3Ph Inverter to 1Ph Interconnection

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I used to be a believer in the Leyton 3-1 connection, but someone here pointed out quite effectively that although it seems to draw equal current from all three phase wires, the phase angle means that it is far from drawing equal power from the three legs.
This gets back to the question I had in the 2017 thread that was never fully addressed: 3 phase balanced loading unity power factor gives constant power delivery, while single phase unity power factor gives a cyclic power delivery at twice the AC frequency. So can standard transformers perform the subcycle power time shifting required to convert one into the other?

If not, then that reflects your comment above, that while the current waveforms on the 3 phase side may be of equal magnitude, they must not be all in phase with their respective voltage (i.e. 120 degree apart from each other), as the power delivery waveform must remain cyclic at twice the AC frequency.

Cheers, Wayne
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
This gets back to the question I had in the 2017 thread that was never fully addressed: 3 phase balanced loading unity power factor gives constant power delivery, while single phase unity power factor gives a cyclic power delivery at twice the AC frequency. So can standard transformers perform the subcycle power time shifting required to convert one into the other?

If not, then that reflects your comment above, that while the current waveforms on the 3 phase side may be of equal magnitude, they must not be all in phase with their respective voltage (i.e. 120 degree apart from each other), as the power delivery waveform must remain cyclic at twice the AC frequency.

Cheers, Wayne
A very nice analysis approach!
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
A very nice analysis approach!
Thank you, but it's predicated on the statement that "standard transformers can't perform subcycle power time shifting". And I don't know whether that's true or not, no one has actually agreed or disagreed with the statement, either now or in the 2017 thread. My understanding of transformers is rather ad hoc.

Cheers, Wayne
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I've been one of the strong disbelievers in validity of the Leyton 3-1 connection for balancing single phase loading on a 3 phase system. However I would note that this connection bears a striking similarity to the 'double delta' connection which is used to connect 3 phase alternators to provide single phase power.

Regarding transformers providing the necessary power time shifting, then would depend upon having some significant amount of energy storage built into the transformer. There clearly is _some_ magnetic energy storage in a transformer, but I don't believe a standard transformer stores enough energy magnetically to perform the desired conversion.

A transformer coupled to suitably sized capacitors could do it, as could a transformer which incorporates a rotating secondary (generally we call these 'rotary phase converters' :) :) ), and I suppose that a transformer with extra inductive energy storage could provide the necessary energy storing phase shift.

-Jon
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Thank you, but it's predicated on the statement that "standard transformers can't perform subcycle power time shifting". And I don't know whether that's true or not, no one has actually agreed or disagreed with the statement, either now or in the 2017 thread. My understanding of transformers is rather ad hoc.

Cheers, Wayne
The magnetizing current of a transformer and the energy associated with it are small compared to the energy transferred by mutual inductance between the primary and secondary. The magnetizing current simply sets up the preconditions for using the magnetic coupling.
Some special purpose devices like constant voltage transformers use a tuned LC circuit, but they have enormously large windings and core compared to a standard transformer of the same rating.
Just to formalize it, yes, standard transformers cannot perform subcycle power time shifting.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
However I would note that this connection bears a striking similarity to the 'double delta' connection which is used to connect 3 phase alternators to provide single phase power.
I was reading about 12 lead generator connections in an overview from some manufacturer which noted that the available kVA in the double delta single phase configuration was about 50% of the available kVA in 3 phase configurations. That, along with a comment I read about one of the transformers in the Leyton configuration needing to be oversized, are in accordance with the idea that the resulting 3 phase loading is not balanced.

Cheers, Wayne
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
My understanding of transformers is rather ad hoc.
My understanding of transformers is a little more than ad hoc.

I have never seen this transformer configuration discussed any place other than this forum. Does anyone know of any peer reviewed papers analyzing it?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I was reading about 12 lead generator connections in an overview from some manufacturer which noted that the available kVA in the double delta single phase configuration was about 50% of the available kVA in 3 phase configurations. That, along with a comment I read about one of the transformers in the Leyton configuration needing to be oversized, are in accordance with the idea that the resulting 3 phase loading is not balanced.

