Fault current in YN/Yn0 transformer

[dfdt]

Could any please confirm or contradict my understanding regarding distribution of fault current between hv and lv winding of a YN (ungrounded)/YN(grounded) transformer for phase to ground fault on lv side.As permy understanding the fault current distribution on hv side will be 1:1:2 ratio for phase to ground to fault on lv side

 

[HamburgerHelper]

GE's Why's of Wyes

https://www.google.com/url?url=http...TnyB9Q&usg=AFQjCNHE4Hoa8fQvCBWdbvWNFKgmhlbjzA

 

[HamburgerHelper]

I think this thread is the same as what you are asking.

http://www.eng-tips.com/viewthread.cfm?qid=184543

 

[dfdt]

thanks for link to but it does not answer my question which is distribution of fault current on ungrounded star side for single line to ground fault on LV side which is grounded star.I have no doubt that fault current will flow on HV side but wasn't sure how will it spilt between the HV phases i.e will it be in the ration on 2:1:1 or just between two phases of HV side?

 

[mgtrp]

Your question is answered, along with quite a lot of explanation in the the previous thread that HamburgerHelper referenced. The theory of why is covered in the GE paper.

The short answer is: virtually no fault current will flow, unless the transformer is a three-limb core form type, in which case the phantom delta effect means that some current will flow. The fault current in the phantom delta case shouldn't be as much as it would be if you had a Grd-Y/Grd-Y or Delta/Grd-Y connection.

Cheers,
mgtrp

 

[dpc]

mgtrp is correct. When evaluating how fault current flows across the transformer windings, a couple of easy rules that can help:

There must be a complete path for the current to flow.

Ignoring exciting current, if current is flowing in a secondary winding there must be a corresponding (opposing) current flowing in the primary winding.