Auto transformer dangers

[timselectric]

These inverters have built in short protection.
But I agree, there should be a 2 pole breaker between the inverter and transformer. (Which also protects the loads, after)

 

[RCinFLA]

These inverters have built in short protection.

Yes, they are called output IGBT devices.

 

[timselectric]

Yes,
I lost 2 TV's and a DVR, for my security system.
But, the inverter protected itself and shut down.

 

[cdevidal]

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.

I'm struggling to picture what you're describing, would you please share a drawing? Even a crude back-of-the-napkin drawing would help.

 

[Bud Martin]

Yes, they are called output IGBT devices.

View attachment 92868

WOW! What is the make and model of this blown inverter?

 

[John Schmidt]

I'm struggling to picture what you're describing, would you please share a drawing? Even a crude back-of-the-napkin drawing would help.

See https://diysolarforum.com/threads/auto-transformer-dangers.38820/page-2#post-494846

 

[cdevidal]

See https://diysolarforum.com/threads/auto-transformer-dangers.38820/page-2#post-494846

Afraid that didn't help clear things up. I'm hoping to use a 5000ES 240V Euro inverter in North America, using two plug-in Rockstone 3000W autotransformers, and the loads would be run from extension cords. Air conditioner is a 220V mini-split, 110V loads run off the two autotransformers. I'll remove the bonding screw inside the inverter.

Here's what I have in mind. I want to avoid scenario #2 that @RCinFLA describes. I believe I am able to by virtue of the fact that the 110V loads don't run through the panel, and the breaker only leaves GND in an overcurrent condition.

Not pictured: Normally closed 85C thermal control switches placed inside the autotransformer enclosures to kill the power should they overheat.

Will this avoid scenario #2?

 

[RCinFLA]

The picture is a 5kW, 240vac HF inverter. All the Chinese inverters have pretty much same circuit configuration.

If you get a stuck (welded contacts) on the AC input connect relay and turn on AC input breaker while inverter is running, the inverter output can be running out of phase with AC input and with welded relay, wammo. Welded relay contacts are usually the result of grid dropping multiple times during use months which momentarily overloads inverter until relay releases from grid. It puts a lot of stress on relay contacts every time grid drops out.

Normally, the AC input connect relay would not close until inverter adjusted its phase and AC voltage to match AC input.

 

[RCinFLA]

Afraid that didn't help clear things up. I'm hoping to use a 5000ES 240V Euro inverter in North America, using two plug-in Rockstone 3000W autotransformers, and the loads would be run from extension cords. Air conditioner is a 220V mini-split, 110V loads run off the two autotransformers. I'll remove the bonding screw inside the inverter.

Here's what I have in mind. I want to avoid scenario #2 that @RCinFLA describes. I believe I am able to by virtue of the fact that the 110V loads don't run through the panel, and the breaker only leaves GND in an overcurrent condition.

Not pictured: Normally closed 85C thermal control switches placed inside the autotransformer enclosures to kill the power should they overheat.

Will this avoid scenario #2?

View attachment 92996

For 240vac loads, there is no current supplied by autotransformer.

If the two sides of 120 vac loads are exactly balanced, there is also no current supplied by isolation transformer. This would be a rare case but it is usual that there is some balancing of 120 vac loads on either 120vac phases. Like 15 amp load on one side and 25 amp load on other. In this case auto transformer only supplies the difference of 10 amps.

I mentioned kitchens since they are normally the usual location of heavy 120 vac loads. Most kitchens AC outlets are on 20 amp breakers.
Coffee pots, microwave oven, toaster ovens, all heavy 120 vac loads and often run simultaneously.

 

[cdevidal]

For 240vac loads, there is no current supplied by autotransformer.

Correct, but the 240vac air con is only one portion of my system. Have a look at my graphic, I've also got 110V loads on the right side.

 

[cdevidal]

If the two sides of 120 vac loads are exactly balanced, there is also no current supplied by isolation transformer. This would be a rare case but it is usual that there is some balancing of 120 vac loads on either 120vac phases. Like 15 amp load on one side and 25 amp load on other. In this case auto transformer only supplies the different of 10 amps.

Is this also the case with two autotransformers, as I have?

Back to the original question. Would it be possible for you to draw out what you were describing in scenario #2?

 

[RCinFLA]

Is this also the case with two autotransformers, as I have?

Back to the original question. Would it be possible for you to draw out what you were describing in scenario #2?

I will draw it up tomorrow. Think of what would happen if you feed 240vac from inverter to L1 & L2 into panel, without any autotransfomer.

This is same as issue #2 when inverter is feeding 240vac into panel L1 & L2 through one dual pole breaker and second dual pole breaker from autotransformer also independently going into panel and autotransformer breaker is opened. There will not be the autotransformer connected to provide neutral in breaker panel so all 120vac loads in panel are screwed and the neutral will get pulled to one side or other toward L2 or L1 phase depending on 120vac loads. Each 120vac load can see anything from zero vac to 240 vac.

 

[cdevidal]

I will draw it up tomorrow. Think of what would happen if you feed 240vac from inverter to L1 & L2 into panel, without any autotransfomer.

