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

[PVfarmer]

1. 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.

2. 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

3. 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.

4. 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

1. SO...subsets...meaning...run all of the 240V/heavier/motor loads off of one of those three and use the other 2 for 120V?
Or...use two for the 240V and one for the 120V loads?
The second one makes sense offhand- there's one big 240V load that's a bunch of 240V motors, other 240V loads are more "spaced out", welder here, compressor there, and then the 120V is the smallest (amp wise) of the group.

2. Neat! However, it's a barn, not a data center or hospital, so would "hexaphase" be...over-engineering it?

3. Got it. So if one side of the xfmr has a neutral and the other doesn't...there's no transfer?

4. OK. So then the high leg IS contributing, and not just "unused"?

Thanks!

 

[PVfarmer]

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

2. 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:

3. 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.

4. 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. That's not a surprise. Umm...?
When a PV system is connected to a load center by a breaker, the problem that can occur is overloading the busbars (see figure 1). For example, if the main disconnect and busbars are rated at 200 amps, and the PV system is capable of producing 40 amps, it is then possible to pull 240 amps off the busbars via feeders and branch circuits, if the users of the system connect enough equipment simultaneously. Section 705.12(D)(2) requires that the “sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.” So, 240 amps are allowed onto busbars rated at 200 amps. But,
705.12(D)(7) requires that PV breakers be located at the opposite end of the busbar from the utility feed. Figure 1 shows why. If the PV 40-amp breaker is located adjacent to the utility feed, the sum of amperage on the busbars immediately after that breaker can be 240 amps, too high for the metal of the busbar. But, if the breaker is located at the opposite end, this cannot occur, since power is fed from two different directions. As we will see, a similar scenario can exist when tapping into feeders with a PV system.
http://iaeimagazine.org/magazine/2013/09/16/pv-load-side-feeder-taps-compliant-or-not/

2. OK. But...this seems legit? Off the top of my head, more of the motors have the boxes than don't.
Most motors have a small box on the side where power connections are made. Inside there are winding leads that can be re-arranged to accommodate various input voltages.

208 is from a Wye connection at the transformers, IE each transformer has a common connection to make a neutral, and the other pole is the power output, this gives 120 volts single phase, 208 single phase, and 208 3 phase. A delta has the transformers in a configuration where each pole connects to the next transformer and is also an output, and will give 120 single phane, 240 single phase, and 240 3 phase. a neutral comes from the center tap of one transformer.

You can use the 240 motor as wired in 208 but it will be at reduced power. change the internal connections (and most larger motors are set up this way) and you get full power at 208.

Simply call an electrician after you verify there is a bundle of wiring that can be changed around, to do it for you, because your asking this type of question is proof you may be asking for trouble if you attempt it yourself, and do it wrong.
http://www.iforgeiron.com/topic/25836-3ph-208v-vs-240v-motor-question/

3. The dang thing does trip, that's the problem. Not often, and not sure if it's too much load or imbalance. The electrician has been suggesting upgrading for a while. But it happens rarely- more often than losing power, but not an everyday thing. It could be as simple as a 240V load in the house kicking on when the barn is at full load at 240.
More good news- I *think* the POCO pays for the xfmrs on the pole. But I'm not a lawyer either...however, I am a lot cheaper than one. I can read the stuff, that's most of it. You just have to skip over the jargon. There's just so much of it!

4. You don't have to balance the load because... the two sides of the transformers are separated by a gap? As in...if the xfmr gap deals with the harmonic "discrepancies" between the two sides, it does the same for the same problem with phases?
Or are we back to "I have no idea what you mean" again?

 

[ggunn]

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.

I am not a motor expert, but isn't the speed of a synchronous AC motor determined by the frequency of the AC? I know if you restrict the movement of a AC fan, even a little bit more than its design allows, the motor will draw more current, overheat, and eventually burn up. Running a motor at too low a voltage may do something similar.

