Ideal xfmr(s) for connecting 480/277Y inverters (60kW total) to 12470/7200 grid

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thanks. got it.

thanks. got it.

MV = Medium Voltage, i.e., 12470/7200V in this case.

I'm also using a program I paid for, but not sure about saying "hey try this it's great"...is that OK on this forum?


The existing service, as you told us in the first post, is 240/120 split phase. You are considering asking them to change your service to 3 phase 480/277 because it suits your desire to use Tripowers. (Why do you want to use those? I'm still not exactly sure.)
What transformer(s) they use to bring that to the service point pretty much doesn't need to be your concern.

I did say single phase 120/240 in the first post, but now I know it's single phase three wire 120/240.
The service is being upgraded because it (at 25kVA) can't handle 60kW of 480/277V.
It is sort of a concern because the POCO isn't paying for the xfmrs, BUT they might be able to get a better price on something...better, if that makes sense.

Tripowers are nice because you can:
1. Go with close to 800V DC or use the same panels, cut the strings in half and run at close to 400, and they work just as well either way. Which is handy, say you have a spot with a weird amount of panels, they're a lot more "adjustable", it seems.
2. The company's overall setups are pretty simple, and make sense (somewhat) to me, who is somewhat new to this.
3. It seems like you can overload them with panels like crazy, and they love it, which is good for the solar conditions around here.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I mean 3 single phase 7200V to 480V transformers there.
As far as the Tripowers, they seem to have the best features, and are a lot more flexible in short.

I misunderstood earlier; I thought you were talking about three three phase transformers, one for each Tripower.

Yes, I am totally clueless- I'm not the electrician here, or the POCO. The electrician (Master's license) would prefer if I deal with the POCO.
Basically, again, I want to be able to suggest something to the POCO fellow (who is quite nice) that may be less $$ or better some other way.

This is all about feed-in tariffs, there are all kinds of procedures and complications- 60 kW happens to be one of those points where it's "less complicated".

Via a feed in tariff is one way to interconnect ahead of the meter, but less complicated in what way?

Three single phase xfmrs wired in Y would connect to a 480Y/277 MDP...how?
Or if line side, to the inverters...directly?
Is there a problem having the 480Y from the inverters go BOTH out through the junction box AND in to the load through a 120/240V xfmr (paid by customer)?

I'm not sure that what you meant to say is "junction box". What sort of enclosure are you talking about? If you are saying that you want to connect the Tripowers BOTH in a FIT arrangement with the POCO AND to your loads, no, you cannot do that. One connection is ahead of the meter and the other is behind it. You cannot interconnect in two places except if it is two separate systems (see below for my suggestion).

I do not know of a way to make 240V split phase from 480V three phase with a transformer. Maybe a 277V to 120V transformer on two phases? Perhaps someone with more transformer experience will chime in.

The output of the PV is a lot more than any loads, 90% of the time. So it seems it would make sense to feed the PV into the load at 120/240, then the "unused" part goes straight into grid.
I don't know if you can do that. See above.

I guess I got the 3 single phase idea here on this forum!
" from what I understand on such a distribution system most of your pole top type transformer banks would usually be connected wye on the primary side - mostly because they build the banks out of commonly stocked single phase transformers that are connected line to neutral on the primary side."
3rd comment in this thread.
http://forums.mikeholt.com/showthread.php?t=161313

If you are determined to have PV functioning during outages, my advice would be to get a pair of Sunny Islands and a 240V Sunny Boy (NOT a Tripower) connected to them through a protected loads panel behind the meter, and connect your Tripowers ahead of the meter through 480V to MV transformer(s) to MV in a FIT arrangement with the POCO. 480V three phase behind the meter in a 240V split phase service is IMO going to be nothing but trouble.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I'm also using a program I paid for, but not sure about saying "hey try this it's great"...is that OK on this forum?
Recommending a program that you are not selling is fine on this forum.

I did say single phase 120/240 in the first post, but now I know it's single phase three wire 120/240.

That's the same thing.

