Safe Grid Use of the 5000ES and transformer

[JohnGalt1717]

Hi, I'm new to the form... and honestly I could only spend 6+ hours trying to parse through the 800+ posts on this thread looking for an answer. All of the answers and replies get intermixed with "that will work" right after someone says that a different solution will not work. I understand some options cannot be made to work... but that doesn't mean no option will work.

To help summarize the past 800+ posts and narrow the answer for people who already have all or part of an SPF 5000 ES system:

• SPF 5000 ES from SS, or circuit board ground screw removal (no internal bonding between ground and neutral on AC input, which seems to be something someone can confirm with a DMM)
• 120VAC loads required, but not specific to any transformer(Growatt At5000, the solar edge transformer, or isolation transformer)

What are safe options for a system configuration to use the US grid as a backup and or input?

X # of Growatts that either have the ground screw removed or come from SS. (best is to take the main board out and neuter the pin that the ground screw connects to so that there's no chance of arc if you're using ones not from SS)
1 or more Growatt auto-transformers (or someone else's doesn't matter as long as it takes 240V in and outputs a 120V neutral center tap) Generally 5000 watts is enough for an even remotely well balanced house.
240V incoming from Grid WITHOUT neutral (just both hot legs) into the Growatt AC IN
240V outgoing from the Growatt.
Make sure that you have the phases aligned. IE: Red in/Red Out, Black In/Black Out and every inverter in parallel lined up in your panel so that all phases are in sync for both grid and inverting (the inverters will correct but still...) or you'll get a hard switch from the Growatt that UPSes etc won't like.

If you are going to have some sort of switch that bypasses the Growatt and goes directly back to the grid power, then you MUST either have the auto-transformer still hooked up and NO NEUTRAL FROM THE GRID OR you MUST switch the neutral too using a 3 pole, double throw switch ($$$$!!!)
In short, you're fine as long as the grid's neutral is not connected anywhere while the growatts are also connected.

 

[Tony Scott]

The Growatt 5000es is made for the U.S. market and is sold by other retailers, do not try and modify the wrong model.

 

[JohnGalt1717]

The Growatt 5000es is made for the U.S. market and is sold by other retailers, do not try and modify the wrong model.

The ES is everywhere but the US. 5000US is US model.

 

[Tony Scott]

Call Growatt, the ES of which have three when this whole mess started.

 

[msllarson2]

X # of Growatts that either have the ground screw removed or come from SS. (best is to take the main board out and neuter the pin that the ground screw connects to so that there's no chance of arc if you're using ones not from SS)
1 or more Growatt auto-transformers (or someone else's doesn't matter as long as it takes 240V in and outputs a 120V neutral center tap) Generally 5000 watts is enough for an even remotely well balanced house.
240V incoming from Grid WITHOUT neutral (just both hot legs) into the Growatt AC IN
240V outgoing from the Growatt.
Make sure that you have the phases aligned. IE: Red in/Red Out, Black In/Black Out and every inverter in parallel lined up in your panel so that all phases are in sync for both grid and inverting (the inverters will correct but still...) or you'll get a hard switch from the Growatt that UPSes etc won't like.

If you are going to have some sort of switch that bypasses the Growatt and goes directly back to the grid power, then you MUST either have the auto-transformer still hooked up and NO NEUTRAL FROM THE GRID OR you MUST switch the neutral too using a 3 pole, double throw switch ($$$$!!!)
In short, you're fine as long as the grid's neutral is not connected anywhere while the growatts are also connected.

Is there a wiring diagram posted or otherwise that could demonstrate this solution?

 

[msllarson2]

The Growatt 5000es is made for the U.S. market and is sold by other retailers, do not try and modify the wrong model.

Can't this concern be resolved by any user with a DMM to identify if each unit they have is a US variant or a European variant?

• 'Continuity' between any of the 3 inputs = Europe (Modification needed for US grid input)
• 'Open' between each of the 3 inputs = US (No conversion)

As always, correct me if I got any details wrong but please don't forget to address the main question.

 

[timselectric]

In short, you're fine as long as the grid's neutral is not connected anywhere while the growatts are also connected.

