Going off-topic here. But what would be nice is better integration of micro inverters. I have a small roof. There are a few extra positions spots where I could put one or two panels. Doesn’t really make sense for a separate MPPT or AC string inverter but a microinverter would fit the bill. Those Enphase units look cool.
Presumably the Fronius and the MPPTs are similar in that respect. The Fronius has a limit on its output current, but you can oversize the PV array to more consistently be able to hit that maximum output current. The MPPTs are the same. They have an output current limit, and (to some degree) will allow a lot of oversizing on its input current.
Both my 150/45 MPPTs are oversized by about 25% on a nominal system voltage of 48V.
Haha! I have ample West facing roof and very limited North. So I similarly matched my pool pump with the PV and heating to the afternoon. Though this time of year I have so much spare power, I run it from around 9am to 6pm. Have a 750W-ish pump.
You learn something every day.
I knew the Fronius is a grid tied inverter, I knew Fronius and Victron play along very nicely, but it’s nice to get a totally fresh perspective on yet another way to skin a cat.
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Yup. All that is really needed is for those providers to make some kind of Sunspec implementation on their “data manager” or whatever hub solution they use to tie it all together. It is quite common to also have a cell phone app for residential solutions.
They’d only have to implement models 1 (mandatory, common information), 101 (single phase inverter) and 120 (nameplate ratings). And then maybe, if they wanted to, model 123 (immediate controls), if you want to control power levels.
I’d think implementing a power limiter on micro-inverters could be an interesting project. You have to share the limit between a large number of units, in such a way that those with better sun get more of it, and so forth.
(I happen to know a way of doing that rather efficiently, but I’m not sharing 
).
So @Sarel.Wagner with this setup of yours, can the Fronius and Quattro still work in parallel to provide for peak loads of say 10, 12 kW should it be needed?
The Quattro will of course use battery power and the Fronius PV, but is it possible?
You can use the combined power in an off-grid scenario, but it could be very unstable.
As large loads switch on & off, things scale up & down, but if the change is big or fast enough you see a wobble on the AC side. If that wobble is big or unstable enough, the grid-tie may trip and takes about a minute to resynchronise before providing power. During that time the Quattro has to carry everything. Even if it doesn’t trip, you’re really straining it, so the grid-tie might never synchronise until loads reduce.
Not an ideal situation. The Fronius has a “Micro-grid” mode which makes it more forgiving of AC wobbles so you have some headroom, but fundamentally that setup does not like running in the red.
Also, during a cloudy day, the Fronius might provide very little usable power, leaving you with only the Quattro in any case.
Edit: On-grid, this actually allows you to get away quite cheaply: you can run a larger Fronius (for money-saving in the day) with a small Multiplus + battery (for load-shedding only), but only with the Fronius on the INPUT of the MP. During LS your Fronius switches off and only your essentials run.
The compromise of losing solar during load-shedding is obvious, but it could be a lot cheaper if your essentials are truly only essentials.
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As per mariusm, in a Micro grid, theoretically yes, practically, don’t go there. For my system and Utility grid switched off, I try to never exceed 6500W for the AC1 output. This allows the Fronius to deliver all it can, up to 8200W and use that to carry my load and charge batteries at the same time.
The Fronius is quite a sensitive beast, mine is set to MG, still it dislikes large load swings. In Utility grid switched on mode, the grid stiffness takes care of absorbing the large swings and the Fronius is perfectly happy and stable.
In the inadvertent case where the system exceeds the 8kVA on the Fronius, normally the Quattro does make up the difference, but I stay well away from that.
Groetnis
Indeed. You set the Country Code to MG50 (Microgrid, 50Hz). You must do this anyway if you put the Fronius on the output of the Multi, or it will never be completely stable.
That’s what I’ve done. The PV-inverter is on the input side, and represents about a third of my total capacity. If I lose that during load-shedding, that’s fine. The big loads that I need the extra capacity for already cannot run from the small 3kVa inverter
Your next step would obviously be to get a huge generator to provide the 50Hz signal to your home’s grid and have the necessary inertia to absorb large load swings until the Fronius and pick it up again. 