The service is being upgraded because it (at 25kVA) can't handle 60kW of 480/277V.
It is sort of a concern because the POCO isn't paying for the xfmrs, BUT they might be able to get a better price on something...better, if that makes sense.
Hold on there. If you are going with a FIT, then you don't need a service upgrade; your feed from the inverters will be outside the service. The MV to 240V transformer is not part of the service; the service starts at the meter. If the issue is that the MV transformers on the pole aren't big enough, then the conductors feeding it may be too small as well. Here's another thing: if you upgrade the service to 480/277V from what it is now, then you can service 120V loads with a 480V/208V three phase transformer, but three wire 240V loads are going to be problematic.

Tripowers are nice ...
Yes, they are, but AFAIK you cannot connect a Tripower to a 240V single phase service no matter what size it is.
 
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Via a feed in tariff is one way to interconnect ahead of the meter, but less complicated in what way?

You have to deal with an additional layer of "paperwork" over 60kW, which is maybe related to the amps/services available? Over 60kW becomes a process twice or 3 times as long.
For many various reasons, long story...

I'm not sure that what you meant to say is "junction box". What sort of enclosure are you talking about? If you are saying that you want to connect the Tripowers BOTH in a FIT arrangement with the POCO AND to your loads, no, you cannot do that. One connection is ahead of the meter and the other is behind it. You cannot interconnect in two places except if it is two separate systems (see below for my suggestion).

I'm going by the "example" diagram from my POCO, the inverters go from the "AC Combiner" through an internal AND external diconnect, and into the "junction box", the JB is between the meter and the MDP.


I don't know if you can do that. See above.

I meant (in italics) into the load through a xfmr, which would go 480 to I think 208Y/120, as there is no 3 phase 240V machinery or motors.

The output of the PV is a lot more than any loads, 90% of the time. So it seems it would make sense to feed the PV into the load at 120/240, then the "unused" part goes straight into grid.


Recommending a program that you are not selling is fine on this forum.

me-
I did say single phase 120/240 in the first post, but now I know it's single phase three wire 120/240.
That's the same thing.
??

three wire 240V loads are going to be problematic.


Yes, they are, but AFAIK you cannot connect a Tripower to a 240V single phase service no matter what size it is.

Helioscope- you get a month for free- it is pretty great.

So then 240/120 split phase and 120/240 single phase three wire are...the same thing?

Do you mean 3 wire or 3 phase 240 loads?

The Tripower is connected to the xfmr, 480/277 to 208/120 which is connected to the load. So the current wiring for the load section can stay the same, with a new MDP? Because you's still be getting the 240V from two legs of 120V.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Helioscope- you get a month for free- it is pretty great.
Thanks. I am just now starting to get into Helioscope.

So then 240/120 split phase and 120/240 single phase three wire are...the same thing?
Yes.

Do you mean 3 wire or 3 phase 240 loads?
Three phase 240 is a completely different animal. It's delta only (no neutral) and incompatible with 240 single/split phase.

The Tripower is connected to the xfmr, 480/277 to 208/120 which is connected to the load. So the current wiring for the load section can stay the same, with a new MDP? Because you's still be getting the 240V from two legs of 120V.
No, sorry. With 208/120 three phase the 120V loads will be OK, but the 120V legs are 120 degrees apart so two legs only gives you 208V. You can't run 240V single phase loads from it and you cannot connect it to a 240V single phase service, which is what I have been saying...
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Too late to edit so I will just repost with corrections:

Helioscope- you get a month for free- it is pretty great.
Thanks. I am just now starting to get into Helioscope.

So then 240/120 split phase and 120/240 single phase three wire are...the same thing?
Yes.

Do you mean 3 wire or 3 phase 240 loads?
Three phase 240 is a different animal. It's delta only (no neutral) and incompatible with 240 single/split phase except (maybe) when the service transformer is a high leg transformer with a center tapped grounded neutral between phases A and C.

The Tripower is connected to the xfmr, 480/277 to 208/120 which is connected to the load. So the current wiring for the load section can stay the same, with a new MDP? Because you's still be getting the 240V from two legs of 120V.
No, sorry. With 208/120 three phase the 120V loads will be OK, but the 120V legs are 120 degrees apart so two legs only gives you 208V phase to phase. You can't run 240V single phase loads from it and you cannot connect it to a 240V single phase service, which is what I have been saying.

