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Auto transformer dangers

I'm running this setup, with no concerns.
You just have to deal with the couple of caveat's.
I have zero regrets. And am very satisfied with what I am getting, for the price I have paid.
It is certainly forcing me to learn a lot. Solar seems simple on the surface. I didn't have many concerns with bringing two 480vDC strings into my on-grid SolarEdge inverter, due to the use of the optimizers that limit the output to 1vDC per panel until activated by the SE inverter. Watching Ian nearly do some arc welding with his solar panels will make me be more careful when I go to hook the panels up to the Growatt. Likewise, need to be careful with the 48v EG4 batteries as well. Lots of power.
 
Either way works fine. The object is to stop current flowing to the loads, if the neutral is lost. And, protect the transformer from overcurrent. Either option will accomplish the goal.
This goal can actually be accomplished , in a simpler manner. With a single pole 25 amp breaker for the neutral. And a relay breaking the power, controlled by the 120 volt output.
 
On a DIY forum, I've yet to see anyone that is truly qualified!

I think some/many of us are qualified to safely design and do, what some others are not.


Hmm, a good application for surplus power, and I have a couple transformers salvaged from microwaves.

Censorship, the only way to save lives:

 
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My regret is don't get the autotransformer before is gone from all shops :(, now i only have 240v, still waiting for the autotransformer.
 
This goal can actually be accomplished , in a simpler manner. With a single pole 25 amp breaker for the neutral. And a relay breaking the power, controlled by the 120 volt output.

Even simpler, connect auto transformer to a sub-panel, directly without breakers. Feed that panel with a 25 amp breaker. Put all 120V loads on that panel.

My regret is don't get the autotransformer before is gone from all shops :(, now i only have 240v, still waiting for the autotransformer.

Any transformer with pair of 120V windings will do. Many have a pair of 240V primary, and a pair of 120V secondary. Just wire the secondaries in series and you've got an auto-transformer. Check eBay.
 
This goal can actually be accomplished , in a simpler manner. With a single pole 25 amp breaker for the neutral. And a relay breaking the power, controlled by the 120 volt output.

Actually, I think you may want the neutral to carry 50A.
And I'm not sure, but you might have a chicken-and-egg problem there. How about a sketch?
 
Even simpler, connect auto transformer to a sub-panel, directly without breakers. Feed that panel with a 25 amp breaker. Put all 120V loads on that panel.
I don't think that this would protect against transformer failure. Unless, I'm missing something.
 
Actually, I think you may want the neutral to carry 50A.
And I'm not sure, but you might have a chicken-and-egg problem there. How about a sketch?
With the Solar Edge 5k transformer. It's rated for 32 amps L to N, and 21 amps L to L.
 
Although, I wouldn't push it to the extreme.
Which is why I chose a 25 amp breaker for the neutral.
I generally stay under 7 amps, unbalanced.
 
I don't think that this would protect against transformer failure. Unless, I'm missing something.

True, not against transformer failure. It protects the 120V loads from tripping of breaker feeding transformer, because if breaker trips it shuts off power to loads as well as transformer.
If transformer fails shorted, breaker trips so loads are protected. If transformer fails open, they aren't, but I think that failure is extremely unlikely.

With the Solar Edge 5k transformer. It's rated for 32 amps L to N, and 21 amps L to L.

If transformer would be protected with a 25A 2-pole breaker for L1 and L2, then it would be delivering 50A on N when 25A goes through each "L".
So long as its two windings each come out at N with wires as fat as L1, L2, it will be fine. If the windings connect together and come out with a single wire the size of one Ln, then it wouldn't be.
 
Although, I wouldn't push it to the extreme.
Which is why I chose a 25 amp breaker for the neutral.
I generally stay under 7 amps, unbalanced.

25A on the neutral is 12.5A on each of L1, L2.
Way under the transformer's rating, but 7A unbalanced is a very small amount.
 
I believe that my solution, covers all bases. But, there's always something missed. lol
 
Just that it doesn't utilize all the the autotransformer capacity.
And make sure you won't get a relay stuck with welded contacts.
 
Just that it doesn't utilize all the the autotransformer capacity.
And make sure you won't get a relay stuck with welded contacts.
The auto-transformers only job, is to cover the unbalanced load. And, my goal is to keep that as close to zero as possible. The 7 amps, comes from uncontrollable cycles of certain loads. Like refrigerator compressors, and furnace blower motor.
 
Has anybody checked their inverter DC idle current without the auto transformer and no load on inverter then check inverter idle current with auto transformer across 240 vac inverter output also with no additional load on inverter output?
 
