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What is factor affecting insulation resistance of transformer?
Surface condition of bushing
Oil quality
Test voltage and its time
All of these
All of these
Insulation resistance (IR) in transformers is not an inherent constant but a variable measurement highly dependent on external and internal conditions. Factors such as surface contamination, moisture content in the transformer oil, and the stress duration of the applied DC test voltage significantly alter the leakage current path.
Insulation resistance (IR) in transformers is not an inherent constant but a variable measurement highly dependent on external and internal conditions. Factors such as surface contamination, moisture content in the transformer oil, and the stress duration of the applied DC test voltage significantly alter the leakage current path.
RinsтАЛ=ItotalтАЛVDCтАЛтАЛ тАФ Relationship between test voltage and measured current
PI=R1minтАЛR10minтАЛтАЛ тАФ Polarization Index representing insulation health
The IR test relies on applying a high-voltage DC (usually 500V-5000V) and measuring the resulting leakage current. Surface conditions (bushing dirt) provide a parallel path for current, lowering resistance. Transformer oil quality (dielectric breakdown strength and impurity levels) dictates the internal insulation capability. Furthermore, because insulation exhibits capacitance and absorption phenomena, IR values increase with time (polarization index), making the test duration and voltage level critical parameters.
Surface tracking on bushings creates low-resistance paths due to dust, moisture, or salt deposits.
Oil dielectric strength decreases with moisture, sludge, and particle contamination, leading to lower IR values.
The Megger test requires specific time intervals (e.g., 60 seconds) because the charging current must decay before the true insulation resistance can be measured.
Temperature significantly affects readings; IR values must be corrected to a reference temperature (usually 20┬░C or 75┬░C).
Detects early stages of insulation degradation.
Non-destructive testing method when performed within voltage limits.
Sensitive to environmental humidity and temperature fluctuations.
Cannot detect local winding faults or turn-to-turn shorts.
Routine preventative maintenance of power transformers.
Post-installation verification of insulation integrity.
Standard: IEEE 62 or IS 2026 guidelines are typically followed for interpreting IR values.
The polarization index is used to determine the dryness of the transformer insulation.
D is correct тАФ The insulation resistance of a transformer is a dynamic measurement influenced by external surface conditions, internal dielectric health (oil), and the specific parameters of the test procedure.
Always remember that insulation resistance is inversely proportional to temperature; a small increase in temperature can lead to a significant decrease in IR values.