You could transform the 480 from the Tripowers to 240 3 phase and interconnect with a 240 high leg service if the POCO would tolerate the imbalance from the loads only drawing from the A and C phases, and understand, every time you run through a transformer you have losses. If you transform from 480/277 to 208/120 to go through the SI's and then up to 240 delta to interface with a 240 high leg service transformer you take a hit every time. And you will have grounding issues, I'll wager.
 
there's some good news

there's some good news

If you have a combiner panel, it should go to a single 3-phase circuit that would be your service. What transformer(s) they use to bring that to the service point pretty much doesn't need to be your concern.

Well, first of all, I hired a PE. He said, "this is doable".
SO that's good!

I hear what you are saying, but I'd like to politely disagree- trying to avoid the first dot below here, which is what the situation is as it stands.
They want Y-delta, or delta-Y, but not so much Y-Y. "Company acceptance review" sounds...extremely annoying.
It could turn out to be "oh, you're using Tripowers, that's fine", but if it takes a month to get that answer...

So, the distribution around here is 12470Y/7200. 3ph service is 3 cans on the pole, so those are presumably usually three 7200V to XXXvolt xfmrs wired together in Y on primary side and delta for the 3ph secondary.
Therefore, wiring the 480/277 inverter output into those 3 POCO xmfrs could go any of these three ways below...but what's the best?
The second dot seems to be saying "we'll go ahead and wire our side in delta because your inverters are Y output, not a problem".

And then the 480/277 to 120/240 xmfrs for the load could be three of these in the pdf, right?
They'd each take a leg to neutral from the inverters, so three 277V to 120/240V xmfrs could then be joined at the MDP, giving you a 100% balanced draw/ load?
Does that make any sense?
:blink:

http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram32.pdf

Effectively Grounded, Four-wire Multi-grounded 3-phase Wye EPS:
The Company requires that all interface transformers be configured to have a
wye connected primary winding with a fully insulated neutral, and the secondary
winding to have a delta connection.
• If infeasible, an alternative that is subject to Company acceptance review is a
primary wye grounded - secondary wye grounded transformer requiring a
grounded source permitted under specified conditions.
• Any DG or aggregate DG below 500kW in a Customer’s facility may be
permitted to utilize a primary delta - secondary wye grounded transformer.
• Where any DG or aggregate DG is 500kW or greater in a Customer’s facility,
ungrounded transformers (i.e. primary delta or Yg-Yg with ungrounded
source) are not permitted on any Company-owned effectively grounded, fourwire
multi-grounded 3-phase wye circuits.
 
You can't run 240V single phase loads from it and you cannot connect it to a 240V single phase service, which is what I have been saying.

You could transform the 480 from the Tripowers to 240 3 phase and interconnect with a 240 high leg service if the POCO would tolerate the imbalance from the loads only drawing from the A and C phases, and understand, every time you run through a transformer you have losses. If you transform from 480/277 to 208/120 to go through the SI's and then up to 240 delta to interface with a 240 high leg service transformer you take a hit every time. And you will have grounding issues, I'll wager.

Okay then, so (see previous comment to ben) take three legs to neutral from the Tripowers at 277V, and use three of these for the load?
Combine them at the load MDP as said @ ben?
What's the deal with the little empty circles here (which mean neutral) being on opposite sides (to left/to right) of the primary/secondary here?
It just means each side has its own neutral and they don't interact because of the "gap" in the xfmr, which is a good thing for overall balance?

http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram32.pdf
 
this is helpful

this is helpful

If yours is the typical facility, most of your transformers are loaded at about 35%. A few of them are overloaded, and you can really feel the heat rolling out of the vent (if a dry type). Some sources suggest loading every transformer to some value, such as 85%, to achieve maximum efficiency. It’s not that simple, however.
Don’t be a slave to matching your calculated load and the transformer spot on. You probably can’t do that anyhow, and energy efficiency is one of many considerations. It’s also worth noting that as you get closer to the optimum loading value, the increase of efficiency tends to diminish. So if the manufacturer says this model is at maximum efficiency when loaded to 88%, an application that loads it to only 75% is still far more efficient than one that loads it to only 35%. The key is to avoid grossly oversizing or under-loading the transformer.
http://ecmweb.com/electrical-testing/make-energy-transformation

The way I see it, if the efficiency of your transformers for your loads is important because you've paid for the power at the meter, and less efficient means your $$ are turning into heat...
Then, if the main goal is to export power and the load is secondary to that goal, the POCO transformers are also important.
Yes, you are getting paid for what goes out from the meter, and the electric loss is on the POCO side, but the heat loss means less power going out overall, so less $$ coming in.
I suppose it's sort of the same idea as overloading inverters, but sort of the inverse- getting the UNDERloading ratio right.
If the xfmr(s) are too big, they'll waste power when idle, and if too small run hot and not last as long while losing power at the same time, probably the latter is worse.
I think I said something before about xfmr efficiency curves...