 

[ggunn]

1. That's not a surprise. Umm...?
When a PV system is connected to a load center by a breaker, the problem that can occur is overloading the busbars (see figure 1). For example, if the main disconnect and busbars are rated at 200 amps, and the PV system is capable of producing 40 amps, it is then possible to pull 240 amps off the busbars via feeders and branch circuits, if the users of the system connect enough equipment simultaneously. Section 705.12(D)(2) requires that the “sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.” So, 240 amps are allowed onto busbars rated at 200 amps. But,
705.12(D)(7) requires that PV breakers be located at the opposite end of the busbar from the utility feed. Figure 1 shows why. If the PV 40-amp breaker is located adjacent to the utility feed, the sum of amperage on the busbars immediately after that breaker can be 240 amps, too high for the metal of the busbar. But, if the breaker is located at the opposite end, this cannot occur, since power is fed from two different directions. As we will see, a similar scenario can exist when tapping into feeders with a PV system.
http://iaeimagazine.org/magazine/2013/09/16/pv-load-side-feeder-taps-compliant-or-not/

2. OK. But...this seems legit? Off the top of my head, more of the motors have the boxes than don't.
Most motors have a small box on the side where power connections are made. Inside there are winding leads that can be re-arranged to accommodate various input voltages.

208 is from a Wye connection at the transformers, IE each transformer has a common connection to make a neutral, and the other pole is the power output, this gives 120 volts single phase, 208 single phase, and 208 3 phase. A delta has the transformers in a configuration where each pole connects to the next transformer and is also an output, and will give 120 single phane, 240 single phase, and 240 3 phase. a neutral comes from the center tap of one transformer.

You can use the 240 motor as wired in 208 but it will be at reduced power. change the internal connections (and most larger motors are set up this way) and you get full power at 208.

Simply call an electrician after you verify there is a bundle of wiring that can be changed around, to do it for you, because your asking this type of question is proof you may be asking for trouble if you attempt it yourself, and do it wrong.
http://www.iforgeiron.com/topic/25836-3ph-208v-vs-240v-motor-question/

3. The dang thing does trip, that's the problem. Not often, and not sure if it's too much load or imbalance. The electrician has been suggesting upgrading for a while. But it happens rarely- more often than losing power, but not an everyday thing. It could be as simple as a 240V load in the house kicking on when the barn is at full load at 240.
More good news- I *think* the POCO pays for the xfmrs on the pole. But I'm not a lawyer either...however, I am a lot cheaper than one. I can read the stuff, that's most of it. You just have to skip over the jargon. There's just so much of it!

4. You don't have to balance the load because... the two sides of the transformers are separated by a gap? As in...if the xfmr gap deals with the harmonic "discrepancies" between the two sides, it does the same for the same problem with phases?
Or are we back to "I have no idea what you mean" again?

Are you just asking all this because you are curious, or are you trying to use our answers to design the system? If it is the latter, I would in the strongest terms encourage you NOT TO DO THAT. We only have access to what you tell us, which is at best incomplete information. This is a nontrivial design situation and someone who really knows what he (or she) is doing needs to look at all aspects of it to come up with the best solution. One thing which is clear from what you have posted here (and I do not wish to be unkind) is that that person is not you. You said earlier that you had hired a PE. That is very good; let him (or her) design your system.

 

[jaggedben]

2. OK. But...this seems legit? Off the top of my head, more of the motors have the boxes than don't.

Again, you'll need to look at every motor or other appliance on a case-by-case basis.

3. The dang thing does trip, that's the problem. ...

Okay, that clears that part up. You still may have more than one option of how to move forward, such as upgrading the existing 240V service and getting a 2nd service for the PV. You should let your PE advise you on what would be necessary so you can accurately compare costs.

4. You don't have to balance the load because... the two sides of the transformers are separated by a gap? As in...if the xfmr gap deals with the harmonic "discrepancies" between the two sides, it does the same for the same problem with phases?
Or are we back to "I have no idea what you mean" again?

Let your PE explain it to you. And don't drive him/her crazy with questions if it's not necessary to get the job done. You're the developer here, right? Focus on goals, money, and cost/benefit. Let your PE and electrician explain the tech stuff to you.

I am not a motor expert, but isn't the speed of a synchronous AC motor determined by the frequency of the AC? I know if you restrict the movement of a AC fan, even a little bit more than its design allows, the motor will draw more current, overheat, and eventually burn up. Running a motor at too low a voltage may do something similar.

