3Ph Inverter to 1Ph Interconnection

pvgreeze

Member
Location
Philadelphia
Occupation
Electrical Engineer
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
 

Attachments

  • 3Ph to 1Ph - SLD Sketch.pdf
    105.9 KB · Views: 12

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
The total instantaneous power delivered from a 3-phase source to balanced loads is a constant over the complete cycle. However, the instantaneous power with single-phase goes to zero twice each cycle. And so where would the power delivered by the 3-phase source (i.e., inverter) go when the single phase voltage waveform established by the POCO service passes through zero?
Also, I would think that the 3-phase inverter would fail to start up if it did not see the correct phase relationships between its output terminals.

I suggest you go ahead and push harder for single phase inverters.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
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?
In a word: no. Why would anyone want to do this?
 

pvgreeze

Member
Location
Philadelphia
Occupation
Electrical Engineer
To briefly summarize, because the customer wants to install both carports and rooftop arrays, roughly ~60kW. If it was just rooftop, then single phase inverters would not be a hard sell. I don't even know what the capacity/size of the service at the site is (I'm just relaying information I've been told from our sales dept) so this is super early in the design process. Just wanted to see if there was any possible solution before going back and saying 'no.'
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
To briefly summarize, because the customer wants to install both carports and rooftop arrays, roughly ~60kW. If it was just rooftop, then single phase inverters would not be a hard sell. I don't even know what the capacity/size of the service at the site is (I'm just relaying information I've been told from our sales dept) so this is super early in the design process. Just wanted to see if there was any possible solution before going back and saying 'no.'
I still don't get why someone wants three phase inversion. Is the $/Watt that much cheaper for 3P? Are they wanting to do the whole site with a single inverter? Would they really want to route DC all over the property? I would give each structure its own appropriately sized split phase inverter and combine them to interconnect.

Someone is making this a lot harder than it has to be. It begs the question of whether they even know what three phase power is.
 
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pv_n00b

Senior Member
Location
CA, USA
You can't connect a grid-tied 3ph inverter to a single-phase service. Just say "no." You might get confused in searching around the internet because it is fairly easy to power 3 phase loads from a single-phase service using a phase converter. None of that will help you with an inverter. Transformers can't convert a single-phase source into 3 phase because they can't create the necessary 120deg phase relationship.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
This sounds like someone read an article in a magazine or watched a youtube video and is now a solar power expert.

I don't see anyway to connect a 3 phase inverter to a single phase grid, short of not using the third wire on the inverter output, if the thing would even work that way.
 

pvgreeze

Member
Location
Philadelphia
Occupation
Electrical Engineer
You can't connect a grid-tied 3ph inverter to a single-phase service. Just say "no." You might get confused in searching around the internet because it is fairly easy to power 3 phase loads from a single-phase service using a phase converter. None of that will help you with an inverter. Transformers can't convert a single-phase source into 3 phase because they can't create the necessary 120deg phase relationship.
Yeah..thanks, I just wanted to make sure I wasn't missing something obvious!

The transformer reps I've talked to all say 'just use one phase off the three phase system and transform that' but obviously I can't do that with a three phase inverter without losing the output from two of the phases...I don't know why that is such a hard concept for them to grasp haha. Oh well, like I said, just wanted to make sure I wasn't overlooking something
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This sounds like someone read an article in a magazine or watched a youtube video and is now a solar power expert.

I don't see anyway to connect a 3 phase inverter to a single phase grid, short of not using the third wire on the inverter output, if the thing would even work that way.
It wouldn't.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Yeah..thanks, I just wanted to make sure I wasn't missing something obvious!

The transformer reps I've talked to all say 'just use one phase off the three phase system and transform that' but obviously I can't do that with a three phase inverter without losing the output from two of the phases...I don't know why that is such a hard concept for them to grasp haha. Oh well, like I said, just wanted to make sure I wasn't overlooking something

You'd think that solar related stuff would be half of the questions that transformer reps get these days, and they'd be getting a clue. As ever, I overestimate my fellow humans. :rolleyes: :p
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
There is a very weird transformer connection that can be used to supply single phase loads from a three source. It is called a 'hoppy hookup' and can be built from 2 single-phase units. It does not draw balanced currents from the three phase source, the common leg is 1.73X the other two, which makes its usefulness extremely rare.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
There is a very weird transformer connection that can be used to supply single phase loads from a three source. It is called a 'hoppy hookup' and can be built from 2 single-phase units. It does not draw balanced currents from the three phase source, the common leg is 1.73X the other two, which makes its usefulness extremely rare.

Diagram?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
There is a very weird transformer connection that can be used to supply single phase loads from a three source. It is called a 'hoppy hookup' and can be built from 2 single-phase units. It does not draw balanced currents from the three phase source, the common leg is 1.73X the other two, which makes its usefulness extremely rare.
But it does not work in reverse and it does not spread the load power equally over all three phases.

Sent from my Pixel 4a using Tapatalk
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
It is called a 'hoppy hookup' and can be built from 2 single-phase units. It does not draw balanced currents from the three phase source, the common leg is 1.73X the other two, which makes its usefulness extremely rare.
The reference I found to this (Square D catalog) says it gives eg 120/208V 3-wire. So it seems like it's basically open delta - open wye, if that's reasonable terminology.

Cheers, Wayne
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
There is a very weird transformer connection that can be used to supply single phase loads from a three source. It is called a 'hoppy hookup' and can be built from 2 single-phase units. It does not draw balanced currents from the three phase source, the common leg is 1.73X the other two, which makes its usefulness extremely rare.
Scott T?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
A crucial question that has not been answered is why anyone would want to do this. Anyone who has a clue about three phase power, that is.
 
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There is a very weird transformer connection that can be used to supply single phase loads from a three source. It is called a 'hoppy hookup' and can be built from 2 single-phase units. It does not draw balanced currents from the three phase source, the common leg is 1.73X the other two, which makes its usefulness extremely rare.


No one has mentioned the Leyton connection, which been discussed a few times on this forum. There doesn't seem to have been consensus that it works however.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
No one has mentioned the Leyton connection, which been discussed a few times on this forum. There doesn't seem to have been consensus that it works however.
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.
And of course it only function in current division if the souce is on the three phase side. That is not possible for grid interactive PV because the only available initial excitation is single phase.
 
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