This is same as issue #2 when inverter is feeding 240vac into panel L1 & L2 through one dual pole breaker and second dual pole breaker from autotransformer also independently going into panel and autotransformer breaker is opened. There will not be the autotransformer connected to provide neutral in breaker panel so all 120vac loads in panel are screwed and the neutral will get pulled to one side or other toward L2 or L1 phase depending on 120vac loads. Each 120vac load can see anything from zero vac to 240 vac.

I think I'm safe then by virtue of the fact that the 110V loads won't go through any breakers at all but are directly connected to each autotransformer's neutral by extension cords. But I'd like to see your drawing to verify.

 

[cdevidal]

This is what I mean. My two autotransformers look like this. I'd plug the 110V loads directly into the 110V outlet on the front panel.

 

[copec]

Afraid that didn't help clear things up. I'm hoping to use a 5000ES 240V Euro inverter in North America, using two plug-in Rockstone 3000W autotransformers, and the loads would be run from extension cords. Air conditioner is a 220V mini-split, 110V loads run off the two autotransformers. I'll remove the bonding screw inside the inverter.

Here's what I have in mind. I want to avoid scenario #2 that @RCinFLA describes. I believe I am able to by virtue of the fact that the 110V loads don't run through the panel, and the breaker only leaves GND in an overcurrent condition.

Not pictured: Normally closed 85C thermal control switches placed inside the autotransformer enclosures to kill the power should they overheat.

Will this avoid scenario #2?

View attachment 92999

The 240v side would be considered EU power still and so would still need the EU neutral-ground bond, and the mini-split needs to be EU power (wiring) compatible. Otherwise a center-tap neutral is still required for safe operation as otherwise there will be no low resistance alternative fault paths.

Using those transformers if the high side keeps the EU neutral-ground bond is fine. Both of us use a neutral and hot for our common receptacles, theirs is just higher voltage. However we use a center neutral for our 240v appliances. So US 120v to EU 220/230/240v is okay, so is EU to US 120v, but not EU to US 240v

 

[Hedges]

I think your loads connected to output of autotransformers will protect against the "lost neutral" problem.
But it will still be subject to issues of defining ground, and possibly autotransformer trying to balance the grid.

These autotransformers were designed to be fed from a utility connection with one grounded neutral and one hot. You're going to feed it two hots. You may get a hot neutral out of the deal, and will probably not get a neutral/ground bond.

 

[cdevidal]

I think your loads connected to output of autotransformers will protect against the "lost neutral" problem.
But it will still be subject to issues of defining ground, and possibly autotransformer trying to balance the grid.

I don't understand the implications of defining ground, sorry. Kinda new at this. But the autotransformer balancing the grid should be mitigated by the 10A upstream breakers and thermal switches no?

These autotransformers were designed to be fed from a utility connection with one grounded neutral and one hot. You're going to feed it two hots. You may get a hot neutral out of the deal, and will probably not get a neutral/ground bond.

What would be the implications of this? I don't understand sorry.

 

[cdevidal]

The 240v side would be considered EU power still and so would still need the EU neutral-ground bond, and the mini-split needs to be EU power (wiring) compatible. Otherwise a center-tap neutral is still required for safe operation as otherwise there will be no low resistance alternative fault paths.

Using those transformers if the high side keeps the EU neutral-ground bond is fine. Both of us use a neutral and hot for our common receptacles, theirs is just higher voltage. However we use a center neutral for our 240v appliances. So US 120v to EU 220/230/240v is okay, so is EU to US 120v, but not EU to US 240v

Should I just sell the Growatt and get one for US power? I spoke with Ian from Watts247 on the phone today because I had bought it from him. He didn't think I needed to sell it, but perhaps he didn't fully understand my setup.

 

[copec]

I think your loads connected to output of autotransformers will protect against the "lost neutral" problem.
But it will still be subject to issues of defining ground, and possibly autotransformer trying to balance the grid.p

These autotransformers were designed to be fed from a utility connection with one grounded neutral and one hot. You're going to feed it two hots. You may get a hot neutral out of the deal, and will probably not get a neutral/ground bond.

I use a similar one that I rewired as an actual center-tap autotransformer, inside it is wired so that the neutral from EU power is passed as neutral to the US outlet, and the voltage cut in half from the EU hot leg with a center-tap.

 

[Hedges]

Once upon a time, in a land where we live today, appliances had metal chassis connected to one slot of the electrical outlet called "Neutral", and one slot going to the power switch. Plugs had a wide blade and a narrow one. If wired backwards, cabinet of the TV, etc. carried 120VAC.

Switches usually only interrupt the hot, not the neutral.

Cheap portable inverters don't bond "Neutral" to ground, so it may not be a fatal problem.
But we like to make our AC wiring comply with code.

It would be good to at least use 3-prong grounded appliances, and make sure the ground pin of socket is grounded to your electrical system.

Should I just sell the Growatt and get one for US power? I spoke with Ian from Watts247 on the phone today because I had bought it from him. He didn't think I needed to sell it, but perhaps he didn't fully understand my setup.

I would rather see compliant, listed equipment.
UL or equivalent listing involves extensive testing, eliminates lots of issues that may occur with either poor quality or designs by people not good at thinking of what could go wrong.

Isolation transformer seems like a reasonable way to make do with these 220V or 240V inverters.