Need Help with reversed engineered AC Coupling...

[Israel Keys]

Hey guys,
My name is Israel and I'm a Brit who has just moved to Africa to do some Missionary & Charity work. We have a sizeable property here that we are trying to run off-grid (electricity here is very unreliable).

We've been building slowly and currently have 20.7kw of solar panels (3 different arrays) with another 9.85kw to come soon (adding a 4th array and expanding one of the previous 3). We expanded gradually and currently have 3 different inverters totalling 27.5kva (with a final 15kw to come with the extra panels). We also currently have 72 kwh of LiFePo4 with a final.14.4 to come

The property is 3 phases, and each phase handles a different section of site. Each inverter handles a different phase, and the 4th inverter will split on phase into 2. We got each component relatively cheap by waiting for good deals and jumping on them. This brings me to what I need advice with.

I recently found a deal for a Solax X3 Mega G2 grid tie inverter in the UK. It's a 3 phase 60kw (not kva) inverter with 6 built-in MPPTs and can take 12 different PV Arrays. The going rate was just under 3k POUNDS (not dollars), but I snagged it for 800 pounds (it was from a job the customer cancelled). I was so excited to get it I didn't realise it was a TRUE grid-tie in the classical sense:

1. No battery connectivity (at least not without horsing around with it ) and
2. It shuts off when the grid is down

Well, the grid is down over half the time over here. So, I've been looking to see if I could do AC or DC coupling and watching SEVERAL YOUTUBE videos. I'm already stuck with the inverter as I can't return it. Also, the 60kw will mean we can run the whole property without load watching (as we currently do). It will also mean the capacity on each phase can be shared with other phases (which we currently can't do as each of the existing inverters is a different size, so we can't parallel them).

As I said, this system was built stage by stage utilising amazing deals as they presented themselves so we could insist on everything being uniform. Sorry for the dissertation, but I'm hoping that the finest brains in DIY Solar can give me some counsel on how to proceed.

My current ideas are a crazy mishmash of both AC and DC coupling, but I'll save that till AFTER you reply so I don't affect your ideas. Thanks in advance

 

[DougfromdaUP]

I’d start advertising that your inverter is for sale. It sounds like a good deal for someone in the proper situation to use its features.

 

[Israel Keys]

I’d start advertising that your inverter is for sale. It sounds like a good deal for someone in the proper situation to use its features.

I'll take that under advisement lol

 

[Zwy]

It can be done by building a micro grid. You need inverters that can accept AC coupling. Some members here have experience with AC coupling, I looked into it but decided it wasn't a good fit for me.

I'm not certain what you have for voltage there, my assumption is Euro 220-240V at 50hz?

And what do you mean by a phase handles a different section of the site? Just a single Euro 240V phase to a structure?

What do you currently have for inverters?

 

[Zwy]

I’d start advertising that your inverter is for sale. It sounds like a good deal for someone in the proper situation to use its features.

It can be done with micro grid. The system will need inverters capable of AC coupling.

 

[DougfromdaUP]

I didn’t say it to just be funny or snarky. I think it would be best if you get the right equipment instead of try to make a grid-tie only inverter work in your situation. It sounds like you could get your money out and buy something suitable.

 

[solar8484]

Any microgrid solution would cost way more than 800 pounds. Selling the inverter and buying a proper off-grid inverter with similar power capacity would cost less but likely still way more than 800 pounds.

 

[Hedges]

Here's what I did, similar but with 30 kVA 3-phase grid-tie inverter.

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...

diysolarforum.com

The European model 230V Sunny Island might work for you without the step-up transformers I had to use with my 120/208Y system to get 277/480Y.

It would still be limited to 36kW of GT PV inverter if you have one island of 3x Sunny Island. A larger system can be made interconnecting several.

The other guys are correct that you'll spend less buying inverters which fit your application than putting in a 3-phase AC coupled system, but that can be done if it makes sense. The SMA equipment would set you back $15,000 or so. Other brands offer similar 3-phase capability, not all support AC coupling well.

Does your 3-phase GT PV inverter support frequency-watts, where it reduces power output when line frequency increases? If it doesn't, forget about AC coupling to it.

https://www.solaxpower.com/uploads/file/x3-mega-g2-datasheet-en.pdf

"Certification VDE4105; EN 50549; AS 4777.2; VDE4105; IEC 61727; IEC 62116; IEC 61683; IEC 60068; EN 50530; NB/T 32004"
I don't know those standards; in the US, "Rule-21" can include frequency-watts.

 

[Israel Keys]

Thanks everyone for the responses so far... Im taking them all in good faith (including the advice to sell the inverter). I'm just being stubborn cause I'm naturally a problem solver and I'd like to solve this one if at all possible.

If not, then I'll accept defeat and cut my losses.

I was wondering if I could use one of my existing inverters (it has an inbuilt 100A charge controller) with a charging-dedicated PV array (different from my GT inverter). Lets call this the "Battery Inverter" then disable the grid export function on the GT inverter.

I'd then use the "Battery Inverter" as my "grid" and couple its output to my GT Inverter's grid input... (my GTI would handle all the other PV arrays).

 

[Israel Keys]

It can be done by building a micro grid. You need inverters that can accept AC coupling. Some members here have experience with AC coupling, I looked into it but decided it wasn't a good fit for me.

I'm not certain what you have for voltage there, my assumption is Euro 220-240V at 50hz?

And what do you mean by a phase handles a different section of the site? Just a single Euro 240V phase to a structure?

