Transformer Turn Ratio test intermittent failure

[LBolt]

We have 3 8333kva 44kv-7200V single phase transformers connected in Delta primary and Wye secondary. We had a percentage differential relay operate and a dip on our system voltage. The transformers were broken loose and a TTR test was performed on each. Two nulled the meter at ratio the correct value 6.111. The last nulled at approximately 3. The tap was changed an still measured 3. The transformer has been sent to the shop and untanked. No apparent problem and all the tap ratios are measuring very closely to the calculated values. Has anyone ever experienced anything like this?

 

[busbar]

Assuming the unit was oil insulated, was a DGA test conducted?

 

[sngpl]

Were any megger and doble (C&DF) tests performed?

 

[sparky1976]

What type of your TTR test equipment, have you try to put low voltage eg.220V to your 44kV terminal and chek the voltage on your 7.2 kV terminal.