Phase-earth fault in YNd11 step-up transformer while operating at no load

[BAHADIR AKBAL]

Hello everyone
There is a YNd11 step-up transformer in a factory. When the star point of the transformer is grounded, the protection relay gives a phase ground fault approximately 20 minutes later. The same problem occurs when the delta winding of the transformer is completely unloaded. The star side is at 33 kV, the delta side is at 6.3 kV. If the star point is not grounded, it does not give a fault. The fault current is reported by the protection relay as approximately 30 A. It is a great risk for the star side to work without grounding. However, when grounding is done, the protection relay trips. Do you have any information on this subject? Can the 3rd harmonics in the network or in the transformer's idle operation reach the level of 30 A?

 

[waross]

Unbalanced primary phase voltages or pri9mary phase angles.

 

[BAHADIR AKBAL]

In these cases, I think the protection relay opens immediately. However, approximately 20 minutes after the power is applied, the protection relay opens the breaker. 20 minutes is a very long time in such cases. Could there be another reason?For example, a partial discharge in a cable. Can this last for 20 minutes?

 

[davidbeach]

An overcurrent element just above pickup can take an excruciatingly long time to trip.

 

[dpc]

What type of relay? If it's a digital relay, you should be able to review the fault data including the current phasor data. Make sure the CT polarities are correct. If the relay is summing the phase current numerically, it can nuisance trip if one of the phase ct's is wired incorrectly.

 

[waross]

Any unbalance in the incoming power and the delta will cause a neutral current.
This may be misinterpreted as a ground fault be the protection.

 

[Kiribanda]

Could you please share with the members,
1) SLD of the protective relay scheme of the transformer?
2) Which relay has operated?
3) Its settings?