According to Transformer Tester Professionals, acidity testing of transformer oil is an important diagnostic procedure used to assess the condition of the oil and detect deterioration that may affect the performance and life of the transformer. Oil oxidation, moisture and ageing can form acidic compounds that lead to corrosion, sludge formation and reduced insulation.

1. Purpose of Acidity Testing

• Detect oxidation byproducts (e.g., carboxylic acids).

• Prevent corrosion of transformer internals (windings, tank).

• Avoid sludge buildup, which impairs cooling and insulation.

• Ensure compliance with standards (e.g., IEC 60296, ASTM D974).

2. Test Methods for Acidity in Transformer Oil

A. Titration Method (ASTM D974 / IEC 62021-1)

Principle: Neutralize acidic components in oil using a potassium hydroxide (KOH) solution.
Procedure:

1. Sample Preparation:

• Collect oil sample in a clean, dry glass bottle (avoid contamination).

• Filter if particulates are present.

2. Titration:

• Mix oil with an isopropanol-solvent (to dissolve acids).

• Add phenolphthalein indicator (turns pink at neutral pH).

• Titrate with 0.1M KOH until a pink color persists for 15 seconds.

3. Calculation:

• Acidity = (Volume of KOH × KOH normality × 56.1) / Oil sample weight (mg KOH/g).

Limits:

• New oil: ≤ 0.03 mg KOH/g

• Used oil (acceptable): ≤ 0.1 mg KOH/g

• Critical action needed: > 0.3 mg KOH/g

B. Colorimetric Test (Quick Field Test)

• Uses pH-indicator strips or pre-packaged test kits.

• Less precise than titration but useful for rapid field assessments.

3. Interpretation of Results

4. Causes of High Acidity

• Oxidation (exposure to oxygen at high temperatures).

• Moisture ingress (accelerates acid formation).

• Overloading/overheating of transformer.

• Contamination (dust, metal particles).

5. Mitigation & Maintenance

• Oil Reclamation: Remove acids via clay filtration or ion-exchange resins.

• Oil Replacement: Necessary if acidity exceeds limits.

• Breather Maintenance: Use silica gel breathers to prevent moisture ingress.

• DGA (Dissolved Gas Analysis): Pair with acidity tests for comprehensive diagnostics.

6. Standards & References

• IEC 60296: Specifications for unused mineral insulating oil.

• ASTM D974: Standard test method for acid number by titration.

• IEEE C57.106: Guide for transformer oil maintenance.

Conclusion

Regular acidity testing helps extend transformer life and prevent failures. For accurate results, use titration (ASTM D974) in labs and quick tests for field checks. If acidity exceeds 0.3 mg KOH/g, take immediate corrective action.

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