Connecting 480 3ph inverters to 480 Delta--208/120 transformer

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jim dungar

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Even if connecting them to a 480V delta xfmr would work in some way, I'd think that even if an inverter went kaput, even for some other reason, the 480V delta thing would void the warranty.

I rarely design systems with the intent being to just 'keep the warranty in effect'.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I didn't say one HAD to- in fact, it's the NOT recommended way, on pages 6 and 7. I meant: you'd have to go from 480/277V TRIPOWER inverters to a 480/277 >> 480 delta xfmr.
Are you saying you did that 480/277 to 208/120 thing with one three phase xfmr? Because I can't seem to find one- a single 3 phase 208/120 step up to 480/277, sure, but...
I did it with a bank of AE 480/277Y inverters which interconnect the same way as a Tripower. The transformer was not at all hard to find; I just called up... I believe it was Eaton... anyway, I just called up the transformer manufacturer and told them what I needed to do and they gave me a part number. This stuff is really not all that complicated.

You know, you can save yourself a lot of headaches when you are dealing with transformers by calling the companies that make them and talking to their apps engineers. They know a lot more about how to make happen what you want to happen than you (or I, for that matter) do.
 
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If the OP's inverter maker says it's OK to use 480V delta, great.

Got the threads crossed here- we're talking about Tripowers on this one, not Solectrias.
SMA in fact doesn't say delta is OK at all. They do make the Tripower MLX60 for 480V delta connections.

Connecting a 480/277V inverter to 480 delta makes no sense to me- you'd have to connect L1 + L2 to xfmr H1, L2 + L3 to H2, and L3 + L1 to H3, and forget the neutral.
Even if possible, why would you want to do that?

I just called up the transformer manufacturer and told them what I needed to do and they gave me a part number.

I'd be really surprised if they recommended 480V delta there.

As far as the OP's situation, not sure about the details, but I don't think anyone has suggested getting 480/277 service instead of 208/120.
I personally think you have to consider the total load vs. PV output numbers.
If the loads are going to be using up most of the PV output, you'd want to go Tripowers 480V(but really 277V) >> 208/120 xfmr.

If the PV output is considerably more than the loads, it'll be going straight to the grid most of the time- why step it down to 208/120 first?
If the 208/120 service is 200A and supplied by 75kVA of xfmrs, the POCO might say "ok, 75kVA of 480/277 is 90.3A, that's plenty for 48.2A max of Tripower output."
Then the step down to 208/120 xfmr will run less overall.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I just called up the transformer manufacturer and told them what I needed to do and they gave me a part number.

I'd be really surprised if they recommended 480V delta there.
So would I. Why would they? My inverters were 480/277Y.

My larger point is that rather than slog through data sheets trying to figure out what would work (and taking the chance that I would buy the wrong expensive part, or cause a fire, or do something equally horrendous), I called the transformer company and talked to an applications engineer. I got an expert in the field who told me exactly what I needed for my system.
 
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I got an expert in the field who told me exactly what I needed for my system.

I hear you- but at the same time, like jim said the inverter guys might not know everything about xfmrs, so it follows that the transformer people might not say "oh sure, those inverters" right off the top of their heads.
Point being, you could have both inverter and xfmr guys in agreement, and still get a "no, you can't do that" from the POCO.

side note:
intent being to just 'keep the warranty in effect'.

Sure, but designing in accordance with the warranties can be a requirement for financial incentives (gov. grants, POCO PBIs, etc).
 

jaggedben

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Solar and Energy Storage Installer
Then you haven't done a very good design job.

Actually I think PVfarmer has a valid point (for once). Some utilities have rules about DG that just don't really make any engineering sense. In some cases it's an overly broad principle that does not need to be applied to every system.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
Some utilities have rules about DG that just don't really make any engineering sense. In some cases it's an overly broad principle that does not need to be applied to every system.

