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Auto Transformer use with 3 Phase Inverter Setup

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Just thinking out loud here. If I was to arrange a few growatt 5000es's in a 3 phase configuration to feed my shop, and wanted a neutral for 120v use. I know I can use a isolation transformer, but can I use 3 single phase auto transformers like the ones Sig Solar sells?

I would connect one to a and b phases, and another to b and c phase, and another to a and c phase. Would this work to provide a neutral?

Would I have to do all 3?

I know an isolation transformer is the "right way" but I was just pondering this in my head. Thanks
 
With three phase 'Y' connection with center of 'Y' neutral for common autotransformers connection point. The neutral of inverters should also be connected in common to center of 'Y'. The inverter common neutral with center neutral 'Y' arrangement is the only way to create three phases with inverters.
 
As RC says, 3x inverters in "Y" has a neutral. You have either 120/208Y or 230/400Y
In the case of 120/208Y, an auto-transformer can take 120V L-N and make 120/240V L-N-L split-phase
In the case of 230/400Y, you could use a transformer with three identical (or similar) windings to get 115/230V with an extra 115V hanging off one leg. But more likely use an isolation transformer.
Isolation transformer, could take in either 230V or 400V, the latter using two inverter outputs but drawing current at a phase angle.

I have tried 120V to 120V isolation transformers, using L1 as one phase of 120/240V split-phase, and taking a 120V "vector" fed by L2 into transformer primary, isolation with secondary wining making a 120V "vector" off L3. That makes something like a split-phase 120/240V in line with L1. But it is off a bit in voltage and phase depending on load. It might work for what you want to drive, but it didn't behave nice to synch my 3-phase inverters with single-phase grid.

If you have 3-phase delta, e.g. 240V delta with no ground reference, a "zig-zag" transformer is common way to generate neutral.
Alternative is "T"; one transformer with two identical 1:1 windings for 120V & 120V generates midpoint between two legs say L2 and L3. Then a second transformer 2:1 for 138V & 69V (approx?) between L1 and that midpoint generates neutral.


Most transformers aren't as nice to drive with inverter as you might expect.


You can use transformers with 240/480V primary, 120/240V secondary to make some of these configurations. It works best if "240V" primary windings are driven with 120V, and then two "120V" secondary windings connected in series for "240V" makes 120V. Or, drive one "240V" primary winding with 120V, and second "240V" primary winding makes 120V.

But anything 3-phase prefers perfect symmetry, which is lost when one phase is generated from another. Delta vs. WYE connection of transformers and motors may affect how much excess current gets drawn. Just for transformers, I'm getting nicer behavior with three isolated transformers rather than a single 3-phase E-core transformer.
 
Shows WYE connection.
Is that 230/400Y? European inverter, I assume so. Use an isolation transformer, either 230V or 400V input.

If it was 120/208Y, auto-transformer could be used to make 120/240V split-phase. But I don't think that is the voltage your inverters operate at.


Either way, refer to what I wrote about applied voltage, transformer current, saturation. Some transformers, or operating at reduced voltage, will give better results.
 
Thinking out loud....

What voltage would I get n to l1, n to l2, n to l3?

What voltage would I get l1 to l2, l1 to l3, l2 to l3?

I only have single phase power to my house, I was hoping to use these 3 to charge batteries from grid, or gen if I go completely off grid. Is this possible as they show inputs from l1, l2, and l3, and neutral.
 
Is your 5000es 240V?

In that case, N to L1 is 240V, N to anything is 240V.
L1 to L2 is 415V = 240V x sqrt(3)

With 3x 120V Sunny Island, I take in 120V single phase from grid, the two slaves make 120/208Y 3-phase.
But if you want 3-phase, a VFD is the way to go (at least for motors.)
 
Not bonded, could wire as 240V delta, and use auto-transformer to make that 120/240V high-leg delta.

How well does 5000es behave with low PF due to phase-shifted current?
Single inverter L-N with transformer would be in phase for near 1.0 PF if driving resistive load.
A load connected L1-L2, whether driving isolation transformer or auto-transformer to make 120/240V high-leg delta, the current would be 60 degrees out of phase.
 
The Growatt SPF-5000-ES is 230/400 in 3-phase configuration.
A SS autotransformer would give you 200v to neutral.
 
A 120/240V auto-transformer connected to 400V might fry, would likely at least draw a high peak current every phase.
Connected to 230V, center tap would give you 115V to neutral.

You could get a 240/480V to 120/240V isolation transformer, connect 230V L-N and get 115/230V split phase from secondary (ground the center tap).
However, typical transformer meant to be powered from the grid goes part way into saturation every phase, and this will increase losses of your inverter. It was optimized for cost, not highest efficiency in a battery powered system.

I would rather apply 230V to a 480V primary winding. In that case, the 120V + 120V series-connected secondary will only make 115V total. So I might try center tap of 240V + 240V primary to get one 115V phase, and 120V + 120V secondary to get other 115V phase. All windings in series as auto-transformer.
That won't make perfectly balanced 115/230V split-phase, because voltage droops under load and secondary starts out about 3% high. But should be close enough.
 
I was originally planning on going 3-phase for my system. To save on wiring. But, it just wasn't feasible to do in a cost effective manner. Also puts you back in to a balancing act with the loads.
Single phase was the best way to go, in the end.
 
I have 6 inverters in parallel feeding the house and shop now single phase. I can pull more than 30,000 watts easy with ev cars, heat pumps, air comp. ect. So was thinking of separating house from my shop, but still using the same battery storage for both.

Then I remembered I have a few pieces of 3 phase equipment in storage that I could use if I went 3 phase for the shop. Now I am not so sure that plan is that great in the end.
 
I have 6 inverters in parallel feeding the house and shop now single phase. I can pull more than 30,000 watts easy with ev cars, heat pumps, air comp. ect. So was thinking of separating house from my shop, but still using the same battery storage for both.

Then I remembered I have a few pieces of 3 phase equipment in storage that I could use if I went 3 phase for the shop. Now I am not so sure that plan is that great in the end.
If you have equipment that runs on 3-phase. Then it might be worth getting an isolation transformer.
But I have very little need for actual 3-phase. So, it wasn't cost effective for me.
 
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