger. Also what should be the megger voltage when measuring IR of these transformers having a primary of 6 or 11KV. LV is 420 volts 3ph 4 wire with neutral grounded.
Please refer to any standard where this issue is discussed.
The Polarization Index indicates the quantity of contamination in the windings. The IEEE Standard 43-
2000. describes the procedure for IR and PI tests. The Polarization index measurement can be conducted in VPI windings. The dirt which has penetrated the winding is what gets polarized.
For windings with 6.6 kV voltage a 5 kV megger (or more correctly an insulation tester) should be used. While 11 kV machines should be tested with a 5 kV insulation tester