The POCO rules make not "make engineering sense", but the design still needs to conform to them. There is no sense building a system that cannot be used.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I hear you- but at the same time, like jim said the inverter guys might not know everything about xfmrs, so it follows that the transformer people might not say "oh sure, those inverters" right off the top of their heads.
Point being, you could have both inverter and xfmr guys in agreement, and still get a "no, you can't do that" from the POCO.
I respectfully submit that the transformer apps engineers have a much better shot at getting it right than someone who doesn't have a whole lot of experience in dealing with either transformers or inverters.
 

iwire

Moderator
Staff member
Location
Massachusetts
I respectfully submit that the transformer apps engineers have a much better shot at getting it right than someone who doesn't have a whole lot of experience in dealing with either transformers or inverters.

Or a DIY who likes to read.
 
ggunn-
the transformer apps engineers have a much better shot at getting it right

Sure, I hear you, not trying to argue- but the xfmr guys aren't going to do the math/run same simulations that the grid people do.
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Originally Posted by jim dungar

Then you haven't done a very good design job.
Actually I think PVfarmer has a valid point (for once). Some utilities have rules about DG that just don't really make any engineering sense. In some cases it's an overly broad principle that does not need to be applied to every system.

:blink:
Um...thanks, jb!
Could you give an overly broad example of something like that? Just out of curiosity.
I'd like to add to that- as well as rules that might not make sense, the POCO documents have parts that are sort of "left open", like the below.
There's another part of another doc that says over 50kW of PV "may have to be 3 phase...please consult the Company...".


So I'll change this:
"no, you can't do that" from the POCO.

to: the POCO might say "there's a a better way to do that in that particular area due to interface/balance issues".
As in, the farther out in the sticks a project is, the more important the single vs. 3 phase thing becomes. (also perhaps why they try not to offer delta service anymore?)

At the same time, you (or customers) don't really want to rewire a customer's entire system.
If the system in place is 120/240V, you'd probably just go with the same voltage inverters, unless of course you're going 50kW or more and "have to" (by POCO) go 3 phase.

My point being: standard services are 120/240, 208/120, and 480/277.
These "rules" (or guidelines) below could all be explained more clearly. Pretty sure it's saying you can connect PV to any service you want, but...what does "expand" mean here? I read it as "increase xfmr capacity".
So if you already have any of the bolded, you can add a PV system as long as the output doesn't exceed the xfmr capacity you have- if you want to go bigger, you get a new service.

But it's just plain confusing- they don't mention WHY 2400, 4160 and 4800V are still available.
Then the last line below kinda reads as if ONLY standard services are available, when it mentioned 2400V etc two lines before!

Further examples:
Someone has an old non-standard 240V delta service which is 30kVA (3 x 10kVA bank) and wants to install PV.
You might be able to use somewhere around 20-25kVA of inverters with that. It could be three 7000w single phase 240V inverters or a single 20kW three phase inverter stepped down thru xfmr.

But you sure couldn't install 40kVA of inverters- you'd need at least 50kVA of xfmrs for that, or you'd need at least 75kVA of xfmrs for 60kW of inverters.

Now..if there was 75kVA of 240 delta service, getting 60kW of 480/277V inverters and stepping every watt down to 240V might not make sense.

But with 75kVA of 480V delta service, the same 60kW-480/277V to 480 delta might just be enough of an improvement (over losses from stepping down to 240V, other costs, etc) to work....or you'd want a 480V delta inverter instead.

Or....you could get new standard service of 480/277- but standard is 200A @ 480/277V- that's kinda way to much for the same 60kW of 480/277V inverters (which are 72.4A max output).

