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Transformer oil, also known as insulating oil, is a critical fluid used in oil‑filled transformers and electrical equipment. It serves multiple purposes: electrical insulation, heat dissipation, and arc extinction. Understanding transformer oil properties, types, and quality indicators is essential for ensuring safe and reliable transformer operation.
Transformer oil is classified into three temperature grades based on pour point (freezing point):
| Grade | Pour Point | Application |
|---|---|---|
| 10# | −10°C | Warmer climates (e.g., tropical regions) |
| 25# | −25°C | Temperate regions with moderate winters |
| 45# | −45°C | Cold climates (e.g., northern China, Siberia) |
China's transformer oil standard GB2536 was originally developed based on the former Soviet Union‘s GOST standard (ΓΟCT10121) and the IEC296 standard. Therefore, these three grades cover most temperature conditions worldwide.
Transformer oil performs three essential roles:
The oil fills the space between live parts (windings, leads, and core) and prevents electrical short circuits. Its high dielectric strength maintains insulation integrity under high voltage.
Transformer oil absorbs heat generated by the core and windings. As hot oil circulates to the radiator or tank wall, it transfers heat to the surrounding air. Therefore, the oil keeps the transformer within safe temperature limits.
In oil circuit breakers and on‑load tap changers, switching contacts produce arcs. Transformer oil decomposes under the arc‘s high temperature, generating gas that creates pressure and helps extinguish the arc quickly.
| Property | New Oil (Normal) | Warning Signs |
|---|---|---|
| Color | Light yellow | Dark red → oil oxidation/deterioration |
| Odor | Odorless or slight kerosene | Burnt → overheating; Sour → severe aging; Acetylene → electrical discharge |
| Transparency | Clear with blue‑violet fluorescence | Loss of fluorescence → moisture, impurities, or free carbon |
✅ Practical rule: Regular visual inspection of oil color and clarity is a simple first‑line check for oil quality.
The flash point is the lowest temperature at which oil vapor ignites when a flame is applied. It indicates oil volatility.
Therefore, a high flash point reduces fire risk and indicates good oil quality.
| Parameter | Value |
|---|---|
| Density (at 20°C) | ≈ 895 kg/m³ |
Moisture degrades the electrical properties of transformer oil and reduces insulation performance. The less moisture, the better.
| Condition | pH Value |
|---|---|
| New oil | ≥ 5.4 |
| In‑service oil | ≥ 4.2 |
A lower pH indicates acidic compounds, which can corrode metal components.
The acid value measures the amount of organic acid in the oil. It is expressed as mg KOH per gram of oil (mgKOH/g).
| Oil Condition | Acid Value Limit |
|---|---|
| New oil | ≤ 0.03 mgKOH/g |
| In‑service oil | ≤ 0.10 mgKOH/g |
⚠️ High acid value + moisture = accelerated corrosion of copper, steel, and insulation. An increasing acid value over time is a reliable indicator of oil degradation.
Dielectric strength is the most important electrical characteristic of transformer oil under high voltage. It measures the oil‘s ability to withstand electric stress without breaking down.
Test method: A standard oil cup with a 2.5 mm electrode gap is used. The voltage at which breakdown occurs is recorded as the breakdown voltage. The average of six breakdown tests gives the final value.
Therefore, oil purification (filtration, degassing, and drying) is essential to maintain high dielectric strength.
| Parameter | New Oil Standard | In‑Service Limit |
|---|---|---|
| Flash point | ≥ 135°C | ≥ 130°C |
| Moisture | ≤ 20 mg/L | ≤ 35 mg/L |
| Acid value | ≤ 0.03 mgKOH/g | ≤ 0.10 mgKOH/g |
| pH | ≥ 5.4 | ≥ 4.2 |
| Breakdown voltage | ≥ 35 kV | ≥ 30 kV |
Regular oil testing ensures early detection of problems and extends transformer life.
Contact a Xinghe representative today to learn more about our Transformer.