Has anybody checked their inverter DC idle current without the auto transformer and no load on inverter then check inverter idle current with auto transformer across 240 vac inverter output also with no additional load on inverter output?
4 watts difference, is what I measured.
 
There are three primary issues associated with using an auto transformer.

1) Some European 230v inverters neutral-ground to one side of their 230 vac output. This is absolute no-no for U.S. split phase, requiring the European inverter to be modified to disconnect the neutral-ground from one side of its 230vac.

2) Many folks incorrectly wire the 230 vac inverter to breaker box through dual 240v breakers and separately wire their auto transformer also through another 240v dual breaker in panel with auto transformer center tap neutral going to breaker panel neutral bus bar. If auto transformer breaker is opened while inverter is still supplying 230vac to panel you have a floating neutral situation in panel. 120v ac loads neutral will be free to float between L1 to L2 side dependent on how many 120vac loads are turned on each L1, L2 side. Good way to blow out a lot of 120 vac equipment.

Auto transformer must be connected directly to 240vac inverter output (with separate dual pole small breaker box between inverter and auto transformer) and L1, L2, and neutral from auto transformer brought into electrical panel with L1 and L2 fed into a single dual 240vac breaker. Center tap neutral can be directly connected to panel neutral bus bar.

3) When having connected grid pass through on split phase 240/120vac through inverter and neutral from grid is passed through to neutral output of inverter (auto-transformer center tap) any 120vac voltage imbalance from grid L1-N or L2-N will try to be corrected by auto-transformer. Voltage imbalance of 120vac sides may be caused by your neighbors 120 vac loads that share the same utility pole transformer. If you have a good quality grid pole transformer and grid connections it may not be a significant issue. You may have 2-3 amps of auto transformer load current caused by grid imbalance that will eat up a little bit of auto-transformer load capacity.

Other: These small auto-transformers being sold (approx 8" box size) are really only capable of maybe 2kVA to 2.5 kVA. This is normally okay as any of your 120vac loads that are on opposite L1-L2 side phase will not contribute any current through auto-transformer if the amount of 120 vac loads on each L1,L2 sides are similar. For example, if you have 3 kVA 120vac loads on L1 side and 2 kVA on 120 vac L2 side load, only 1 kVA balancing current is carried by auto-transformer. If you have a 6kVA inverter and 120vac load L1 side with 4 kVA and no 120 vac load on L2 side the auto-transformer will over heat since it must carry the 4 kVA balancing current. This is an unusual use case but it is possible. Be careful of carelessly wired kitchen with multiple 20 amp breaker outlets wired to same L1 or L2 phase side.

240 vac load contribute no loading on auto transformer. So any high current loads should be converted to 240vac if possible. A water pump is a good example of motor that can be rewired easily to run on 240vac instead of 120vac.
Prior to hearing about all of the possible issues with using auto-transformers, the diagram below shows what I was planning to install. In fact, it is already cleanly mounted on the black plywood!

If I'm understanding things right, I'm all set with #1 because my screw was removed (or never existed).

I'm all set with #2 because my auto-transformer is not connected to the panel from a separate breaker.

I'm all set with #3 because I don't use the grid pass through. I used a separate 48v charger purchased from Signature Solar.

Note 1: I'm breaking the 120 rule.
Note 2: The breaker in the Reliant A/B switch marked 60A from the Growatt will be changed to a 25A. Redundant I guess, but it can't hurt.

Question 1: I should install a 25A dual pole between the output of the Growatt SPF 5000 inverter and the auto-transformer, correct?
Question 2: Do you see any issues with the way I have the neutral or ground wiring?


Thank-you everyone for your help!
 

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Prior to hearing about all of the possible issues with using auto-transformers, the diagram below shows what I was planning to install. In fact, it is already cleanly mounted on the black plywood!

If I'm understanding things right, I'm all set with #1 because my screw was removed (or never existed).

I'm all set with #2 because my auto-transformer is not connected to the panel from a separate breaker.

I'm all set with #3 because I don't use the grid pass through. I used a separate 48v charger purchased from Signature Solar.

Note 1: I'm breaking the 120 rule.
Note 2: The breaker in the Reliant A/B switch marked 60A from the Growatt will be changed to a 25A. Redundant I guess, but it can't hurt.

Question 1: I should install a 25A dual pole between the output of the Growatt SPF 5000 inverter and the auto-transformer, correct?
Question 2: Do you see any issues with the way I have the neutral or ground wiring?


Thank-you everyone for your help!
Looks good but you should put a two pole breaker between inverter AC out feeding autotransformer. An auto transformer winding could develop a short between its winding and its core/case so the AC breaker would protect against that. Other possible failure mode on transformer is inter-winding short between winding layers on transformer. This will put a heavy load on inverter.
 
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