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AC coupling for a small community

A

Ai4px

Solar Enthusiast
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Searching for a brain storming session.... Let's say I have a dozen tiny houses on my property. Each house has it's own inverter, batteries and solar. But we want to link the houses together. Recently I posted that I was planning to link my daughter's tinyhouse to ours via a 600v/240v transformer. So let's say I've created a utility line that circles the property using 600v (or 7500v... doesn't matter). If I connect one inverter as master and use it's frequency controlled output as the "pilot signal" for the community, then other inverters can connect as they desire to the "grid". The problem arises when 100% of my dozen tiny houses are connected and I can't frequency shift fast enough and end up with 12x6kw of solar inverters back feeding my "master" unit.

Also, the tiny house inverters would see the community grid varying in frequency and probably drop their connection or never connect at all. So if I simply installed a relay on the output of each tiny house inverter to connect to that community grid, there's the question of making sure we are in phase before allowing the connection, and these inverters wouldn't follow the phase anyway since each would think it was the "master".....

Ideally, I'd love to have a tiny grid that the various tiny houses could latch on and give/take power as needed. But the devil is in the details. Any ideas?
 
Assuming USA. Fronius or SMA Energy or new old stock/used SMA Sunny Boys coupled with SMA Sunny Islands or Victron (the ones with extra AC inputs; the Quatros I think).

That would decentralize you daytime solar collection and conversion to AC. For battery storage and nighttime DC to AC conversion, I would centralize that using the aforementioned Sunny Islands or Victron. The Victron can be bough and tied together for much more output than the Sunny Islands (which are a shade under 100A @ 240…probably more than enough for a small hippy compound though).

The new SMA Energy would decentralize your batteries too…I am not a fan of this for offgrid. I think they did this to comply with new NEC crap (limitations on battery bank sizes per building/area…which probably makes sense for Joe Suburbanite, not so much when you have a dedicated outbuilding (and semi sacrificial in the event of magic smoke followed by flames) for electrical).

I don’t know if anyone on here has had any experience with the Sunny Energies as of yet.

I don’t think this is something you would want try to homebrew. SMA cut their teeth on this.
 
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I don't know yet how well Sunny Boy Smart Energy with PV & battery would AC couple to a central system. Or how well other hybrids would. But it is an interesting approach.

You could make an 18kW 3-phase cluster of Sunny Island or 24kW split-phase cluster, supplying the village.
Each house could have Sunny Boys, up to 36kW or 48kW respectively.
There could also be DC coupled PV, and generator, at the Sunny Islands & battery.

If hybrids could work at each home, that would add battery and inverter capacity.

You can also do a "multi-cluster" of up to 12 Sunny Islands in 3-phase, 72kW worth. $$$

You can use transformers, but you're better off with more "ideal" transformers than the typical utility sort because those draw excessive no-load current.
 
@sunshine_eggo is correct. The master inverter that is creating the grid needs to be as powerful as all of the grid tied inverters added up.

How much power does each tiny house need?

Maybe make it a split hybrid system. So we have 12 little houses and each one has 6 KW of solar panels on the roof. That is a whole lot of solar power. My full size house has a total of just 6,800 watts of solar panel. But let's go with this and see what can be done.

How about we use 4 of the EG4 18K-PV inverters. Each group of 3 houses is connected to a single inverter. 6KW on each roof x 3 matches the 18KW solar input rating of the 18K-PV which has 3 MPPT controllers. Run the cables between the 4 inverters as needed to run them in parallel mode to stay in sync. The AC "grid" can then tie all 12 homes together. 48 KW of inverter power available at all times to all of the houses.

Having 4 separate battery banks is not ideal, but I have an idea for that as well. Also run a loop of DC cables between the 4 systems. Maybe just 50 amps breakers to deal with any current imbalance. And then you can use just #4 awg wire. The resistance of the wire will act as a buffer to allow current to balance the banks, but not pull too much current if the voltages differ a little. If it goes way out of balance, the breaker to the DC loop will open and eliminate any fire hazard.
 
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wpns said:
Does that work over long distances? Can you convert to fiber and run it between houses?
Click to expand...
Single mode fiber optic Power Delivery??? FODP? new USB standard...lolz...
 
Wow.. so many responses.. THANK YOU.

The system will be based in the USA. Actually I have two different setups in mind....
1)This idea is actually on behalf of a friend who wants to grow micro greens/ hydroponics. He has 45 acres and wants to create 6-10 grow houses. For him a centralized system with a set of batteries would be fine. He may use 600v/240 transformers to move power over distance just like I'm thinking of doing to get power to my daughter's tiny house down by our pond.

2)the hippie commune, each house should have it's own battery and each person should be mindful of how they use power after dark. Don't mind sharing AC coupled power between tiny houses when the sun is shining. After all, once my batteries are full the power would go unused anyway. But I also dont want someone leaving their stove on all night and running down the community batteries. So I think each house should have it's own batteries, PV and inverter. This gets so complex that I may just give up on it.... but it's a shame to have unused PV power that could be AC coupled from 5 of the huts and have one guy who needs to finish topping his battery for the evening. But I absolutely want to separate the houses after sundown so we don't end up with a tragedy of the commons situation.
 
Ai4px said:
Wow.. so many responses.. THANK YOU.