Are you just asking all this because you are curious, or are you trying to use our answers to design the system? If it is the latter, I would in the strongest terms encourage you NOT TO DO THAT. We only have access to what you tell us, which is at best incomplete information. This is a nontrivial design situation and someone who really knows what he (or she) is doing needs to look at all aspects of it to come up with the best solution. One thing which is clear from what you have posted here (and I do not wish to be unkind) is that that person is not you. You said earlier that you had hired a PE. That is very good; let him (or her) design your system.

I'm sure you are correct about the motors. (Here I am thinking of DC motors:slaphead:. Shows you how often I work with the other kind).

I'm going to bow out of this discussion for the same reasons you stated in your last post. I've already put my foot in my mouth a couple times and led the discussion in unhelpful directions. I daresay I don't normally do that. :ashamed1:

 

[PVfarmer]

Are you just asking all this because you are curious, or are you trying to use our answers to design the system? If it is the latter, I would in the strongest terms encourage you NOT TO DO THAT. We only have access to what you tell us, which is at best incomplete information. This is a nontrivial design situation and someone who really knows what he (or she) is doing needs to look at all aspects of it to come up with the best solution. One thing which is clear from what you have posted here (and I do not wish to be unkind) is that that person is not you. You said earlier that you had hired a PE. That is very good; let him (or her) design your system.

It's the first, really.
And I hear you, but..
"For the one line diagram, I have no concerns with the actual electrical design...considering all the legwork and input you've done my bet is we can do this... "

the PE guy is ^^, believe it or not, cool with getting help, and appreciating it, so...why not? The system is 90% designed....Helioscope even gives you the AutoCAD drawing of the panels. Which ain't cheap!

 

[PVfarmer]

Again, you'll need to look at every motor or other appliance on a case-by-case basis.

Okay, that clears that part up. You still may have more than one option of how to move forward, such as upgrading the existing 240V service and getting a 2nd service for the PV. You should let your PE advise you on what would be necessary so you can accurately compare costs.

Let your PE explain it to you. And don't drive him/her crazy with questions if it's not necessary to get the job done. You're the developer here, right? Focus on goals, money, and cost/benefit. Let your PE and electrician explain the tech stuff to you.
I'm going to bow out of this discussion for the same reasons you stated in your last post. I've already put my foot in my mouth a couple times and led the discussion in unhelpful directions. I daresay I don't normally do that. :ashamed1:

Don't blush- it's obviously my fault if you said the wrong thing!
The electrician who's going to do the actual wiring in fact said transformers are "voodoo magic".
"You still may have more than one option.."
Many more! Hence the confusion.

This is what I'm trying to avoid.
At least three serious problems with auto transformers must be addressed. First, auto transformers are confusing. :dunce: Most technicians who successfully install them consider them magic. For a recent project, 6 PE's at two engineering firms argued for months on sizing the auto transformers for an air compressor.
http://www.pdhonline.org/courses/e162/e162.htm

This is a good paper:
http://www.pdhonline.org/courses/e162/e162content.pdf

 

[winnie]

Keeping in mind that this is random discussion to answer your curiosity, and _not_ any sort of design suggestions!:

1. SO...subsets...meaning...run all of the 240V/heavier/motor loads off of one of those three and use the other 2 for 120V?
Or...use two for the 240V and one for the 120V loads?
The second one makes sense offhand- there's one big 240V load that's a bunch of 240V motors, other 240V loads are more "spaced out", welder here, compressor there, and then the 120V is the smallest (amp wise) of the group.

If you have 3 phase 480/277V, you can use a _single phase_ transformer to take one of the 480V branches and derive 120/240V, and use this single 120/240V to feed both your 120V and your 240V loads. The problem here is that your 3 phase loading would not be balanced.

If you take _three separate_ single phase transformers, each connected to a separate 480V branch, then you will derive _three separate_ 120/240V supplies. Each of these supplies could be used to power both 120V and 240V loads. You would have 3 separate panels, each providing a separate 120/240V supply to separate loads. If each of these _separate_ panels had similar loading on it, then your three phase loading would be balanced.

2. Neat! However, it's a barn, not a data center or hospital, so would "hexaphase" be...over-engineering it?

over-engineering would be a mild word for using this sort of setup! You mentioned that the solar array would be on the order of 60KW, and significantly larger than the normal loading. You would have 3 separate relatively small transformers, would have problems dividing up the loads evenly because each load would be such a large fraction of the whole, .... IMHO you are better off with a single 'normal' 480 to 208/120V three phase transformer.