I hope this helps prove my point that...it isn't always "simple"! :D

To rephrase that- the best standard service for any customer usage (which is also what the premises are wired at) might not be the same as the best service for same customer's PV ouput/grid connection, so if the PV output is 2x or 3x the usage...you'd match the inverter(s) to the service and use step-down xfmr(s) for the usage. PV is fairly new and all systems are different sizes and/or equipment- there aren't really "standards".

shall consult with the
Company to obtain a standard single or three phase 60 Hertz service at an appropriate delivery voltage.


shall not expand the use of such service, except in
very limited circumstances


3.8 SERVICES NO LONGER STANDARD
Non-standard services include, but are not limited to: 25 Hertz, 2 phase systems, 2 wire 120 volts,
240 volts delta, 460 volts wye, 480 volts delta, 600 volts delta, 2400 volts, 4160 volts or 4800 volts
services.
While 2400 volts, 4160 volts, or 4800 volts are no longer standard, they may still be available
at certain locations; consult the Company.


Customers now receiving non-standard service shall not expand the use of such service, except in
very limited circumstances at the sole discretion of the Company.
Customers with an existing non-standard service requesting a service change shall consult with the
Company to obtain a standard single or three phase 60 Hertz service at an appropriate delivery voltage.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
ggunn-
the transformer apps engineers have a much better shot at getting it right

Sure, I hear you, not trying to argue- but the xfmr guys aren't going to do the math/run same simulations that the grid people do.
How do you know that? The transformer apps engineers have to deal with utilities all the time. In my case a single phone call got me all the info I needed and my utility company had no problem with their solution. Once again, I think you are making this look a lot harder and more convoluted than it actually is in the real world. This stuff is not all that complicated and we deal with it all the time.
 
How do you know that? The transformer apps engineers have to deal with utilities all the time. In my case a single phone call got me all the info I needed and my utility company had no problem with their solution. Once again, I think you are making this look a lot harder and more convoluted than it actually is in the real world. This stuff is not all that complicated and we deal with it all the time.

Not trying to argue- my point was that the POCO are the only people who have the data about their own system robustness or lack thereof at the PCC/interface point.

For instance- the "preliminary info" from the POCO in one case says it's a 12.47kV radial feed and also that three phase service / available within ¼ miles = N/A, meaning it's available at zero distance, or they'll run 3 lines/12.47 delta to the final drop/pole instead of the existing two lines, one 7200V and one neutral.
That ain't much to go on.

At the same time/case- the overhead 120/240V drop is going to change from pole/overhead >> 120/240 main disconnect to pole/underground >> 480/277V main, which doubles the distance there (from 100 to 200 feet or so)
So when the POCO says that 4160V service is "possibly still available, please inquire", and also that customers can provide xfmrs...

A xmfr from PV output voltage up to 4160V does kind of makes sense just because of that 200 feet between main disconnect and the POCO pole/xfmr/12.47kV, and maybe some other reasons, too.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Not trying to argue- my point was that the POCO are the only people who have the data about their own system robustness or lack thereof at the PCC/interface point.

For instance- the "preliminary info" from the POCO in one case says it's a 12.47kV radial feed and also that three phase service / available within ¼ miles = N/A, meaning it's available at zero distance, or they'll run 3 lines/12.47 delta to the final drop/pole instead of the existing two lines, one 7200V and one neutral.
That ain't much to go on.

At the same time/case- the overhead 120/240V drop is going to change from pole/overhead >> 120/240 main disconnect to pole/underground >> 480/277V main, which doubles the distance there (from 100 to 200 feet or so)
So when the POCO says that 4160V service is "possibly still available, please inquire", and also that customers can provide xfmrs...

A xmfr from PV output voltage up to 4160V does kind of makes sense just because of that 200 feet between main disconnect and the POCO pole/xfmr/12.47kV, and maybe some other reasons, too.

The subject of this thread is how to connect a 480/277Y inverter to a 208/120Y service.

If the combined current at 208/120V fits into the MDP under the 120% rule feed a breaker. If it's more than that and less than the size of the service build a line side tap. If it's bigger than the service you have to do something else beyond the scope of this thread but there is no reason to transform down to 208/120Y if you are going to transform up to MV to interconnect
 
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