Transformer Trip

Is there no Senior Electrical Engineer at the facility?

Usually when a transformer trips on differential something bad has happened inside. I assume that those past-fault readings are recorded by the digital relay just prior to the protection scheme tripping all connections.

The readings are instantaneously the same because there was probably a three phase to ground fault internally, remember that fault conditions greatly distort the normal phasor relationships that occur under normal conditions.

Why a three phase fault? Look at the magnitudes which are also all the same +/-, that indicates a symmetrical fault. Open up a good text on Symmetrical Components and check out the analysis and phasor diagrams for the different types of faults on that type of transformer.

Very important, Don't re-energize the transformer until a DGA (Dissolved Gas Analysis) and an internal inspection has been carried out!!!!

Thank you Mr.Ram for your informative reply. We have carried out all the tests for transformer(insulation resistance- 4 G ohm, BDV - 50KV, Turns ratio test) found OK. then we charged transformer again tripped on differential 3 times, then we Bypassed differential and charged the transformer. Transformer run for 10 days with out any trip. Then transformer tripped on Buchholzs trip. Then we inspected the transformer thoroughly found WATER CONDENSED INSIDE BREATHER PIPE. we removed the pipe and cleaned with hot blower and charged transformer. Now running Ok no problem. Thank you for your support .

Consider yourself very lucky! You still haven't found the spot where the fault occurred, and even though you seem satisfied that the oil is OK, you also haven't found how all that water that condensed got into the transformer to begin with.

In the States we would have done a DGA, filtered/treated/dried the oil, and determined what caused the differential to operate. There's probably a layer of water at the bottom of the tank, and the next time the same ambient conditions arise you may find yourself redoing this exercise.

Because it IS a three phase short circuit fault within the transformer.