3 Phase Transformer SIngle Phasing at Secondary-Effects on Primary?

[ThePunisher]

I would like to throw this question as all single phasing literature I read always presents blown primary fuses.

What if:

I have step-down transformer 69-13.8 kV and primary is DELTA protected by primary fuses and secondary WYE-Solidly grounded and is also protected by fuses (typical in a small distribution utility.

If one phase of the three phase 13.8 kV secondary power lines, is open-circuited (by a blown secondary fuse or physically cut), will the single phasing condition (2 lines operation) results to sufficient primary current to trip the upstream 69 kV primary protection relay's overcurrent element?

What if: The one phase conductor of secondary 3 phase lines (before the line side of secondary fuse) cuts and hits ground. Will the primary 69 kV protection relay trip due to line-to-ground fault?

Thank you all, this is also related to one post I made earlier ("cause of trip query")

 

[waross]

This depends on too many factors to speculate.
What are the respective fuse types and ratings?
What are the breaker settings?
What are the secondary loads?
Are there significant motor loads?
When the conductor was cut, was it shorted before it went open?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

During single phasing on secondary side if we consider the two surviving phases are carrying rated current, the current in one of the phases on the primary will be 115% of rated primary current (i.e. 15% overload in one of the phases).
In case of single phase to ground fault on secondary the current reflected will be 58% of the reflected fault current in two corresponding phases on primary. Depending on the earth fault current magnitude (as it is subject to soil conditions / earth loop impedance) the fuses / phase fault protection elements of protective relay on transformer primary may operate.