Transformer Buried delta protection 1

[cesgibis]

Hi,
We are specifying a 220/33kV transformer as Y-Y with a tertiary buried delta for stabilisation purposes only. The transformer is 115MVA, the primary solidly earthed and the secondary is resistance earthed to limit the current to 400A. I am preparing a protective diagram and I am not sure how is this tertiary winding typically protected. Do we need to have a CT on this winding and what protection function/relay needs to be applied for the protection of this winding ? Thanks for your assistance.

 

[slavag]

87T and buchholz relay are protected this winding.
You do not need any additional protection.

 

[Kiribanda]

cesgibis,
Your transformer is having a buried tertiary. Therefore, normally one corner of the DELTA tertiary winding is connected to the NEUTRAL line of the primary inside the tank. As a result the DELTA wdg is inside the protected zone covered by the 87T unit protection. That means for a ground fault on the DELTA tertiary the 87T will respond & trip but not for turn to turn or phase faults of the tertiary. Since the tertiary is buried no extra protection is required.

But if it is taken out from the tank and used for station service power supply then a CT scheme is needed to detect any overcurrent condition.

 

[prc]

In case of buried delta, it is a good practice to bring one corner of delta outside through two bushings and connect to ground earth. It will enable checking of winding continuity, resistance and IR of tertiary winding.
In case of bank formation (ie three single phase units) the open delta voltage will be monitored through three voltage transformers to detect accidental earthing of any phase, which if left un attended, can form in to a LL fault on tertiary.

Sensitivity of 87 T for buried tertiary faults may be limited.