480V vs 208V inverters

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Zee

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Some info needed on commercial systems. I do mainly resi.

IN GENERAL
For commercial, what is the most common AC Voltage in the USA?
How common is 208 Vac Wye?
If, as I read, 208 is the most common commercial voltage why are "all" solar inverters 480V/277V?
Is it standard practice in comm. solar to install a transformer to get to your particular AC V?

SPECIFICALLY
Right now, I am tasked with re-powering an (E) 28 kW-DC, 15 yr old system with an (E) 208 V Wye Xantrex inverter (30 kVA) and separate ground mounted isolation transformer.
I am having a helluva time finding a good inverter match.

To make it harder, The DC circuit is combined on roof into a single #2/0 conductor circuit.
So, I need a single inverter (not two).
I also need a single DC input on that inverter that can handle that amperage. Many 30 kW inv.s seem to have multiple inputs.
 
Have you evaluated what it would cost to replace the single feed circuit with a multiple wire circuit instead and divide the array output at the combiner location?
It may be better in the long run to incur this incremental expense to allow more choice in the configuration of the rest of the system. Otherwise your limitation to either a single MPPT input or several MPPT inputs slaved to single master for Vmp setting may make it impractical to use available modern inverters.
Note that some inverters with two or more sets of input terminals allow you to configure them either as two separate MPPT inputs or parallel them into a single MPPT input.
 
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I would say 208V services are much more common than 480 in terms of number of buildings with each. I have always been a bit baffled by the lack of offerings in 208 inverters - both string and central. IIRC, the best string you can do is a fronius 15KW symo. That will do 1kv strings, and is probably the only 208 string inverter out there that does 1kv strings.

If you didnt see it, in this thread https://forums.mikeholt.com/showthread.php?t=197410 from a little while back I ranted against using 480 inverters with transformers to connect to 208 services. Obviously though, depending on the size and wire run, just the wire savings could make it worth having a tranny.

I concur with Goldie about seeing what it would take to split that combined DC up. You would have more inverter options, and likely multiple MPPT's
 
Zee said:
Some info needed on commercial systems. I do mainly resi.

IN GENERAL
For commercial, what is the most common AC Voltage in the USA?
How common is 208 Vac Wye?
If, as I read, 208 is the most common commercial voltage why are "all" solar inverters 480V/277V?
Is it standard practice in comm. solar to install a transformer to get to your particular AC V?

SPECIFICALLY
Right now, I am tasked with re-powering an (E) 28 kW-DC, 15 yr old system with an (E) 208 V Wye Xantrex inverter (30 kVA) and separate ground mounted isolation transformer.
I am having a helluva time finding a good inverter match.

To make it harder, The DC circuit is combined on roof into a single #2/0 conductor circuit.
So, I need a single inverter (not two).
I also need a single DC input on that inverter that can handle that amperage. Many 30 kW inv.s seem to have multiple inputs.
Click to expand...

A light commercial application will likely be built with a 120/208V service, and this is the default service voltage unless there's an advantage to having 277/480V. Even larger commercial/industrial applications, will most likely have a 120/208V grid somewhere on the premises, due to 120V needed for receptacle loads. 277/480V would be the main service voltage if most loads need 277V or 480V, while a step-down transformer might handle ~10% of the building loads that need 120V.

In spite of this fact that 120/208V services are more common, inverters for these grids are less common because of economic reasons. Generally, voltage is cheap while current is expensive. Voltage is "paid for" with insulators like plastic and ceramics, while current is "paid for" with metal conductor/semiconductors and heat sinks. A 208V 3-phase inverter will be about 40% the power of an otherwise equivalent 480V 3-phase inverter. Case and point, compare SolarEdge's 14.4kW inverter with their 33.3kW unit. Same physical size, same output amps, same input amps & string qty, same housing, same heat sink. The 33.3 kW unit has more economy of scale on its cost, and the 33.3 kW has the capability of higher DC operating voltage, which gives you an advantage on the DC side of system design. For this reason, 480V grids are the standard solution for an array not electrically connected to a building, which puts economic pressure to manufacturer 480V inverters in greater numbers.

It used to be that a transformer was part of the inverter. The winding ratio would allow you to build roughly the same inverter, and adapt it to ether grid voltage per how its internal transformer is built. Or by changing the taps on an adjustable transformer, as was the practice with string inverters. With transformerless inverters, that is no longer the case. You would need an external transformer to step from a 480V inverter to a 208V building grid.
 
