PI of transformers

What is P.I?

P.I is for the Polarization Index. This index is used when the insulation is measured.

It is a ratio which is defined by

P.I = (Insulation value measured at 10 minutes) / (Insulation value measured at 1 minute)

Why is it important?

Moisture may be absorbed within the insulation and condensed on the dirty connection surfaces. For very moist and dirty windings, the relatively constant surface leakage component of the current will predominate over the time varying components, so that the total current will rapidly reach a near steady-state. Thus, to help determine how dry and clean the insulation is, the Insulation is usually measured after 1 minute and after 10 minutes. A high value of P.I indicates the good condition of the insulation system.

What should be the value of P.I?

This value should be more than 1.5.

-MS