Valet - worse-case scenario for max PV string voltage is combined Voc plus a 1.15 multiplier (add 15%) to take into account Voc temperature coefficient impact at low temperatures (reduced temperature of modules gives an increase in voltage output).
yeah well, I like to er on the side of caution, particularly when giving interweb advice to someone in Aus.
I've no idea what temps they get there, and IMO 15% is a bit tight (we've installed a fair bit in Inverness and known it to be -20 outside and bright sunlight, which would give +14.85% Voc on most mono panels, which adding in the panel tolerances could well take it into over voltage territory if right at the limits - I assume the 15% rule of thumb was determined by southerners)
get yourself some red and blue tape*, mark up the leads at both ends throughout the circuit, solve the problem at source instead of risking it all on the reverse polarity protection... though I'd be amazed if the fronius didn't have it (not used em myself)
*I prefer to differentiate between DC and AC colour coding
Thank you for your advice Gavin.
Ill try tapes. In Aus temp where we are temp always 28+ so a bit hot and panels get hot as well. So we went for Panasonic 250 HIT. At one point Fonius 30 had reading 3499w on 2.5 kw system so do you think it was possible or it was inverter glitch. Or inverter stopped to overproduce which may cause clipping.
Many electronic transformerless inverters clip the power automatically when it reaches a certain level. But this one has a transformer so potentially should be no limit?
Optimum output on this inverter as per manual is 3000w. I did check on Fronius manual and it has polarity protection. So in my case dos not mater if polarity was swapped? As long as not swapped on panels?
What is the best way to test panels before installation on the roof. Iam planning to use multimeter with test cables for MC3 connectors on the ends. Any suggestions, precautions I should take? On Panasonic 250w
You can test the panels indiviually using an inexpensive meter so lang as it can take 10 amps and 50 V DC.
You do two tests
1) Voc = Volts OPen Circuit, i.e. just measure the voltage across the panel - depending upon the light conditions and so long as you line the panel up to the sun, you should get a reading similar to that on the Panel Specification Sheet.
2) Isc = Current Short Circuit, and yes we do mean short circuit make sure you put it thoruigh the 10 Amp shunt on the multimeter and join the ends together. ! DON't Us this to do the string test though, you'll blow it up! The current is HIGHLY dependant upon the light/sun's output though so don't worry if this is way off, a bright sunny day with the panel at the correct angle is the only time you'll get the Isc quoted on the Spec sheet and you'll need an Irradiance meter to check it anyway.
You may find it worthwhile to get a push switch rated at 10 Amps and put it in the circuit that way you wont be pulling apart a 'live circuit'
When you've installed the string you should also do a Voc and Isc on the whole string - unless you've got a DC clamp meter don't do the Isc test, (7 or 8 amps at 300 or so volts..) also make sure your voltmeter is rated to 10 x Voc - and that is the reading you should be looking for. Be VERY careful with the DC cables after the panels are connected in a string you are working with very high DC voltages. You can get big sparks and bangs from shorted DC cables - just remember you can melt a screwdriver across a car battery and that's only 12V!!
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