Solar wye/wye transformer

[pv_n00b]

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.

There are a couple of utilities in the NE that require Ygyg transformers to be used to pass the loss of phase test. A Dyg transformer isolates the inverter from a loss of phase on the utility side and so it would fail the test. I Ygyg transformer passes the loss of phase through to the inverter side. But anywhere else I would go Dyg.

 

[electrofelon]

Most of the commercial PV inverters I work with can use a neutral but do not require it. As delivered there is a strap bonding the neutral lug to ground that must be removed if a neutral conductor is run to the inverter.

The use of a neutral conductor to an inverter has always been a bit of a mystery. I have always despised the neutral conductor as it seems unnecessary and is a waste of money and resources. I have been out of commercial PV for several years, so maybe this is changed , but in the past it seemed like most manufacturers would discourage skipping the neutral conductor even if the inverter was capable of working fine without it. I have never been able to clear answer on why this was and why using a neutral seems to be so common. In the past people on this forum have said it is required for UL 1741, but that doesn't make any sense because they are some inverters that do not need it so that seems to blow that theory.

 

[pv_n00b]

There is a reason that in a standard MV transformer build the X0 and H0 are bonded together internally and only a single X0/H0 terminal is brought out. Because it's bad mojo to ground one and not the other.

 

[pv_n00b]

The use of a neutral conductor to an inverter has always been a bit of a mystery. I have always despised the neutral conductor as it seems unnecessary and is a waste of money and resources. I have been out of commercial PV for several years, so maybe this is changed , but in the past it seemed like most manufacturers would discourage skipping the neutral conductor even if the inverter was capable of working fine without it. I have never been able to clear answer on why this was and why using a neutral seems to be so common. In the past people on this forum have said it is required for UL 1741, but that doesn't make any sense because they are some inverters that do not need it so that seems to blow that theory.

Why is it needed? Well, there were some 3 phase inverters in the past that used it as a load-carrying conductor. I remember one 3 phase inverter that was connected in a WYE configuration and would turn on the phase inverters separately to match the inverter load to the PV array output. I don't know of any inverters today that do that, maybe some large central inverters still do it. That's the only load carrying situation I have seen, the other uses have to do with voltage detection. If you want to detect loss of phase voltage you have to be able to measure the line to neutral voltage. There used to be a couple of inverters that would allow a 3 wire and EGC connection as long as it was to a grounded WYE system. They just used the EGC as a substitute for the neutral and did the phase voltage measurement from line to EGC.
Now, connection to an ungrounded delta service is pretty new for most inverters. No neutral is needed there, no loss of phase detection, just monitoring the L-L voltages.

 

[pv_n00b]

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

A Ygyg transformer with a solid ground will not usually provide effective grounding. Effective grounding requires a specific combination of impedances from line to ground that will limit the overvoltage from the PV system in the event of a ground fault on an isolated line, and a solidly grounded Ygyg typically does not do that. But if you put a specially designed grounding transformer in the system then you can have effective grounding since a Ygyg transformer is basically transparent to ground fault current and allows the grounding transformer to set the impedance to ground.

 

[bwat]

As fate would have it, I had very similar laments on this topic a year ago yesterday:

Why Why Wye Wye

I must need to revisit some of my old text books. If there's a Yg-Yg step up transformer from a 3 phase generator where the generator neutral is NOT grounded, if there's a single line to ground fault on the distribution side of the transformer, how is it possible to have over-voltage on the...

forums.mikeholt.com

The questions were somewhat answered, but not completely closed out.

 

[electro7]

Here are a couple photos of 2 of the sites nameplates. Tertiary winding??

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[synchro]

Here are a couple photos of 2 of the sites nameplates. Tertiary winding??

No, it doesn't show a tertiary winding. And so you should connect the utility neutral to X0.

 

[electro7]

Copy. Thank you!

Im breaking the XO bond to ground and connecting nuetral to XO.

Thanks again!

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[ggunn]

Next time use a delta - wye.

 

[wwhitney]

What does "HV is Grid-Side Voltage" mean after "Suitable for Bi-Directional Use"? Is it a problem that LV is going to be used for the grid-side?

