Resources and Tools

Saturday, October 22, 2011

Measurement of insulation resistance on a power transformer

Measurement of insulation resistance

These measurements aimed to determine the condition of insulation between the windings to ground or between two windings. A common method is to provide a dc voltage and represents the condition of the unit megohm insulation. Measured insulation resistance is a function of leakage current that penetrates through the insulation or through the leak in the external surface. Insulation resistance testing may be influenced temperature, humidity and leakage paths on the external surfaces such as dirt on the bushing or insulator. Megaohm meters typically have a capacity of testing 500, 1000 or 2500 V dc.
Mega Ohm meter gauge

Polarization Index Test

The purpose of the polarization index test is to ensure the equipment is operated or even feasible to do overvoltage test. Polarization index is the ratio of insulation resistance at the 10th minute by minute to 1 with a constant voltage.

Total current that arise when providing a dc voltage steady state consists of: 
1. Charging current due to the nature of the measured capacitance of the insulation. This flows down from the maximum value to zero very quickly.
2. Absorption of molecular charge current due to shifting in isolation. These transient currents disappear to zero more slowly
3. leakage current is a conduction current in insulating concrete. Leakage current varies depending on the test voltage. Also includes a leakage current due to leakage at the surface due to contamination.


R = insulation resistance (MΩ)

C = 1.5 for oil filled transformers at a temperature of 20o C 30.0 to untanked oil-impregnated transformers

E =  voltage rating (V) on the inter-phase delta connection, the phase neutral star connection

kVA = rating capacity of the tested winding.

Insulation condition based on the index of polarization


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