Tan delta values of a Generator

[appunni]

Sir,
In one of our 144MVA generator, we got tan delta value as given below:
1.Phase-to-Phase values:
R-Y: 40%
Y-B: 38%
B-R: 20%
2. Phase to Neutral values:
R-N: 2%
Y-N: 3%
B-N: 3%
All measurements were with 6.5KilotVolts AC supply.
Last year, the phase-to-phase values were around 11% and phase to neutral values were around 1%. When we tested tan delta values of other 144MVA units, we got all values below 2%.
What is the reason for high tan delta values? What are the remedies to be taken? I request for your detailed comments. Advance Thanks.
With regards,
Appunni.

 

[jbartos]

Suggestion: Tan delta (or dissipation factor), if correctly measured, phase-to-phase values increased noticeably. It may be due to:
1. Rapid generator aging caused by the higher temperature of operation
2. Increased relative humidity of the cooling media
3. The higher harmonic or interharmonic content in the generator produced current due to nonlinear loads that impacted the winding insulation
4. Etc.
The systematic troubleshooting of the above items is recommended.