If the grids L1 and L2 are being used. The grids neutral must also be used. All current must return to the source. If you don't provide the proper path, it will try to find another path. (Ground conductor, metal structures, people)

 

[Hedges]

All of the answers and replies get intermixed with "that will work" right after someone says that a different solution will not work. I understand some options cannot be made to work... but that doesn't mean no option will work.

240V incoming from Grid WITHOUT neutral (just both hot legs) into the Growatt AC IN
240V outgoing from the Growatt.


If you are going to have some sort of switch that bypasses the Growatt and goes directly back to the grid power, then you MUST either have the auto-transformer still hooked up and NO NEUTRAL FROM THE GRID OR you MUST switch the neutral too using a 3 pole, double throw switch ($$$$!!!)
In short, you're fine as long as the grid's neutral is not connected anywhere while the growatts are also connected.

... and I'm going to say that does not work safely.

As I've written before (among the 600+ replies to this thread, or the 6 (?) other threads on same topic),
there will be "objectionable" current in ground wire (if connected back to grid), or neutral will take on voltages higher than ground, or "ground" will be higher than actual ground, which you may even discover while sitting in a bath tub.

Only judicious application of isolation transformers will make it work correctly and safely.
Simplest is an isolation transformer on the output. I'd rather put that on input and have auto-transformer on output; these can be sized for less no-load power consumption (something I've been studying and experimenting with extensively for 3-phase step-up applications.) Transformers, especially that will handle the power and not waste much power are not cheap!

If the grids L1 and L2 are being used. The grids neutral must also be used. All current must return to the source. If you don't provide the proper path, it will try to find another path. (Ground conductor, metal structures, people)

Exactly!

Except if you do the following, which is the safest way: Connect grid to 240V primary of an isolation transformer. Connect secondary to Growatt.
What I'm not sure of, can't say without seeing a schematic of Growatt, is if we connect transformer center-tap to neutral of growatt, use an auto-transformer after Growatt, or ...

 

[timselectric]

Transformer after the Growatt is better.
240v grounded system through the Growatt to the isolation transformer. Bonded transformer secondary for a grounded system after.
This way everything is protected by a grounded system.

 

[Hedges]

It just means you need a transformer big enough for all 240V loads as well as 120V.
(could have saved money and losses with one 240V load panel, and a breaker-protected 120/240 load panel with auto-transformer.)

There are a few auto-transformers offered for inverters. Got any leads on efficient isolation transformers for use downstream of a battery inverter?
What I'm finding is excessive no-load current draw because they're size to save on cost, and partially saturate so current spikes up.

Like this waveform driving a 20kVA, 200V delta transformer with my 120/208Y inverters. It shows 6.3Arms per 120V leg. That's 750 VA of reacative and real power per 6.6 kVA of transformer, similar to what the GroWatt would see:



My solution is to run windings at half their intended voltage. If a transformer has two 240V primary windings, I put 120V across each.
The transformer for which I captured the above waveforms was meant to take in 200/230/460/575V delta on the primary and produce 277/480Y secondary.
What I've done is to rewire primary as WYE, and I apply 120/208Y to the 200V windings, so 460V windings act as auto-transformer and make 277V. Still good for 20 kVA due to happenstance (ratios of applied/designed voltage and auto-transformer step-up ratio.)
That dropped current to 1/4 as much.



High reactive load on an inverter will increase its no-load loss.

 

[timselectric]

Too far out of the box thinking for me.
But I appreciate it in theory.

 

[msllarson2]

It just means you need a transformer big enough for all 240V loads as well as 120V.

Does it need to be one big transformer, or can you stack them in parallel like the 5000 ES to increase capacity as your system increases?

How do you size the transformer(s), do you:

• Match the size of the Transformer(s) with the size of the Inverter(s)?
• Match the size of the Transformer(s) with the max surge of the Inverter(s)[to accommodate motor start up, etc.]?
• Some other way?

 

[msllarson2]

Transformer after the Growatt is better.
240v grounded system through the Growatt to the isolation transformer. Bonded transformer secondary for a grounded system after.
This way everything is protected by a grounded system.