It is common in the industry to oversize the PV array by using a PVto- inverter sizing ratio of around 1.15. After all, a well-designed system will typically have end-to-end system losses of about 15%–16%. Oversizing the array ensures that the inverter is driven to its maximum output, at least during the best sun hours of the day. System designers who are looking at a 20-year design life for the system will usually size the array-to-inverter using a 1.2 to 1.25 ratio. Some PV system integrators even routinely use a 1.3 ratio.
http://solarprofessional.com/articl...to-inverter-sizing-ratio?v=disable_pagination
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The way I see it, if the efficiency of your transformers for your loads is important because you've paid for the power at the meter, and less efficient means your $$ are turning into heat...
Then, if the main goal is to export power and the load is secondary to that goal, the POCO transformers are also important.
Yes, you are getting paid for what goes out from the meter, and the electric loss is on the POCO side, but the heat loss means less power going out overall, so less $$ coming in.

The way GT inverters work lets them export the same amount of power (essentially all that is available from the PV input) regardless of the voltage at which they are operating. So IR voltage increase at the inverter because of losses upstream of the meter will not have any effect on the exported power that the meter reads or the amount of power that the GTIs deliver.
 
The way GT inverters work lets them export the same amount of power (essentially all that is available from the PV input) regardless of the voltage at which they are operating. So IR voltage increase at the inverter because of losses upstream of the meter will not have any effect on the exported power that the meter reads or the amount of power that the GTIs deliver.

I think I get that- the 480/277 GT inverter uses the neutral to balance the phases. So as long as the xfmr isn't sized "wrong", going by this graph, the "losses" are negligible:

http://forum.allaboutcircuits.com/p...ph1.gif&hash=e68ab59b7f3c30c8a2e878e7b78905a3

Hmm. That seems to be saying high efficiency tp-1 xfmrs are good from 20 to 60%, but then 80 degree xfmrs are better over 60% or so.
And that you wouldn't want say an 80 degree xfmr loaded to only 20%.
No idea where that pic is from however.

Thanks!
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
So, the distribution around here is 12470Y/7200. 3ph service is 3 cans on the pole, so those are presumably usually three 7200V to XXXvolt xfmrs wired together in Y on primary side and delta for the 3ph secondary.
Therefore, wiring the 480/277 inverter output into those 3 POCO xmfrs could go any of these three ways below...but what's the best?
The second dot seems to be saying "we'll go ahead and wire our side in delta because your inverters are Y output, not a problem".

You need the secondary to be a wye or your Tripowers won't operate. Confirm that with SMA if you want to be sure, but that's the connection listed on the datasheet.

I'm assuming they are referring to primary and secondary in the conventional manner of the primary being on the utility side. Just to be clear, you need your side to be a wye or to choose a different inverter. It looks to me like bullet two is your only option and it's good you're under 500kW.

And then the 480/277 to 120/240 xmfrs for the load could be three of these in the pdf, right?
They'd each take a leg to neutral from the inverters, so three 277V to 120/240V xmfrs could then be joined at the MDP, giving you a 100% balanced draw/ load?
Does that make any sense?
:blink:

Not really. Whether the loads are balanced depends on which loads are turned on. And while I'm not the transformer expert here, I believe your 240/120 transformers would be hooked phase-to-phase on the 480V source.

And also, you're not taking any thing from the inverters, per se. The inverters (I believe) won't transmit any current on the neutral; that will come from the utility transformers. The Tripowers will deliver balanced 3-phase power. Any imbalance needs to be handled by the utility as they would normally handle it.

