The main function of transformer oil is insulating and cooling of the transformer. It should have the following properties:
- High dielectric strength
- Low viscosity
- Freedom from inorganic acids, alkali, and corrosive sulfur
- Resistant to emulsification
- Freedom from sludging under normal operation
- Rapid settling of arc products
- Low pour point
- High flash point
Enemies of insulating oil are:
- Oxidation Oxidation is the most common cause of oil deterioration. Careful and routine vacuum dehydration to remove air and water is essential to maintaining good oil.
- Contamination Moisture is the main source of contamination. It tends to lower the dielectric strength of the oil and promote acid formation when combined with air and sulfur.
- Excessive Temperature Excessive heat breaks down the oil and will increase the rate of oxidation. Avoid overloading the transformer.
- Corona Discharges Sparking and local overheating can also break down the oil and produce gases and water.
The following table lists the physical properties for insulating oil.
| Property | Specification | Typical | ASTM No. |
|---|---|---|---|
| Color | 0.5 max. | 0.5 | D 1500 |
|
0.100 dielectric gap (@ 60 Hz) 0.40 dielectric gap (@ 60 Hz) 0.080 dielectric gap (@ 60 Hz) |
30 kV min. 28 kV min. 56 kV min. |
35 kV 30 kV 60 kV |
D 877 D 1816 D 1816 |
| Neutralization number | 0.03 mg max. | 0.01 mg | D 974 |
| Free or corrosive sulfur | None | None | D 1275 |
| Flash point | 293°F (145°C) max. | 302°F (150°C) max. | D 92 |
| Pour point | -40°F (-40°C) max. | -67°F (-55°C) | D 97 |
|
Viscosity max. cST @ 212°F (100°C) Viscosity max. cST @ 104°F (40°C) Viscosity max. cST @ 32°F (0°C) |
3 max. 12 max. 76 max. |
3 10 70 |
D 88 D88 |
| Moisture content | 35 ppm max. | 20 ppm | D 1533 |
| Specific gravity @ 60°F (15.5°C) | 0.910 max. | 0.890 | D 1298 |
| Inorganic chlorides or sulfates | None | None | D 878 |
| Interfacial tension | 40 dynes min. | 45 dynes | D 971 |
|
Power factor @ 60 Hz, 77°F (25°C) Power factor @ 60 Hz, 212°F (100°C) |
0.05% max. 0.30% max. |
0.01% 0.10% |
D 924 D924 |
|
Oxidation stability, sludge after 72 hr Oxidation stability, sludge after 164 hr Oxidation stability, total acid after 72 hr Oxidation stability, total acid after 164 hr |
0.15% 0.30% 0.5 mg KOH 0.6 mg KOH |
0.10% 0.20% 0.2 mg KOH 0.3 mg KOH |
D 2440 D 2440 D 2440 D 2440 |
| Aniline point | 145.4–181.4°F(63–83°C) | 167°F (75°C) | D 611 |
A more recent measure being used to evaluate oil is the particle count distribution (PCB). This provides information on the size and number of particles in the oil. Additionally in some countries the presence of PCBs is still prevalent. The quality of insulating oil and the routine purification by vacuum distillation is most important in assuring that the high quality of insulating oil is maintained.
References
1. Pearce, H.A. “Significance of Transformer Oil Properties.” Electric Insulating Oils. STP 988 H.G. Erdman Ed. ASTM, 1988.