I'm assuming you could use this design to have multiple options to maximize flexibility:

• Have a transfer switch that moves the 240 from the output and when needed bypass the inverters and feed the transformer directly without worrying about the ground
  • (Not efficient on grid, but good if needed to keep the lights on if the solar system needs maintenance)
• Replace grid input with a grounded generator and still have the system retain it's own ground.
  • This might even allow a transfer switch to change between generator and grid power feeding the inverters.

 

[Hedges]

Parallel might work, but could share a bit unevenly. Especially if different models. I only occasionally see an eBay lot with several of one model.

Nominally same VA rating as inverter ought to work.
At full load, it is going to run HOT. Look at the labels; some are rated to run at 150C or 200C (innermost windings; I expect outside of enclosure to be safe to touch.

You don't need to size it for surge. A transformer rated 3% puts out +3% voltage under no-load, -3% under full load. That would drop to -9% under 2x load, which is probably all your inverter can deliver. If your 5kW inverter could deliver 25kW surge, then -27% under typical 5x starting surge for a motor.

A heavier transformer may run at lower temperature rise, and lower no-load current. A higher VA rated transformer will run cooler; power dissipation in winding goes as square of current, so 7.5 kVA transformer carrying 5kVA would be (2/3)^2 or 4/9 as much power dissipated.

If you want an auto-transformer, don't try to use two 120V secondary windings. They can draw much higher current than you expect.
I recommend getting a 240/480 to 120/240 transformer, connecting the two 240V primary windings in series, and using them for 120/240V auto-transformer. Unfortunately, you're not likely to find one that can be used at 1/2 voltage as an isolation transformer. If you find one for 240/480 primary and 240/480 secondary, that would do it. Used at half voltage (full current), it will only be for half its VA rating.

It's OK to be inefficient on grid, which is why I'd rather have a transformer on input to inverter. Inverter on output has to be powered the entire time inverter operates.

 

[msllarson2]

(could have saved money and losses with one 240V load panel, and a breaker-protected 120/240 load panel with auto-transformer.)

What do you mean by this?
This statement feels like what Signature Solar recommends in their video and schematic. They seemed to suggest that having a 2 pole breaker on the output makes a difference when an over current on either leg will trip the other side... or cuts power to the input or something.

Is there a variation/tweak you can make to keep the neutrals/grounds happy and safe, while not having 240v on one 120 leg or the other?

• Like having a multi-pole breaker with one pole connected to the output, and the other half going to the input of the auto transformer, and the other side connected to the output?

... or is an isolating transformer before or after the only way to safely handle the ground/neutral issue under off-nominal cases (bad load balancing, switching breakers, short to ground, short circuits at any point, etc)

 

[Hedges]

True, sounds too much like SS's recommendation.

I haven't seen an auto-transformer schematic for the Growatt that I'm entirely happy with (when it is connected to grid.) A small isolation transformer could be used for a small loads panel. A large isolation transformer could work for full output.

An autotransformer is half the material, half the cost of an isolation transformer of same VA rating. Also half the loss.
If largest imbalanced 120V load is smaller than total wattage of inverter, a separate panel could be dedicated to just those loads, fed by a smaller auto- or isolation-transformer.

That makes it cheaper.

An undersized transformer can be protected with a breaker. But you want the breaker to disconnect the 120V loads as well, not just the transformer that creates neutral. So if I used an auto-transformer I would connect it to the panel without a breaker, and feed the panel with a breaker sized to protect the transformer.


I think switching of auto-transformer centertap (but not switching neutral) is what's needed, but don't think that is supported by the Growatt design.

 

[msllarson2]

It's OK to be inefficient on grid, which is why I'd rather have a transformer on input to inverter. Inverter on output has to be powered the entire time inverter operates.

I'm assuming this would just be a 1:1 isolating inverter, and then use an auto-transformer (like the Growatt one) on the output side, or is there a better way to get split phase on the backside?

