What is the tanδ test, and what do the readings indicate?
The tanδ (dissipation factor/power factor) test measures dielectric losses in the insulation system under AC voltage. An ideal capacitor has zero loss; higher tanδ indicates aging, moisture, or contamination.
Typical values (Oil-impregnated paper insulation at 20°C):
|
tanδ (%) |
Condition |
Interpretation |
|
< 0.3 |
Excellent |
Dry, clean insulation |
|
0.3 – 0.5 |
Acceptable |
Slight aging or moisture |
|
0.5 – 1.0 |
Marginal |
Significant degradation |
|
> 1.0 |
Poor |
Immediate investigation required |
What high tanδ can reveal:
• Moisture ingress into paper insulation or oil
• Carbonization from partial discharge activity
• Oil contamination (sludge, dissolved decay products)
• Overall insulation aging over service life
Recommended Standards: IEC 60247 (oil), IEC 60422 (oil maintenance guide), IEEE C57.152 (bushings).