Solar wye/wye transformer

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electro7

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Northern CA, US
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Electrician, Solar and Electrical Contractor
I have a 208V wye on the utility side and 480V wye on the PV side. Both have neutrals XO and HO. I did run a nuetral conductor from the utility to XO. The transformer is bonded on the XO side but I have no continuity between HO and ground. I want to make sure I ground and bond this correctly. I am thinking I should disconnect the neutral landed on the XO and add a bonding jumper from HO to the can and grounding electrode. Does that sound correct?

The EGC on the 208V side would be connected to the HO as well in this case.


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I am not sure whether the 208 neutral should be run and connected to XO, maybe someone else will comment on that, but XO should definitely NOT be bonded / connected to ground. HO should be bonded and connected to an acceptable GES like any SDS.
 
Okay. I did remove the XO nuetral but the XO terminal is still bonded to ground. I may need to remove the bonding strap to make that disconnect, if its needed.

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With a wye:wye transformer the secondary neutral to ground voltage _may_ be unstable if the primary neutral is not connected. There may also be problems with high impedance to ground fault currents.

The reason is that with a wye:wye transformer each secondary phase is coupled to only one primary phase. Unbalanced current on the secondary needs to be matched by unbalanced current on the primary, which might not be possible without the primary neutral.

However a wye:wye transformer can be built with additional core limbs or a tertiary delta winding to solve this issue, so the decision to connect the primary neutral is very dependent on the specific installation.

Jon
 
This is very different from a wye to delta configuration, where connecting the primary neutral can cause severe problems and even equipment damage.
 
I think what I dont want is both XO and HO bonded to ground, correct?

At first the HO was not bonded to ground and I was concerned that if there were a ground fault on the 480V PV side it wouldnt clear. I feel better about the HO being bonded to ground along with the EGC from the 208V side.

The question is if I should remove the XO bond to ground and re-terminate the nuetral from the 208V service to the XO terminal. I think thats the question. Let me know what you think.





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Its a 480V 3 phase solar system with a 2 pole circuit for the communication and a single pole circuit for lighting- phase a,b,c being used for these.

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electro7 said:
I think what I dont want is both XO and HO bonded to ground, correct?

At first the HO was not bonded to ground and I was concerned that if there were a ground fault on the 480V PV side it wouldnt clear. I feel better about the HO being bonded to ground along with the EGC from the 208V side.

The question is if I should remove the XO bond to ground and re-terminate the nuetral from the 208V service to the XO terminal. I think thats the question. Let me know what you think.





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Since the neutral point of the primary wye may carry substantial imbalance load (although far less with the PV than with actual loads) the X0 shoud be solidly bonded to the input neutral wire and disconnected from transformer frame ground. Even though the EGC is capable of carrying significant current with low VD, it would all be objectionable current.
 
electro7 said:
I think what I dont want is both XO and HO bonded to ground, correct?
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I believe the X-0 should be connected to the utility neutral, and H-0 should be connected to the solar neutral.

Yes, X-0, which is bonded, and H-0, which should be bonded, will inadvertently tie the neutral together.
 
LarryFine said:
Yes, X-0, which is bonded
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But bonding X-0, which is on the utility side here, would be a violation of 250.24(A)(5) "A grounded conductor shall not be connected to normally non–current-carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article."

Cheers, Wayne
 
The last grounded Y to grounded Y I installed was for a PV system. We eventually had to remove the ground jumper on the building side neutral while leaving it connected on the invertor side. We were having erratic GF tripping on the building breaker.
 
wwhitney said:
But bonding X-0, which is on the utility side here, would be a violation of 250.24(A)(5) "A grounded conductor shall not be connected to normally non–current-carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article."
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I'm not saying that X-0 or the neutral should be connected to the "normally non–current-carrying metal parts."

I'm saying that X-0 should be connected to the source neutral conductor, which is bonded back at the service.
 
If the primary side X0 of this transformer is connected to the supply neutral (see my comment above, such connection is a maybe) then X0 will eventually be connected to grounded metal. This X0 should _not_ be direct bonded to an EGC or GEC.

It is connected via the grounded circuit conductor (AKA neutral) to the source transformer X0 (possibly the utility transformer) which will be bonded to a GEC.

Jon
 
electrofelon said:
I wish people would just install delta->wye transformers for these. It just seems like some people think the wye-wye is "more grounded" or some other silly reasoning.
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I thought that the only reason to install a wye:wye was specifically because the utility required it for _their_ 'effective grounding' requirements.

Hv+Lv can probably much better explain what utility 'effective grounding' means, but I believe it involves voltage stability on multi-grounded systems, rather different than the single point grounded systems used in building wiring.

-Jon
 
winnie said:
I thought that the only reason to install a wye:wye was specifically because the utility required it for _their_ 'effective grounding' requirements.

Hv+Lv can probably much better explain what utility 'effective grounding' means, but I believe it involves voltage stability on multi-grounded systems, rather different than the single point grounded systems used in building wiring.

-Jon
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Jon I think you are correct that the only reason to install a wye wye is if required by the utility. National grid (my utility) here in central NY has these "effective grounding" requirements for larger PV systems.. They typically require a zig zag transformer, even if the serving distribution system is a wye and the Poco transformer is a grdY/grdY. I do not actually know if they would require that a customer owned transformer, if used, be a wye wye, but I would not be surprised.
 
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