Depending one whether you really have any 240V loads that can't operate at 208V, a 480/277 to 208/120 transformer might be an option for better balancing the load. If your loads are simple appliances (e.g. heaters), they might be fine at 208, just less effective. I mean, you're wanting to radically change the service voltage here, you might want to be prepared for the possiblity that it will complicate the operation of the loads and require some replacement of equipment. If this is a feed-in tariff, why not just get a second service for it?
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Okay then, so (see previous comment to ben) take three legs to neutral from the Tripowers at 277V, and use three of these for the load?
Combine them at the load MDP as said @ ben?
What's the deal with the little empty circles here (which mean neutral) being on opposite sides (to left/to right) of the primary/secondary here?
It just means each side has its own neutral and they don't interact because of the "gap" in the xfmr, which is a good thing for overall balance?

http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram32.pdf
Didn't you say you hired a PE? If so, forget everything we've been saying and do what he tells you to do. It's his license on the line, so he'll set you straight, and he will be privy to a heck of a lot more detailed information than we've been getting from you. You'll have to have a PE stamp, anyway, so you may as well let him design it.
 
You need the secondary to be a wye or your Tripowers won't operate. Confirm that with SMA if you want to be sure, but that's the connection listed on the datasheet.
Yep, that I've got.

3 Incompatible transformers with STP TLUS-10
Sunny Tripower TLUS needs a neutral conductor, so transformers with delta topology in the low voltage side are not
compatible for direct connection of the inverter (see figure below e.g. YNd1).


It looks to me like bullet two is your only option and it's good you're under 500kW.

But one of these in the temco links would work, right?
Since the $$ comes in due to what goes out through the meter, this might be the right idea- it's what they want, and when they say "may be permitted" (for bullet 2) that doesn't mean WILL BE!
Simpler is always better.
The top of page 4 here:
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf

Is the same as the wiring diagrams for these here, right?
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0326.html
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0426.html

Wiring for the pricey one:
http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram19.pdf

Whether the loads are balanced depends on which loads are turned on. And while I'm not the transformer expert here, I believe your 240/120 transformers would be hooked phase-to-phase on the 480V source.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0120.html

But if there are 3 of those ^^^ single phase 15 kVAs, all 3 going into a single....pardon me...busbar? Of the MDP? Then all 3 will supply any loads equally.
If there were a bunch of random circuits going into the 3 xfmrs unequally, that would be unbalanced. If the load draws on all three equally, they're balanced, I'd think.

Depending one whether you really have any 240V loads that can't operate at 208V, a 480/277 to 208/120 transformer might be an option for better balancing the load. If your loads are simple appliances (e.g. heaters), they might be fine at 208, just less effective. I mean, you're wanting to radically change the service voltage here, you might want to be prepared for the possiblity that it will complicate the operation of the loads and require some replacement of equipment. If this is a feed-in tariff, why not just get a second service for it?

So...what IS the deal with that 208/240 thing? The heavy load is all 240 motors, single phase that run off the 2 legs of 120. THAT causes imbalance!
They'd overheat running at 208 volts, right?
But...if 208 motors can run at 240 just fine, can't 240 motors run at 277V?
:dunce:

It's not just a tariff- the 120/240 200A service now is a hair too small, so might as well upgrade it to fit the PV, when the PV output will be twice the yearly usage.
If the service was adequate, the 2nd meter thing would be the idea.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So...what IS the deal with that 208/240 thing?

The two 120V legs of a 240/120V split phase service are 180 out of phase, so between them it's 240V, while the two 120V legs of a 208/120V three phase service are 120 degrees out, so you only get 208V between them; is that what you are asking?

The heavy load is all 240 motors, single phase that run off the 2 legs of 120. THAT causes imbalance!
They'd overheat running at 208 volts, right?
But...if 208 motors can run at 240 just fine, can't 240 motors run at 277V?
:dunce:

It's not just a tariff- the 120/240 200A service now is a hair too small, so might as well upgrade it to fit the PV, when the PV output will be twice the yearly usage.
If the service was adequate, the 2nd meter thing would be the idea.

You cannot connect a 480/277V three phase inverter to a normal (not a high leg) 240/120V split phase service no matter what you do with transformers. No way. No how. The size of the service is irrelevant. Maybe you can connect to a 240/120V three phase high leg service through a delta (on the 240V 3 phase side) and wye (on the 480V 3 phase side) transformer, but grounding the derived neutral on the wye side will likely be an issue.
 
