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Insulation Test
Insulation testing on transformer windings: megger resistance, PI/DAR determination and polarisation analysis, with engineer-signed reports.
Insulation testing measures the resistance of transformer windings to ground and between each other with a megger (insulation tester), and assesses the insulation system for moisture, contamination and ageing through polarisation index (PI) and dielectric absorption ratio (DAR) determinations. A single resistance value gives partial information; examining the time-varying current (absorption) distinguishes whether the insulation is "wet, dirty or aged". The test applies to oil and dry type transformers, on HV and LV windings.
Measurement Logic: PI and DAR
- Insulation resistance (R60): Resistance at the 60th second under a defined DC test voltage; corrected to temperature by reference tables. The value is compared against manufacturer data and previous years.
- DAR (R60/R30): Ratio of the 30 and 60 second readings; a quick directional indicator. DAR ≥ 1.4 is generally acceptance (dry clean insulation).
- PI (R10min/R1min): Ratio of 1 and 10 minute resistances; measures the polarisation process. PI in the 2.0-4.0 band is good per insulation class; a fall marks moisture or contamination. In aged paper insulation PI may stay high — separating wet-dirty insulation reads the absolute R60 and the trend together.
- Stepped voltage: On suspicious results, the variation of absorption current with voltage is followed to separate bulk from surface leakage.
Procedure Steps
Before the test the winding must be de-energised, discharged to ground and ambient temperature-humidity recorded; protection-electronics (thermometer circuits, PT100, lighting) are disconnected to protect them from test voltage. Measurement sequence: HV-EV, EV-LV, HV+EV-LG, LV+EV-LG combinations; at each stage the 10-minute PI protocol is taken holding the duration fixed. On oil units the correction factor is minimal when oil temperature is near the reference; extreme cold or heat distorts interpretation. Discharging the windings after the test is a safety requirement.
Interpretation, Legislation and Outcome
Low R60 + low PI: moisture/contamination (drying-conditioning or bushing cleaning needed). Low R60 + high PI: surface leakage (porcelain soiling, oil film). A sudden fall over time: suspicion of internal moisture ingress or cracked-bushing leakage — verified with oil moisture and related tests. The test sits under Law No. 6331 and the High Current Installations Regulation duties; method reference IEEE C57.12.90 / IEC 60076-1 and, for field practice, the acceptance bands of IEEE Std 43. Measurements are reported with a signed protocol (device serial and calibration data) and entered into the trend file. This test is in our Transformer Tests and Analyses group; for complete test sets see Cell Insulation Test, or contact us for your programme.
FAQ on Insulation Test
It includes insulation resistance (IR, with a megohmmeter e.g. 5 kV), polarisation index (PI-dielectric absorption-DAR), capacitance and tan delta (dissipation factor) and, where needed, stepped DC voltage or on-site VLF-oscillometric partial discharge (PD) scanning. These measurements give the contamination-moisture-ageing and layer-weakness state of the winding insulation; IR-PI show the moment, tan delta-capacitance the frequency-based degradation trend.
The transformer must be fully de-energised-earthed-isolated, and the bushing-winding-body ties opened per the test topology. In IR-PI, voltage-pressure-surface-leakage current influence, humidity-temperature correction and charge-discharge times are managed, with the guard terminal suppressing surface leakage. Tan delta runs with a test tower or portable bridge under earth-balancing-overvoltage precautions. Results are corrected to the oil-temperature reference.
PI-tan delta values are read against standard-manufacturer thresholds and the past-year reference baseline: falling PI-rising tan delta-capacitance drift marks the moisture-contamination-degradation trend; extremely low IR or localized PD points to a layer-weakness fault position. The trend, not a single number, is the basis. On abnormality the advanced test-furan-DGA-drying verification chain enters.
Where falling insulation resistance or clear PD activity appears together with rising tan delta, the transformer goes into a drying-vacuum-degasification-filtration or repair-change plan and is not energised before the verification measurements repeat. Loading before moisture-contamination is pulled inside limits can start an internal discharge-puncture-interturn short chain. The report carries the limit table, trend curve and action suggestion; records enter the OHS-energy file.