Insulation property of cast resin type instrument transformer will gradually deteriorate across the years. Factors of deterioration are as follows. Deterioration will be caused by both single factor and multiple factors.
|
| Description of Insulation deterioration | Factor | Process | |
| Heat deterioration | Heat | Oxidation and Pyrolysis ⇒Decrease in mechanical strength, Increase in water absorption |
|
| Electric field deterioration | Partial discharge | Void (Crack, Peeling, Bubble) |
Oxidation, Perforation ⇒Decrease in insulation thickness ⇒ Breaking |
| Treeing | Projection, Foreign material | ||
| Stress deterioration | Heat stress, Heat cycle stress,Vibrational stress | Crack, Void ⇒Voltage deterioration |
|
| Environmental deterioration | Moisture, Dust, Dirt, etc. | Defacement, Moisture absorption ⇒Decrease in insulation resistance, Tracking |
|
[Source: the report of the Institute of Electrical Engineers of Japan, part2, No. 225]
Such defective insulation, caused internally in resin, as Cracks,Peeling or Voids do not have self-healing property.The defective insulation will become worse in many cases.Insulation resin ,directly exposed to the air ,is easily affected by environment and has also "tracking" which is specific problem of organic material.The degree of the deterioration will differ in each case.
Therefore, we recommend the renewal of the cast resin type instrument transformers used over 15 years. The above mentioned renewal period is not guaranteed value but is based on the case that daily or annual check are performed correctly.
|