Generator PI value problem

[edison123]

I have an 11 KV, 1 MVA VPI insulated generator stator which was thoroughly overhauled and dried out. While the PI (10 min / 1 min) value of individual phases (with other two phases grounded) is more than 2, the PI value for three phases together with respect to ground is only 1.1. Can anyone explain how this could happen ? ( All values were taken at the same temperature and with the same megger at the same voltage).

 

[edison123]

kantor,

Your posts are appreciated and I am sorry if my reply gave any other impression. I was just trying to clarify that PI is normally defined at 10 min IR/ 1 min IR.

In my case, the PI for individual phases is different from three phases together, which contradcits the above theory.

The idea of PI is to discard the capactive current and take the resistive and absorption currents, which are affected by moisture. So a one minute to ten minute ratio seems to make sense.