Adding an off grid system w/ battery to current on-grid

[mrfrodo]

Hi All,
I have a 3-Phase Sungrow 20kw (SG20RT) Grid-Tied(ongrid) inverter connected to 43 solar panels of Jinko-Ntype 580W via 3 strings(14,14,15) (Total 24,940W)
Problem: When electricity/grid goes down or any 1 of 3 phases goes down, the inverter/solar system doesnt work.
(we have a common problem of loadshedding(power cuts) and phase cuts in our country)
I designed the system myself by reading online and had a professional electrician do all the work like installation, wiring, earthing etc etc. Everything is working fine and I am selling my excess kwh units to the power company

Now, I want to add an off grid inverter of around 10/12 kw and 4 batteries of 5kw each;
So when grid is affected/outage and during peak hours(6pm-10pm when electricity unit cost is higher) , I want to be able to enjoy my solar & battery backup.

I am looking for design and technical details on how to go about this? such as what solar strings I would need and automatic switch over etc.
(I dont wanna Hybrid inverter, since I just installed this sungrow on grid and hybrid inverters are double the cost. And also power company will have issue since I cannot change the on-grid setup without prior approval)

 

[Hedges]

You can put in a separate single phase or 3-phase backup system, hybrid or off-grid, configured to not backfeed. It would not use your existing PV panels.

You can put in a 3-phase battery inverter which will AC couple with your existing grid-tied inverter, making your only local grid.
This works best if your grid-tie inverter has a feature we call "frequency-watts", output power ramps down as frequency increases.
Without that, it will cycle on and off, not as nice but possibly OK.

Switching high voltage DC strings between inverters is normally not done.

The one I'm most familiar with is SMA Sunny Island. Three 6kW inverters are configured for 3-phase, 230/400Y. (In the US I have 120/208Y)

Sunny Island 4.4M / 6.0H / 8.0H | SMA Solar

www.sma.de

Some other brands we use here, such as SolArk, do the same. Related brand Deye sold elsewhere may or may not have same feature.
SolArk does work with GT PV inverters which don't do frequency watts, but recommends 50% or more PV be DC coupled to SolArk; that way it can ramp DC coupled PV up/down and simply turn AC coupled on/off with a relay. But that would be changed configuration you should get utility approval for.

 

[DIYrich]

You say you don't want a hybrid because it is double the cost. Now you have to buy a 2nd inverter because you didn't buy one in the first place.

Your current inverter does not take batteries, which is why it does not work when the grid is down.

The problem with asking your question on this forum is that most of us are in the USA, and you have a 3-phase European system. Best i can guess Is buy a victron system and put it in 3-phase configuration. You may be able to find a Deye inverter designed for 3-phase European power.

I suggest an all-in-one so you do not have to figure out how to connect the parts. A 3-phase isolation switch may be expensive, but are included in many all-in-ones. You may be able to run your existing strings of panels into the new inverter. You need to check.

 

[mrfrodo]

You can put in a separate single phase or 3-phase backup system, hybrid or off-grid, configured to not backfeed. It would not use your existing PV panels.

You can put in a 3-phase battery inverter which will AC couple with your existing grid-tied inverter, making your only local grid.
This works best if your grid-tie inverter has a feature we call "frequency-watts", output power ramps down as frequency increases.
Without that, it will cycle on and off, not as nice but possibly OK.

Switching high voltage DC strings between inverters is normally not done.

The one I'm most familiar with is SMA Sunny Island. Three 6kW inverters are configured for 3-phase, 230/400Y. (In the US I have 120/208Y)

Sunny Island 4.4M / 6.0H / 8.0H | SMA Solar

www.sma.de

Some other brands we use here, such as SolArk, do the same. Related brand Deye sold elsewhere may or may not have same feature.
SolArk does work with GT PV inverters which don't do frequency watts, but recommends 50% or more PV be DC coupled to SolArk; that way it can ramp DC coupled PV up/down and simply turn AC coupled on/off with a relay. But that would be changed configuration you should get utility approval for.

This is the spec sheet of my on-grid inverter. So can this be used with a 3-phase battery inverter, to make my own local grid?

 

[Hedges]

Probably not. I think it is only grid-tie.

No battery input, just PV input. Although if connected to HV battery like EV it might backfeed that into grid.
(some people have done that with a hybrid.

Maybe if it had the right firmware it could make a grid from PV alone.

The right way to do it is buy an inverter meant to be grid-forming. There are quite a few. Some even AC couple to GT PV inverters.