If it is an auto transformer creating the 120VAC, I imagine you would need to take certain extra precautions when wiring (ie 2 pole circuit breakers or something like I mention above) to prevent an over voltage like Ian demonstrates with his light bulbs in his video.

 

[Hedges]

Yes, 1:1 isolating transformer between grid and inverter lets you make your own neutral and bond it to ground.
A 240/480 to 120/240 transformer, at least 5kVA if that's what your inverter can pass through, would do it.
Higher VA rating is good. Shipping is expensive. I picked one up; at 265 lbs plus a frame, was very difficult to get into the pickup. A strong guy did most of the work. Unloading was easy with engine hoist. Cost me about $400 to $500. I've seen some on the East coast for less, but with shipping costs more.

You don't want 120V loads to ever have the auto-transformer disconnected from them. That's why I would hardwire the transformer to a breaker panel, no breakers to disconnect it. Then limit current going to the panel. e.g. Connect the transformer to panel's main lugs, and backfeed the panel through a 30A breaker. (backfed breakers should be secured against coming loose.)

I learned that over-voltage of a light bulb trick by rewiring my parents' house with the power on (add a branch to my workbench.) That's what happened when I opened neutral. Ian should have known better than to let a breaker opening do that, but maybe it was on purpose, to earn more than the bulbs cost him (or to teach.)

 

[timselectric]

I'm assuming you could use this design to have multiple options to maximize flexibility:

• Have a transfer switch that moves the 240 from the output and when needed bypass the inverters and feed the transformer directly without worrying about the ground
  • (Not efficient on grid, but good if needed to keep the lights on if the solar system needs maintenance)
• Replace grid input with a grounded generator and still have the system retain it's own ground.
  • This might even allow a transfer switch to change between generator and grid power feeding the inverters.

For that scenario I would have that transfer switch bypass the inverters and transformer. Without the inverters you don't need the transformer.

 

[timselectric]

The problem with placing the transformer ahead of the inverter is that the inverter is not always connected to the AC input. And therefore you wouldn't have a neutral at the output. (Insert bad things here)

 

[Hedges]

If you put an isolation transformer ahead of the inverter (same situation as using an isolated 240V generator), you just have to use auto-transformer to derive neutral all the time. It avoids issue of paralleling auto-transformer with 120/240V grid, and makes inefficiency of typical isolation acceptable, because not a load on battery/PV system when grid is down.

 

[timselectric]

If you put an isolation transformer ahead of the inverter (same situation as using an isolated 240V generator), you just have to use auto-transformer to derive neutral all the time. It avoids issue of paralleling auto-transformer with 120/240V grid, and makes inefficiency of typical isolation acceptable, because not a load on battery/PV system when grid is down.

So you are talking about putting an isolation transformer ahead of the inverter? And an autotransformer after the inverter?
That's more expensive, but would work.

 

[??????]

Parallel might work, but could share a bit unevenly.

Just wanted to say to be careful with this; two midpoint transformers wired in parallel may be in a permanent state of trying to balance each other due to manufacturing variances between the two (even in identical models), causing high neutral currents (in theory). Preferred solution here is a single, larger transformer.

 

[timselectric]

I'm just using an autotransformer after the inverter. And switching it off automatically when in bypass mode. I get the best of both worlds this way. But not all units can be configured to do it.

 

[Hedges]

So you are talking about putting an isolation transformer ahead of the inverter? And an autotransformer after the inverter?
That's more expensive, but would work.

Can be cheaper, because large selection of 240/480 to 120/240 transformers available used.

The auto-transformer could then be small, dedicated to 120V loads.
For used, two 240V primary windings in series should make a good 120/240V auto-transformer.

I'm just using an autotransformer after the inverter. And switching it off automatically when in bypass mode. I get the best of both worlds this way. But not all units can be configured to do it.

Yes, switching external auto-transformer seems to be the solution.
Maybe could be implemented with a hack, tapping some wires of the internal relay. But bad things can happen if control wires go open-circuit. I like fail-safes. An overvoltage disconnect could serve as redundant protection.

By the way, what wire gauge connects the neutral of your auto-transformer ?
Compared to L1/L2, and available amperage or breaker rating?