Cheers, Wayne
Generator windings behave and can be configured differently than transformers with primary and secondary windings.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I understand that we are a bunch of engineers and tradesmen who love to pound theoretical issues into greasy spots in the dirt, but the question I keep asking is that in the real world, aside from whether it is possible or not, why would anyone ever want to interconnect a three phase inverter with a single phase service?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I understand that we are a bunch of engineers and tradesmen who love to pound theoretical issues into greasy spots in the dirt, but the question I keep asking is that in the real world, aside from whether it is possible or not, why would anyone ever want to interconnect a three phase inverter with a single phase service?
Because you bought the wrong inverter already and can't deal with admitting you made a mistake. ;)
 
I understand that we are a bunch of engineers and tradesmen who love to pound theoretical issues into greasy spots in the dirt, but the question I keep asking is that in the real world, aside from whether it is possible or not, why would anyone ever want to interconnect a three phase inverter with a single phase service?
Well, in all seriousness, there are some advantages to three phase inverters, so I can understand the temptation. Much larger sizes available, 1kv strings (actually fronius single phase does do 1kv), 480 voltage if that's what your service is.......
 

Carultch

Senior Member
Location
Massachusetts
I understand that we are a bunch of engineers and tradesmen who love to pound theoretical issues into greasy spots in the dirt, but the question I keep asking is that in the real world, aside from whether it is possible or not, why would anyone ever want to interconnect a three phase inverter with a single phase service?

One reason you might want to do this, is if you are at a location where you only have a single phase service, and can't upgrade it to a 3-phase service. You might have an interest in large scale energy storage, where single phase equivalents of battery energy storage systems just aren't available at the scale you need. Not an unlikely interest, since being at the end of a long distance single phase feeder might be a location with poor grid stability, where BESS is of interest for better resiliancy.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Well, in all seriousness, there are some advantages to three phase inverters, so I can understand the temptation. Much larger sizes available, 1kv strings (actually fronius single phase does do 1kv), 480 voltage if that's what your service is.......
Well, of course, but the OP is talking about a residential rooftop PV system, so 1000V is not on the table, and the interconnection is 240/120V single phase, which is what this thread is about.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
One reason you might want to do this, is if you are at a location where you only have a single phase service, and can't upgrade it to a 3-phase service.
The original question is about a residential single phase service; I can't say that they don't exist anywhere, but I have never seen a residence with a 480/277V service or even that needed one.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Well, you did say this:



😁
Yes, I did say that, and I still don't see it. IMO, any benefits that could be derived from such an implementation would be overcome by the expense and sheer kludginess it would entail. To put it into technical terms, it would suck. :D
 

tbuller

Member
Hello all,

Thank you for all your help with my previous questions. I'm sure this question has been posted before, but I could not find any succinct threads or answers.

My question is this; how (or 'is it possible') to connect a three phase inverter to a single phase service? I've seen plenty of literature online for the opposite (1Ph inverter to 3Ph service), but I have a project that is looking to interconnect a 480/277V 3Ph inverter output to a 240/120V 1Ph service. I've put out a few lines to transformer reps that I work with, but I feel like this forum will have more expertise in the full breadth of the design problems.

I know in a typical electrical system, I could use a 277V - 240/120V single phase transformer, but obviously with this type of solar system there is more of a need for phase balancing.

I've attached an extremely rudimentary sketch with what I see as the only possible option, which is using three transformers to balance all three phases into a single phase balanced service. I'm sure it is riddled with errors, but this is just a high level concept sketch, so go easy on me!

And yes, I'm pushing for just using single phase inverters, but for now, I need to design three phase inverters to a single phase service. Any and all help is appreciated. Thanks!! -pvgreeze
pvgreeze, Here are a few things to consider. The inverters are not identical and their outputs will likely not be synchronized with each other unless there is a connection between them to do that. Assuming that they can be synchronized the other issue is converting the three phase to single phase. To get full power out of the 480V transformer each single phase transformer would need to be on a different phase of the 480v system. Unfortunately this would make their outputs out of phase with each other so they could not be connected directly together in a single phase panel. You only have a couple of options really. One is to buy a phase converter which I don't think is a good option and the other is to do one single transformer (single phase 480V to 240v/120V). Unfortunately you will not be able to get full power out out of the inverters since you will only be using a single phase. The last thing that needs to be considered is whether or not you are synchronized with 240V/120V system that you are connecting to. Here again you will likely need some synchronization equipment (probably another inverter) to make that happen.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
...the other is to do one single transformer (single phase 480V to 240v/120V). Unfortunately you will not be able to get full power out out of the inverters since you will only be using a single phase.
No. You cannot use a single phase of a three phase inverter and leave the other two phases open circuited. The inverter will not even start up.

This whole scenario is ill advised and most likely conceived of by someone who hasn't a clue of how three phase power works. I believe that we have flogged this dead horse into a greasy spot on the ground.
 
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