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Grid Neutral interconnected to out Neutral? Interesting finding about MUST and possibly other Voltronic clones.


meetyg

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Sorry for the long title.

But a fellow forum member from another forum has found something interesting about his Voltronic clone, which also happens to work in my MUST inverter: I'm talking about the fact that these inverters interconnect the IN/OUT Neutrals automatically, when grid power is present (but not necessarily in use).

Please see his post:

https://diysolarforum.com/threads/ground-neutral-bonding-for-voltronic-clone-hybrid-inverter.56245/#post-735971

I'm excited about his finding, because it helps us solve the N-G bonding issue with these inverters. As you know these don't do dynamic N-G bonding when inverter is in battery/solar mode (powering the loads).

There has always been a discussion whether it's OK to combine the Neutrals on such inverters. But now I guess the manufacturer has given us a hint: It more than OK, since it happens automatically! 

In summary, the solution to N-G bonding would be to make sure grid connection is available (I.e. no RCD or MCB disconnecting the Neutral IN), and at the output, make an N-G bond using a Normaly closed contactor, powered by the grid. So when grid power exists, it will open the contactor (so not to have multiple N-G bonds).

But when grid power goes out, the NC contactor will close.

 

Was this "feature" known to the community?  I wonder on what other inverters this exists...

Edited by meetyg
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25 minutes ago, meetyg said:

There has always been a discussion whether it's OK to combine the Neutrals on such inverters

Maybe I dont fully understand your statement, but connecting the in neatral to out neatral is not only OK, its the only way to do it when the inverter is in line mode. Unless you introduce a 5kw isolation transformer (very expensive) there no other way in the modern bidirectional transformerless inverter. And quite frankly there is no reason not to connect in neatral to out neatral when grid is active. 

When the inverter is in line mode, the input neatral is passed thru to the output, and no earth bonding is done, because the grid is already bonded elsewhere. 

When the inverter is battery mode, the grid live and the grid neatral is disconnected by safety relays and the output neatral is bonded to earth. We have now formed an island. 

EDIT: note on some inverters there is no automatic bonding inside the inverter. This done then  externally with a relay bond. 

 

Edited by BritishRacingGreen
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41 minutes ago, meetyg said:

Was this "feature" known to the community?  I wonder on what other inverters this exists...

The installer is responsible and the COC official is liable for  making sure the  line and battery mode is correctly done for neatrals, wether the chosen inverter performs it aitomatically or not. 

 

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36 minutes ago, BritishRacingGreen said:

When the inverter is battery mode, the grid live and the grid neatral is disconnected by safety relays and the output neatral is bonded to earth. We have now formed an island. 

Well, that's what I'm saying: We always think that when the inverter is in battery mode, the physical grid connection is disconnected via the internal relays. But what this guy found out is that the Neutral is not always switched! 

When I tested this on my MUST inverter, I had grid connection, but the inverter was supplying power from the battery (SBU output setting). The LCD display showed that it identified a grid connection, but loads were being powered from battery (not line/bypass mode).

There was still connection between grid neutral and out neutral in this case.

Edited by meetyg
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11 minutes ago, meetyg said:

Well, that's what I'm saying: We always think that when the inverter is in battery mode, the physical grid connection is disconnected via the internal relays. But what this guy found out is that the Neutral is not always switched! 

 

 

The modern inverter can still be in line mode (grid available) but still use battery / solar power  as a selected priority mode. Thats ok, but grid is on, so its in line mode, and neatrals are connected. Nothing wrong with that. 

But when grid is unavailable we are in full battery mode, and now we disconnect the neatrals and bond earth. 

Edited by BritishRacingGreen
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3 minutes ago, BritishRacingGreen said:

The modern inverter can still be in line mode (grid available) but still use battery / solar power  as a selected priority mode. Thats ok, but grid is on, so its in line mode, and neatrals are connected. Nothing wrong with that. 

OK, so that's the fact that I (and others I guess) weren't aware of.

Again, this little fact makes it alot easier to make a solution for external N-G bonding. Not all inverters today use the dry-contact to signal a grounding box, unfortunately. Some older Axperts had the option at least in the settings. Newer inverters (mainly Voltronic clones) have a dry-contact but don't have the menu option to use it as a grounding box signal.

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9 minutes ago, meetyg said:

OK, so that's the fact that I (and others I guess) weren't aware of.

Again, this little fact makes it alot easier to make a solution for external N-G bonding. Not all inverters today use the dry-contact to signal a grounding box, unfortunately. Some older Axperts had the option at least in the settings. Newer inverters (mainly Voltronic clones) have a dry-contact but don't have the menu option to use it as a grounding box signal.

Yes, its semantics that  confuses us many a time in this solar industry. 

In fact, as you be aware, the inverter can blend grid with battery (and solar) seamlessly. But to accomplish that the grid must be tied. Even in the Axpert and offgrid clones. 

 

Edited by BritishRacingGreen
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1 hour ago, meetyg said:

Again, this little fact makes it alot easier to make a solution for external N-G bonding. Not all inverters today use the dry-contact to signal a grounding box, unfortunately. Some older Axperts had the option at least in the settings. Newer inverters (mainly Voltronic clones) have a dry-contact but don't have the menu option to use it as a grounding box signal

Yes, even if a given inverter does not entertain any form of bonding, it can still be independantly done merely by energizing a relay with grid, and using a normally closed contact to anchor earth to neatral. But it does rely on the prerequisite that the inverter disconnects the neatrals when grid becomes unavailable. I think most if not all inverters comply. 

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