Hi
I presently have a grid tied dc coupled three phase system consisting of
- 3 x multiplus 3kVa
- 6kWp pv panels
- 2 x pylontech US3000C (7kWh)
I have got my hands on a Fronius SYMO 6.0-3-M and another 5kWp of PV panels. I would like to use these to expand my system into an AC coupled system.
My question is: will there be a problem with my relatively small battery size if I install the Fronius and the extra 5kWp on the AC in side of the multiplus (getting 11kWp in total)? As far as understand the issue with the minimum battery size it is related to overcharging the battery in case of a sudden load change (and also to charge the inverter capacitors when powering up). Is this also an issue if the Fronius is connected on the AC in side of the Multiplus? Then there is no need of slow frequency shifting by the Multiplus in case of a sudden load change and the excess PV power from the fronius can instead be fed directly to the grid. Or what? Is there something I have not understood?
Minimum battery size recommended is 12kw for a 3 phase 3k system, with pylontechs
Pv on ac in side doesnt matter for that, fronius can be controlled by the GX over network, excess pv will feed to grid directly
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Yes. You will get overshoot, they can’t sink the current.
It will work for a while, but after a few years you will have ballons for cells.
hi @lxonline . Thanks for you reply. Can you explain in a little more detail as I am not sure I understannd the concept? Thanks
You have 9kVa of inverter on a 48v system.
Your battery bank needs to be able to supply at least 188A.
2x us3000c is 2x 37A.
If you have a load on and it disconnects and the pv is really productive there will be a current overshoot.
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Thanks once more for you reply. Does this also count when I am grid connected and the fronius is on the AC in side of the victron inverters? In this case I would assume that the excess pv power would go straight into the grid instead of the batteries in case if a sudden load change
Ahh. Are you referring to my existing system? Arentou saying my battery is already undersized as it is now?
Yes the battery bank is undersized.
The 1:1 ratio does not apply if the ac pv is on the input.
Ok. thanks again. I understand what you are saying but still not why. There is somewhere my knowledge is lacking.
Below I have summed is my reasoning
In normal operation
My inverters can supply 3 x 2400W = 7200W
My batteries can supply 48V x 74A = 3552W (BMS controlled)
In this case the rest (app 3700W) will be supplied by the grid. Or what?
During start up
I turn on one inverter at a time so the batteries can supply the capacitor charging current.
In case of sudden load change
If the load suddenly drops while the battery is full and the PV is producing at full power (6kW in this case) will the excess power not be supplied to the grid instead of overcharging the batteries?
Yes undersized for your system. Your nominal discharge should at least match the inverter discharge current needs.
What happens if you are working the system and the grid drops out? Or is rejected for whatever reason?
What happens when there is an overload - overloads happens for reasons that are not aways related to loads (electical faults/shorts or an appliance that has developed a problem such as a pump or motor).
The battery bank ideally should bb3 able to provide the fault current needed for the inverter system to shut down before the bank cuts out. It is healthier for all components involved.
Are you planning on feeding back to grid?
If you cant feed back to grid, the system absorbs this…
What happens here depends on what you have set your system can do.
I am already feeding back to the grid and I live in Denmark where the grid is up an running 99,99% of the time so grid outage is very unlikely. So do you think it is still a problem? Will short circuits and other overloads be supplied by the grid? And will over production in case of sudden load change be sent to the grid?
When you feedback to grid CCL on the battery is ignored.
If the battery is undersized they develop problems.
All i am saying is from experience i have seen pylons do not handle that well.
But in anycase you seem to have made your mind up.
If you do decide to get AC PV. Install it kn the AC on side.
I have not made my mind up I was just trying to really understand the issue. I think I will trust your expertise and not expand the system any further as the batteries seems to be undersized already. Thanks for clarifying before I made it even worse.
Your original feeling that is should be here is correct. You had already seemed decided but had concerns. You do need to either leave it - add more battery or try and limit the way the battery is being charged. Something that ESS wont do when feedback is enabled.
What you can do is set try your inverters to not charge from grid or have low charge current (in ve config). But this won’t help with the DC MPPTs
https://www.victronenergy.com/media/pg/Energy_Storage_System/en/configuration.html#UUID-c8bc0feb-a414-272e-f508-6f25206571de
Or you could begin with a bank upgrade then add more solar.
Let’s say I add two more pylontechs and make my existing system good. Can I then add more ac coupled PV on the AC in side? Or should I then add even more batteries?
You would need a total of 5 us3000 to meet minimum dis/charge requirments.
Simple maths
3 x 3kva = 9kVa
9000w/48v = 187.5A
187.5A /37A (battery charge/discharge nominal) is 5.
If you do that then that will be a solid long lasting system even with the 1:1 ac pv on the output feeding through the system.