Transformer trip on inrush

[Prot123]

25MVA YNyn0d1, source side ungrounded transformer tripped on Instantaneous EF during energisation. The waveform signature revelled inrush, but the question is how did neutral current appear on the supply side when the star point is floating?
Appreciate any thoughts

 

[dpc]

How is the earth "fault" detected? Probably through summation of the three phase currents in the relay. There can be some CT error during the very high inrush current, especially as the current will not be equal in all three phases depending on the voltage phase angles on energization. Unequal error results in phantom "neutral" current.

 

[sasalu]

Hi dpc I have some queries , Erath fault protection will operate on Fundamental component, During energization high harmonic content will be there. In our system also many Ungrounded star winding transformer are in service with only source transformer star neutral grounded. Earth Fault protection are provided on summation principle in numerical relay. what should be the criteria for setting the Earth fault IDMT and Inst pick up. while i am going through the settings in case of 12.5 MVA Trf Inst earth fault setting is 250 A 20 ms delay. Voltage level 33 kV.

 

[RRaghunath]

The Earthfault current pickup setting of 250A seems fair. May be you should add a slight delay (~100ms) just to allow the protection element to stabilise on rare occassions when the inrush is unusually high.
Alternatively, you may use 2nd harmonic (in phase currents) detection feature in numerical relay to block the earth fault protection.

 

[dpc]

sasalu,

Are you actually having nuisance tripping on inrush with those settings? They seem pretty safe. You could set an inverse time overcurrent element with a lower pickup, and then an instantaneous set higher.