Hello, I have a single phase PV system withy a Fronius Primo 8.2 and 9kW of solar panels. As a rule we generate more power than we use, and that includes charging our two electric cars (using Fronius Wattpilot). I plan to add a Multiplus II (probably 8kvA) with Cerbo GX and Pylontech batteries so that we are largely independent of grid power.
I do not need or want to install more than 4 us5000 batteries. As I understand it, that would limit me to 6kW of AC-coupled solar PV.
Can/should I configure the Fronius so that it cannot output more than 6kW of power? Or, alternatively, reconfigure the panels so that only 6kW of panels are connected to the Fronius? The remaining 3kW of panels would be connected to a DC-coupled Victron MPPT.
My installer wants me to buy at least 6 us5000s and a 10kVA Multiplus II, but I don’t feel I need this and would prefer one of the two approaches outlined above. What do you think?
As an aside, die to the limitations of the grid here we cannot inject more than 2kW into the grid without adverse consequences.
guystewart
(Guy Stewart (Victron Community Manager))
2
Those settings are designed for maximum reliability, so your installer is always going to want to follow them strictly because then there is no problems for them.
The grid mostly solves it, and you could work around it limitation completely by installing your AC PV inverter on the AC input side of the multi, if your grid connection approvals process allows for that.
The issue then is you have no PV power during a grid outage, but then that also solves the issue of too much power on the AC bus if the grid drops out while the AC PV is powering a big load + while also exporting on a full battery, which is one of the reasons that 1:1 rule exists.
The safe answer is stick strictly to the rule and size inverter and batteries to specification.
Anything beyond that is going to be riskier. How much risk you take, such as software limiting to the AC PV, just increasing battery size but not PV size, etc etc thru all the variables is just up to you and the installer to agree.
And to agree who is liable to spend the time to fix it if something goes wrong.
There is no advice from Victron to do anything over the 1:1 line, because for us that is also safe and well known. Others sometimes choose to push it at their own risk.
guystewart
(Guy Stewart (Victron Community Manager))
3
I’ll add something further, that this is also a long term decision.
Lithium batteries ability to handle high charge and discharge currents fades over time.
So even if the system is tested and works great brand new, a few years later when the batteries, and their individual cells are weaker, the cracks might start to appear as individual cell voltage warnings and alarms during high load transition moments.
The super tricky problem then is maybe there is an issue with the battery that might have happened anyway, and been covered by warranty, but now because the system has been installed outside of manufacturers specifications fingers start to get pointed around.
Your idea to limit the output of the AC PV seems reasonable, but that is also going to have transitory spikes of excess power when the sun suddenly appears from behind a dark cloud (sunburst effect) and the AC PV cannot regulate fast enough and you get a very high moment of full panel production.
The factor 1.0 rule can be circumnavigated by tuning the frequency knee points of both the fronius and the victron to suite the system. If set and tested correctly there is no risk to the batteries and can even work much better then simply following the factor 1 rule. I can dig up my settings as a start point if you want to go that way. The main help was increasing the inital frequency when curtailment starts in the victron so there is little to no delay to power reduction. Then tune the rest from there.
This is pretty advanced and not for the inexperienced but there is a way.
Thanks for the replies. The “sunburst effect” is something that I had not considered, and I guess that could cause an issue in some scenarios. If I could get someone to swap their 6kW Fronius Primo for my 8.2, that would solve the problem. But surely keeping the 8.2kW Primo, but with only 6kW of panels attached, would also make the system compliant with the Factor 1.0 rule without adding additional batteries?
That is interesting, but unless I can find a local installer who is prepared to implement and support this, I would be reluctant to adopt this approach. I think the Victron recommendations do err on the side of caution, but this is done for a reason…
Yes liability and the buuuuut yoooooou said (imgaine the whining voice there). No one likes that
Get a 10kVa MP2 and then the issue with fronius goes away.
Most ideally you still will want a black start dc mppt in the end. But baby steps here.
For the pylontec capacity fade is an issue. If you want a 10 year design life without noticing an issue particularly on capacity go with the installer recommend bank size. (Will still be good bank with a 10kVa.)
