Can differential protection operate while its only one side is energized or being energized (HT side in this case)? When trying to energize a 132/11KV t/f from HT side when its differential protection operated while its LT side breaker was still open? Was it because of starting inrush current or t/f tap position?(It has an auto tap changer on HT side & while it was being energized tap position was on negative side)
It would help to use full words instead of abreviations. I believe what you are asking about is differential protection schemes for high tension (HT) transformers (T/F). I am just guessing on this one, but with the low side (11kV) open, initial closing of the high side breaker could have any of a number of conditions present. Theoretically inrush current would be at a minimum but for the time following the closure of the high side breaker, the core structure would have substantial ringing due to the no load condition. This would reflect some rather interesting waveforms back to the differential sensors and would likely be detected as a fault. This would be a question better asked of the software engineer that designed the controls. In normal (loaded) operation, I would suspect internal arcing (fault condition) to produce similar waveforms.
It is also possible that without a load, the tap selector may have tried to adjust for output voltage. This “hunting” would have also introduced waveforms that could be interpreted as fault conditions.