If there is no visible damage or the protection system has not been activated (i.e. gas alarm, pressure relay, differential relay), then there is a high chance that the transformer has withstood the event. A sample of the oil should be immediately taken and full electrical testing be done including ratio, resistance, insulation resistance and power factor. An excellent test to confirm that there has been no geometric movement in the windings is the Sweep Frequency Response Analysis (SFRA) test. An optional but very final confirmation is also possible with a site high voltage induce test with partial discharge.
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