question

botje avatar image
botje asked

strings and panels on an mppt RS 450/200 with Fronius symo on output

Hello,

My installation:

- 3x MPII 5kVA,
- 30 or 45kWh of EVE batterys in 2 or 3 Seplos BMS enclosures,
- 1x MPPT RS 450/200,
- 1x Fronius Symo 8kW inverter,
- 48 Longi 415Wp = 19,68kWp


The fronius would be placed after the Multiplusses in series.


Via mppt calculator i did different scenario's:

- 4 * 10 panels on the 450/200 and 8 panels in one string on the fronius;

1667596909024.png

1667596937969.png

with the excel in the 450/100:

1667596956317.png


Here i have 40 panels of 415 Wp = 16,6kWp on one 450/200 with a power of 11kW. Could someone explain to me if the ratio between kWp and kW has to be respected, or if the "90% to small" can ben believed?


Furthermore the excel gives me the notification that : "charge power is reduced to max MPPT tracker power".


With the other panels (only 8) i have 3,32kWp on my inverter. This is kinda small i guess, knowing that the charging current of the 450/200 is being limited on 200A?




- 4 * 9 panels on the 450/200 and 12 panels in one string on the fronius;


1667597138628.png


1667597224123.png

Here i have 36 panels of 415 Wp = 14,94kWp on one 450/200 with a power of 11kW. Here i am at 82%.


The excel doesn't give me the errorcode: "charge power is reduced to max MPPT tracker power".


With the other panels (12) i have 4,98kWp on my inverter. This is kinda small i guess, knowing that also here the charging current of the 450/200 is being limited on 200A?


- 4 * 8 panels on the 450/200 and 16 panels in one string on the fronius.



1667597373875.png


1667597381525.png

Here i have 32 panels of 415 Wp = 13,28kWp on one 450/200 with a power of 11kW. Here i am at 72%.


With the other panels (16) i have 6,64kWp on my inverter. This is kinda small i guess, knowing that also here the charging current of the 450/200 is being limited on 200A?


- 3 * 10 panels on the 450/200 and 18 panels in one/two strings on the fronius.


1667597555700.png



Here i have 30 panels of 415 Wp = 12,45kWp on one 450/200 with a power of 11kW. Here i am at 90%.

Furthermore the excel gives me the notification that : "charge power is reduced to max MPPT tracker power".


With the other panels (18) i have 7,47kWp on my inverter.


Is it better to use lesser strings (3 in stead of 4) and more panels per string, or use all the strings and put lesser panels per string?


Alle the panels would be put on the south side of a building with all the same amount of sunlight in 4 rows of 12 panels.




Multiplus-IIMPPT ControllersFronius
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3 Answers
wkirby avatar image
wkirby answered ·

You could put 28 (11.62kW) on the MPPT RS (4 strings of 7 panels).
Then 20 (8.3kW) on the Symo (2 strings of 10 panels). Symo will make 8.2kW max and your inverter power is 15kVA, so it will be happy.

In both of those configurations you are only slightly over the MPPT RS PV capacity and slightly over the Symo PV capacity. It will be rare to reach the maximum power levels in reality.

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botje avatar image
botje answered ·

Thank you very much for the quick awnser.

Some additional questions:


- Why does the mppt calculator tell me that one tracker is only used for 90% when i use 10 panels? see image above.

- The power of the mppt is placed in (11)kW. The power of the panel is expressed in kWp. Those two aren't the same, but yet you do use them like that in your assesment. I don't want to be rude, i only wish to understand...

- In your post you tell me to use 4 mppt's with 7 panels each. Is that a better solution than using 3 mppt's with 10 panels? The difference in used panels is minimal, but i guess there is a difference. What is better and why?

- Will the power of the fronius be used to charge the batteries when there not full? (again, he will be placed after the 3 multi's.

- The reason i use a fronius, is to for one use the ac power directly when there is some sun and otherwise to inject the power to the net when the batteries are full with more effeciency. May i presume those are two valid reasons to do this?

Thank you very much :)

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wkirby avatar image wkirby ♦♦ commented ·

Peak power would rarely be reached.

From you list of panels and equipment I thought is best spread the panels as evenly as possible over the MPPT and Symo capabilities.
There are four trackers on the MPPT, why would you only use three? Maybe you have future plans to add another string to the fourth tracker at some point?

With PV Inverter support enabled, the power of the Symo will be used to power loads on AC-OUT side first, then extra power will be used to charge the battery and then out of AC-IN to the network if the battery is full.

You installation is very similar to this one:
https://www.victronenergy.com/blog/2021/03/02/victron-training-in-a-cabinet-ess-off-grid-3phase/

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botje avatar image
botje answered ·

Indeed, for future plans of perhaps placing some panels on the other side of the building ... So therefore the questions, what is better... 3*10 panels or 4*7 panels. Or is it just the same?


Some fuse questions:


- The panels in a mppt (dc/dc) need to be fused. But what is the rating of the fuse/DC circuit breaker to be used?

Isc = 14,94A. So a circuit breaker rated for DC use or a fuse of 16A?


- How does one determine the rating of the (mega)fuse from mppt to +busbar?

Is it the max value of the mppt + some spare. So for the 450/200: 200A, so 250A?


- How does one determine the rating of the (mega)fuse from busbar to one battery?

The batteries are 280Ah EVE cells that will be discharged at max 0,5C. So 140A. So 200A?


- How does one determine the rating of the (mega) fuse from busbar to multiplus 5kVA? I found this in the multiplus manual

1667678614940.png

So per multiplus a (mega)fuse of 200A for a max current of 70A? Why not 125A? Has it some to do with inrush currents?


- How does one determine the rating of the circuit breaker at the AC IN of the multi's? In the manual i found this:

1667679020004.png

but from a very respected builder of victron systems I found this. Those are 3kVA Multiplusses, but it's still half the rating described linecurrent ...

1667679091865.png




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