P.I
P.I
POLARISATION INDEX (PI) MEASUREMENT
The PI measurement is a ratio metric test used to assess the performance of insulation. Moisture may be absorbed within the insulation and/or condensed on the connection surfaces, which is often dirty. 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 IR is usually measured after 1 minute and after 10 minutes. The PI is the ratio of ten minutes reading to one-minute reading. Since this being a ratio, it is independent of temperature and no temperature correction is required.
The value of PI generally shall be above 2. In general, a high value of PI indicates the good condition of the insulation system. If the PI values are less than normal value or shows large deviations from the previous measured values then immediate corrective actions need to be taken. Steps should be taken to dry the insulation and remove the contamination before proceeding with other tests.
In some cases the measured PI values may be misguiding. For example, if multi-layered insulation fails in one of the layers, while the others retain high resistance, the effect on the test current will tend to increase the PI value, masking the possible problems from surface leakage caused by dirt and contamination. Also, for older asphalted windings, a PI greater than six or seven may indicate that it is thermally degraded. If the IR value is more than 1000MW , the PI becomes an erratic indicator.