3. Got it. So if one side of the xfmr has a neutral and the other doesn't...there's no transfer?

Current only flows where there is a closed circuit path, and each transformer breaks this path, so each transformer secondary neutral can be connected together or to ground, without creating a complete path where current will flow. In fact you are generally required to 'ground' the neutral of each new transformer.

4. OK. So then the high leg IS contributing, and not just "unused"?
Thanks!

Sort of. If you have a delta transformer, then both coils connected to a given terminal contribute output to that terminal. So if you take a delta transformer and ground the midpoint of one coil (creating a 'high leg') then all three coils will contribute to the output of the grounded coil. The 'high leg' output is incidental to this; in fact for generators being used to supply single phase loads the normal connection is called 'double delta', which doesn't create the 'high leg' but does use the 'ungrounded coils' to help supply the single phase loads.

Something else you should keep in mind: motors are essentially transformers. If you change the voltage or configuration of your electrical system supply, you should consider replacing motors rather than adding transformers to make existing motors work.

-Jon

 

[PVfarmer]

You would have 3 separate panels, each providing a separate 120/240V supply to separate loads. If each of these _separate_ panels had similar loading on it, then your three phase loading would be balanced.
over-engineering would be a mild word for using this sort of setup! You mentioned that the solar array would be on the order of 60KW, and significantly larger than the normal loading. You would have 3 separate relatively small transformers, would have problems dividing up the loads evenly because each load would be such a large fraction of the whole, .... IMHO you are better off with a single 'normal' 480 to 208/120V three phase transformer.
--
Sort of. If you have a delta transformer, then both coils connected to a given terminal contribute output to that terminal. So if you take a delta transformer and ground the midpoint of one coil (creating a 'high leg') then all three coils will contribute to the output of the grounded coil. The 'high leg' output is incidental to this; in fact for generators being used to supply single phase loads the normal connection is called 'double delta', which doesn't create the 'high leg' but does use the 'ungrounded coils' to help supply the single phase loads.

Hey, thanks, I think this is making sense now- there's no real way to balance the three 120/240 panels, it would be two 120 and one 240, and the 240 doesn't run much but it draws almost all of the 200 amp service.
you are better off with a single 'normal' 480 to 208/120V three phase transformer.
So...is "normal" an auto xfmr or harmonic mitigating....can't seem to find "normal" here.
And- if there are possibly going to be THREE in total 120/240 xfmrs coming from the grid, that's 6 lines of 120V, so wouldn't 2 xfmrs for the PV to grid part make sense, if they're putting out three legs each, to also = 6 lines?

http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0823.html

So if you take a delta transformer and ground the midpoint of one coil (creating a 'high leg') then all three coils will contribute to the output of the grounded coil.

That I follow- but how does it relate to this? The last sentence of this clip below just loses me.
https://onedrive.live.com/?id=CBCE5...EF&group=0&parId=CBCE51590CC26FEF!655&o=OneUp

 

[winnie]

you are better off with a single 'normal' 480 to 208/120V three phase transformer.
So...is "normal" an auto xfmr or harmonic mitigating....can't seem to find "normal" here.

Normal would be a standard three phase distribution transformer.
This is an 'isolation' transformer, meaning that there is no electrical connection between primary and secondary coils.
Primary voltage would be whatever is on the utility side of things, secondary voltage is whatever you need for utilization.

For example:
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0275.html
This is a 208V primary, 480/277V secondary. This is what you would use if the utility was providing a 208/120V service, and you needed to get 480/277V for your inverters.

On the other hand, if the utility were providing 480/277V service, which your inverters were using directly, and you needed to get 208/120V for your local loads, then you would use:
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0303.html giving you 208/120V three phase (note this is a smaller transformer, I just picked a random one with the proper voltage configuration)

And- if there are possibly going to be THREE in total 120/240 xfmrs coming from the grid, that's 6 lines of 120V, so wouldn't 2 xfmrs for the PV to grid part make sense, if they're putting out three legs each, to also = 6 lines?

If you go the route of having a 208/120V service from the utility, the utility will provide you with 3 'hots' and a 'neutral'. Each hot will be 120V to neutral, and 208V to another 'hot'. The transformers that the utility has on the pole _might_ be capable of 120V use or 120/240V use, but that is not relevant to the customer. The utility will provide standard three phase wye (3 hots plus neutral).

