Victron’s MPPT calculator results in Voc of 254V at -20°C.
The datasheet of the 250/*-series says:
Maximum PV open circuit voltage
250 V absolute maximum coldest conditions
245 V start-up and operating maximum
May or should I connect this PV-string to a 250V-MPPT or is the Voc-limit really that strict?
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This is the long story…
my current PV-setup is:
solar panels with these parameters (STC-values, standard test conditions (Irradiance 1000 W/m , Cell temperature 25°C and air mass of AM1.5):
Voc = 38.67V
Temp. coeff. of Voc = -0.275 %/°C
51,2V batteries
2x MPPTs 150/45
from each MPPT, 2x (+ and -) 10m connection 6 mm² to the roof.
There y-connection to 2x 3 panels.
Vmppt was never >105V (within 4 years). But when disconnecting modules and MPPT, Voc is about 20% higher than Vmppt.
On the wires from the MPPT to the roof, voltage-drop is relatively high, about 7% e.g. of 2200W = 150W respectively 300W over both MPPTs. Thus, the temperature within the hermetic closed overvoltage-protection-box can get quite high.
I think about switching from 2x mppt 150/45 à 2x 3 panels to 1x mppt 250/100 à 1x 6 panels..
With this change, voltage-drop would decrease to 1,6%, from 300W to 70W.
But the problem is, that Voc can slightly be above 250V.
Victron MPPT voltage input limits are hard limits that must not be exceeded under any conditions, else risk potential component failure that would not be warrantable.
Better to re-design the array to be certain that the array will fall within the published limits of the device.
As a general rule, I wouldn’t use a 250 V regulator in the first place, let alone push it to its limit… But something doesn’t add up with the figures you’ve provided. A 10 m length of 6 mm² cable has the same resistance as a 1 mm² cable that is 3.3 m long. That’s 0.018 × 3.3 = 0.06 Ω. Even at a current of 25 A, that would result in a voltage drop of only 1.485 V, equivalent to 37 W. In that case, troubleshooting is probably the first step. The Y-splitters are the first suspects. But there have also been reports of mysterious cables…
I made a typing mistake. The correct version is:
I think about switching from 2x MPPT 150/45 à 2x 3 panels to 1x MPPT 250/100 à 2x 6 panels (running over 2 cable pairs from the MPPT to the roof)
You’re right, I had entered the data incorrectly in the Victron Toolkit app: under normal operating conditions, my two MPPTs, each with 3x 2 modules, currently have 100V × 22A at full load (2,200W per MPPT) over a 20m cable length (up and down) — a 3.1% voltage loss. With two MPPTs and a total of 4,400W, 135W is lost as a result.
If I now run 6 modules through 6 mm² cable, the voltage doubles and the current is halved.
Power loss = I^2 \* R => halving the current reduces the power loss to 1/4.
During normal operation, 6 panels would run with 200V Vmppt, far away from 250V-limit.
The “only” problem is switching off the panels during a sunny very cold day (-20°C). Because then Voc would increase to 254V.
When reconnecting the MPPT to the panels, the voltage would slightly exceed the limit for a short time, until MPPT start working respectively dropping the voltage to operational 200V.
Why do you want to switch from 2x MPPT 150/45 to 1x MPPT 250/100 in the first place?
from 2x MPPT 150/45 to 1x MPPT 250/100
I would certainly consider the following points regarding the 254 V at -20°C issue (personally, not from a “Victron perspective”):
Voltage drop from the roof? Would Voc ever actually reach the MPPT?
-20°C is already very pessimistic. Is that even realistic for your location? The panels are also “attached to the house”, which radiates heat. So if it’s -20°C outside, the panels won’t necessarily be at -20°C yet.
But regardless of that—if the only reason is to reduce the number of MPPTs (?), you could also switch from 2x MPPT 150/45 to 1x MPPT 150/100. I think your 2x3 string should be fine in terms of current, right?
If you naturally want to switch to a 6S string because of the “losses”, then that doesn’t help, of course—but in that case I would add the following: Have you ever worked out how long you would have to operate those few watts of “lower losses” before replacing two intact MPPTs became worthwhile?