What do you currently have for inverters?

Yes we are 220-240v and 50Hz down here

 

[solar8484]

I'd then use the "Battery Inverter" as my "grid" and couple its output to my GT Inverter's grid input... (my GTI would handle all the other PV arrays).

What is the specs of your battery inverter system? Can your battery inverter handle up to 60kW of GTI power for charging battery? You would also likely need 120kWH of battery in order to accept up to 60kW of charging power for any practical duration. And when the battery is full then you need to curtail GTI quickly (e.g. frequency-watt control if supported by the battery inverter and GTI) or need somewhere to divert the up to 60kW of GTI power so it doesn't blow up your battery inverter.

 

[Israel Keys]

What is the specs of your battery inverter system? Can your battery inverter handle up to 60kW of GTI power for charging battery? You would also likely need 120kWH of battery in order to accept up to 60kW of charging power for any practical duration. And when the battery is full then you need to curtail GTI quickly (e.g. frequency-watt control if supported by the battery inverter and GTI) or need somewhere to divert the up to 60kW of GTI power so it doesn't blow up your battery inverter.

Currently have 72kwh of battery but upgrading to 86 soon. I assumed (perhaps in error) that all I'd need to account for (coming into the battery inverter and batteries) would be the wattage from the panels (not the whole 60kw of the GT inverter).

Am I wrong?

Also my proposed plan above kinda means the only charging that the batteries will see is from the dedicated array to the Battery Inverter... The rest of the PV will only ever work in handling my loads during the day. I'm planning to disable the GT from backfeeding to the Battery Inv

 

[Israel Keys]

It can be done by building a micro grid. You need inverters that can accept AC coupling. Some members here have experience with AC coupling, I looked into it but decided it wasn't a good fit for me.

I'm not certain what you have for voltage there, my assumption is Euro 220-240V at 50hz?

And what do you mean by a phase handles a different section of the site? Just a single Euro 240V phase to a structure?

What do you currently have for inverters?

Realised I didn't fully answer your questions. We wrired the property such that each phase handles a floor. It has 3 floors and a basement. So Phase 1 is the top floor, phase 2 the middle floor and phase 3 the ground floor and basement.

So currently each of our smaller hybrid inverters is controlling a floor

 

[Zwy]

Currently have 72kwh of battery but upgrading to 86 soon. I assumed (perhaps in error) that all I'd need to account for (coming into the battery inverter and batteries) would be the wattage from the panels (not the whole 60kw of the GT inverter).

Am I wrong?

Factor 1 Rule applies. https://www.victronenergy.com/live/ac_coupling:start

The 60Kw of power has to go somewhere.

Also my proposed plan above kinda means the only charging that the batteries will see is from the dedicated array to the Battery Inverter... The rest of the PV will only ever work in handling my loads during the day. I'm planning to disable the GT from backfeeding to the Battery Inv

Off grid with AC coupling you will need to have the batteries used to handle the excess not consumed by loads.

 

[400bird]

To build this micro grid, you need battery inverters that are capable. More importantly, sized to handle all of the incoming AC from that monster 60kw inverter. Typical this means you need 60kw of battery inverter.

 

[solar8484]

Currently have 72kwh of battery but upgrading to 86 soon. I assumed (perhaps in error) that all I'd need to account for (coming into the battery inverter and batteries) would be the wattage from the panels (not the whole 60kw of the GT inverter).

The max GTI power is limited by the panel power capacity but there are other factors (e.g. anti-islanding disturbance injection power) that are often based on the max rating of the GTI. Assuming 86kWH battery with no other charging power source, the safe max charging power would be ~40kW. What's the power capacity of your battery inverter?

 

[Ampster]

I think it would be best if you get the right equipment instead of try to make a grid-tie only inverter work in your situation.

I agree. AC coupling can be tricky, especially in a country where access to equipment is not convenient. Also AC coupling works with GT inverters that comply with some standards. In the US we have UL 1741 and I know nothing about the GT inverter you purchase.

 

[Israel Keys]

The max GTI power is limited by the panel power capacity but there are other factors (e.g. anti-islanding disturbance injection power) that are often based on the max rating of the GTI. Assuming 86kWH battery with no other charging power source, the safe max charging power would be ~40kW. What's the power capacity of your battery inverter?

Sorry guys, been ill for a while. My largest battery inverter is 15kva. I also have 2 10 kva unit and a 7.5kva unit. I'm stressing I have multiple in case it's best to couple them in 3 phase mode as opposed to single phase mode

 

[Israel Keys]

To build this micro grid, you need battery inverters that are capable. More importantly, sized to handle all of the incoming AC from that monster 60kw inverter. Typical this means you need 60kw of battery inverter.

It's a 3 phase inverter which means (theoretically) I can couple 3 different battery inverters (one per phase) right? If so, then putting my largest 3 battery inverter3s together, I can get up to 35 kw...

Truth is I highly doubt I'll ever exceed 35kwh at any one-time. I got the inverter cause I had a deal to get it at less than a third of the price. So it was cheaper than getting a 30kwh one

 

[Israel Keys]

Factor 1 Rule applies. https://www.victronenergy.com/live/ac_coupling:start

The 60Kw of power has to go somewhere.


Off grid with AC coupling you will need to have the batteries used to handle the excess not consumed by loads.

I have 86kwh of batteries now... Also I believe the excess power that needs to be consumed is what the PV generates, not just the max inverter capacity right...?