Grounding a 24v GroWatt off grid but with AC backup

dmartz

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Sep 19, 2021
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I'm a newbie here, learning.

I just purchased a GroWatt SPF3000 24v unit with a 5kWh LifePO4 battery bank (coming). My starting point will be to add a smaller 4 panel solar array, but mostly use AC grid power (one way) to top up the batteries. The climate here has painfully low solar hours for 2 months a year, so the grid supply is necessary.

Grounding is where I am getting confused. Is the AC input in with its included ground wire, grounding the entire system, or do I need to deal with that separately? It seems to me that if my AC input was a generator, then I would require an independent grounding.

I'm sure someone has written about this specific setup somewhere, but can be hard to find since most write ups talk about fully off grid without a backup AC input.
 

FilterGuy

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I'm sure someone has written about this specific setup somewhere, but can be hard to find since most write ups talk about fully off grid without a backup AC input.
Getting good grounding info on the Growatt systems has been extremely difficult. I feel Growatt is nearly criminally negligent in their documentation about grounding.

Having said that this is my best understanding of the situation:

NOTE: I have studied everything I can find on Growatt grounding, but I have never used one. Therefor take the following with a bit of skepticism.

I am 99.99% sure that the AC-in Ground, AC-out Ground and the case ground lug are all bonded together electrically.

Where things get very hazy is what happens with Neutral-Ground bonding. This is what I *think* is going on.

* I am pretty sure that when there is power at the AC input, there is no Neutral-ground bond inside the Growatt but the AC-out will 'see' any neutral-ground bond that exists on the AC input. This means that there should be no Neutral-Ground bond 'downstream' of the growatt when the AC-in is powered.

* I *think* there is also no Neutral-Gound bond inside the growatt when there is no AC in and it is running off the inverter. However, when runing off of inverter, I don't think the AC out Neutral is connected to the AC in Neutral, so it does not 'see' the neutral ground bond from the AC-in.
This means an Neutral-Ground bond must be added when on Inverter (but removed when powered from AC in. What a PITA!!!

The only good news is that the growatt appears to have a dry contact output that can be programmed to control a rely that will create the N-G bond when the unit is on the inverter.

@NH renewables recently posted information about this for a 48V Growatt system.
 

dmartz

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Let's see how well I'm learning about this.

There is an AC ground (green or bare) at the transformer, another at my home, and the metal boxes of equipment are all connected. AC neutral (white) and AC hot (black) carry the flow of electricity. The neutral and ground follow the same path, mostly, but the ground line has higher resistance when it reaches the earth direct.

Neutral must come from the same source as the hot line to complete the circle of AC, and since the ground is another neutral path but with a direct to earth exit, switching between grid and battery must also switch ground, neutral, and hot.

What happens then if the growatt is given a ground direct to earth of its own? Does that allow the grid to ground direct if needed and the inverter to do the same, maintaining each systems independence? I recall Will Prowse arguing against that because it risks damaging the equipment in a lightning strike. I'm trying to understand what his passionate concern was about for that, and how it may or may not be relevant here.

FilterGuy, you are saying the AC input and AC output share the same ground which would really screw this up. This definitely needs more digging to figure out, and I'm guessing there must be a way to run an experiment to (safely) test this out.
 

FilterGuy

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Grounding is a lot more complex than most folks realize.... To make it worse, the term "ground" is often used in places that have nothing to do with earth ground.

You may want to review the 4 Grounding resources I posted particularly the 1st two. (I originally set out to document it in a single resource but it got so long I had to break it up to two... then three and finally I ended up with 4 separate papers.)

The following is the preamble (with links) to the papers.

The subject of grounding is a complex, multifaceted subject, that is often treated as an after-thought but needs to be considered from the beginning of the design and build process of any DIY Solar/Battery Project.

This is a 4-part series on grounding basics.

  1. AC & Household
    Part 1 covers the basics of grounding for household AC systems. Even if you are familiar with house wiring, it is suggested that the reader review this paper to become familiar with the terminology concepts and practices of grounding.
  2. Stationary Systems
    Part 2 introduces the grounding principles of DC wiring, inverters and multiple power sources.
  3. Solar Panels
    Part 3 is a short overview of how to properly ground the frames and mounting racks of Solar arrays.
  4. Mobile Systems
    Part 4 goes through designing the grounding scheme that addresses the unique situations encountered in a mobile system.
 

dmartz

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Sep 19, 2021
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Thanks FilterGuy! I just downloaded your PDF's and will start reading straight away.
 
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