What is the polarization index of a single phase dry type transformer winding that shows insulation resistance values of 1200 megohm after one minute and 2200 megohm after ten minutes?

To determine the polarization index, one can utilize the insulation resistance values measured at different times. The polarization index (PI) is calculated by taking the insulation resistance value at ten minutes and dividing it by the insulation resistance value at one minute.

In this scenario, the insulation resistance after one minute is 1200 megohms, and after ten minutes, it is 2200 megohms. When you calculate the polarization index using these values:

PI = Insulation Resistance at 10 minutes / Insulation Resistance at 1 minute

PI = 2200 megohms / 1200 megohms

When you perform this division, you find that the result is approximately 1.83. This value indicates the health and effectiveness of the insulation in the transformer winding.

A polarization index value of around 1.83 suggests that the insulation is in good condition, as a higher PI indicates better insulation. Generally speaking, a PI of 1.0 to 2.0 is considered acceptable for well-functioning insulation systems in transformers.

Understanding the polarization index is crucial for evaluating the insulation quality of transformer windings and ensuring they are safe and effective for operational use.