ZNyn transformer and earth faults on secondary

[RRaghunath]

I understand from Westinghouse protection handbook that there would not be any currents for earth faults on secondary side of ZNyn transformers. Could some one tell me what is the practice followed for protecting the secondary system in such case against earth faults.
I am talking of an existing system with 33kV/415V ZNyn transformer. There is current transformer in the secondary neutral and an earth fault relay connected to it. Does the above mean that the current based existing protection is futile and will need to be replaced.
Thanks in anticipation. Raghunath

 

[jbartos]

Suggestion: Depending on the transformer kVA rating, the secondary wye may have:
1. Solidly grounded neutral
The earth fault relay connected to the CT, in this case, could be used for tripping the transformer or for an alarm indicating a high neutral current.
2. High-resistance grounding could be also used if grounding calculation results permit it. This grounding scheme permits a ground fault for a certain period of time depending on the insulation level, e.g. 100% is good for 60 seconds, 133% insulation level is suitable for 1 hour, and 173% insulation level is suitable for an indefinite period of time.

 

[RRaghunath]

Thanks Jbartos for the response. But my problem is that even with primary and secondary neutrals solidly grounded, there would not be any earth fault current on secondary. Raghunath

 

[nraju]

Is there a restricted earth fault protection on both primary and secondary sides?

 

[Shortstub]

If you are referring to the 1979 book "Applied Protection Relaying", dedicated to Dr. J.L. Blackburn, then, there is a solid connection from the transformer's neutral-to-ground.

If not, can you identify your reference more fully?

 

[busbar]

As far as low-side loads/faults/imbalance are concerned, couldn’t you treat the ZNyn transformer as if it were YNyn?

 

[jbartos]

Suggestion to faulty (Electrical) Feb 17, 2003 marked ///\\But my problem is that even with primary and secondary neutrals solidly grounded, there would not be any earth fault current on secondary.
///This probably assumes that the primary ground connection is perfect and will stay perfect during the existence of the transformer. What if the line conductor on the secondary makes an electrical contact with the conductive earth?\\