So if you take a delta transformer and ground the midpoint of one coil (creating a 'high leg') then all three coils will contribute to the output of the grounded coil.

That I follow- but how does it relate to this? The last sentence of this clip below just loses me.

I could not load the clip on 2 systems, so I did not see it.

I believe that if you have a transformer with a center tapped delta secondary, and only use the center tapped leg to supply single phase loads, then the effective rating of the transformer is roughly 70% of its nominal 3 phase rating. (So if you had a 30KVA transformer and were using it for single phase loads only on the center tapped leg, then you could only supply 21KVA without overloading the transformer.)

-Jon

 

[PVfarmer]

Primary voltage would be whatever is on the utility side of things, secondary voltage is whatever you need for utilization.
if the utility was providing a 208/120V service, and you needed to get 480/277V for your inverters.

On the other hand, if the utility were providing 480/277V service,

If you go the route of having a 208/120V service from the utility, the utility will provide you with 3 'hots' and a 'neutral'. Each hot will be 120V to neutral, and 208V to another 'hot'. The transformers that the utility has on the pole _might_ be capable of 120V use or 120/240V use, but that is not relevant to the customer. The utility will provide standard three phase wye (3 hots plus neutral).

I could not load the clip on 2 systems, so I did not see it.
I believe that if you have a transformer with a center tapped delta secondary, and only use the center tapped leg to supply single phase loads, then the effective rating of the transformer is roughly 70% of its nominal 3 phase rating. (So if you had a 30KVA transformer and were using it for single phase loads only on the center tapped leg, then you could only supply 21KVA without overloading the transformer.)

-Jon

Here's another try at the clip.
A funny thing happened- a guy that installs xfmrs for a living is my neighbor!
He seems to think 3 phase service is too much $$ and 600 amps of single phase would be the right way to go. He also said 50 kVA is the biggest xfmr they put up on poles.
So, having 200A of 120/240 now, with a 25 kVA xfmr, it might be two more of those, or one 50 kVA 120/240.
That would mean going... 480/277 from the inverters...3 phase...so go down to 208/120, that's 3 lines of 120 to neutral.
There would be 6 lines of 120/neutral from 3 POCO xfmrs, (or 4 lines if 2 POCO xfmrs), so... 3 would be more balanced.

As far as the load, why can't those 3 lines of 120/N feeding the load go ... 1 for 120 loads and 2 for 240V loads?

edit: Dang, that's hard to read. I'll try again.
um.... this?
http://1drv.ms/1NznumC

 

[ggunn]

Here's another try at the clip.
A funny thing happened- a guy that installs xfmrs for a living is my neighbor!
He seems to think 3 phase service is too much $$ and 600 amps of single phase would be the right way to go. He also said 50 kVA is the biggest xfmr they put up on poles.
So, having 200A of 120/240 now, with a 25 kVA xfmr, it might be two more of those, or one 50 kVA 120/240.
That would mean going... 480/277 from the inverters...3 phase...so go down to 208/120, that's 3 lines of 120 to neutral.
There would be 6 lines of 120/neutral from 3 POCO xfmrs, (or 4 lines if 2 POCO xfmrs), so... 3 would be more balanced.

As far as the load, why can't those 3 lines of 120/N feeding the load go ... 1 for 120 loads and 2 for 240V loads?

edit: Dang, that's hard to read. I'll try again.
um.... this?
http://1drv.ms/1NznumC

I doubt seriously that the POCO is going to let you interconnect a three phase inverter to three services, if that is what you are saying.

 

[PVfarmer]

I doubt seriously that the POCO is going to let you interconnect a three phase inverter to three services, if that is what you are saying.

I hear ya, but it would (I think) be one service with one or 3 xfmrs.
On the application, the POCO asks what size inverters, how many, and also "is the customer providing a xfmr?", in addition to theirs.
So...
Ideally:
the inverters are putting out 3 ph/4 wire, 3 legs of 277V-N.
This would be the type of thing to step up to grid V, too big for the pole so it would have to go on the ground.
http://www.temcoindustrialpower.com/products/Medium_Voltage_Transformers/T38593.html

As you can see, that's a lot of $$.
The POCO has warehouses full of ones like this:
http://www.temcoindustrialpower.com/products/Medium_Voltage_Transformers/T38421.html

so even though those two are around the same price on that site for one 12.47kV to 480/277 3 ph vs. three 7200V to 120/240 1 ph models, the smaller ones would be less $$ since the POCO is providing them, and they have thousands ready to go.
There are also 7200 V to 277V 1 phase models.
Whichever, I'd think the POCO would prefer the PV output to be balanced between the 3 main feeders?
Since the majority of the power is going to be going out...

