240v Split Phase with 4x Victron Multiplus and/or Autotransformer?

[WolfbaneAries]

Currently I am running 2 Multiplus II 48/3000/35s in split phase configuration. My problem is that despite my efforts to balance the loads in the breaker box, at times enough power is drawn on one leg to put the respective multiplus into overload while the other multi is just sitting there.

In order to solve this problem I'm looking at using an Autotransformer to balance the loads. My question is about how to configure this setup keeping in mind that I would like to stack additional multiplus units down the road for more power output. Should I:

A: Keep the two multiplus units in parallel split phase configuration and use the autotransformer for load balancing, then down the road add two additional multiplus into the configuration or

B: Reconfigure the multis to series single phase configuration feeding ~6kVA 120v to the autotransformer which in turn provides the 240v split phase, then down the road add an additional multi or two to provide more 120v power to the autotransformer?

The advantages I see for option B are that 1: I can add just one Multiplus into the system to increase my total available power rather than needing to purchase two at a time and 2: both multis can provide 35a of charging to the batteries if/when I need to run the generator, but I'm curious to know other's thoughts.

 

[sunshine_eggo]

The AT has a 28A limit on the Neutral. I don't entirely understand the implications beyond that obvious restriction.

Parallel at the same voltage/phase makes your wiring pretty critical. The inverters don't coordinate load sharing. They output based on load/resistance. It's critical that your AC out wires be 1) not too thick and 2) the same length. Having too little resistance in your AC out wiring makes small changes in resistance more significant.

Check out link #5 in my signature.

 

[solar8484]

The AT has a 28A limit on the Neutral. I don't entirely understand the implications beyond that obvious restriction.

That just means the max load imbalance allowed between the 2 120V legs is 120V*28A=3360VA.

 

[sunshine_eggo]

That just means the max load imbalance allowed between the 2 120V legs is 120V*28A=3360VA.

That's the "obvious restriction." Given that you feed the AT with the N at the N, I don't know that there are additional limitations.

 

[solar8484]

That's the "obvious restriction." Given that you feed the AT with the N at the N, I don't know that there are additional limitations.

Assuming "additional limitations" refer to the AT, I don't believe there is other limits beyond the 28A. However, the 28A AT neutral limit could potentially restrict overall system config. I think OP's option B (step up config) would be limited to absolute 28A load current on the step-up 120V leg not directly powered by inverters even if the inverters could provide more power. In contrast, OP's option A (load balancing config) the 28A limit would not apply to the absolute load current on either 120V legs, but only to the load current difference (imbalance) between the 120V legs.

 

[sunshine_eggo]

Assuming "additional limitations" refer to the AT, I don't believe there is other limits beyond the 28A. However, the 28A AT neutral limit could potentially restrict overall system config. I think OP's option B (step up config) would be limited to absolute 28A load current on the step-up 120V leg not directly powered by inverters even if the inverters could provide more power.

That's my concern.

In contrast, OP's option A (load balancing config) the 28A limit would not apply to the absolute load current on either 120V legs, but only to the load current difference (imbalance) between the 120V legs.

Yep.

I'm going to reach out to my dealer and see what he says. Option B is appealing as MP + AT is cheaper than MP+MP.

 

[WolfbaneAries]

Well I've got an AT on the way and I'm going to proceed with option A. I think that with ~3000va of load balancing the two MPs will do the job for quite a while and then down the road I can add to the system as needed. Thanks for your comments!

 

[sunshine_eggo]

Well I've got an AT on the way and I'm going to proceed with option A. I think that with ~3000va of load balancing the two MPs will do the job for quite a while and then down the road I can add to the system as needed. Thanks for your comments!

Did you get the 100A or 32A model?

At least that option is well understood. As long as you don't exceed (28A * 120V=) 3360W imbalance on the legs, the AT should help.

If it turns out that option B isn't hindered by the 28A neutral limit, then you can still go that route as well.

 

[Hedges]

I expect an autotransformer connected in parallel with a split-phase source to only partially rebalance. Resistance of the transformer and resistance of the source would determine now neutral current splits.

I ran a test with loads on split-phase inverters and auto-transformer at end of an extension cord; it carried only a modest percentage of the neutral current. If both loads and auto-transformer were on a breaker panel with longer wire run to inverters it would probably carry more.

If inverters are used to produce 240V, neutral not connected, then auto-transformer derives a neutral and carries 100% of the imbalance. But only if neutral-ground bond is not made in two different locations, because then ground wire carries part of the neutral current.

Isolation transformer would solve the problem of objectionable ground current and perfectly balance load across two 120V inverters stacked for 240V (but once you parallel several MultiPlus, keeping those balanced is on you.) Problem is it will be difficult to find a nicely behaved 240V primary, 120/240V secondary transformer. The commonly available ones draw excessive no-load current and present a nasty inductive load on inverters. I have some possible work-arounds for that, but producing symmetric 120/240V output would require two typical utility transformers.

If you can live with maximum 28A imbalance, and if it is OK to have Multiplus neutral unbonded, using your new auto-transformer to derive neutral seems best way to go. If neutral of inverters connects to center-tap of auto-transformer, use a clamp ammeter to check how current is split. Maybe it will work well.