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The two 120V legs of a 240/120V split phase service are 180 out of phase, so between them it's 240V, while the two 120V legs of a 208/120V three phase service are 120 degrees out, so you only get 208V between them; is that what you are asking?

Nope, I get that now. I meant- you can run a 208V motor at 240V and it's good. Running a 240V motor at 208 makes the motor overheat and be less efficient, because it's drawing more amps to compensate for the lack of voltage...or something like that?

You cannot connect a 480/277V three phase inverter to a normal (not a high leg) 240/120V split phase service no matter what you do with transformers. No way. No how.

What's wrong with taking each leg-to-neutral of the 480/277 inverter, so you've got 3 legs of 277V (all connected to the same neutral) running into three single phase 277 to 120/240 xfmrs?

Maybe you can connect to a 240/120V three phase high leg service through a delta (on the 240V 3 phase side) and wye (on the 480V 3 phase side) transformer, but grounding the derived neutral on the wye side will likely be an issue.

You lost me at "grounding the derived neutral". Why "derived"?
And wouldn't the neutral connect to a neutral on the 480 side of the xfmr, with no neutral on the 240 delta side...so grounding the 240 side would seem to be an issue...but it's just that the neutral that you tap into the one side of the delta "triangle" with is also a ground in that scenario, right?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What's wrong with taking each leg-to-neutral of the 480/277 inverter, so you've got 3 legs of 277V (all connected to the same neutral) running into three single phase 277 to 120/240 xfmrs?

You cannot change the timing of the phases with transformers. No matter what you do, your inverters have three phases 120 degrees apart and a 240/120V split phase service has two phases (sort of) 180 degrees apart.

me said:
Maybe you can connect to a 240/120V three phase high leg service through a delta (on the 240V 3 phase side) and wye (on the 480V 3 phase side) transformer, but grounding the derived neutral on the wye side will likely be an issue.

You lost me at "grounding the derived neutral". Why "derived"?
And wouldn't the neutral connect to a neutral on the 480 side of the xfmr, with no neutral on the 240 delta side...so grounding the 240 side would seem to be an issue...but it's just that the neutral that you tap into the one side of the delta "triangle" with is also a ground in that scenario, right?

No, a 240 high leg has a center tapped coil between the A and C phases as a grounded neutral so you get split phase 240/120V from A - N - C and the B phase is the "high leg" - 208V from B to neutral. You cannot use that neutral for a 240V to 480V wye to wye connection because the voltage between it and B is different from that to A and C; your transformer on the 240 side must be delta with no ground. You can't ground a phase (corner ground) because the center tap is grounded and you can't do both.

The 480V wye side of a delta wye transformer derives a neutral from the three phases and you must ground it for the inverter to operate, but the derived neutral may not be exactly at the same potential as the grounded neutral on the 240 side, so current may flow through the ground, which could be problematic. It might work and it might not; you'll need someone with more transformer experience than I to tell you if it will or not.

I say again, let your PE design this system.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
What's wrong with taking each leg-to-neutral of the 480/277 inverter, so you've got 3 legs of 277V (all connected to the same neutral) running into three single phase 277 to 120/240 xfmrs?

Nothing wrong with this, except that you end up with three _separate_ 120/240 systems. Each system has a different phase angle, meaning different timing of the zero crossings of the AC cycle. You would have a heck of a time supplying your current 120/240 loads, unless you could easily divide them up into subsets.

This sort of 'triple 120/240' system is also called 'hexaphase', and is sometimes used to reduce harmonic issues from computer systems and the like

You lost me at "grounding the derived neutral". Why "derived"?
And wouldn't the neutral connect to a neutral on the 480 side of the xfmr, with no neutral on the 240 delta side...so grounding the 240 side would seem to be an issue...but it's just that the neutral that you tap into the one side of the delta "triangle" with is also a ground in that scenario, right?

Every transformer derives its output from the changing magnetic field in the core. If the transformer has a neutral, then this is a 'derived' neutral.

A 'high leg delta' has a center tap in one leg of the secondary (as you describe). This secondary becomes the 'base' of your 120/240V system. The benefit here is that the 'side legs' provide some of the power for your 120/240V system, so your single phase 120/240V consumption gets somewhat distributed over all three phases.