So...
1. Most $$ = inverters straight to 480/277 xfmr, stepped up to 12.47kV. And another 480/277 to 120/240 for the load. (OR 208/120- my neighbor the xfmr guy seemed to think running 240 motors off of 208 is no big deal. Just rewire the motors or get small boosters for the ones you can't)
2. Less $$ (maybe) = inverters to three 277 to 7200 1 ph step up xfmrs. And another for the load.
3. Still less $$ = inverters stepped down to 120/240 or 208/120 feeding the load and the grid -> three 25kVAs xfmrs on pole (2 more of what's up there now), 7200V each.
4. Least $$ = inverters down to 120/240, one 75kVA up to 7200 xfmr for grid on ground, going into ONE of the three mains. (That seems wrong on both sides)

 

[PVfarmer]

well. here it is right on page 209!

well. here it is right on page 209!

found this thread:
http://forums.mikeholt.com/showthread.php?t=145205


which lead to this,
http://www.rd.usda.gov/files/UEP_Bulletin_1728F-804.pdf

SO! That narrows the options down to...SIXTEEN!

SINGLE-PHASE, CONVENTIONAL TRANSFORMER
(DEADEND POLE)

G2.1 (G210- ) TWO-PHASE TRANSFORMER BANK
OPEN-WYE PRIMARY
OPEN-DELTA, 4 WIRE SECONDARY

G2.1G TRANSFORMER / METER CONNECTION GUIDE
THREE-PHASE, OPEN-WYE - OPEN DELTA
FOR 120/240 VOLT POWER LOADS

 

[winnie]

Here's another try at the clip.
A funny thing happened- a guy that installs xfmrs for a living is my neighbor!
He seems to think 3 phase service is too much $$ and 600 amps of single phase would be the right way to go. He also said 50 kVA is the biggest xfmr they put up on poles.

It is very common for a power company to use a set of 2 or 3 single phase transformers to create a 'three phase bank' and provide a three phase service. So if they set 3 25kVA transformers on the pole, you get a 3 phase 75kVA service.

If you get a single phase service, then you will either need to set up your inverters to provide single phase power, or you will need to have some method of storing energy to convert three phase to single phase. The energy storage is only for a fraction of an AC cycle, but it is necessary because three phase power is delivered continuously (the peaks and zeros of the 3 phases are at different times; with single phase voltage, current and power all rise to a maximum and then fall to zero twice per AC cycle)

So, having 200A of 120/240 now, with a 25 kVA xfmr, it might be two more of those, or one 50 kVA 120/240.
That would mean going... 480/277 from the inverters...3 phase...so go down to 208/120, that's 3 lines of 120 to neutral.
There would be 6 lines of 120/neutral from 3 POCO xfmrs, (or 4 lines if 2 POCO xfmrs), so... 3 would be more balanced.

As far as the load, why can't those 3 lines of 120/N feeding the load go ... 1 for 120 loads and 2 for 240V loads?

You need to learn more about how 3 phase power works. The L-N voltage might be 120V, but the L-L voltage changes depending upon the phase angle (timing) of the 120V AC cycle.

With an ordinary single phase service, you try to balance your 120V loading, so that your aggregate 120V loads look more like a 240V load, and then you use the L-L supply for your 240V loads as well. You don't try to separate things out.

With an ordinary 208/120V three phase service, you again try to balance your 120V loads on all three legs, then for higher voltage you use a connection between two legs, which gives _208V_.

The paper that you posted the link to is about connecting utility transformers to properly balance voltages in fault conditions. I think that is something for you to let the PE focus on. The transformer used in the paper would need to be made with _three winding_ transformers, which the POCO likely will not supply. But if you could get 7200V:120/240V:277V transformers, then a single set of transformers could provide 480/277V wye for the inverters _and_ 240/120V delta for your existing loads. Very unlikely that this approach will be used, since the POCO will want to use transformers that they have, but an interesting theoretical possibility.