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The "standard" voltage rating of the services you run into will pretty much depend on the size of the facilities you work with. Typically once the current at a service hits a certain point, and this varies by utility, the utility will force a change in the service. It usual progresses something like 240/120V split phase to 240/120V 3ph or 208/120V 3ph then to 480/277V 3ph. After that you get into primary voltage service for really high ampacities.

If you deal mostly with smaller commercial buildings and smaller PV systems then for you most services will be 240/120V split phase with some 240/120V or 208/120V 3ph once in a while. If you deal mostly with larger buildings and campuses then you will mostly see 480/277V 3ph with the occasional 240/120V or 208/120V 3ph and then you use a transformer.

Worst thing in the world is trying to connect a large PV system to a 208V service. Crazy number of parallel conductors, maybe a cable duct between the transformer and the service SWBD. If it's a supply side interconnection, and it almost always is, then you have to land all that in way too small an area.
 
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Thank you gentlemen for the excellent and diverse info!

Looks like I have to "uncombine" the DC input AND add a 480 to 208 VAC transformer.

Although,
I can skip transformer by getting 2 @ 15 kW (Solectria or Fronius Symo) inverters that are available in 208 VAC output.

Still comparing prices, features, LABOR etc
 
So after weeks of delay and digging, it looks like i cannot easily un-combine the 11 DC strings.
So i need an inverter with a single DC input.

The Fronius SYMO fits the bill, due to what it mentions as "direct pass through avaialable" for pre combined strings)
But, largest SYMO is 24 kW.
Array size is 31.79 kW (10 or so years old, Mitsubishi 170W mod.s)

SE facing at approx 30 degree pitch.

1. What will I lose annually in AC output? and... by "I", I mean customer :D

2. Yaskawa (AKA Solectria) has a 28TL inverter which I prefer as 28 kW inverter would take 32 kW array - no prob.
They mention "fuse bypass available" but I am not sure what that means..... and how many amps allowed on DC input?

FWIW - The array rated Imp is 76A on 1/0 conductors.
 
Never mind above questions. Fronius SYMO inverter it is.

Is there a commonly used 24 kW, 480 to 208 VAC transformer in the solar biz?
Any particular features or any xformer with above rating will do?
 
Zee said:
Never mind above questions. Fronius SYMO inverter it is.

Is there a commonly used 24 kW, 480 to 208 VAC transformer in the solar biz?
Any particular features or any xformer with above rating will do?
Click to expand...

You just need a typical dry ventilated 208 Delta to 480/277 wye. Looks like it's got to be 30 KVA. I have also worked on a few projects where autos were used, but I don't think there is any efficiency or cost advantage with that much of a voltage change. An iso would likely have taps which could come in handy.
 
electrofelon said:
You just need a typical dry ventilated 208 Delta to 480/277 wye. Looks like it's got to be 30 KVA. I have also worked on a few projects where autos were used, but I don't think there is any efficiency or cost advantage with that much of a voltage change. An iso would likely have taps which could come in handy.
Click to expand...

That topology is atypical, and is subject to the risk that the installer makes an oversight when ordering, and gets it backwards. So you are looking at a somewhat custom transformer, rather than the run-of-the-mill dry-type transformer used in buildings that have both voltage systems. Usually, the delta is on the 480V side, and the wye is on the 120/208V side. But given inverters that need a neutral, you'll need a wye system on the 480V side in some form or another.
 
Carultch said:
That topology is atypical, and is subject to the risk that the installer makes an oversight when ordering, and gets it backwards. So you are looking at a somewhat custom transformer, rather than the run-of-the-mill dry-type transformer used in buildings that have both voltage systems. Usually, the delta is on the 480V side, and the wye is on the 120/208V side. But given inverters that need a neutral, you'll need a wye system on the 480V side in some form or another.
Click to expand...
I hear that stated often, but I don't consider that an uncommon transformer at all. I run into them in the wild (for non solar applications) now and again and it's a standard stock item, ready to ship :thumbsup:
 
Thank you, both.

Yes, I am told I need a neutral.

The building does have a 208Y/120 3PH 4W system.

