Question About Preventing Grid Backfeed

[jeff88]

I'd like to clear up a few assumptions and get a clear understanding based on those assumptions being confirmed (or not).

To my understanding:
Hybrid inverter: has the ability to backfeed. Turning off the setting can still backfeed a very small amount of power when turning off a large load, for example.
Off-grid inverter: physically cannot backfeed.

If this is the case then an off-grid inverter would work for a system without an IA with the utility.

On the other hand, if an off-grid inverter can still backfeed that minor amount, would a setting like always inputting a small amount of grid power stop that "leak"?

 

[timselectric]

To my understanding:
Hybrid inverter: has the ability to backfeed. Turning off the setting can still backfeed a very small amount of power when turning off a large load, for example.
Off-grid inverter: physically cannot backfeed.

If this is the case then an off-grid inverter would work for a system without an IA with the utility.

Your understanding is correct.

 

[DIYrich]

Mostly right, that is the difference between a grid interactive inverter vs off-grid inverter.

A hybrid inverter can be either, the eg4 18kpv and eg4 6000xp being hybrid examples of each.

Also, a non-hybrid inverter can be either grid interactive or off-grid. Growatt min 5000tl-x is grid interactive, but cannot charge. Micro inverter is one way grid interactive.

 

[ChrisG]

A hybrid inverter can be either, the eg4 18kpv and eg4 6000xp being hybrid examples of each.

Thought the 6000xp was off-grid. There is no option to sell back. Mine can only run from AC Grid passthrough OR battery/pv. They are never connected together in a hybrid way.

 

[hwy17]

Didn't we establish recently that you have to run in SBU or SUB mode to avoid the inverter ever paralleling to the grid? I forget which one.

 

[ChrisG]

Didn't we establish recently that you have to run in SBU or SUB mode to avoid the inverter ever paralleling to the grid? I forget which one.

Those modes don’t exactly exist on the 6000xp. There are methods to mimic those.

 

[timselectric]

Thought the 6000xp was off-grid.

Correct

 

[cs1234]

Anything with a UPS mode that is fast enough to keep your PC from shutting off can possibly backfeed can't it? Including some regular AGM included inside UPS's? Or does it depend on the internal design of the inverter? Are some of them capable of keeping their internal inverter synced up to the grid signal for fast switch over back and forth, but not possibly leak even the tiniest trace amount?

I was pretty sure we came to the conclusion that the only way to "guarantee" no backfeed is to do double conversion.

 

[timselectric]

Anything with a UPS mode that is fast enough to keep your PC from shutting off can possibly backfeed can't it?

No

Or does it depend on the internal design of the inverter?

Yes
In order to export, it must be running in parallel with the grid.

 

[cs1234]

No

Yes
In order to export, it must be running in parallel with the grid.

Is the problem only when an inverter does any type of supplementing of the incoming AC from the grid? Say adding battery or solar to cover some of the loads. You have to be either all or nothing otherwise the possibility for feedback is there when a load drops?

So if it's an inverter that just monitors the grid signal, keeps it's inverter ready to switch, but just passes it through to the loads.. that's fine. It can then pass the loads over to the internal inverter when the grid goes down or when desired, and then swap them back to the grid at a time of it's choosing with no feedback chance?

 

[ChrisG]

Only actual evidence I have that the 6000xp I have doesn’t send anything back to grid is: Neighbor hooked up a Solark and POCO swung by the same day and made him disconnect. My XP has been running for months without incident.

 

[ricardocello]

In the Victron world, the following must be true to prevent any grid feedback:

• ESS Assistant (or older Hub assistants) must not be installed on the MultiPlus or Quattro.
• PowerAssist feature must be disabled.

UPS mode should be fine. Lots of people use the Victron gear as critical load UPS systems.

 

[cs1234]

In the Victron world, the following must be true to prevent any grid feedback:

• ESS Assistant (or older Hub assistants) must not be installed on the MultiPlus or Quattro.
• PowerAssist feature must be disabled.

UPS mode should be fine. Lots of people use the Victron gear as critical load UPS systems.