Cheers, Wayne

 

[electro7]

You kidding, never going wye wye again, why do it?

Anyways, removed what I thought was the bonding strap and I still have continuity from the case and XO. Im now taking out the mounting bolts to see if that is whats making the connection. I may need to add rubber washers??

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[synchro]

What does "HV is Grid-Side Voltage" mean after "Suitable for Bi-Directional Use"? Is it a problem that LV is going to be used for the grid-side?

Cheers, Wayne

It means there are taps on the HV side and not the low voltage side as in the other transformer shown. As long as the grid voltage is balanced with an appropriate voltage level (volts per turn) it should be OK. But it does have limitations.

 

[GoldDigger]

It means there are taps on the HV side and not the low voltage side as in the other transformer shown. As long as the grid voltage is balanced with an appropriate voltage level (volts per turn) it should be OK. But it does have limitations.

For what it is worth, the nameplate does not show a removable jumper, but it does not show the neutral-EGC bond at all.
For the OP's reference there are two problems, one small and one potentially (sic) serious, with using LV for the grid side:
Normally the taps on the grid side windings can be used to adjust to the grid voltage such that for grid volatage on the high end there will be no risk of core saturation. With the taps on the inverter side that is not possible. But if the transformer is designed conservatively or if the grid voltage is close to nominal it will not be an issue.
The smaller inconvenience is that with the taps on the inverter side it is slightly less intuitive which taps to choose for a given grid voltage.

 

[wwhitney]

It means there are taps on the HV side and not the low voltage side as in the other transformer shown. As long as the grid voltage is balanced with an appropriate voltage level (volts per turn) it should be OK. But it does have limitations.

Ah, thank you. So the second nameplate is a better choice for this application. That part number has "BK" in it instead of "KB" in the first part number.

Cheers, Wayne

 

[electro7]

Well I think I am back to square one. Not sure how to break the XO bond. Going back to bonding HO and leaving the XO neutral off. What was the problem with that again- unbalanced current back through the XO, and without the XO nuetral it doesnt have anywhere to go? What would happen then?

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[jaggedben]

Not when the invertor is the source.

Yes, but not all invertors are grid-tied.
I have done several that involve PV, battery walls, hydrogen generators, and natural gas generation and provide backup power as well as cogeneration.

If the inverter is not supposed to island then the neutral can be derived from the utility transformer as Jon described. If it does have to island it can be derived from another transformer within the island. It does not have to come from a wye-wye.

 

[wwhitney]

Well I think I am back to square one. Not sure how to break the XO bond.

My understanding is that the consensus here is that you must break that bond in order to use that transformer with the grid on the LV side.

Have you checked the bonding on the transformer in your second photo? Since it is labeled "LV is grid-side voltage" perhaps X0 is not bonded.

Cheers, Wayne

 

[electrofelon]

There is a reason that in a standard MV transformer build the X0 and H0 are bonded together internally and only a single X0/H0 terminal is brought out. Because it's [,color=red] bad mojo [/color] to ground one and not the other.

There are a number of reasons grdY/grdY transformers are used by power companies where the distribution system is an MGN. One of them is ferroresonance, but sure for the sake of this discussion, we can just say, "it's bad mojo" is an acceptable answer.

 

[electrofelon]

A Ygyg transformer with a solid ground will not usually provide effective grounding. Effective grounding requires a specific combination of impedances from line to ground that will limit the overvoltage from the PV system in the event of a ground fault on an isolated line, and a solidly grounded Ygyg typically does not do that. But if you put a specially designed grounding transformer in the system then you can have effective grounding since a Ygyg transformer is basically transparent to ground fault current and allows the grounding transformer to set the impedance to ground.

And I can't help but think some/most/all of this "effective grounding" stuff is hogwash or voodoo. The reason I say this is I have done multi megawatt PV systems for Georgia power that had no such requirements, but then other utilities (such as national grid in NY and new England) where they require zig zag transformers for "effective grounding" on all their utility scale PV systems. Does national grid really know something that Georgia power doesn't??? Is the luminiferous aether in Georgia different than it is in NY???