What is the significance of Polarization Index (PI) and Dielectric Absorption Ratio (DAR) in assessing the health of generator insulation?

Polarization Index (PI) and Dielectric Absorption Ratio (DAR) are important indicators of the overall condition of a generator's insulation system, providing insights into its dryness and contamination levels. These tests are performed offline by measuring the insulation resistance at different time intervals after applying a DC voltage. The Polarization Index is the ratio of the insulation resistance measured after 10 minutes to the insulation resistance measured after 1 minute (PI = R10min / R1min). A high PI value (typically above 2) indicates that the insulation is relatively clean and dry. A low PI value suggests that the insulation is contaminated with moisture or other conductive materials. The Dielectric Absorption Ratio is the ratio of the insulation resistance measured after 1 minute to the insulation resistance measured after 30 seconds (DAR = R1min / R30sec). DAR provides a quicker assessment of insulation condition than PI. Like PI, a high DAR value (typically above 1.4) indicates good insulation condition, while a low value suggests contamination or moisture. Both PI and DAR are valuable tools for trending the condition of generator insulation over time. A significant decrease in PI or DAR values can indicate developing insulation problems and prompt further investigation or maintenance actions. These tests are particularly useful for detecting the presence of moisture, which can significantly reduce the dielectric strength of the insulation and lead to premature failure.