-Jon
 

jaggedben

Senior Member
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Solar and Energy Storage Installer
But if there are 3 of those ^^^ single phase 15 kVAs, all 3 going into a single....pardon me...busbar? Of the MDP? Then all 3 will supply any loads equally.
If there were a bunch of random circuits going into the 3 xfmrs unequally, that would be unbalanced. If the load draws on all three equally, they're balanced, I'd think.

You're so far off on the 'single busbar' I can't even follow what you're thinking.

So...what IS the deal with that 208/240 thing? The heavy load is all 240 motors, single phase that run off the 2 legs of 120. THAT causes imbalance!
They'd overheat running at 208 volts, right?
But...if 208 motors can run at 240 just fine, can't 240 motors run at 277V?
:dunce:
Nope, I get that now. I meant- you can run a 208V motor at 240V and it's good. Running a 240V motor at 208 makes the motor overheat and be less efficient, because it's drawing more amps to compensate for the lack of voltage...or something like that?

You have it exactly backwards. Look up Ohms law. A heater at 208V will heat up less than at 240V. A motor will run slower, perhaps too slow to fulfill its function.

If you want to know whether any appliances or machinery you have that operate at 240V can make the switch to 208V (or 277V), you'll have to look up the ratings and instructions for each specific appliance. I probably shouldn't have implied that this is an option worth pursuing. Forget I mentioned it. :slaphead:

It's not just a tariff- the 120/240 200A service now is a hair too small, so might as well upgrade it to fit the PV, when the PV output will be twice the yearly usage.
If the service was adequate, the 2nd meter thing would be the idea.

Again, you could keep the first service for the existing loads and get a 2nd service installed. I thought the existing service was just too small for the PV, not for the existing load. If you're main breaker isn't tripping and you're not planning to add loads then the existing service isn't too small. I would probably just keep it unless it turns out that that it's somehow cheaper not to.

What's wrong with taking each leg-to-neutral of the 480/277 inverter, so you've got 3 legs of 277V (all connected to the same neutral) running into three single phase 277 to 120/240 xfmrs?

Nothing wrong, but those would be three separately derived systems and I believe would be pretty pointless if only one or two were necessary for the load.
You don't have to balance the load.
 
1. You cannot change the timing of the phases with transformers. No matter what you do, your inverters have three phases 120 degrees apart and a 240/120V split phase service has two phases (sort of) 180 degrees apart.

2. No, a 240 high leg has a center tapped coil between the A and C phases as a grounded neutral so you get split phase 240/120V from A - N - C and the B phase is the "high leg" - 208V from B to neutral. You cannot use that neutral for a 240V to 480V wye to wye connection because the voltage between it and B is different from that to A and C; your transformer on the 240 side must be delta with no ground. You can't ground a phase (corner ground) because the center tap is grounded and you can't do both.

3. The 480V wye side of a delta wye transformer derives a neutral from the three phases and you must ground it for the inverter to operate, but the derived neutral may not be exactly at the same potential as the grounded neutral on the 240 side, so current may flow through the ground, which could be problematic. It might work and it might not; you'll need someone with more transformer experience than I to tell you if it will or not.

4. I say again, let your PE design this system.

1. So this then...
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0393.html
Which is wired like this...
http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram26.pdf

...would be perfect, wouldn't it? Running in reverse of course. A hair bigger than the current 25 kVA service, and the three 480/277 phases are going directly into the three 240V phases.
So any imbalance on the load/240 side would stay there?

2. So not using the "wild leg" isn't a problem then, just run the whole load from the A-N-C 120/240 and forget the B leg is there. If so, great! That sounds like the answer.

3. "SMA recommends "Dyn_" transformers with a delta connection on the medium-voltage side and a star connection on the
low-voltage side with a neutral point that leads outward (see figure below e.g. Dyn1)."


Page 4-
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf

If going from LV wye to MV delta is the same "process" as going 480/277 wye to 240 delta...and looking at the wiring diagram from Temco there, there's no neutral on the delta side- the Y neutral "leads outward" (which is really "inward", bc it's going in reverse), but there's no N on delta side, so...the N being grounded on the load/delta side solves that transfer problem, doesn't it?

4. That's where Helioscope comes into play. It lowers the bill when I give the PE the setup, instead of him doing the whole thing from scratch.
It involves less hours if he can just say "ok, that's what you want, looks good, just change this one small thing..."
 
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