Didn't we hear that Victron's auto-transformer is NOT symmetric, i.d. it's windings were made so 120V input would generate a second 120V phase, +3% or maybe +5% no-load, -3% or maybe -5% full load? Such a transformer, if connected in parallel with L1/N/L2 of 120/240V MultiPlus system, I think will fight to get its own way. Not what you want.

You're trying to balance 120V loads. How much 240V do you have? You could configure MultiPlus for 120V, connect all in parallel. You would have no issue balancing 120V because there would only be one. Use auto-transformer to create L2 for 240V with limited current.

 

[sunshine_eggo]

If inverters are used to produce 240V, neutral not connected, then auto-transformer derives a neutral and carries 100% of the imbalance. But only if neutral-ground bond is not made in two different locations, because then ground wire carries part of the neutral current.

That's the prescribed Victron config. Two 120V inverters in 120V/240V split phase config with their neutral connected to each other, but not to the AT. Ground relay in both inverters disabled (open) with the AT providing the N-G bond.

You're trying to balance 120V loads. How much 240V do you have? You could configure MultiPlus for 120V, connect all in parallel. You would have no issue balancing 120V because there would only be one. Use auto-transformer to create L2 for 240V with limited current.

That's the question with his option B. There's a 28A limit on the AT N. AT is fed 120 on L1 and N. Does that limit only apply to the AT output? One would hope so, but I've reached out to my dealer to confirm.

 

[Hedges]

I've looked at their autotransformer schematic.
If you get the 100A model, it can pass up to 100A 240V through L1 and L2, and create 28A on N.
You could of course tap L1 and L2 before auto-transformer, but that would have to go to 240V loads only, not a split-phase panel with 120V loads that gets derived N. The auto-transformer protects L1 and L2 at 100A, protects N at 28A, and disconnects all three if overloaded (so loads shut down instead of suffering lost neutral.) If I recall correctly it also has temperature sensor for shutdown. So it gives you all the auto-transformer can give, and protects itself.

"B: Reconfigure the multis to series single phase configuration feeding ~6kVA 120v to the autotransformer which in turn provides the 240v split phase, then down the road add an additional multi or two to provide more 120v power to the autotransformer?"

Oh, maybe he did mean straight 120V output from inverters.
Be sure to feed appropriate phase of auto-transformer, the lower turn-count one, so derived phase is slightly higher no-load voltage.

I didn't pay attention to how it gets connected when fed 120V, generating 120/240V. Seems tricky how you would connect inverter, auto-transformer, split-phase breaker panel so as to not experience lost phase. You don't want to feed L1/N into breaker panel and have L2 disconnected, because with 120V loads on L2 and 240V loads between L1 & L2, those experience brownout.

Victron's schematic does look to me like it can cause the lost-phase brownout problem. I don't like that.

https://www.victronenergy.com/upload/documents/Datasheet-Autotransformer-32A-and-100A-EN.pdf

"series single phase" I thought he meant each MultiPlus was to be 120V single phase, not 120/240V split-phase. Then stack two for 120/240V split-phase.

Many ways to skin a Victron. Having two 120V outputs that can be connected either series or parallel is convenient. SolArk does the same. This lets a single inverter provide split-phase, or they can be used for 120/208Y three-phase.
Unfortunately Victron doesn't appear to have programmable output voltage sag with current, which would facilitate load sharing when paralleled.

 

[Hedges]

I think I've figured out how to best use it when creating split-phase from 120V single phase.
Connect Neutral from inverter straight to breaker panel, also to auto-transformer 28A neutral terminal.
Connect L1 from inverter to auto-transformer 100A L1 breaker. Tap other side of 100A breaker and feed to L1 of breaker panel.
Connect auto-transformer 100A L2 breaker to L2 of breaker panel.

Inverter can feed up to 100A on L1 & N for single-phase 120V loads on breaker panel.
If auto-transformer disconnects due to 28A current limit or over-temperature, it also interrupts both 100A poles, shutting off all loads.



Yes, I think the Victron more detailed drawing has it correct. This one shows 32A, but 100A would do the same. Appears to be a 2-pole breaker with 3rd shunt trip.

https://www.victronenergy.com/upload/documents/AT-3-split-phase-120V-to-120-240V-with-Quattro-120V.pdf

 

[sunshine_eggo]

Well that answers that. Awesome!

 

[G00SE]

So does that change the load capacity at all of the two 3ks?

 

[Hedges]

There will either be a limitation on 120V (unbalanced) loads, or on 230V loads. Because autotransformer is rated for less than two MultiPlus.
Maybe one configuration will support all intended loads. If not, "We're gonna need a bigger transformer!"

 

[WolfbaneAries]

Hooray! Now my wife can run her iron and microwave at the same time!!!

 

[ChrisG]

Hooray! Now my wife can run her iron and microwave at the same time!!!
View attachment 170133

View attachment 170134

So you are still running 2 MPs in splitphase (Your option A)?

 

[WolfbaneAries]

Correct. That seems to be the best option.

 

[ChrisG]

Correct. That seems to be the best option.

That’s a damn even split on each leg! Impressive.

 

[WolfbaneAries]

AT does what it says it does!