PI Value of 6.6 KV 450 KW Motor

I can't tell just where you are measuring in each case, but 2.4/1.4 ~~ sqrt 3.

Each phase of cable star point removed and as one phase cable as single measured along with windings of that phase is measured PI Value is 2.4 and when all cable of each phase star point joined together and measured as 1.4 PI value with one cable phase as the PI value of motor normally measured as complete phase as star or delta windings connected.What is the reason for the above

A lack of understanding of how the PI values are calculated. When you place your test leads on 1 winding, only the leakage current between that winding and the surrounding metal is measured. When you connect the neutrals together you now have increased the surface area of insulation in contact with the metal; more surface area equals more distributed impedance in parallel equals a lower PI value. However there's more to calculating the PI than just Ohm's law, you also have to account for the DAR (Dielectric Absorption Ratio), temperature, humidity, time, etc.

Please consult your copy of the latest revision to IEEE 43-2000, this article may help, also check the manual that came with your tester for more insights.

http://electromotores.com/PDF/InfoT%C3%A9cnica/EASA/Using%20Polarization%20Index.pdf

J; Here is the path to a very informative whitepaper on PI testing. As I do not know anything about what the testing methods used were nor what prior testing was performed, it would be best if you review this documentation then react accordingly.