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Grounding TxFo, why ZZ instead of Ygd?

Grounding TxFo, why ZZ instead of Ygd?

Grounding TxFo, why ZZ instead of Ygd?

(OP)

Hi,

I'm wondering this question for a while, why ZZ instead of Ygd for a grounding transformer?

I think that the ZZ doesn't have to be as big as the Ygd for the same fault current, so maybe a cost benefit.  Do you know something else, and is it really a cost benefit since Yd are more common?  I search in old thread for this, but didn't really find why ZZ are often used.

If you have  good reference which explain how to size the grounding transformer, it'll be appreciated.

thanks

RE: Grounding TxFo, why ZZ instead of Ygd?

You're right - the only benefit to a zigzag grounding transformer over a wye-delta transformer is that the zigzag would be smaller for the equivalent amount of desired ground current.  This probably means reduced core losses as well.  
 
The wye-delta works perfectly well for a grounding transformer and we often recommend this approach since they are so much easier to get (and replace).  

 

RE: Grounding TxFo, why ZZ instead of Ygd?

(OP)

thanks dpc for your reply.

I have an other question regarding this.  In the Westinghouse T&D book, you have a factor that you apply for determining the kva of the transformer according to the time before the fault is clear; 10sec, 1min, 2min, 3min...


But, for the star delta connection, you haven't a factor for a 10 sec time rating, only for 1min and more.  Is there a reason for that?  For my situation, the time to clear the fault will be pretty quick, so a 10 sec and less rating should be appropriate.  A zigzag could have more benefit if i use a 10 sec rating instead of 1min for a Yd.
 

RE: Grounding TxFo, why ZZ instead of Ygd?

I think it is just because zigzags have traditionally been used for grounding.  The typical specification for a zigzag grounding transformer does not have a specific kVA rating.  

If you are buying a new wye-delta strictly for grounding purposes, I would include the desired short-time neutral current rating in the specification.  And make the specified kVA rating "nominal" if the transformer will never need to carry any load.   One advantage of using the wye-delta is that it could be loaded up if that makes sense to do.  A wye-delta-wye with a buried tertiary can also serve as a ground source.  

 

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