Analysis of HV Bushing (138KV) PF and Capacitance test results

[adi 3291]

After having a recent maintenance testing we observed the following C1 test results. The capacitance values are satisfactory but the PF values were increased for some bushings, when compared to Name plate values mentioned in the report.

We also observed change in oil color in the 34.5Kv bushing sight glasses.

My understanding is higher PF indicates the contamination and deterioration of Bushing’s insulating materials.

Would like to understand other's comments based on the test results and what can we do to mitigate this?

 

[Kiribanda]

1) Can you please upload a photo of the name plate of one of the 34.5kV bushings showing the PF & C1, C2 values?
2) Are these 34.5kV bushings rated for the full load rating of the transformer or your anticipated full load of the plant?
3) Can you please upload the data sheet of 34.5kV bushing?

 

[adi 3291]

Following are the C2 results. Included the C1 results previously.

HV bushing is 138KV

LV bushing 34.5 KV

The bushings are rated for full load capacity of the plant.

 

[Kiribanda]

Then it looks like the transformer is 100% loaded for 24/7 so that bushing oil is oxidized.

 

[prc]

Please check what type of bushing. If OIP (oil Impregnated condenser bushing) the general recommendation is if PF increases to 2 to 3 times the nameplate value, bushings shall be replaced. You cannot do anything at the site to improve these values.
When PF increases to 2 times, better to conduct the test at narrow band frequency ( 40-400 HZ) If the bushing is defective (esp moisture ingress) you will find an excessive increase in PF at low frequency. Change the bushing without further waiting.
Please see clause 7.3 of IEEE C57.152-2013 Diagnostic testing of Transformers