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Victron Multiplus II 5000 with Fronius Primo


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Hi All,

Looking to install a Victron system, and trying to understand the impact of connecting a Fronius Primo unit on the AC Output. If the Victron is Mutiplus II 48/5000 can a Fronius 5.0-1 e installed and still honour the 1:1 rule?

What is the total output expected of this system? Is is only the max of the Multiplus? The max of the Fronius? Or the sum of Multilplus and Fronius?

Thanks,
Grant

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2 hours ago, Aminal said:

can a Fronius 5.0-1 e installed and still honour the 1:1 rule?

I believe so, yes.

2 hours ago, Aminal said:

What is the total output expected of this system?

In theory it is the sum of the two... BUT... a PV inverter trips quite easily if the voltage isn't perfectly stable or the frequency drifts too fast, all things that are likely to happen at high power levels when everything is working hard. For that reason, it is best to size the Multi for the full load. So in practice, the max is the rating of the Multi.

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3 hours ago, Aminal said:

Fronius Primo unit on the AC Output.

35 minutes ago, plonkster said:

So in practice, the max is the rating of the Multi.

Question?

If Grid is available and load is 4000Watt, the Fronius will try and produce the 4000 watt, so limit is the Fronius because the multi can pass through. 

If grid is not available the limit is the Multi... and whatever the Fronius cant supply the Multi will take from the Batteries... 

35 minutes ago, plonkster said:

In theory it is the sum of the two... BUT

Can it ever be?

Edit: Wait it can... or close to it at least... 

If the load is 4000 Watt and the Fronius can supply 4000 watt, the Multi has no load, so if you switch on another load it will draw from the batteries, and if you keep on  adding it will reach its own limit. (In Theory) 

Would love to test this...

 

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12 hours ago, Jaco de Jongh said:

If Grid is available and load is 4000Watt, the Fronius will try and produce the 4000 watt, so limit is the Fronius because the multi can pass through. 

If the grid is available, then the backfeed relay on the Multi is closed and the PV-inverter is practically connected to the grid, and there is no size limitation. The size limitation is there for situations where the grid is disconnected (which of course can happen at any time, hence the 1:1 rule).

The Multi is a low-frequency inverter, so in this setup where the PV-inverter is pushing power backwards, the Multi is essentially a battery charger with a transformer and a synchronous rectifier (not completely, but kinda). What follows then, in an off-grid (or grid-outage) situation, is that the PV-inverter is connected directly to your battery bank via a transformer. It then follows that this transformer should be sized to handle the power produced  by the PV-inverter... and voila... 1:1 rule.

But when the grid is there, then whatever cannot be handled by the transformer can be pushed into the grid, BUT as I already said, the grid may fail at any time.

Let me expand a little bit about my "sum of the two" comment. That is in theory, and it assumes some things. But let me list some points (this assumes AC-coupled PV running off-grid):

1. The PV-inverter is continually checking the quality of the waveform it is tied to, and will disconnect if this becomes unsatisfactory.

2. The higher the power level that the Multi works at, the more the quality of the waveform suffers, especially if the power demand is not constant.

3. Ergo, the harder the Multi works, the higher the likelihood that the PV-inverter might disconnect.

4. Fronius inverters in such a setup are configured with country code MG50 (microgrid 50Hz), which gives it wider margins and make it less likely to disconnect, but it will still disconnect at times.

5. So imagine a 5kva Fronius coupled to a 5kVA Multi. In theory, if you are running 5kVA of loads, then the PV-inverter is supplying 100% of the load and the Multi is just humming along. But this assumes your PV is perfectly stable. In practice it is not, it fluctuates a bit, the MPPTs are hunting a bit, so the Multi is also supplying a bit of load, then charging the battery a bit, back and forth ad infinitum.

6. Imagine the load pushes up to 5500W. So now the PV-inverter is making 5kw (or maybe a bit less) and the Multi is bringing in the remainder <1kw... but again, this only works if the load and the sunshine are more or less stable. There are control loops everywhere keeping this stuff going. If a cloud passes past the sun right now the load on the Multi increases, the voltage sags for a few milliseconds (as we've seen in the past). So maybe this 5500W is sustainable as long as the PV-inverter does not disconnect, but...

7. Imagine the PV-inverter does disconnect. It will now take 60 seconds for it to reconnect. The Multi will probably be able to carry 5500W of load for 60 seconds, but remember that under this load it is not making a very clean waveform... the PV-inverter might take longer than 60 seconds to finally sync with this waveform.

8. So the question becomes, at what kind of load will things be stable... should I really exceed the 5kw limit in this example if I want it to be stable...

9. Which leads me to this conclusion: That yes, the combination may be able to handle loads somewhere between the capacity of the Multi and the combined capacity of the two... BUT... please don't design around it, because you will be disappointed in the result.

Edited by plonkster
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Thanks for the rapid feedback. 

I'm not sure if my thinking is right, but this setup seems to be a better option than just the Multiplus II with two x 250/70 MPPT's for not much of a price difference?

19 hours ago, Jaco de Jongh said:

Would love to test this...

@Jaco de Jongh Might be a very expensive test? :)  If I pull the trigger on this, happy for you to guide me through the test (assuming it won't be a test to destruction?)

Is the Dyness Powerbox 9.6kWh a good option for the above? i.e. Do they play nicely together?

 

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@plonksterThanks for the detailed explanation. This kind of info has been difficult to find, even on Fronius and Victron's sites/forums. 

The options for single-phase above 5kW seem limited, hence the need to look at some kind of stacking. I have recently done renovations and split my DB to carry essential (light and some plugs), non-essential (oven, washing machine and tumble dryer and kitchen plugs) and the geyser (a small 150lt as the balance of my water is on gas geysers).

The intention is to convert the geyser and remove the 3kW draw.

I run an Efergy Engage and seem to peak at about 8kW if the geyser is removed. This is day-time only. Night time usage is low as I only run low power led's.

Looking for a versatile fit. Victron seems to give the most options for scaleability?

 

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