Ground Fault through Y-Yg Transformer

[synchro]

For a wye-delta transformer in a customer facility, it is normally appropriate to avoid connecting a neutral to the center point of the wye primary.
I believe tertiary delta windings are typically rated for a fraction (for example, 1/3) of the kVA rating of the primary and secondary windings. I think this is analogous to having a reduced size neutral, because the current in a tertiary is also related to the amount of load current imbalance. And so I think there is even a stronger case than with a wye-delta to avoid connecting a neutral to the center point of the wye primary.

 

[tortuga]

I think this drawing depicts a NEC compliant wye-wye setup. I am unsure why they use a wye-wye?
but I think this (other than OP has a 208 secondary) is what the OP was originally describing :

 

[D. Castor]

Tertiary delta should provide zero sequence current for L-G faults on the secondary side. But with the primary wye neutral point grounded, there may still be some current that flows in the 480 V neutral as well.

 

[bwat]

I think this drawing depicts a NEC compliant wye-wye setup. I am unsure why they use a wye-wye?
but I think this (other than OP has a 208 secondary) is what the OP was originally describing :
View attachment 2581021

Yes, that's pretty much spot on other than the 208V and the transformer also has the delta tertiary.

It's sounding like H0 may not be necessary to be connected (and/or preferred to remove), but I can't come up with a good reason why this would be the reason for the GF trips. Right?

 

[tortuga]

I think its purpose of that third 'delta' / middle winding AKA 'tertiary winding' is just internal to provide a path for current to flow if the secondary must supply an unbalanced load instead or a fault, so it replaces the primary neutral?
My current hypothesis is the H0 point of the primary winding cannot be connected to the primary system neutral if there is that 3rd winding. You consult the manufacturers litature on that that just my guess.

If the tertiary where not provided, like in my drawing, then my *guess* is a primary neutral would be required,
I wonder if in the 2-winding wye wye I posted will any unbalanced load would result in the neutral point on the secondary side shifting & the voltages become unsymetrical even with the primary neutral?
In the one I posted there is no L-N load only load is 240 but there is both a 3-phase 240 load and single 240 1-phase L-L lighting. But there could be a L-G fault and then I wonder if the 480 GF main would see the GF?
My apologies for posting just more questions than answers.

 

[wwhitney]

If you have a 480V wye grounded wye source, feeding a 480/277V wye: 208/120V wye transformer, with a _neutral_ from the source to the transformer H0, and then H0 and X0 connected together, _not_ connected to the GEC or EGC, and then the X0 neutral continuing to loads, then I think that the _physics_ will work correctly, but I think you bump into some serious code issues.

I'm not aware of any code issues. You just have a non-SDS. None of the SDS requirements apply.

Cheers, Wayne

 

[synchro]

Can you read the real time ground fault current Ig from a display on the breaker feeding the transformer?

 

[bwat]

Can you read the real time ground fault current Ig from a display on the breaker feeding the transformer?

It's a Schneider Micrologic 6.0P trip unit. Looking at the manual, it looks like I can show A, B, C, and N, but not the net/residual (or calculated GF) from what I can tell.

 

[synchro]

It's a Schneider Micrologic 6.0P trip unit. Looking at the manual, it looks like I can show A, B, C, and N, but not the net/residual (or calculated GF) from what I can tell.

From what I see in the manual for the 6.0P, the main screen only shows A, B, C, and N. But if you go to the "metering" menu and select "instantaneous" current, it looks like the calculated residual ground fault current should be displayed (next to the lightning bolt to ground symbol ). But the unit in question would have to be checked out to see if it will display the residual current.

 

[bwat]

From what I see in the manual for the 6.0P, the main screen only shows A, B, C, and N. But if you go to the "metering" menu and select "instantaneous" current, it looks like the calculated residual ground fault current should be displayed (next to the lightning bolt to ground symbol ). But the unit in question would have to be checked out to see if it will display the residual current.

Interesting! Which version of the manual are you looking at? The one I grabbed from SE website (rev 7/2020) doesn't show the sample screen for instantaneous. Just that it "Displays current for A, B, C and Neutral (depending on network type)" for the Instantaneous display. Might be something we just have to go to the site to check out.

From here:

MicroLogic 5.0P and 6.0P Electronic Trip Units User Guide | Schneider Electric

This instruction bulletin provides information on display navigation, trip unit configuration, operation, and trip unit, rating plug, and battery...

www.se.com

 

[synchro]

Check out pg. 59 of the manual from the link that you provided.
In the figure 24 you showed, the box with [ Ia, Ib. Ic, In ] inside is still a menu selection, and you have to hit enter to get the display that includes the ground current.