Adding Autotransformer to 4 x SPF 3000TL LVM-48P

[steviep19]

I have 4 SPF 3000TL LVM-48P inverters.

I've had phase balancing issues with these since day one, but only with 2 inverters on one leg and two inverters on the other. I'd see lights flickering, the inverters resetting due to voltage fluctuation etc.

Weirdest part is I gave up on having one large system, and settled for two 120v systems, two inverters on each leg capable of outputting 6000 watts. This has worked perfect, no flickering lights no issues, but I really want 240v.

One option which could work for me is to still have two separate systems, but 1 inverter on each leg, paralleled for split phase. For some unknown reason this works great too. The only problem is I have to be really careful with my loads, since I only have 3000w available on each leg per pair. This is why I went with 120v system.

I recently saw Signature Solar has a growatt autotransformer that can handle 5000 watts of imbalance. I'm thinking I could use this autotransformer to create a neutral, and only pass L1 and L2 from the growatt inverters. My only problem, is I live in the NorthEast and I fall back to the Grid often in the winter. Are autotransformers safe to use for the neutral, when the Growatt inverters are in bypass and just passing the gird through? Also, seems to be some confusion, at least on my part of how to wire the neutral, as well as neutral/ground bonding issues when on grid.

In my current setup the growatts have a relay which does not bond neutral/ground, if it detects the grid input, but a relay bonds neutral to ground if not.

 

[ricardocello]

Autotransformer neutrals are tricky things. Be sure to look at the older autotransformer posts here, there are lots of them.
If you were totally off-grid, there would be fewer issues.

Before you go through autotransformer pain, are you sure you have to be that careful with your loads?
Have your inverters overloaded during actual use?

The problem comes if the autotransformer neutral is connected directly to your grid neutral.
The autotransformer actually tries to balance L1/L2 all the back to the transformer on the pole.
Sometimes this can include your neighbors, depending on housing density.

Electrically, the autotransformer would be in parallel (L1, L2, N) with the transformer on the pole, separated by some length of unknown impedance wiring, with most of the loads at one end. Don't do that.

In my case, I am setup for grid use, with an ESS as a backup.
I actually switch the house neutral via a big relay from the 120V inverter neutrals to the autotransformer neutral when grid power is lost.
Thus, phase balancing occurs only when inverting.
The inverters disconnect L1, L2 from the grid when inverting, so the problem described above does not occur.
I've got critical load sub-panels for for the output of the inverters.

The relay has to switch < 16 msec, or the neutral will be floating. I've got failsafes to ensure the neutral will never ever float.
The autotransformer L1 and L2 are always connected (wasted power draw seems to be about 30w).

As for neutral/ground bonding, due to the way I've wired things, I rely solely on the ground/neutral bond screw in the main panel.

If all of this sounds overly complicated, it is, but it does work for me.

 

[Hedges]

I recently saw Signature Solar has a growatt autotransformer that can handle 5000 watts of imbalance.

Not according to my reading of the data sheet. I think it is only 1500VA of imbalance!

First of all, I'd rather see the autotransformer switched, so neutral is either provided by grid or provided by centertap of transformer.
If autotransformer is connected across grid, that may or may not be OK depending on how much imbalance and how hard it tries to rebalance. I'm not 100% clear on the switching of this Growatt, but I think it does have overload protection, which is good.

I tried an auto-transformer on output of stacked 120V Sunny Islands, and it only rebalanced a percentage of the load. That was with 10's of feet of wire to the auto-transformer. So wire resistance will protect it from drawing as high a current.

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Spec sheet for the Growatt indicates that like Victron, two models have different amperage pass-through on L1 & L2 but same neutral current. In the case of Victron, 32A or 100A pass through, and 28A max continuous on neutral.

GroWatt data sheet says 5000W L-N, but I think that is misleading.

https://signaturesolar.com/content/documents/GROWATT/ATS%2011400T-US%20Spec%20%281%29.pdf

At the bottom, it says, "The rated power of secondary side L1-N and L2-N could up to 5kVA respectively, and the power difference between the two split phases can not exceed 1.5kW."

For model ATS5000T-US, "21A@240Vac"
21A x 240V = 5040VA. That's not 5040W unless PF = 1.0 (well, 0.99) as it would be for a resistive load. Some inverter vendors quote watts based on 0.8 PF, so 4kW.

1500W / 120V = 12.5A (or rather, 1500VA / 120V = 12.5A)

According to their data sheet, neither ATS5000T-US nor ATS11400T-US can support 5000VA 120V load on line side (e.g. L1-N) with zero on the other (e.g. L2-N). Can only support 1500VA imbalance. Which is probably less than your stacked inverters can handle, whether two of them or four.

I think your stacked 120V inverters are best operated within their limitations, but I don't know what causes the flickering.
Using them as 240V with an isolation transformer theoretically could allow all wattage on either 120V leg or 240V, but it opens another can of worms (what available transformers can actually accomplish that nicely for small inverters.)

The Victron auto-transformer is more capable, in my not-so-humble opinion.
Although at 27.5 lbs. vs. 33 lbs., it actually weighs less. I think it has a thermostatically controlled fan, allowing higher power handling.

By the way, the Victron auto-transformer began life as an isolation transformer, and the second winding has a few more turns to produce higher voltage no-load, sagging to lower voltage with load (as is normal practice, maintaining +/- some percentage regulation. Used as an auto-transformer, that's great for stepping up 120V to 120/240V split-phase. Stepping down 240V to 120/240V, one leg loaded heavier would start out high, sag low. But he other leg would start out low, sag lower. And, paralleled with a split-phase source like two stacked inverters or the grid, it would fight to unbalance it according to its turns ratio.

Where the GroWatt auto-transformer data sheet says up to 5000W L-N, that appears to mean assuming you have 3500W on the other L-N. Better turn on a couple hair dryers on the other circuit so you don't overload the transformer, trip its breaker, and cut all your power. That's no way to live.

If you want a fully capable auto-transformer, pick up a used 25kVA 240/480 to 120/240V transformer. (Hope this is for stationary application, not mobile.) They weigh about 350#, and can be had for about $350 and up (local pickup; freight will cost more than the transformer.)
Wire primary windings in series, and apply 240V where 480V was expected (at rated current but half voltage, transformer could deliver 12.5kVA). Tap off one of the two windings to get 120V (double current at half voltage, 12.5kVA available). 15kVA transformer would work too, delivering 7.5kVA, but with 4x 3000W inverters, less than 100% could be used on one leg. Be sure to size wires and breaker panel correctly, because this doubles current coming from inverters.

 

[steviep19]

It seems the only way to really use the auto transformer, is off grid. Once you introduce the grid, you have to rely on relay's to switch back and forward, between the two neutrals, and that's just asking for trouble.

It's just odd that Growatt's inverters work great in split phase with two inverters, but the second you introduce the second pair, everything goes bad.

 

[solar8484]

The Goodwe AT supports 4.8kW (40A) imbalance.

 

[Hedges]

Yes, that one does say 40A imbalance at 4800VA in the listing and data sheet. Also 48.5 lbs.
(took a bit of searching to find a page with the details.)

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