Oil transformer problme (emergency ) 2

[emadshaaban1]

hello
An electrical oil transformer of 35KVA step up 11KV/20KV has tripped by
-bucholoz relay -differentiation protection -50N
An Insulation test for windings gave : 170 Kohm in sec / 4Mohm in primary
Oil Dielectric test tripped in 13KV..
after oil regeneration done :
Oil Dielectric test now tripped in 40KV but the windings Insulation still 170Kohm for sec and 4Mohm for Pri.
my questions are :
1)Are the Dielectric oil test is enough to tell the oil is good or I need to make other test?
2) How can I confirm that the windings has low insulation not because of the oil?

 

[davidbeach]

Did you do a dissolved gas analysis? What about a turns ratio test?

 

[panter]

Emad,
if you had Buchholz relay trip signal transformer is in a serious problem and oil is not reason for low insulation resistance .
You can connect 3 x 380V to High voltage *20kV* side and measure the current and voltage on secondary connections.
Probably there will be big differences and that will be a sure sign that there is a problem on windings.
When spilled gas came from the Buchholz rele and You can light it up with cigarette lighter that is also first and reliable sign of short circuit and burning inside transformer .
Our standard for transformer oil analysis is minimum 120kV per cm and should further do gas analysis to know what to do with him .

 

[prc]

The test for checking winding integrity after a differential trip is low voltage single phase excitation current measurement,followed by TTR&DGA.

 

[emadshaaban1]

Thank you for your fast reply..
I will apply 380v in 20kv side but do I need to short the 11kv winding to measure the current or I measure the voltage only .

 

[emadshaaban1]

and you may know that the trtansformer is internally connected in (Y/delta) for (11/20kv)

 

[panter]

Emad,
I thought that you can measure voltage on open secondary *11kV* side and current on primary *20kV* side of transformer.
If you get good results, then you can think about a short circuit on secondary but then current on both sides will be more serious so you should obtain technical assistance of some experienced colleagues .
Good luck