More seriously, I have no technical background in these things, but I do think there should be a market for adding flywheel based storage to micro grids to provide some inertia to the system. Would it not help with taking the instantaneous load off these inverters?
Yes, flywheels would help a lot, but once you start doing the calculations of how big a mass you need and the angular speeds involved, it becomes less viable. The tolerances involved is the biggest hurdle - that makes the cost go up way beyond lithium storage.
Always wondered how long that wheel can spin and store power. Have not tried it but my old cycling wheel does not spin very long before it stops.
Are these flywheels inside a vacume?
Yes they are, so there is problem nr1. Secondly they run on magnetic, non contact bearings, expensive all by emselves….
Groetnis
Not a flywheel, a synchronous condenser.
It is just a synchronous machine that is run up to speed and worked neither as a motor nor a generator.
It requires an induction motor to run it up to as close to synchronous speed as possible, thereafter the DC is applied and the machine locks into synchronous speed (with no slip like an induction motor has).
This can supply sufficient spinning mechanical inertia at 50Hz to absorb system bumps.
There are constant losses that would have to be supplied by the existing generation.
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What is the problem we are trying to solve? Will the solution cost less than an extra multiplus?
A Multiplus won’t solve the “wobble” problem, it takes to long to respond. Putting more in parallel won’t change that unfortunately. Not even a larger one. Surviving the wobble != mitigating it.
The synchronous condenser provides the robustness by being difficult to shift around, even by inconsiderate neighbours on the AC line.
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I had a dream like this once… Spinning yellow lights, first glowing fast, then slower. Woke up with quite the headache.
Might’ve been a ES tut…
Is that really a problem in a microgrid?
I know in Scotland they have a project that uses a flywheel to guard against “wobbles” as you say. It is not used as energy storage, but simply to prevent sudden changes in frequency.
The effect of wobble can be referred to as the consequence of a weak grid, the lack of wobble is in turn the consequence of a stiff grid. Grid strength is directly linked to the impedance of the lines. If impedance is high the strength is weak. If impedance is low the strength is strong. In a typical small Micro grid, the line impedance are not really considered, and the Micro grid could be very weak.
Impedance of the line is inversely proportional to the short circuit current capability at the connecting point. If short circuit current is high strength is high. If short circuit current is low strength is weak and in a typical Micro grid, short circuit currents are very low.
Groetnis
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I think that Scots thing will just be a flywheel attached to a synchronous condenser to keep it at a constant speed. The more weight the more mechanical inertia and therefore the more stability.
I don’t see a micro-grid in the context it is being discussed here, as the combination of a hybrid a PV inverter and a battery. But rather the combination of such a system with another similar but independent system. Sort of like, you and everyone else on the streets solar generation connected at the AC side. ( without an ESKOM connection).
In this instance, I don’t see how a micro-grid could be formed without adding mechanical inertia. (This is of course what ESKOM supplies by virtue of their stable frequency).
Micro-grids can also be quite large, powering a hundred or so houses. See the Victron Blog.
Such a system would keep that grid more stable.
You’re not supposed to be able to shift a “grid” with anything you do on the consumer side. Worst case you trip a breaker or blow up a transformer, but the grid itself remains unmoved. Otherwise it’s a weak grid.
Massive mechanical things moving help to keep a grid stable. Electronic things typically do not, unless they’re massive in cost.
So @Phil.g00’s machine would help the micro-grids’ stability, if they think it’s worth the cost and losses, and they have a big enough problem. But those things provide stability, not power. They consume power to provide stability.
On an individual level, maybe you could have something to absorb the wobbles between your Quattro and your Fronius, to prevent the Fronius from tripping, but it’ll be a lot cheaper to simply get another Quattro. That won’t stabilise the wobbles, but would probably allow you to ride it out. Like I said earlier, surviving the wobble is not the same as mitigating it.