Hi,
I'm talking with a company to install a 3 phase system, solar + batteries, ESS, using 3 Victron Multiplus II 48/5000/70. The guy is insisting on using a 3 phase Fronius for the photovoltaic panels, and he's not able to explain me why he doesn't want to use Victron Solar chargers instead.
In theory, I would prefer that the solar energy goes from DC to batteries DC, right?
What would be the advantage of using an AC coupled Fronius inverter? Easier install for him?
regards,
Sorin
On larger installs, it does make sense to have a Fronius. But there does need to be at least on DC MPPT there for black start/system recovery.
Ideally you would want DC MPPTS for battery charging and AC PV to run loads directly. Charging from AC PV is inefficient, as there are more conversion losses and the Victrons run hotter (so derating) when doing so.
Your system falls in the weird middle ground between large and small.
As a personal preference I would have leaned more on the new RS 450 range. Much easier integration and does have the advantage of not caring if your three phase loads are balanced or not. It does add more install costs on cabling and protection gear though.
For some off grid situations the frequency shifting is also not desirable.
There may be other design considerations for your unique situation that your system designer/installer had in mind when proposing the system for you. So chat to them about the choices and effects of changes for you on your system.
Thank you!
The Fronius will produce to meet the load on the highest phase and produce that on all phases equally. This means unwanted feedback on phases that have lower loads since there is nowhere for the power to go. In some places, this is an unwanted side effect especially if feedback is not allowed. In that case DC coupled PV is better.
Maybe this publication is helpfull for you.
I have tried everything (watching Victron Professional Trainings, Fronius webinars, reading forums) to better understand what would be the system design criteria to choose Fronius (or any other AC coupled PV Inverter) or Victron MPPT (od any other DC coupled PV Charger).
So far I was able to summarise in my head the following pros and cons for each solution:
AC Coupled PV
+ More effective when there is direct need for AC load (example: a factory running during the day)
+ More effective when there is direct need to feed back to Grid (example: a solar array)
- Less effective at charging excess PV to house battery
- Keeps Multiplus busy and hot when charging excess PV to house battery
- Requires Battery + DC coupled MPTT to keep running during grid outages
DC Coupled PV
+ More effective at charging house battery
- Assuming house battery is full, less effective supplying direct AC loads or feeding back to grid
- Keeps Multiplus busy and hot when feeding excess PV back to grid
- Small house battery + large loads could cause battery ripple and shorten life of batteries?
Could you please elaborate more about what should be taken into account in general when designing systems with AC/DC coupled PV?
In my concrete example, I currently have 9.1kWp PV using Victron MPPT to charge 14.2kWh batteries and supply house loads and feed excess to grid.
I have another 5.4kWp of PV ready to be installed (to cover whole roof) and am deciding between installing another MPPT or Fronius and am kind of lost which one to choose.
I'd also potentially add 7.1kWh of battery in the longer run depending on how the system behaves.
what's your experience please?
thanks, Martin
Nice summary of the training.
I would use DC PV. Direct to battery, easier to integrate, comms less of an issue. End user friendly. Easier to self troubleshoot as well. No need to worry about the 1:1 ratio with ac pv.
Dc mppts require a bit more dc cabling, fusing etc though. So there is a bit more cost there on install.
Plus in house loads (particularly when grid disconnects) fronius has flickering in some conditions, I can say is annoying at times.
I would also add a point about AC coupled inverters providing improved overall AC capacity when the sun is out. I have two parallel MPii 5000s and 5 small MPPTs charging the battery directly so total AC capacity from them and the battery long term is something like 8.8kw. That works nicely most of the time but if I have the car on charge at 7kw then I would only have 1.8kw of headroom, less if things are hot. If you want to run an AC load or two above that then you are drawing from the grid. I also have about 4kw of AC coupled capacity. That means the total AC capacity when the sun is out rises to 12.8kw or so which is enough for us.
I have spent more time thinking about this and one reason why choose Fronius over MPPT is to increase overall system peak throughout in case of strong sun. Multiplus 2 3000 in my case allows inverting of only 10A, and if sun provides more and batteries are full there is no way for additional PV power to go anywhere, neither to ev charger, nor back to grid. Adding Fronius allows keeping the Multiplus 2 sized down, and utilizing additional PV directly without need of inverting.
Probably obvious to anybody reading it now, but it took me some time to realize that Multiplus inverting power is the bottleneck if only using MPPT.