So basically, all or nothing. You use the grid in raw form, or not at all. (It should be able to charge batteries while doing pass through I suppose)

 

[ricardocello]

So basically, all or nothing. You use the grid in raw form, or not at all.

UPS mode synchronizes with the 50/60Hz from the grid so it can switch within a cycle.
But it does not enable the bidirectional flow of current back to the grid.

@cs1234 What middle ground do you think is possible?

 

[cs1234]

@cs1234 What middle ground do you think is possible?

None. There is no middle ground. If you alter the incoming grid signal by "adding" to it in any way, using power of your own that didn't come from the grid, you run the risk of them finding out.

 

[ricardocello]

None. There is no middle ground. If you alter the incoming grid signal by "adding" to it in any way, using power of your own that didn't come from the grid, you run the risk of them finding out.

Right, because as soon as you start “adding to it”, you need to be infinitely fast to know when to stop “adding to it”.
And that is not physically possible.

Though I think if a power systems engineer really tried to minimize the latency in their design, it could be greatly improved over what we have today.

 

[timselectric]

Is the problem only when an inverter does any type of supplementing of the incoming AC from the grid? Say adding battery or solar to cover some of the loads. You have to be either all or nothing otherwise the possibility for feedback is there when a load drops?

So if it's an inverter that just monitors the grid signal, keeps it's inverter ready to switch, but just passes it through to the loads.. that's fine. It can then pass the loads over to the internal inverter when the grid goes down or when desired, and then swap them back to the grid at a time of it's choosing with no feedback chance?

It depends on how it connects to the grid.
If in parallel, there's always a possibility of export.
It's It's an either/ or , through a relay, it can't export.

 

[cs1234]

So. If you want to be able to use grid power and add solar / battery dynamically to it, to supplement it, and you don't have a feedback agreement.. the only way to guarantee the utility mafia doesn't get you, is double conversion.

 

[Robert Mate]

So ... would Victron 48V MultiPlus-II UL1741 3kVA 120V Inverter 35A Charger be a problem?
(7-millisecond transfer time from grid to inverter power & two ac outputs)

​

 

[cs1234]

So ... would Victron 48V MultiPlus-II UL1741 3kVA 120V Inverter 35A Charger be a problem?
(7-millisecond transfer time from grid to inverter power & two ac outputs)

In the Victron world, the following must be true to prevent any grid feedback:

• ESS Assistant (or older Hub assistants) must not be installed on the MultiPlus or Quattro.
• PowerAssist feature must be disabled.

UPS mode should be fine. Lots of people use the Victron gear as critical load UPS systems.

See @ricardocello 's post from above about Victron.

 

[ricardocello]

So ... would Victron 48V MultiPlus-II UL1741 3kVA 120V Inverter 35A Charger be a problem?
(7-millisecond transfer time from grid to inverter power & two ac outputs)

Don’t install ESS Assistant, don’t use PowerAssist.
No feedback, no problem. Great UPS solution.

 

[cs1234]

Don’t install ESS Assistant, don’t use PowerAssist.
No feedback, no problem. Great UPS solution.

Not so great for saving money, via blending / feedback, but yes, good UPS mode.

I wish there was a way we could actually affect change and get tiny amounts of spurious feedback legalized without a permitting process, so long as the device meets the basic UL1741(sabcdedf) regs. I'm pretty sure if brought to a vote, it would win if people knew what we were even talking about.

 

[zanydroid]

If this is the case then an off-grid inverter would work for a system without an IA with the utility.

FWIW off-grid is not formally defined enough to conclude this.

For instance, I recently learned that inverters marketed as off-grid will do zero export in SUB mode, ie parallel operation with grid. Which isn't what I think of as off-grid, but I guess these companies have first amendment rights...

 

[ChrisG]

the only way to guarantee the utility mafia doesn't get you, is double conversion.

assume this means another inverter or chargeverter for charging from grid?

 

[cs1234]

Right, because as soon as you start “adding to it”, you need to be infinitely fast to know when to stop “adding to it”.
And that is not physically possible.

It's like the stock market, it's impossible to truly know when to exit a position for maximum gain. You either exit too early or too late. Unless of course you have some sort of inside track.