-Jon

 

[PVfarmer]

1. It is very common for a power company to use a set of 2 or 3 single phase transformers to create a 'three phase bank' and provide a three phase service. So if they set 3 25kVA transformers on the pole, you get a 3 phase 75kVA service.
If you get a single phase service, then you will either need to set up your inverters to provide single phase power...

2. You need to learn more about how 3 phase power works.
With an ordinary single phase service, you try to balance your 120V loading, so that your aggregate 120V loads look more like a 240V load, and then you use the L-L supply for your 240V loads as well. You don't try to separate things out.
With an ordinary 208/120V three phase service, you again try to balance your 120V loads on all three legs, then for higher voltage you use a connection between two legs, which gives _208V_.

3. But if you could get 7200V:120/240V:277V transformers, then a single set of transformers could provide 480/277V wye for the inverters _and_ 240/120V delta for your existing loads. Very unlikely that this approach will be used, since the POCO will want to use transformers that they have, but an interesting theoretical possibility.

1. ...Yes. Pretty sure it'd be three of these for the inverters. The service wouldn't be 3 phase, it would be 3 one phase lines, under one account/service. Probably two meters.
That would be one 200A service for the load and another 400A (both 120/240) to handle the PV output. Of course the PV would output through all three.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0073.html

2. Yes, that's an understatement and not the only thing either.
But if it isn't really 3 phase, the balancing thing is entirely different.
If it was three of those above going into three 120/240 >> 7200V and also feeding the loads, it would be more balanced than it is now, I reckon.

3. That's what I mean, except skip the the 277 part- the POCO xfmrs would be 7200V>120/240 and feed the load at night, while three of the 277 > 120/240 ones linked above would handle the inverters.

 

[PVfarmer]

If it was three of those above going into three 120/240 >> 7200V and also feeding the loads, it would be more balanced than it is now, I reckon.

I didn't explain that very well.
You'd have all of the neutrals from the inverters connected, those would be connected to all 3 neutrals on their side of the 3 xfmrs.
Each inverter has three 277 L-N legs, A B and C.
Take the As from each intverter and connect them in parallel to xfmr A.
The B legs go to xfmr B.
The Cs to C.
OR three legs from inverter A parallel to xfmr A, same for B/ B and C/C.
But the first one would be more balanced, when it's feeding a load that's connected 240 delta center tapped for 120.

Say xmfr A goes to the A point on the delta, which is a 240V- when the load goes way up, I believe the inverters will feed more amps into that leg, A, because they're all getting the extra load on one of their legs.
If the load is no 240 and all 120 on B and C to neutral, the inverters will run that and equalize the feed to the the grid.
I'm not sure the POCO is really worried as much about small 120V loads at night as they are about large 240V motor loads.

 

[winnie]

1. ...Yes. Pretty sure it'd be three of these for the inverters. The service wouldn't be 3 phase, it would be 3 one phase lines, under one account/service. Probably two meters.

When the power company provides service from 3 transformers, each fed by a different primary line, that is a 3 phase service.

It is of course composed of 3 separate 'hot' lines, with a _common_ neutral line (in the 'wye' arrangement, eg. 480/277V or 208/120V). Each 'hot' to 'neutral' considered separately is a single phase, but taken together they form three phases.

It is very doubtful that you would get 2 _separate_ 120/240V single phase services. In addition to other issues, such a setup violates code.

Your inverters are limited to 480/277V wye. Your choices are:
have the utility supply 480/277V, and then provide your own transformer to give 120/240V,
have the utility supply 208/120V, and then provide your own transformer to give 480/277V
have the utility supply 240/120V high leg delta, and then provide your own transformer to give 480/277V

All of the 480V to 208 or 240V transformers that you are looking at have _delta_ primaries. This means that each individual coil on the secondary side actually loads _two_ of the inverters on the primary side.

-Jon

 

[jaggedben]

...This means that each individual coil on the secondary side actually loads _two_ of the inverters on the primary side.

-Jon

Not if the inverters are still the 3-phase inverters he's been talking about since the beginning of the thread.
I think maybe you meant 'coils' where you said 'inverters'?

 

[ggunn]

Why are you going through all this just so you can use three phase inverters? There are perfectly good single phase inverters out there.