Transformer Oil Testing & Drying Out — Traction Distribution Notes

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Inhibited oil conforming to IS 12463 is used in traction transformers, governed by two real standards — IS-335 for new insulating oils, and IS-1866 (Code of Practice for Maintenance and Supervision of Insulating Oil in Service).

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Exam Focus — Full Tests vs Partial Tests

Full, wide-ranging oil tests should be done ONCE A YEAR. But some specific tests — Breakdown Voltage (BDV) test, acidity tests, and crackle test for moisture — may be carried out at PSI depots/sub-stations ONCE EVERY SIX MONTHS as a lighter condition-monitoring check. This distinction (annual full battery vs 6-monthly partial subset) is a genuinely testable detail.

Oil Purification

Purification removes contaminants — water, carbon deposits, dirt, sludge, dissolved moisture/gases — preserving the most important quality: dielectric strength, which moisture directly affects. Insulating materials in the winding are hygroscopic by nature, absorbing moisture through defective breathers, gaskets, and untreated top-up oil. Purification is done when dielectric strength drops below permissible limits.

Oil purification plants (conforming to RDSO specification) are normally operated from 240V single-phase supply, taken from the 100kVA Station Transformer at the sub-station — the same station transformer referenced in the earlier PSI Fundamentals chapter.

Insulation Resistance During Drying Out — The 4 Real Stages

Temperature and insulation resistance readings are recorded every 2 hours from commencement until full operation completes — plotting these produces a curve with 4 genuinely distinct stages.

Stages A through D

Stage A: Insulation resistance initially DROPS to a low value because of rising oil temperature, up to about 75°C.
Stage B: Resistance remains LOW despite high temperature being maintained — until most moisture from windings and oil has been driven out.
Stage C: Resistance rises gradually and LEVELS OFF — indicating all moisture is driven out and drying is complete. Oil circulation should be discontinued AT THIS POINT.
Stage D: As oil cools off, resistance RISES ABOVE the Stage-C levelling point — because insulation resistance DOUBLES for roughly every 10-15°C temperature drop.

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Exam Focus — When to Stop, Not Just What Happens

The genuinely practical, testable fact isn't just recognising the 4-stage curve shape — it's knowing that oil circulation must be discontinued specifically at the START of Stage C (once resistance levels off), not waited out further into Stage D. Continuing past that point wastes time without improving the actual dry-out result.