The system will be based in the USA. Actually I have two different setups in mind....
1)This idea is actually on behalf of a friend who wants to grow micro greens/ hydroponics. He has 45 acres and wants to create 6-10 grow houses. For him a centralized system with a set of batteries would be fine. He may use 600v/240 transformers to move power over distance just like I'm thinking of doing to get power to my daughter's tiny house down by our pond.

2)the hippie commune, each house should have it's own battery and each person should be mindful of how they use power after dark. Don't mind sharing AC coupled power between tiny houses when the sun is shining. After all, once my batteries are full the power would go unused anyway. But I also dont want someone leaving their stove on all night and running down the community batteries. So I think each house should have its own batteries, PV and inverter. This gets so complex that I may just give up on it.... but it's a shame to have unused PV power that could be AC coupled from 5 of the huts and have one guy who needs to finish topping his battery for the evening. But I absolutely want to separate the houses after sundown so we don't end up with a tragedy of the commons situation.
Click to expand...
1. 3-phase would be much better if you want to move around AC.

2. Sounds too complex. OG SMA was designed to power whole villages in BF Developing Country though.
 
So, as a meta comment.

What OP wants is basically community scale microgrid. That's outside the expertise of the contributors to this forum. 99% of the equipment and knowledge discussed here isn't for that case

In that kind of utility-scale domain, grid forming inverters means something very different than what us primitives here think it means.
 
zanydroid said:
So, as a meta comment.

What OP wants is basically community scale microgrid. That's outside the expertise of the contributors to this forum. 99% of the equipment and knowledge discussed here isn't for that case

In that kind of utility-scale domain, grid forming inverters means something very different than what us primitives here think it means.
Click to expand...
🤷

IMG_0411.png
IMG_0412.png
 
I would use Victron in place of the Sunny Islands though…the cost of a multi cluster boxe could buy a bunch of Victron Quattros.

Though I have no hands on experience with Victron.
 
Ai4px said:
Searching for a brain storming session.... Let's say I have a dozen tiny houses on my property. Each house has it's own inverter, batteries and solar. But we want to link the houses together. Recently I posted that I was planning to link my daughter's tinyhouse to ours via a 600v/240v transformer. So let's say I've created a utility line that circles the property using 600v (or 7500v... doesn't matter). If I connect one inverter as master and use it's frequency controlled output as the "pilot signal" for the community, then other inverters can connect as they desire to the "grid". The problem arises when 100% of my dozen tiny houses are connected and I can't frequency shift fast enough and end up with 12x6kw of solar inverters back feeding my "master" unit.

Also, the tiny house inverters would see the community grid varying in frequency and probably drop their connection or never connect at all. So if I simply installed a relay on the output of each tiny house inverter to connect to that community grid, there's the question of making sure we are in phase before allowing the connection, and these inverters wouldn't follow the phase anyway since each would think it was the "master".....

Ideally, I'd love to have a tiny grid that the various tiny houses could latch on and give/take power as needed. But the devil is in the details. Any ideas?
Click to expand...
Each house it’s own inverter connected to the collective maingrid, batteries centralized?
 
The following assumes that Solar Assistant can:
1) Set "Grid Sell" rate (setting on a Sol-ark)
2) Set Grid Charge Rate (setting on a Sol-Ark)

Install Solar Assistant on each house. Each SA broadcasts to a central "Controller".
Controller can see the status of each Unit (PV production, Load, SOC).
Based upon that information, it can allow certain units to Charge from the "grid", and how many amps it can pull (grid charge rate).
It can also tell a unit how many amps to sell to the Grid (grid sell rate).
For safety, the Master Inverter has "AC PV on Load" (the "grid"), and will charge its own battery to soak up the excess, and can also provide power to the grid. Maybe put an emergency dump load on the Smart Load connection.
 
For the micro-community of TinyHomes, I would ensure each tiny home (TH) has its own off-grid system; fully independent, capable of running just that (TH) household. Their battery-bank is sized to support just their household for design period (3-days or more).

As the group of TH's are near enough to each other, you can now have a centralized off-grid (propane, diesel?) generator sized to meet the needs of all 10 TH's (in terms of recharging each battery-bank, and possibly more in terms of temp household loads). Gen output is fed into each inverter as grid power source, so that's an exercise in one gen distributing some amount of power to 10 TH's, with gen sized properly (75% or thereabouts).

Solar normally feeds each TH, but gen is there if someone either consumes too much overnight, or weather not cooperating. Gen is also emergency power. Anyone not needing gen power, can be notified of gen startup, and everyone can get free big-load power during that period of gen runtime.

Costs of gen/fuel either distributed amongst all 10 TH's each time it runs, or gen downsized to support 1 or 2 TH's, and a surcharge to the TH that needs a refill.

Done (I think) ...
 
CJM said:
Each house it’s own inverter connected to the collective maingrid, batteries centralized?
Click to expand...
that's the thing... batteries are hte most expensive part. I want each house to have and be responsible for their own power use and batteries. I do not want cabin #6 running the community batter down each night and then cabin #3 can't make coffee at 5am for work.

I guess my goal would be to allow those with excess PV power to share the "free" power while the sun is still shining. But after dark, you are on your own.
 
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