Curious in general with transformer terminology:
I noticed you specified a "step up transformer" and you also mention "208V to 480V" and not the other way around (480V to 208V)? I just want to be 100% sure these two indicate line and load and it is fixed.
In other words: solar must be on the load side in general and the utility on the line side?

Silly me, from a theoretical perspective, I always thought transformers would work in either direction.......of course with opposite effect. But it seems the line and load sides are fixed also?
Just theory, but good to know.
 
Zee said:
Thank you, both.

Yes, I am told I need a neutral.

The building does have a 208Y/120 3PH 4W system.

Curious in general with transformer terminology:
I noticed you specified a "step up transformer" and you also mention "208V to 480V" and not the other way around (480V to 208V)? I just want to be 100% sure these two indicate line and load and it is fixed.
In other words: solar must be on the load side in general and the utility on the line side?

Silly me, from a theoretical perspective, I always thought transformers would work in either direction.......of course with opposite effect. But it seems the line and load sides are fixed also?
Just theory, but good to know.
Click to expand...

Generally one should ignore the energy flow direction and just consider the inverter side like any other SDS and load. Transformers do work either direction, but are usually designed to have lower inrush current on the utility side for easier starting.
 
Zee said:
Thank you, both.

Yes, I am told I need a neutral.

The building does have a 208Y/120 3PH 4W system.

Curious in general with transformer terminology:
I noticed you specified a "step up transformer" and you also mention "208V to 480V" and not the other way around (480V to 208V)? I just want to be 100% sure these two indicate line and load and it is fixed.
In other words: solar must be on the load side in general and the utility on the line side?

Silly me, from a theoretical perspective, I always thought transformers would work in either direction.......of course with opposite effect. But it seems the line and load sides are fixed also?
Just theory, but good to know.
Click to expand...

Some (most?) transformers are listed for bidirectional operation but some are not; that's a question for the manufacturer. You'll want your transformer to be delta (no neutral) on the utility side and wye (grounded neutral, separately derived) on the inverter side.
 
ggunn said:
Some (most?) transformers are listed for bidirectional operation but some are not; that's a question for the manufacturer. You'll want your transformer to be delta (no neutral) on the utility side and wye (grounded neutral, separately derived) on the inverter side.
Click to expand...

And just to clarify, I believe that a transformer must be listed/marked as suitable for being "reverse fed" and applies to which side supplies the magnetizing current, not which way the energy (usually) flows. In other words a transformer used for a pv system need not be marked as suitable for reverse feed.
 
YGyg vs. Dyg

YGyg vs. Dyg

Issues to consider:
(1) If the inverter needs to properly sense line-ground voltage to properly implement anti-islanding, you will need to use a YGyg transformer. If you use a Dyg transformer, you lose the ability to properly sense line-ground voltages.
(2) Transformers can move power in both directions but are often rated for excitation and energization from one side. That would be the utility side. The concern is how close the windings are to the core and the resulting inrush current. Consult the manufacturer.
(3) A transformer introduces 24x7 no-load loss and operational I2R loss. Both impact the overall PV system performance. Consider those when selecting a transformer.
(4) It is normal for a 600Vdc system to be used for a 208 or 240V AC connection. At 480V, a 1000Vdc system is used. For a 1500Vdc system, they often operate at 600Vac. Check the voltage rating of the array, if the modules are 600Vdc rated, stick with a 208/240V inverter and skip the transformer.
(5) An autotransformer is YGyg and has a smaller core so core loss goes down. A good choice if you must use 480V inverters on a 208V service.
 
beanland said:
Issues to consider:
(1) If the inverter needs to properly sense line-ground voltage to properly implement anti-islanding, you will need to use a YGyg transformer. If you use a Dyg transformer, you lose the ability to properly sense line-ground voltages.
...
Click to expand...

I'm no expert on transformer engineering but I don't understand how the utility side configuration has any effect on this.
 
jaggedben said:
I'm no expert on transformer engineering but I don't understand how the utility side configuration has any effect on this.
Click to expand...

Same here. I am open to being schooled in this. Inverter could still detect utility L-G voltages as the utility ground/neutral will be solidly connected to the inverter side neutral thru the egc system, so I am skeptical. Also not sure why "some" inverters would want that and others wouldn't. I do recall Mr Gunn stating that he has ran across inverters that speced a wye-wye, but I assumed that was one of those frequent cases of a manufacturer meddling in things they shouldn't care about.
 
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