Trasformer internal fault

[Jed057]

Hi Guru,

we found on transformer 2500 kVA 11kV/0.4 kV,Seal Type,ONAN

Insulation test between secondary with Earth is failed(Reading near zero)but another test such Ratio, DC Resistanceand insulation in primary side are O.K., So what happening with our transformer.

Thanks
Young Engineer

 

[Marmite]

Are you testing the transformer after it has tripped on a fault, or as a precommissioning test after you have carried out some work? Did you disconnect the star point from neutral/ground to do the test? You need to provide much more info for us to have a stab at a decent answer.
Regards
Marmite

 

[TurbineGen]

Sounds to me like someone forgot to disconnect the X0 bushing...

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[VTer]

if wye connected secondary, confirm the neutral to ground connection is removed when you test.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[tem1234]

Also PT connected downstream can give these result. Remove all the fuse of PT.

If not sure, disconnect all the cable or busbar of the 3 phase + neutral directly on the secondary of the transformer before testing it.

 

[stevenal]

If this is a padmount with elbow style connections, the neutral point is likely grounded internally.