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larrytaylor asked

String Size

Hello

This is my 1st post so I hope I abide by all the rules and ask sensible questions! Please don't hesitate to tell me if not....

We are doing a roof conversion at our house in SW England and I have designed a solar system around the impressive Victron product range.

My installer has asked me to confirm the choice of string size I have specified. So I would appreciate any feedback or suggestions from the Victron Community.

Brief description: There are two arrays, one on the main roof (35deg elevation, SSE facing) and one on the garage flat roof (35deg elevation mountings SSW facing). I am using JA Solar JAM60S17-325/MR panels on the main roof and -330/MR on the garage.

The main roof has 2 rows of 6 GSE Integration trays with two removed for integration of Velux windows. This gives 10 panels on the main roof. I did my research and balanced current vs voltage vs cable and specified 4mm2 Class 5 EN 60228 cable with two strings of 4 and 1 string of 2 panels. The strings connect to BlueSolar MPPT charge controllers - 150/35 for the two 4-panel strings and 100/20 for the 2 panel string. Both controllers support a 48V battery. The charge controllers connect to a Lynx Power-In module connected to a Lynx Dustribution module feeding the battery - 48V Pylontech US5000 - and the Victron Multiplus II 5000/70 GX via 100A Mega Fuses. There are 15A fuse lines for the PV string cables, in-line blocking diodes on the +ve lines and DC isolation switches for each string. The current is nominal 15A and the voltage nominal 150V for the 4 panel strings.

The garage array is 2 strings of 4 panels again connecting to a BlueSolar MPPT 150/35 Charge Controller.

My installer asks why not use two 5 panel strings on the main roof. My answer was that I would need to go to 250V charge controllers which are quite a lot more expensive. and I have some flexibility for reconfiguration with the similarity in the main and garage array controllers.

My reasoning for the choice of strings is to have redundancy in the system and some flexibility for future expansion e.g. using a separate charge controller for each string.

Any comments/advice/suggestions would be most welcome.

Thanks in advance,
Larry

MPPT Controllers
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2 Answers
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wkirby answered ·

The biggest problem here is that the open circuit Voltage of a series string of 4x JAM60S17-325/MR modules exceeeds the 150V limit of a 150/35. The Voc of a JAM60S17-325/MR is 40.83V @ 25ºC. At lower temperatures, this Voc will increase.

So, it will be better to purchase a single MPPT 250/70 for two strings of five.
For the garage, two strings of four with a MPPT 250/60.

Alternatively, a MPPT RS 450/100. This has two trackers, so one string of 10 onto one input and a string of 8 into the other input. You'll max out this contriller at about 5kW with a battery Voltage of 50V, but in the UK, it's pretty rare to see the full output of your array anyway. If it is sunny then it's too hot and the PV output is reduced due to the temperature of the PV modules.
This option is not for you if you value resilience as you mentioned.

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larrytaylor answered ·

Hi wkirby. Very useful reply thank you indeed.

Larry

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