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Polarization index is measured to check__________
Vibration level
Oil quality
Moisture in insulation
All of above
Moisture in insulation
The Polarization Index (PI) is a diagnostic test performed on electrical insulation (typically for transformers, motors, or generators) to evaluate the presence of moisture, contamination, or degradation. It is defined as the ratio of the insulation resistance measured at 10 minutes to the insulation resistance measured at 1 minute during a constant DC voltage application.
The Polarization Index (PI) is a diagnostic test performed on electrical insulation (typically for transformers, motors, or generators) to evaluate the presence of moisture, contamination, or degradation. It is defined as the ratio of the insulation resistance measured at 10 minutes to the insulation resistance measured at 1 minute during a constant DC voltage application.
PI=R1minтАЛR10minтАЛтАЛ тАФ Calculation formula for Polarization Index
When a DC voltage is applied to insulation, the total measured current consists of capacitive, polarization (absorption), and leakage currents. As time progresses, capacitive and absorption currents decay toward zero, leaving only the steady-state leakage current. In healthy, dry insulation, the resistance increases over time due to the absorption effect, resulting in a PI value typically greater than 2.0; if moisture is present, the leakage current remains high, causing the resistance to remain relatively constant, which lowers the PI value toward 1.0.
PI is a non-destructive testing method used for large electrical machines.
A PI value less than 1.0 typically indicates defective or wet insulation.
The test is highly sensitive to temperature and contamination, not just moisture.
IEEE standards generally suggest a minimum PI of 2.0 for Class B insulation.
Provides a clear trend of insulation health over time.
Less influenced by minor surface dirt compared to spot insulation resistance tests.
Requires significant time (10 minutes per test).
Results can be significantly affected by ambient temperature variations.
Preventive maintenance of large power transformers.
Condition monitoring of high-voltage rotating machine windings.
Standard PI values for healthy equipment are usually between 2.0 and 4.0.
Option B (Oil quality) is tested using Dielectric Breakdown Voltage (BDV) or Dissipation Factor (tan delta) tests.
Option A (Vibration level) is monitored via mechanical analysis sensors, not electrical insulation tests.
C is correct тАФ The Polarization Index is a standard diagnostic ratio used to identify moisture and contamination in electrical insulation systems.
Always remember that PI is a ratio and therefore unitless, making it a reliable comparative index regardless of the specific size of the machine being tested.