Power transformer insulation test 1

[safy2004]

We have old power transformer working since 1976. It is 33/11 kV, we change its oil, and treat all leaking in its body, but when we test its insulation using 5 kV Megger its insulation value was just 80 M ohm, but the temperature was 50 degree centigrade.
According to the tables this value is acceptable because its temperature is very high and its very old, but we are hesitated to energize it with the power because of this low value (80 M ohm), so could any one help?
Thanks in advanced
Kind regards,
Safy

 

[Zogzog]

When did you change the oil? What method was used? What were the oil test results?

 

[safy2004]

Dear Zogzog
According to the attached file in the link below at the last page this value is acceptable, but we are just worried!
Actually we didnt change all the oil at one time but we change it in steps to avoid the leakage of the humidity to the coils, the oil has been filtered and tested and its breakdown value was 70 KV at 2.5 cm gap.
Safy

 

[safy2004]

Sorry the gap is 2.5 mm

 

[prc]

80 meghom at 50C is a very good value with old oil.You can check the water content of oil (sample it at when oil temp is maximum) and find te relative saturation at that temperature.If it is below 50 %, insulation can be considered reasonably dry.

 

[safy2004]

Dear prc
Thanks a lot for your point of view but could you tell me please exactly what is the minimum acceptable water content in PPM in such case
Regards

 

[prc]

As per IEEE C57.06-2006, permissible limit of water content for transformers of rating 66KV and below is 35 mg/kg (ppm).If it is more than this ,dry out the transformer by hot oil circulation.

 

[safy2004]

Thanks prc
your informations are valuable