Transformer Oil Testing; IS:335 for Transformer oil; Transformer BDV values

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Transformer Oil: Functions, Production, Deterioration & Testing Standards

Transformers are the backbone of modern electrical power systems, ensuring efficient transmission and distribution of electricity. While the transformer itself is vital, transformer oil is equally integral for the reliable operation of oil-immersed transformers. Although dry-type transformers are available, oil-filled transformers remain the most widely used across the globe due to their superior dielectric and cooling properties.

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Functions of Transformer Oil

Transformer oil (also known as insulating oil) serves two major purposes:

1. Dielectric Strength – It provides high electrical insulation between energized components, reducing the risk of breakdown.
2. Cooling Medium – Heat generated in transformer windings and core is dissipated through transformer oil, ensuring safe temperature rise.

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Production of Transformer Oil

Transformer oil is a hydrocarbon product derived from crude oil. Its main constituents include:

• Naphthenic hydrocarbons
• Paraffinic hydrocarbons
• Aromatics

The manufacturing process involves refining crude oil into Transformer Oil Base Stock (TOBS), followed by finishing steps to meet international specifications.

Production Steps:

1. Distillation of crude oil
2. Acid treatment for impurity removal
3. Neutralization of acidic residues
4. Water wash for purification
5. Hot air blowing for stability improvement
6. Clay treatment of TOBS → Finished Transformer Oil
7. Hot filtration → Ready-to-use transformer oil

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Deterioration Aspects of Transformer Oil

Transformer oil undergoes chemical and physical degradation over time. The main causes include:

• Moisture contamination
• Chemical decomposition of hydrocarbons
• Oxidation leading to sludge and acids
• Gas contamination from arcing or partial discharge
• Electrical stresses under high voltages
• Thermal stresses from overload conditions
• Accumulation of oxidation products
• Physical contamination (dust, fibers, etc.)

These factors reduce insulation strength, increase acidity, and cause sludge formation, ultimately leading to transformer failure if left unchecked.

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Precautions in Transformer Oil Sampling

Transformer oil testing is highly sensitive to contamination. During sampling:

• Avoid exposure to dust, moisture, or rain/fog.
• Ensure sampler’s hands do not touch the oil.
• Prevent condensation if oil temperature is lower than ambient air.
• Always use clean, dry glass bottles prepared with:
1. Tap water wash
2. Soap solution wash
3. Hot distilled water wash
4. Acetone rinse
5. Oven drying at 110 °C for 1 hour
6. Final vacuum drying

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Transformer Oil Testing Standards

To ensure quality, transformer oils must comply with national and international standards:

• IS-335 (India)
• ASTM D3487 (USA)
• IEC 60296 (International)
• BS-148 (UK)
• JIS-2320 (Japan)

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Key IS-335 Specification Parameters

Property	Test Method	Equipment	Design Value
Appearance	Visual	–	Clear, no sediments
Density	IS-1448 (P16)	Hydrometer	≤ 0.89 g/ml @ 29.5°C
Viscosity	IS-1448 (P25)	Constant temp. bath	≤ 27 cSt @ 27°C
Interfacial Tension	IS-6104	IFT meter	≥ 0.04 N/m
Flash Point	IS-1448 (P21)	Pensky-Martens	≥ 140°C
Pour Point	IS-1448 (P10)	Pour point apparatus	≤ -6°C
Neutralization Value	IS-1448 (P2)	Titration setup	≤ 0.03 mgKOH/g
BDV (Breakdown Voltage)	IS-6792	BDV tester	≥ 30 kV (unfiltered), ≥ 60 kV (filtered)
Dielectric Dissipation Factor (Tan δ)	IS-6262	Tan delta test set	≤ 0.002
Resistivity	IS-6013	Resistivity cell	≥ 1500×10¹² Ω-cm @ 27°C
Water Content	IS-13567	Karl Fischer titrator	≤ 50 ppm
Corrosive Sulphur	IS-335 Annex-B	Oven test	Non-corrosive
Oxidation Stability	IS-335 Annex-C	Oxidation setup	Neutralization ≤ 0.4 mgKOH/g, Sludge ≤ 0.1%
Ageing Characteristics	IS-12177	Ageing oven	Tan δ ≤ 0.2, Acidity ≤ 0.05 mgKOH/g, Sludge ≤ 0.05%

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NABL Requirements in Transformer Oil Testing Labs

• Maintain equipment history cards
• Calculate measurement uncertainty
• Regular calibration from NABL-accredited labs
• Participate in inter-laboratory comparisons
• Perform replicate testing for accuracy

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Case Study: BDV Test Uncertainty Analysis

A sample transformer oil was tested for Breakdown Voltage (BDV):

• Test results (kV): 68, 74, 72, 60, 68, 64 … Average = 69.82 kV
• Type A Uncertainty: 0.818 kV
• Type B Uncertainty (Resolution + Calibration): 1.1115 kV
• Combined Uncertainty: 1.38 kV
• Expanded Uncertainty (95% confidence): ±2.76 kV

📌 Final Result: 69.82 ± 2.76 kV @ 95% CL

This demonstrates the importance of uncertainty evaluation for reliable transformer oil diagnostics.

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Final Thoughts

Transformer oil is not just a filler medium—it is the lifeline of oil-immersed transformers. Regular monitoring, correct sampling, and strict adherence to IS, IEC, and ASTM standards ensure long service life and prevent catastrophic failures. With NABL-compliant testing, utilities can ensure transformer reliability, grid stability, and reduced maintenance costs.