Design using 208V Delta: 480Y/277V transformer connection

[ggunn]

No wye:wye is not the way it is done, I think someone is giving you hog wash. In fact I have never seen or heard of a wye wye being used in <600 systems. Utilities of course use wye:wye extensively but that is a different animal.

FWIW, when I first started doing this (connecting 480V inverters to 208V services) I got conflicting advice from inverter manufacturers. Some said wye to delta, some said wye to wye. In each case I followed their recommendations and both ways worked and are still working as far as I know.

But wye to delta is a lot more likely to come off the shelf (so to speak) and running a neutral from the service is a waste of materials, so if for no other reason I now use wye to delta for these installations.

 

[pv_n00b]

FWIW, when I first started doing this (connecting 480V inverters to 208V services) I got conflicting advice from inverter manufacturers. Some said wye to delta, some said wye to wye. In each case I followed their recommendations and both ways worked and are still working as far as I know.

But wye to delta is a lot more likely to come off the shelf (so to speak) and running a neutral from the service is a waste of materials, so if for no other reason I now use wye to delta for these installations.

I've noticed this too, inverter manufacturers are not consistent. Add autotransformers into the mix and it get's even more confusing. Then there is the isolation issue. Wye-Delta transformers isolate the inverter and prevent the inverter from detecting a ground fault on the service and shutting down in some conditions. A Yg:Yg or autotransformer will pass the ground fault through and allow the inverter to sense and respond to it. There is a whole list of other considerations, many which do not apply to smaller PV installations under 1MW.

 

[ggunn]

I've noticed this too, inverter manufacturers are not consistent. Add autotransformers into the mix and it get's even more confusing.

Something that rose to bite me with autotransformers is that they generally lack tap points to fine tune the voltage. I used autotransformers for a few jobs only to find that sometimes the utility line voltage was at the top of their range, and that plus the voltage drop in the conductors would occasionally push the voltage at the inverter up out of its operating window, which necessitated the addition of buck-boost transformers to bring it back in. Lesson learned.

 

[electrofelon]

I've noticed this too, inverter manufacturers are not consistent.....

Which is likely just overthinking by the inverter manufacturer. I say this because we have the utility transformer in the mix too which we have no control over, and may not even know how it is connected. Of course for liability and professional reasons, I would follow what the manufacturer says, even though I doubt it matters from an operational standpoint.

As Winnie mentioned, there may be utility requirements that dictate the transformer arrangement. National grid, who I think doesnt know what they are doing all the time and are in this "effective grounding" cult, has required a zig zag transformer when their transformer was already Yg:Yg.

 

[beanland]

Sensing Voltage

Sensing Voltage

One of the reasons to use a YGyg (208Y/120V-480Y/277V) transformer is to allow the inverters, connected to the 480Y/277V, to sense line-ground voltages on the 208Y/120V service. This is an important part of the anti-islanding scheme. If you use a 208V delta winding, you lose the ability to sense line-ground faults on the 208V system. So, even though they have their problems, YGyg transformers are often used.

That said, a 3-leg YGyg transformer will have problems with harmonics because there is no zero-sequence path for magnetic flux. Using a 4-leg or 5-leg core solves that problem. It is like using three single-phase transformers.

 

[electrofelon]

One of the reasons to use a YGyg (208Y/120V-480Y/277V) transformer is to allow the inverters, connected to the 480Y/277V, to sense line-ground voltages on the 208Y/120V service. This is an important part of the anti-islanding scheme. If you use a 208V delta winding, you lose the ability to sense line-ground faults on the 208V system. So, even though they have their problems, YGyg transformers are often used.

I am skeptical. If it's "an important part of the anti islanding scheme" then why is it not required? And what is the issue with a line to ground fault on the side opposite the inverter?

 

[pv_n00b]

I am skeptical. If it's "an important part of the anti islanding scheme" then why is it not required? And what is the issue with a line to ground fault on the side opposite the inverter?

Well, the NEC does not care about anti-islanding, that's a UL Standard issue. And the UL Standard does not care about how the inverter is interconnected, that's an NEC issue. Round and round we go.

 

[jaggedben]

Well, the NEC does not care about anti-islanding, that's a UL Standard issue. And the UL Standard does not care about how the inverter is interconnected, that's an NEC issue. Round and round we go.

:? I think electrofelon meant 'why is it not required by UL?' Whether it's an important part of the method depends on the method, right?

 

[electrofelon]

Well, the NEC does not care about anti-islanding, that's a UL Standard issue. And the UL Standard does not care about how the inverter is interconnected, that's an NEC issue. Round and round we go.

A few things: I disagree that the NEC doesnt care about anti islanding. Although that term in not explicity used, see 705.40 and 705.42. Also if that specific transformer connection was important for anti islanding, why do some/most manufacturers not say (which as JB notes UL could require) we must use a wye:wye connection?

 

[pv_n00b]

A few things: I disagree that the NEC doesnt care about anti islanding. Although that term in not explicity used, see 705.40 and 705.42. Also if that specific transformer connection was important for anti islanding, why do some/most manufacturers not say (which as JB notes UL could require) we must use a wye:wye connection?

The NEC version of "disconnect on loss of primary source" is really just the tip of the iceberg of anti-islanding. It provides no details because the NEC assumes the details are in UL 1741. So the NEC is like, "I care darling, but don't bother me, let's talk interconnections to death instead."

Don't assume because something is not in a UL standard that it might not be important. Keep in mind that UL standards are developed by volunteers with limited time, limited resources, and the knowledge bank is limited to whatever knowledge each STP member brings to the table and cares to share. I think based on what they are working with they doing a bang up job, but I do not think they are all knowing and cover every base. This is compounded by the fact that PV inverter electronics are super complex, there are many ways to do one job and manufacturers are very protective of how they do things. It's just really complex coming up with standards for these systems that are not basic black box tests.

As to why the type of transformer might not be considered, the UL Standard concerns how an inverter reacts to the inputs on it's AC input terminals. It's a product standard and the product is the inverter. If there are other devices between the inverter and the POCC that's not considered, for good or bad.